soilless culture || chemical characteristics of soilless media

6

Chemical Characteristics

of Soilless Media

Avner Silber

6.1 Charge Characteristics

6.2 Specific Adsorption and Interactions betweenCations/Anions and Substrate Solids

6.3 Plant-induced Changes in the Rhizosphere

6.4 Nutrient Release from Inorganic and OrganicSubstrates

References

ABBREVIATIONS

AEC – anion exchange capacity;CEC – cation exchange capacity;HA – [Ca5(PO4)3OH];OCP – [Ca4H(PO4)32.5(H2O)];PW – solution P concentrations;PZNC – point of zero net charge;PZSE – point of zero salt effect;SAI – specifically adsorbed ion;�TCP – [Ca3(PO4)2];ZPT – suspension pH prior to the addition of protons or hydroxyls.

209

210 Chapter 6 Chemical Characteristics of Soilless Media

The surfaces of particles and other solids in horticultural substrates bear permanentand/or variable electrical charges. The surface charge properties and the dissolutioncharacteristics of the substrate solids are important, since they affect the ionic com-position of nutrient solutions. Ordinarily, the chemical properties of substrates areestablished prior to their use for growing plants; therefore undisrupted, homogeneoussolid phases are addressed. However, throughout the growth period organic compoundsexcreted from plant roots or resulting from decomposition processes accumulate in thesubstrate (Tate and Theng, 1980; Huang and Violante, 1986; Tan, 1986; Silber andRaviv, 1996). Thus, the surfaces of the newly formed solids become heterogeneous andthe chemical properties of the mixture may be significantly different from those of thesingle, well-defined component that was characterized before use (Silber and Raviv,1996). Plants growing in soilless culture typically have smaller root-system volumethan plants growing in soil culture. Therefore, the root density of soilless-grown plantsis higher so that such plants have a greater impact on the rhizosphere than soil-grownplants. Despite extensive research relating to irrigation and fertilization managementor plant growth in soilless culture, the literature regarding the chemical properties ofsoilless media is scarce. Although peat, perlite, stone wool and mixtures of organicwith inorganic materials are abundant in the horticulture industry, the literature regard-ing the chemical properties of these media is insufficient. Tuff is a volcanic materialused as a substrate for horticultural crops in Italy, Spain, France, Turkey and Israel,and had been the subject of considerable research with respect to chemical propertiesas a horticultural substrate. Thus tuff is used in this chapter as a model for illustrationand elucidation of the chemical processes taking place in soilless media systems.

6.1 CHARGE CHARACTERISTICS

The surfaces of solids in horticultural substrates of volcanic or organic origincarry permanent and/or variable positive and negative electrical charges. Permanentnegative charge results from isomorphous substitutions (Gast, 1977; McBride, 1989,2000; Sumner and Miller, 1996; Sposito, 1989, 2000), that is substitution of structuralcation by lower valency cation that has the same coordination number and size inlayer-silicates (common examples: Si4+ by Al3+ or Al3+ by Mg2+). The extent ofcation adsorption to the surfaces is referred to as the cation exchange capacity (CEC;cmol kg−1) and is used to characterize the cation exchange properties of the medium(Gast, 1977; Sposito, 1989).

The CEC of several inorganic and organic horticultural substrates are presented inTable 6.1. These values represent CEC measurement of intact materials prior to use,and it is important to note that plant growth may affect the chemical properties of thesubstrate solids. CEC values of soil components are usually referred by weight (i.e.,cmol kg−1). However, soilless production system involve a relative small fixed rootzone volume and therefore it is more practical to relate substrates CEC to volumeunit (cmol L−1). By using volume-based comparison it is possible to properly compareboth ‘heavy’ and ‘light’ media.

6.1 Charge Characteristics 211

TABLE 6.1 Cation Exchange Capacity (CEC) of Several Horticultural Substrates

CEC

Substrate cmol kg−1 cmol L−1 Reference

Inorganic Substrates

Perlite 25–35 2–4 Dogan and Alkan (2004)Stone wool 34 5 Argo and Biernbaum (1997)Tuff 10–60a 10–60 Silber et al. (1994)Clinoptilolite (zeolite) 200–400 400–800 Mumpton (1999)

Organic Substrates

Coconut coir 39–60 2–4 Evans et al. (1996)Peat 90–140 7–11 Puustjarvi and Robertson (1975)Pine bark 98b 10 Daniels and Wright (1988)Compost 160–180 15–20 Inbar (1989)

a CEC in pH 7, tuff has variable charge surfaces as shown in Fig. 6.2.b CEC in pH 7, pine bark has variable charge surfaces as shown in Fig. 6.4B.

The variable charge (both negative and positive, depending on the solution pH)is generated mainly from the adsorption of H+ and OH− on solid surfaces suchas metal oxides, hydroxides, microcrystalline silicates (allophane and imogolite), oron functional organic groups (Stevenson, 1994). The effects of solution pH on themagnitude of the surface charge may be determined experimentally by measuring theCEC and the anion exchange capacity (AEC) over a range of pH values. The pH atwhich the AEC is equal to the CEC is referred to as the point of zero net charge(PZNC) (Parker et al., 1979; Sposito, 1981, 1984, 2000) and is frequently used tocharacterize the charge properties of the medium. Note that in all cases, pH is referredto the solutions. An alternative experimental method is to conduct a pH titration curveusing an indifferent electrolyte (Gast, 1977; McBride, 1989, 2000; Sparks et al., 1996).A typical potentiometric titration for woody peat is presented in Fig. 6.1 (Bloom,1979).

The pH value at which a series of titration curves for various electrolyte concentra-tions intersect is referred to as the point of zero salt effect (PZSE; Parker et al., 1979;Sposito, 1981, 1984, 2000). However, common substrates (compost, peat moss, tuff,stone wool) rarely consist of a single, well-defined component. They typically containa mixture that presents both permanent and variable surface charges. The interpreta-tion of the experimental CEC, AEC or pH titration data obtained from these mixturesis, therefore, complicated. Moreover, the existence of additional sources or sinks forH+/OH− derived from the dissolution/precipitation of minerals during the CEC, AECor pH titration analyses makes direct interpretation of analytical results even moreambiguous.


0.8

0.8

H+ A

dsor

bed

me /

gO

H–

Ads

orbe

d m

e /g

3 4

pH = 3.9

5pH

6

0.4

1.2

0.4

1.2

0

0.1N NaCl

0.01N NaCl

1N NaCl

FIGURE 6.1 Acid-base potentiometric titration curves of woody peat. Reprinted from Bloom (1979),with kind permission from the Soil Science Society of America Journal.

The difficulties involved in the determination of the actual surface charge of aheterogeneous material that contains both pH-dependent and permanently chargedsurfaces, together with primary and secondary minerals, can be seen in the case oftuff (Figs. 6.2 and 6.3). Yellow, Red and Black tuff, commonly used in greenhouseproduction, are characterized by the degree of weathering, with Yellow tuff beingmost weathered and Black tuff being the least weathered. The CECs of Yellow, Redand Black tuff were found to be pH dependent, with the CEC increasing by 6.6, 4.2and 1.9 cmol kg−1, respectively, for every unit increase of pH (Silber et al., 1994).

Yellow tuff

Red tuff

pH

CE

C (

cmol

kg–1

)

Black tuff

30

30

60

90

5 7 9 11

FIGURE 6.2 Cation exchange capacity versus pH for black, red and yellow tuffs from northern Israel.Reprinted from Silber et al. (1994), with kind permission from Elsevier Science.


90

50

NaCl

ZPT

10

–30

Aci

d/ba

se a

dded

(cm

ol k

g–1)

(A)

0.1 M

0.02 M

0.006 M

320

10

–10

0

7 9 115

pH

Net

cha

rge

(cm

ol k

g–1)

(B)

NaCl

0.1 M

0.02 M

0.006 M

FIGURE 6.3 Potentiometric titration curves of yellow tuff: (A) acid/base addition (0.1 N HCl andNaOH, respectively) as functions of pH and NaCl concentrations, tuff/solution ratio of 1/40, equilibriumperiod of 1 week. ZPT is the suspension pH prior to the addition of acid or base; and (B) net surface chargecalculated with Eq. (1). Based on Silber (1991).

The differing values of CEC slope among the three types of tuff reflect differencesin their contents of amorphous materials with variable-charged surfaces. Silber et al.(1994) found, however, that it was not possible to determine the AEC of these tuffsbecause phosphorus released from indigenous Ca–P minerals during their analysis wasre-adsorbed on the tuff surfaces and thus interfered with the AEC determination. TheCEC of tuff at pH 3.5, especially that of the weathered Yellow tuff, was relativelyhigh (35 cmol kg−1) and may indicate the presence of components having a permanentcharge. The pH titration curves of the tuff types differed significantly from thoseof homogenous variable-charged materials. In the cases of the less-weathered tuffs(Red tuff and Black tuff), the pH titration lines obtained with three different NaCl


concentrations overlapped (data not presented). Distinction of an ionic strength effectwas possible only for the Yellow tuff at a pH above 7 (Fig. 6.3A).

The overlap of potentiometric titration curves at low pHs in volcanic material fromChile was attributed to the exchange of added H+ ions with cations associated witha permanent negative charge (Espinoza et al., 1975; Gast, 1977). Wann and Uehara(1978) reported that specific adsorption of P caused overlapping of potentiometrictitration curves of soil; therefore, re-adsorption of P may be partially responsible for theoverlap of the pH titration curves in the case of tuff. Nevertheless, on the assumptionthat H+/OH− consumption occurs in cation/anion exchange, the net charge at each pHvalue can be evaluated by subtracting the net quantity of cations/anions accumulatedin the liquid solution of the substrate (�charge) from the quantity of acid/base addedat the pertinent pH. Hence, �charge (cmol kg−1) is defined as

�charge = (∑cat −∑ anion

)pH − (∑ cat −∑ anion

)ZPT (1)

where ZPT is the suspension pH prior to the addition of protons or hydroxyls(in Fig. 6.3a: 7.1, 7.2 and 7.6 in 0.1, 0.02 and 0.006 M, respectively) and

∑cat

and∑

anion include all the cationic and anionic species in the solution, accordingto speciation calculations. The differences between the quantities of acid/base addedto tuff suspensions, and the net surface charge calculated with Eq. (1) (Fig. 6.3aand 6.3b, respectively), demonstrate that the major quantities of acid/base added to tuffsolutions were exchanged with adsorbed cations/anions or consumed in the dissolutionof indigenous Ca–P minerals and of very fine amorphous particles (Silber et al., 1999).

The CEC of common organic substrates such as peat, pine bark or composts isgenerally high (80–160 cmol kg−1) and is pH dependent (Brown and Pokorny, 1975;Puustjarvi, 1977; Ogden et al., 1987; Daniels and Wright, 1988). The charge is derivedmainly from ionization of COOH groups and, to a lesser extent, from phenolic OH(Stevenson, 1994). The pH effect on CEC of organic material was found to be morepronounced than that of soil inorganic materials (Hallsworth and Wilkinson, 1958;Helling et al., 1964; Ogden et al., 1987; Daniels and Wright, 1988; Stevenson, 1994).The contribution of a unit pH increase to the CEC of soil organic material was foundto be 51 cmol kg−1 of organic C (Fig. 6.4A).

Assuming that organic C constituted 58 per cent of the organic matter (Stevenson,1994), then each unit increase in pH raised the CEC of soil organic matter by29.6 cmol kg−1. Cation exchange properties of pine bark growing media increasedlinearly between pH 4 and 7 and the contribution of a unit pH increase to the CECwas 23.1 cmol kg−1 for particles of size smaller than 0.05 mm (Fig. 6.4B). Note thatthe increase in measured CEC slope of the fine pine bark was close to the calculatedvalue of soil organic matter, and that these were an order of magnitude higher than theaforementioned slopes of 1.9, 4.2 and 6.5 cmol kg−1 per pH unit measured for Black,Red and Yellow tuff, respectively (Fig. 6.2).

The charge characteristics of composts were found to be dependent on compostingtime (Harada and Inoko, 1980; Inbar, 1989; Inbar et al., 1989, 1991; Iglesias-Jimenezand Perez-Garcia, 1992; Saharinen, 1996; Jokova et al., 1997), mainly because oftransformations of organic constituents such as C/N ratio, humic material and lignin


400

350

300

250

200

150

100

120

80

40

0

50

0

4 5 6 7

1 2

Yorg C = –59 + 51Xr = 0.985**

Yclay = 30 + 4.4Xr = 0.979**

2.38 – 6.35

0.05 – 1.19

< 0.05

3

pH

Size fraction (mm)

pH

(A)

(B)

CE

C, m

e./1

00 g

. Org

. C o

r C

lay

CE

C (

cmol

Kg–1

)

4 5 6 7 8

FIGURE 6.4 Effect of pH on the CEC of: (A) soil organic C (Yorg C) and clay (Yclay). Verticallines show standard errors of the individual values. From Helling et al. (1964), with kind permission fromthe Soil Science Society of America Journal; (B) several particle size fraction of pine bark growing media.Based on Daniels and Wright, (1988).

contents during the composting process (Inbar et al., 1989, 1991; Saharinen, 1996).The C/N ratio and the lignin concentration correlated with the composting time (Ravivet al., 1987; Tarre et al., 1987; Inbar, 1989; Inbar et al., 1989, 1991), and could be usedin estimating the CEC (Figs. 6.5A–6.5C) and maturity indices of compost quality.


160

120

80

400 20

Composting Time (days)

Carbon/Nitrogen ratio Lignin (% OM)

CEC = –119.0 e–0.0266X + 185.7

(B) (C)

(A)

Y = 364.6 e–0.1301X + 59.54

Y = –28.22 + 3.800X

r2 = 0.990**

r

2 = 0.976**

r

2 = 0.949**

CE

C(c

mol

kg–1

OM

)

CE

C(c

mol

kg–1

OM

)

40 60 80 100 120 140

200

200

175

150

125

100

75

506 8 10 12 14 16 18 20 22 24 26 25 30 35 40 45 50 55 60

FIGURE 6.5 Cation exchange capacity of compost during the course of the composting process asfunctions of: (A) composting time; (B) carbon/nitrogen ratio; and (C) lignin content. Based on Inbar (1989).

6.1.1 ADSORPTION OF NUTRITIONAL ELEMENTSTO EXCHANGE SITES

The affinity of cations for negatively charged surfaces under equal concentrationin the aqueous portion of the substrate is affected by ion characteristics such asvalence, size and hydration status. The affinity of divalent cations are higher thanthat of monovalent cations because of their greater charge, and usually, the relativeadsorption strength follows the order of hydration, that is, K≈NH4 >Na and Ca > Mg(Barber, 1995). However, concentrations of each of the cations in a typical irrigationsolution for greenhouses crops are not equivalent, with the K concentration commonlyexceeding the concentration of Ca, NH4 or Mg (Adams, 2002, Sonneveld, 2002). Asan example, recommendations for K, NH4, Ca and Mg concentrations in the nutrient

6.2 Specific Adsorption and Interactions 217

solution for soilless production of ornamental and vegetable crops in The Netherlands(Naaldwijk) consist of 5.0, 1.0, 4.5 and 2.5 mmol L−1, respectively (Sonneveld, 2002).Thus, it is expected that exchange reaction, that is displacement of cations from thesubstrate solids by K+, will reduce its concentration and subsequently will increasethe concentration of other cations in the liquids within the substrate. The exchangereaction may be horticulturally beneficial in cases where: (i) concentration of a toxicor detrimental cation such as heavy metals or Na+ in the liquid solution of the growingmedium decline as a result of adsorption or (ii) concentration of a beneficial cation inthat liquid increases as a result desorption. Alternatively, the opposite could occur, withexpected detrimental effects, of cation exchange including: (i) increased concentrationsof a toxic or detrimental cation in the solution followings desorption or (ii) decreasedconcentrations to a deficiency level of beneficial cation followings its adsorption tosubstrate solids.

6.2 SPECIFIC ADSORPTION AND INTERACTIONS BETWEEN

CATIONS/ANIONS AND SUBSTRATE SOLIDS

The solubility and the interactions of nutritional elements with the substrate solidsare associated with ion characteristics such as valence, size and hydration status asdetailed above. The solubility of NO3

−, the major N source for plants grown in soillessculture (Chap. 8), is very high, its affinity to positive charged surfaces is very lowcompared with that of H2PO4

− or SO42−, and therefore, the concentrations of NO3

− inthe substrate solution is nearly unaffected by precipitation and/or adsorption reactions.The effects of microbes on NO3

− availability in the rhizosphere are detailed elsewhere(Chap. 8). NH4

+ is also an N source for plants in soilless production and, similarto NO3

−, generally does not precipitate. Unlike NO3−, the high affinity of NH4

+ tonegatively charged surfaces affects its availability to plant.

Nitrification/denitrification reaction and the effects of NH+4 nutrition on rhizosphere

pH and the availability of other nutritional elements are discussed in Chap. 8. Potassiumis an important nutritional element and is also not involved in precipitation reactions toan appreciable extent. However, as a charged cation, K+ may be adsorbed by negativelycharged surfaces, and this may reduce its availability to plants. Note that the partitioningof K+ as well as other cations between the aqueous and the solid parts of the substratedepends on its relative concentration in the solution and composition of the exchangesites. The affinity of the divalent cations Ca2+ and Mg2+ to negatively charged surfacesis very high (Barber, 1995) and they are highly involved in precipitation reactions,especially with H2PO4

− and SO42−. Actually, it is well accepted that P availability

in the rhizosphere is governed by Ca2+ and Mg2+ activities (Lindsay, 1979). Metalmicronutrients such as Cu, Fe, Mn and Zn are considered as specifically absorbed ion(SAI), their affinity to charged surfaces is very high and considerably higher than othercations. These metals are also highly involved in precipitation reactions so that theiravailability to the plant is very low. Fertigation managements used in the horticulture


industry for guaranteeing the availability of nutritional elements for plants, includingthat of micronutrients, are discussed in Chap. 8.

Phosphorus and zinc were selected below for detailed discussion to illustrate theinteractions of cations and anions with the solid constituents of substrate because:(i) both are important nutritional elements for plants; (ii) under common agriculturalpractices, low availability in the vicinity of the roots of either of these elements oftenlimits crop growth and productivity, even if P and Zn concentrations in the irrigationsolutions are adequate (Marschner, 1995); (iii) ions of both elements are classified asSAIs because of their high affinity to charged surfaces; and (iv) both elements easilyform new solid compounds as a result of precipitation reactions.

6.2.1 PHOSPHORUS

Continuous decline of solution P concentration (PW) following fertilizer applicationis a widespread phenomenon, and temporary P deficiency often restricts crop produc-tivity. PW decreases through two mechanisms: (i) very fast (on a scale of secondsor minutes) electrostatic reactions of adsorption onto the substrate, as a result of thehigh affinity of P ions to charged surfaces; and (ii) slow (on a scale of hours or days)formation of new solid metal-P compounds (with Al, Fe and Mn under acidic pHs andwith Ca and Mg under neutral or basic pHs; Lindsay, 1979).

6.2.1.1 Effect of pH on P Partitioning between Substrate Solidsand Liquids in Tuff Suspensions

Solution pH plays the dominant role on P partitioning between substrate solidsand liquids as presented in Fig. 6.6A for Yellow tuff. Negative values for P retentionindicate that the quantities of soluble P in tuff suspensions were greater than theadded quantities, as a result of dissolution of indigenous hydroxyapatite during theanalysis procedure (Silber et al., 1994, 1999). Retention of P by Yellow tuff at acidicpHs could be fitted to a pH-specific Langmuir isotherm (Fig. 6.6A). The activityproducts of Al3+, Fe3+ and PO4

3− in acidic tuff suspensions (pH 4–7) were below theKsp of variscite (AlPO4.2H2O) and strengite (FePO4.2H2O), respectively, thereforeit is logical to deduce that adsorption reactions governed P solubility in acidic tuffsuspensions. This conclusion is in accord with the high content of amorphous materialsin the Yellow tuff (51.5 g per 100 g of Halloysite-like allophane) having variablecharged surfaces with high P-binding capacity (Silber et al., 1994). However, theformation of an X-ray-amorphous analogue of variscite or AlOHNaPO4 (Veith andSposito, 1977) is also possible. At higher pH values, the Langmuir rule of decliningslope with increased PW was not maintained: at pH 8.3 and 9.3 and at PW above0.46 and 0.31 mM, respectively, an increase in the slope of P-sorption isotherm wasobserved (see arrows in Fig. 6.6A). The increased slope indicates that a change inthe phase-controlling P partitioning took place: increasing P addition rates enhancedthe feasibility of Ca–P mineral precipitation because above pH 7, all the activityproduct of Ca2+ and PO4

3− in tuff solutions exceeded the Ksp of hydroxyapatite(HA; Ca5(PO4)3OH) and shifted towards the solubility line of �TCP (Ca3(PO4)2) or


8

5

2

–1

–9

–12

–15

–1810 12 14 16

0 1

5.19.3

4.2

8.37.3

7.3

8.3

9.3DCPD

OCPβ TCP

HAP

P concentration (mM)

(B)

(A)

P r

etai

ned

(cm

ol k

g–1)

log

H2P

O4– –

pH

log Ca2+ + 2 pH

2 3

FIGURE 6.6 (A) pH-dependent adsorption of phosphate onto yellow tuff (pH values are shownabove each isotherm), constant ionic strength (0.01 M NaCl), tuff/solution ratio of 1/40, 24 h of reaction.Arrows indicate experimental points above the Ksp of OCP; (B) experimental activities in tuff extracts atpH 7.3, 8.3 and 9.3 in relation to the theoretical solubility lines for calcium-phosphate minerals (HAP:Ca5(PO4)3OH, �TCP: Ca3(PO4)2, OCP: Ca4H(PO4)32.5(H2O) and DCPD: CaHPO4.2H2O). Based onSilber (1991).

OCP (Ca4H(PO4)32.5(H2O) (Fig. 6.6B). HA is the most thermodynamically stableCa–P mineral, but more soluble compounds such as �TCP or even OCP temporarilycontrolled P in tuff solutions. This conclusion is supported by the data of Imaset al. (1996) and Salinger et al. (1993), who found that on a time scale of a fewdays, P solubility in the Ca-CO3-PO4 system was controlled by a metastable Caphosphocarbonate phase. Adopting the conclusion of Sanyal and De Datta (1991) that�TCP governs P dissolution in neutral and basic soil solutions, adsorption reactions


may govern P solubility in tuff suspensions at pH 7.3, as long as PW does notexceed 0.9 mM at the pertinent Ca concentration (0.4 mM). At pH 8.3, the PW valuewas below the �TCP line, at 0.09 mM, whereas at pH 9.3 PW was above �TCPsolubility even at zero P addition (Fig. 6.6B). If the solution concentration of Caand Mg are assumed to be 2 mM, which is typically required for plant growth, thenthe calculated PW concentration at pH 7.3, according to �TCP solubility, will notexceed 2.5 �M; its actual value will depend on the CO2 and SO4 concentrations aswell. It is possible to assume, therefore, that adsorption reactions govern PW in tuffsubstrate at basic pHs for a short time (minutes or hours), but as time proceeds, PW isexpected to decline until the activity product of Ca2+, Mg+2 and PO4

3− attains the Kspof a thermodynamically stable compound. Thus, the reaction kinetics must be takeninto account when evaluating the relative importance of adsorption and precipitationreactions in tuff solutions.

6.2.1.2 Effect of solution ionic strength on P solubility

The solution ionic strength (I) may affect P solubility via three mechanisms:

1. Alteration of the ion activity coefficient: increase of I decreases the ion activitycoefficient, in turn increasing the possible concentration of P in the substratesolution.

2. Variation of charge density: the solution ionic strength affects the chargedensity in the diffuse double layer; therefore, the effect of I on P adsorptionwill be pH-dependent. Similarly to the effect of I on the surface chargecharacteristics presented in Figs. 6.1 and 6.3, below the PZSE of the sorbent,increase of I causes a reduction of anion adsorption, whereas above the PZSEthe opposite trend is expected, and increase of I causes increased anionadsorption (Sposito, 1984).

3. Variation in anion/cation concentrations in the substrate solution: increases ofI may affect the release of anions and cations from the solid phases, on whichthey might have been adsorbed or of which they might have been anindigenous part.

The effect of I on PW in Yellow tuff solutions can be used to illustrate thesemechanisms (Fig. 6.7). The activity products of Ca2+ and PO4

3− with no acid/baseaddition (ZPT) were above the HA solubility line but below that of �TCP (Fig. 6.7A).PW in Yellow tuff suspensions at the ZPT decreased as a result of increases in I(Fig. 6.7B). In the light of the conclusions of Sanyal and De Datta (1991) that �TCPcontrols PW dissolution/precipitation reactions, it may be deduced that adsorptiongoverned PW in this pH range. The PZSE of the Yellow tuff was estimated to be 6.2(Silber, 1991), therefore, the increase of PW with decreasing I was consistent withmechanism (ii), described above.

At higher pH the electrostatic potential of the solid surfaces became less nega-tive (Figs. 6.1 and 6.3) and PW increased significantly with decreasing values of I(Fig. 6.7B). An increase in I affected metal concentrations as well (mechanism (iii),


6 8

10 12 14 16

104

500(B)

Yellow tuff + 500 mg P kg–1

ZPT

ZPT

8.9

9.1

9.4

HAP

OCP

β TCP

DCPD

0.006 M

0.02 M

0.1 M

(A)

400

300

200

100

0

–11

–13

–15

–17

pH

P c

once

ntra

tion

(μM

)

logCa2+ + 2pH

logH

2PO

4– – p

H

0.006 M

0.02 M

0.1 M

FIGURE 6.7 (A) pH-dependent ionic strength (NaCl) effect on solution-P concentration in tuffextracts after addition of P at 500 mg kg−1, tuff/solution ratio of 1/40, 24 h of reaction, ZPT is the sus-pension pH prior to the addition of acid or base; (B) experimental activities in tuff extracts at pH 8.9,9.1 and 9.4 in relation to the theoretical solubility lines for calcium-phosphate minerals. Based on Silber(1991).

above), and the Ca and Mg concentrations in tuff suspensions increased (data notpresented). The activity products of Ca2+ and PO4

3− in tuff solutions were betweenthe solubility lines of �TCP and OCP (Fig. 6.7A). Adsorption reactions probablycontrolled PW at high pH but it would be expected that at equilibrium, the PW in 0.1and 0.02 M solutions would decline further, until the activity products of Ca2+ andPO4

3− attained the Ksp of �TCP. In acidic tuff solutions the effect of I on the PW

was insignificant (Fig. 6.7B), probably because of P release from indigenous Ca–Pminerals (Silber, 1991, Silber et al., 1994, 1999) during the analyses.


6.2.1.3 Horticultural Implications

Phosphorus retention by substrates has an important effect on P fertilization manage-ment. For example, a typical substrate quantity used for vegetables or flowers consistof 50–60 kg of tuff m−2. The daily evapotranspiration rate during the growth periodof roses (Rosa × hybrida) in a semi-arid climate is usually 5–10 L m−2; therefore,the daily irrigation supply is 0.1–0.2 L kg−1 of tuff. The recommended P concentra-tion in the irrigation water for cut-rose culture is usually 30–45 mg L−1 (Sonneveldand de Kreij, 1987; Jones, 1997; Sonneveld, 2002), therefore the daily input of P is3–9 mg kg−1. Under this daily P rate and ignoring leaching, the possible PW at pH7.3 according to the P retention curve will be lower than 0.05 mmol L−1 (Fig. 6.6A).In the light of the above data and on the assumption that adsorption reactions controlP availability in tuff suspensions, it is expected that the decline of P solubility in thesolution will be very fast, so that shortly after irrigation, the PW will be deficient, andinsufficient for rose production. Johanson (1978) found that the yield of cut roses fellto 40 per cent following low P application (0.16 mmol L−1), compared with that underan adequate P management (1.3 mmol L−1). Thus, low P concentrations in tuff sub-strates may restrict plant production. If a P concentration of 0.5 mmol L−1 is taken asthe minimum for adequate growth of cut roses, the amounts of P adsorbed by Yellowtuff at pH 6 and 7.3 should be 840 and 600 mg kg−1, respectively (Silber, 1991). Ifthe quantity of P added daily via fertilization is taken to be 4.5 mg kg−1 (0.15 L kg−1

at a P concentration of 30 mg L−1), then with solution pH of 6 or 7.3, at least 187 or133 days would be required, respectively, to fill all the available sites for P adsorptionon the Yellow tuff surface and to attain the desired PW. During this period, cut-rosedevelopment would certainly be impaired. With a solution pH above 7 and Ca and Mgconcentrations assumed to be 1.5–3 mmol L−1, as required for optimal plant growth,P deficiency will restrict plant growth because the activity products of Ca2+, Mg2+

and PO43− are far above the Ksp values of known Ca–P and Mg–P minerals (Lindsay,

1979).The direct outcome of the adsorption and the precipitation reactions detailed above

is that, under equilibrium conditions, PW may be lower than plant requirementsthroughout the pH range of plant growth in tuff substrates. It is important to note thatconditions of P deficiency are not exclusive to tuff media. Precipitation of P with Caand Mg at neutral and high pH is expected to be the general phenomenon in all sub-strates, as long as nutrient concentrations in the irrigation water are kept at adequatelevel for plant growth. Considering the effect of pH on adsorption and precipitationreactions, it is reasonable to expect that optimal P availability should be obtained atpH 6.0–6.5.

P adsorption by organic compounds is well known. High P retention by pine barkmedia have been reported (Cotter, 1979; Yeager and Wright, 1982; Ogden et al.,1987), and low P availability inhibited plant growth (Cotter, 1979). Shortening thetime between consecutive P additions, that is increasing the frequency of fertigation,and inducing non-equilibrium conditions between successive fertigation events maybe a preferable management means to enhance P availability in the rhizosphere and toimprove P uptake by plants (Silber et al., 2003, 2005; Xu et al., 2004). The beneficial


effects of high irrigation frequency on P availability to plant roots are detailed inChap. 8. Alternatively, addition of organic components, such as mature compost, whichcontain humic and fulvic substances, to tuff substrates might significantly reduce Pretention by the substrate (Gottesman, 1989).

6.2.2 ZINC

Zn ions are SAIs that have affinity to charged surfaces. Zn adsorption increases aspH increases, reflecting typical cation behaviour. Zn solubility may be controlled byhopeite [Zn3(PO4)2.4H2O] solubility (Lindsay, 1979). Similar to P, Zn concentrationin the rhizosphere may be lower than plant requirements throughout the pH range ofplant growth in many substrates. The effect of solution-pH on Zn retention in Yellowtuff suspensions is presented in Fig. 6.8.

All the activity products of Zn2+ and PO43− in the tuff solutions were below the

Ksp of hopeite or of any Zn-oxide mineral, therefore the retention of Zn on Yellow tuffsurfaces was related solely to electrostatic adsorption. Addition of Zn to Yellow tuffsuspensions decreased the PW at all the pHs examined (Fig. 6.9). Since the possibilityof Zn precipitation with P was ruled out, it is most likely that the PW decreased as aresult of increased P adsorption, promoted by an increase in the positive charge on tuffsurfaces following Zn adsorption. However, under common agricultural practices PW

is higher by at least three orders of magnitude than the Zn concentration (Sonneveld,2002), therefore, the opposite trend is to be expected, and usually Zn availability isrestricted through hopeite solubility (Lindsay, 1979) and/or the increase of negativecharge that follow P adsorption on the solid phase surfaces (Diaz-Barrientos et al.,1990; Pardo, 1999).

Zn

adso

rbed

(cm

ol k

g–1)

Zn concentration (mM)0.0

6.66.0

5.0

4.2

6

4

2

00.2 0.4 0.6 0.8

FIGURE 6.8 pH-dependent adsorption of zinc onto yellow tuff (pH values are shown above eachisotherm), constant ionic strength (0.01 M NaClO4), tuff/solution ratio of 1/40, 24 h of reaction. Based onSilber (1991).


4

P c

once

ntra

tion

(10–7

M)

0

50

100

150

6

0

250

Zn added (mg kg–1)

500

1000

2000

4000

8pH

FIGURE 6.9 Solution-P concentrations as a function of zinc addition and pH in tuff extracts. Constantionic strength (0.01 M NaClO4), tuff/solution ratio of 1/40, 24 h of reaction. Based on Silber (1991).

Zn adsorption by substrates is expected to follow the CEC values presented inTable 6.1, and therefore some substrates commonly used in soilless culture (tuff,zeolite, compost) exhibit high adsorption capacity for Zn and other micronutrientssuch as Fe, Mn and Cu. Zn adsorption to more inert substrates such as perlite andstone wool is expected to be insignificant (on volume basis); however, plant growthmay alter the surface properties of their solid phases, and subsequently, Zn adsorptionmay be significant even in ‘inert’ substrate as a result of adsorption to dead orinactive roots or to new organic material in the substrate from root decomposition.Thus, under the solution-pH conditions commonly used in soilless production (pH of5.5–7.5), adsorption reactions will reduce the concentrations of metal micro-nutritionalelements to deficiency levels. Bearing in mind the effect of pH on adsorption andprecipitation reactions, it is reasonable to expect that optimal Zn availability shouldbe increased at acidic pHs. Increasing the solution-metal concentration is uselessfor the same reasons as detailed above for phosphorus. Chelating agents such asEDTA (ethylene-diamine tetracetic acid), DTPA (diethylenetriamine penta-acetic acid)or EDDHA (ethylenediamine di(o-hydroxyphenylacetic acid)) are commonly used tomaintain adequate availability of micronutrients (Parker et al., 1995). In addition, as inthe case of P, increasing the fertigation frequency was found to be an effective meansof enhancing metal uptake by plants (Silber et al., 2003, 2005).

6.2.3 EFFECTS OF P AND ZN ADDITION ON SOLUTIONSI CONCENTRATION

Addition of SAI anion or cation, respectively, enhances the negative or positivesurface charge (Sposito, 1984). Silicon is an important constituent of substrate solidsand Si ions have pH-dependent adsorption properties (Hingston and Raupach, 1967;Goldberg and Glaubig, 1988; Hansen et al., 1994; Nartey et al., 2000). Therefore, the

6.3 Plant-induced Changes in the Rhizosphere 225

2.0

Yellow tuff

0.01 M Nacl

0.01 M NaCl + 1000 mgP kg–1

0.01 M NaClO4 + 1000 mgZn kg–1

1.5

1.0

0.5

0.04 6 8 10

pH

Si c

once

ntra

tion

(mM

)

FIGURE 6.10 Solution-Si concentrations as a function of phosphorus and zinc addition to tuffsuspensions. Ionic strength and electrolytes used are indicated inside the Figure, tuff/solution ratio of 1/40,24 h of reaction. Based on Silber (1991).

effects of the addition of SAI anion or cation on solution Si concentration is of interest;Si concentrations increased with decreasing pH in all tuff suspensions (Fig. 6.10)because of the dissolution of volcanic glass and halloysite-like allophane (Silber et al.,1999).

Addition of P to tuff solutions induced an increase of Si concentrations, in accor-dance with the findings of Rajan (1975), Rajan and Perrott (1975), Rajan and Fox(1975), Veith and Sposito (1977), Pardo and Guadalix (1990) that P retention wasfollowed by displacement of SiO2 from aluminosilicates, either because of displace-ment from the surface or because of partial disruption of the structure of the substratesolids. Contrary to the effect of anion addition, Zn addition to Yellow tuff suspen-sions decreased Si concentrations at all the pHs examined (Fig. 6.10). While siliconis generally not considered to be an essential nutrient, it does play a significant rolein plant resistance to stress conditions (Epstein, 1999). Therefore, the effects of P andZn addition on Si concentrations may be of interest.

6.3 PLANT-INDUCED CHANGES IN THE RHIZOSPHERE

6.3.1 EFFECTS ON CHEMICAL PROPERTIES OF SURFACESOF SUBSTRATE SOLIDS

Plant growth may affect the chemical properties of the substrate solids throughthree main mechanisms:

1. changes of the surface charge and chemical properties of the solids through theaddition of specifically adsorbed ions such as orthophosphates in the fertigationnutrients


2. addition of new solid materials, that is organic compounds, mainly because ofroot growth

3. accumulation of root exudates and decomposition products, especially humicand fulvic acids that may cause decomposition of substrate solids (Tan, 1986)and/or modification of the surface charge (Tate and Theng, 1980; Huang andViolante, 1986).

The effect of SAI addition on the chemical properties of substrate solids(mechanism i) is well understood (Sposito, 1984), and as long as the solution-pH isnot extreme (below pH 4) and does not cause substantial dissolution of the solids, thiseffect is expected to be reversible. The direct effect of plant growth on the chemicalproperties of the substrate, that is alteration of the chemical characteristics of themedia resulted from formation of new organic material in the form of roots, is lessunderstood. Generally, the CEC of root dry matter has the same order of magnitudeas soil constituents (10–100 cmol kg−1) and is higher in dicotyledonous species thanin monocotyledonous ones (Blamey et al., 1992; Marschner, 1995). The root CECwas found to be pH-dependent, and adsorption of protons and copper to root cellwalls could be evaluated by the same surface complexation model (single capacitancemodel) used for adsorption to soil (Allan and Jarrell, 1989), suggesting that the samemechanism of adsorption are similar to that of soil. It is, however, doubtful that intactroots contribute to the charge characteristics of a medium, because the high CECvalues reported in the literature were related to root components such as cell walls.Therefore, although the high CEC values may reflect the ‘potential’ CEC, the ‘actual’CEC of intact plant root in vivo is probably lower because of the low accessibility ofions from the external solution to the internal apoplast (Marschner, 1995).

The effects of root excretions and root-induced pH changes or modifications ofnutrient availability in the rhizosphere were reviewed by Marschner and Römheld(1996), Bar-Yosef (1996), Jones (1998) and Hinsinger et al. (2003). The effects ofplant growth on the chemical characteristics of the substrate have been studied bySilber and Raviv (1996). Surface characteristics of Red tuff were evaluated prior (T0)to using it for rose culture as well as after it had been exposed to fertigation solution inthe absence and presence of plants (Tf and Tp, respectively). Extractions were carriedout at various pH levels with solutions differing in ionic strength. The ionic strengthand pH were selected to reflect the relevant conditions at the rhizosphere (Silber andRaviv, 1996). The pH titration lines of intact unused Red tuff (T0) overlapped, anddistinction of an ionic strength effect was not possible (Fig. 6.11A).

Red tuff consists mainly of primary minerals and un-weathered volcanic glass.It contains only 19 per cent of amorphous material that may contribute to thepH-dependent charge (Silber et al., 1994). Therefore, major proportions of addedacids/bases were probably consumed either through exchange with cations/anionsadsorbed on permanently charged sites, through the dissolution of indigenous Ca–Pminerals and very fine amorphous particles, or both, as suggested by Silber et al.(1999). As a consequence, these processes masked the effect of solution ionic strengthon variable-charged surfaces (Fig. 6.11A).


Aci

d/B

ase

adde

d (c

mol

kg–1

)

–30(A)

(B)

–10

10

30

50

703 5 7 9 11

3 5 7

pH9 11

–30

–10

10

30

50

70

0.01 M NaCl

3 5 7 9 11

–30

–10

10

30

50

70

0.1 M

0.01 M

0.001 M

T0

Tf

Tp

FIGURE 6.11 Acid/base addition (0.1 N HCl and NaOH, respectively) as functions of pH and NaClconcentrations, tuff/solution ratio of 1/40, equilibrium period of 1 week. (A) intact unused red tuff; and(B) red tuff after two years of rose growing. The inset curves show a comparison of acid base addition atconstant ionic strength of 0.01 M NaCl to: unused tuff (T0), and tuff irrigated with fertilization solution inthe absence and presence of plants (Tf and Tp, respectively). Based on Silber and Raviv, (1996).

Plant growth induces an increase in the negatively charged surfaces (Fig. 6.11B)analogous to the trend observed after the addition of citric acid to goethite and clayminerals (Lackovic et al., 2003). In addition, the effect of solution ionic strength atpH values above 7 was apparent (Fig. 6.11B), and was similar to that observed forYellow tuff (Fig. 6.3). The increase of negatively charged surfaces of Red tuff in thepresence of plants (Tp) was probably due to the specific adsorption of root-excretedorganic compounds. Two years of irrigating tuff substrate with fertilizer solution inthe absence of plants (Tf), also induced an increase of positive charge on Red tuffparticle surfaces, but this effect was less significant than under plant growth (insetin Fig. 6.11B). Specific adsorption of orthophosphate ions added via fertigation andthe accumulation of soluble low-molecular-weight compounds exuded from roots orreleased during organic matter decomposition in the tuff substrate induced pH decrease


in Tf and Tp solutions, respectively (Silber and Raviv, 1996). In almost all cases,addition of equal quantities of acid or base lowered the pH in tuff solutions, withintact unused tuff being most affected, and tuff exposed to fertigation solutions inthe presence of plants being the least (inset in Fig. 6.11). It is important to notethat all the root-mediated changes in the chemical properties of substrate solids areaffected by the solid characteristics, the plant type and growth stage, and the prevailingagricultural management (rate of fertilization, proportions of fertilizer components,such as the NH4:NO3 ratio, fertigation frequency and the volume of medium availablefor root growth). Morel and Hinsinger (1999) discussed the effect of root-inducedmodifications of P partitioning between the substrate solid and the substrate solutionand concluded that it was notably influenced by root density. They also found thatgrowth of crops with low root density had little or no effect on soil properties overa long period of 80 years (Morel et al., 1994), while one week of intensive plantgrowth with 100-fold increase in root density significantly affected soil properties(Morel and Hinsinger, 1999). Soilless culture is commonly characterized by root-system constraints that differ from those found in conventional soil culture, and thatlead to higher root density of plants in soilless production. The conclusion by Moreland Hinsinger (1999) suggest that in soilless production, plants have a significantlygreater impact on medium properties than plants growing in soil.

The effects of root-mediated pH changes in the rhizosphere are well documented(for review see Nye, 1986 and Hinsinger et al., 2003); they are caused by excretion ofions (mainly H+, OH− and organic bases or acids) which balance the electric charge,following cation or anion uptake by roots, root respiration, carbon exudation or redoxprocesses. All these processes are strongly dependent on plant type and nutritionalstatus (Marschner et al., 1986; Nye, 1986; Neumann and Römheld, 1999; Abadiaet al., 2002; Hinsinger et al., 2003). The effect of plants on the rhizosphere pH islargely related to the chemical properties of the solid phases. Accumulation of solubleorganic acid excreted from roots in the presence of an inert medium may induce a pHdecrease because the adsorption reactions are negligible and the ligand acts as weakacid, whereas, in contrast, the quantities of OH− released following ligand adsorptiononto a substrate with a high surface charge may be significant and much higherthan that of H+ added via ligand dissolution. The effects of tomato (Lycopersiconesculentum Mill.) plants on the pH in leachate from pots filled with various substratesare presented in Fig. 6.12A.

To simulate the effect of roots on the pH of the medium, citric acid, which isan important organic acid excreted from the roots, was added to tuff suspensionsequilibrated at three pH levels (3.3, 4.8 and 6.5) which approximately fit the log K0

of citric acid (3.1, 4.8 and 6.3) (Martell and Smith, 1976–1989; Smith and Martell,1989). At the three pH values examined, the pH rose as a result of increasing citricacid addition, but the size of the change in pH resulting from the citric acid addition(�pH) varied (Fig. 6.12B). The predominant citrate species at pH 3.3 were H3Cit0

and H2Cit− (38 and 60 per cent, respectively, of the total citrate ions in the substratesolution) whereas the monovalent H2PO4

− was the main P species (Lindsay, 1979).Thus, the competing effect of citrate ions was relatively small and, therefore, only


8

7

6Pumice Red tuff Rockwool

Plant

No plant

Watering–pH

(A)

(B)

0.9

0.6

Citric acid added(g kg–1)

1

2

4

8

0.3

0.03.3–3.4 4.7–5.6

pH

Δ pH

pH

6.5–6.7

0.5

FIGURE 6.12 (A) pH in leachates from pots filled with pumice, red tuff and rockwool in the presenceand absence of plants. The solution NH4 N/NO3 N ratio applied was 1:3 and the solution-pH is indicatedby the horizontal line. Based on Bar-Yosef et al. (1997); (B) The effect of citric acid addition to tuffsuspensions equilibrated at three pHs (3.3, 4.8 and 6.5), �pH values represents the difference between actualpH and the initial pH, i.e., before citric acid addition. Based on Silber et al. (1998).

small quantities of OH− were released from tuff surfaces after citric acid addition.In addition, dissolution of indigenous Ca–P minerals during the pH equilibrationprocess and the partial dissolution of tuff surfaces (Silber et al., 1999) probablyadded competing anions. Evaluation of the impact of citric acid on pH changes ateach pH should be based on the quantities of OH− released. The largest quantity ofcitric acid added to tuff (8 g kg−1) at initial solution-pH values of 3.3, 4.8 and 6.5induced increases of 3�1 × 10−10, 1�4 × 10−7 and 1�6 × 10−6 moles, respectively, of


OH− per kilogram of tuff (calculated from solution-pH changes in Fig. 6.12B). Ingeneral, in a multifaceted system consisting of substrate, solution and plant, numerousfactors may have significant impacts on the direction and extent of �pH. Such factorsinclude organic ions excreted from roots or inorganic specifically adsorbed anions(mainly P) added via the irrigation solution, biological processes in the rhizosphere(nitrification/denitrification), surface charge characteristics of the solid phase, rootdensity and, especially, the nature of the plant and its growth conditions.

6.3.2 EFFECTS ON NUTRIENTS AVAILABILITY

The effects of plant growth on the solubility of nutrient elements in the rhizosphereare specific and primarily related to the formation constants between the inorganicions and root-excreted ligands. For example, the formation constants (log K0) of citricand oxalic acids with K are 1.2 and 1.1, respectively, as compared with 4.8 and3.8, 4.7 and 3.4, and 9.9 and 13.0, for the same two acids with Ca, Mg and Al,respectively (Martell and Smith, 1976–1989). Therefore, in the presence of plants andwith the consequent build-up of root-excreted organic acids in the rhizosphere, thesoluble complexes of Ca-, Mg- and Al-organic ligands will be predominant whilethe proportions of K-ligands will be minor. These differences may account for theinconsistency between the effects of plants on solution-K and on solution-Ca in tuffsuspensions (Fig. 6.13): solution-K concentrations decreased while those of solution-Ca and solution-Mg (data for Mg are not shown because of their similarity with thosefor Ca) increased in the presence of plants compared with their concentrations in thesame tuff leached with fertilizer solution (Figs. 6.13A and 6.13B, respectively).

The solution concentrations of Ca and Mg increased as a result of two consecutiveprocesses: increase in adsorption of Ca and Mg by tuff surfaces during the growthperiod and subsequent displacement by electrolyte ions during the titration procedure.The Ca and Mg adsorption increased because of two independent, yet simultaneouslyacting mechanisms: (i) augmentation of the negatively charged surface area (Fig. 6.11)which resulted in increased adsorption of Ca and Mg to inorganic particles and (ii)augmentation of negatively charged soluble complexes (e.g., Ca-citrate) that adsorbedonto positively charged surfaces.

Enhancement of P availability may be the most important outcome of plant growth,because solution-P concentrations are governed by adsorption/desorption and precipi-tation/dissolution reactions; they frequently fall to deficiency levels, as detailed above.Thus, any modification of the surface properties that facilitates an enhancement of Pavailability may be valuable. Numerous studies have found significant enhancement ofP mobilization towards roots as a result of rhizosphere modification by organic acids(mainly citric) exuded by plant roots (Gardner et al., 1983; Dinkelaker et al., 1989;Dinkelaker and Marschner, 1992; Hoffland, 1992; Gerke et al., 1994; Bar-Yosef, 1996;Jones, 1998; Jones and Brassington, 1998; Geelhoed et al., 1998, 1999; Neumann andRömheld, 1999, 2001; Bertin et al., 2003), and recent up-to-date reviews have beencollected by Lambers and Poot (2003). The high root density of plants in soilless pro-duction probably accounts for a greater effect on P availability than that of soil-grown


Ele

men

t con

cent

ratio

n (m

M)

0.03 5 7 9 11

0.3

0.6

0.9

K Red tuff: 0.01 M NaCl

T0

Tf

Tp

3

2

1

03 5 7

pH9 11

Ca Red tuff: 0.01 M NaCl

T0

Tf

Tp

FIGURE 6.13 Potassium and Ca concentrations in three tuff extracts (unused tuff (T0), and tuffirrigated with fertilization solution in the absence and presence of plants (Tf and Tp, respectively) asfunctions of pH at constant ionic strength (0.01 M NaCl). Based on Silber and Raviv, (1996).

plants. Rose growth in tuff substrate induced a considerable increase in solution-P con-centrations compared with that in tuff without plants, irrigated with identical fertilizersolution (Fig. 6.14). Surface reactions probably governed P solubility in the absenceof plants, since the activity products of Al3+, Fe3+ and PO4

3− at pH 4–7 were belowthe Ksp of variscite and strengite, respectively, and that of Ca2+ and PO4

3− at higherpHs were between the solubility lines of �TCP and OCP (only data of Ca–P solubilitylines are presented, inset in Fig. 6.14A). The P partitioning between the solid and theaqueous phases was typical to specifically anions, that is, the concentration increasedwith increasing pH, and there was a clear effect of ionic strength (I). Below the zeropoint of salt effect (ZPSE) an increase of I increased the positive surface charge of thesorbent, whereas above the ZPSE it decreased the positive surface charge, as illustratedin Fig. 6.1. Hence, if P solubility were governed solely by surface reactions, then Padsorption would be expected to increase or decrease with increasing I below or above


1.2–9

–11

–13

–15

–178 10

0.1 M0.01 M0.001 M

0.1 M

Tp

Tf

ZPT EAP

TCP

0.01 M

0.001 M

12 14 16

0.8

0.4

0.0

1.2

0.8

0.4

0.04

4 6 8 10

6

P c

once

ntra

tion

(mM

)

8

pH

(B)

(A)

10

OCPDCPD

logCa2+ + 2pH

logH

2PO

4– – p

H

FIGURE 6.14 Phosphorus concentration as a function of pH at three initial ionic strength (NaCl):(A) tuff irrigated with fertilized solution in the absence of plants (Tf), the inset present the experimentalactivities in tuff extracts in relation to the theoretical solubility lines for calcium-phosphate minerals, ZPTis the suspension pH prior to the addition of acid or base; (B) tuff irrigated with fertilized solution in thepresence of plants (Tp). Based on Silber and Raviv, (1996).

the ZPSE, respectively, as indeed presented in Fig. 6.14A. The overlap around pH 4probably resulted from fast dissolution of tuff surfaces. The clear intersection aroundpH 6.5 (Fig. 6.14A) is consistent with previous results that indicated pH 6.5 to be theZPSE of the Red tuff (Silber, 1991).

In contrast to intuitive thought, plant growth significantly improved P availability inthe rhizosphere. Despite consumption by plants, PW was higher in Tp than in Tf extractsat all ionic strengths, especially at acid pHs (Figs. 6.14A and 6.14B). The higher Caand Mg concentrations in Tp extracts than in Tf ones (Fig. 6.13) did not preventthe PW from being higher as well, probably because the Ca2+ and Mg2+ activities


were low because of organic complexing agents excreted by the roots (Inskeep andSilvertooth, 1988; Grossl and Inskeep, 1991). The effect of I on PW in Tp extractswas almost negligible compared with that in Tf extracts, and no clear intersectioncould be discerned (Fig. 6.14B). The increase of negative-charged surface inducedby plant growth (Fig. 6.11), which were probably caused by specific adsorption ofcarboxylic acids excreted from roots, also reduced P adsorption on tuff surfaces, inaccord with known mechanisms reported in the literature (Nagarajah et al., 1970; Earlet al., 1979; Lopez-Hernandez et al., 1979; Traina et al., 1986a,b, 1987; Kafkafi et al.,1988; Geelhoed et al., 1998, 1999; Jones, 1998; Jones and Brassington, 1998; Moreland Hinsinger, 1999; Lackovic et al., 2003).

6.3.3 ASSESSING THE IMPACT OF PLANTS: THE EFFECTOF CITRIC ACID ADDITION ON P AVAILABILITY

By adding citric acid to tuff suspensions it is possible to assess the role of low-molecular-weight organic acids excreted from roots in P partitioning between theaqueous and the solid parts of the substrate. This also simulates the beneficial effectof plant growth on P availability that was detailed above as shown by Silber andRaviv (1996) (Fig. 6.14). They also found that the isotherms of citric acid adsorptionby Yellow tuff fell as pH increased from 3.5 to 4.9 and to 6.4 (Fig. 6.15) and thatthese curves were pH-specific Langmuir curves. This was consistent with previousfindings of specific adsorption of citrate by variable-charged surfaces (Bowden et al.,1980; Bar-Yosef, 1996; Jones and Brassington, 1998; Geelhoed et al., 1999; Lackovicet al., 2003). On the other hand, it is inconsistent with citric acid isotherms that thequantities of P released is greatest for pH 3.5, intermediate for pH 6.4 and lowest forpH 4.9 (Fig. 6.16).

2

1

00.0 0.3 0.6 0.9

3

3.5Yellow tuff

Citrate concentration (mM)

Citr

ate

adso

rptio

n (c

mol

kg–1

)

4.9

6.4

FIGURE 6.15 pH-dependent adsorption of citric acid onto yellow tuff (pH values are shown aboveeach isotherm), constant ionic strength (0.01 M NaClO4), tuff/solution ratio of 1/40, 24 h of reaction.


P r

elea

sed

(cm

ol k

g–1)

6

4

2

00

0.9

(B)

(A) pH 3.5

pH 4.9

pH 6.4

0.6

0.3

0.00 1

Citrate adsorption (cmol kg–1)2

1 2 3

FIGURE 6.16 Quantities of P released as a result of citric acid addition to yellow tuff, constant ionicstrength (0.01 M NaClO4), tuff/solution ratio of 1/40, 24 h of reaction. (A) initial pH 3.4; (B) initial pH 4.9and 6.4.

Citrate adsorption onto an iron oxide (goethite) surface has been suggested to beas an inner-sphere complex (no molecule of the bathing solvent is interposed betweenthe surface functional group and the molecule unit it bounds (Zelancy et al., 1996))at pH < 7 (Geelhoed et al., 1998, 1999; Harsh, 2000; Lackovic et al., 2003), or asouter-sphere adsorption (non-specific adsorption by electrostatic attraction) at pH 8.8(Lackovic et al., 2003). However, the inconsistencies in the order and slope of thecurves of P release vs. citric acid adsorbed (Fig. 6.16) suggest that an additionalprocess may be involved in the release of P into the substrate solution. It is plausiblethat, in addition to the mechanism of citrate-P competition for the adsorption sites,


citrate addition accelerated the dissolution of indigenous P through the formation ofsoluble metal-citrate complexes. Assessment of the effects of low-molecular-weightorganic acids, such as citric acid excreted from roots, on rhizosphere-P solubilityshould take into account miscellaneous factors such as rate of excretion by plant roots,the involvement of microflora that may secrete or consume organic acids, fertilizationmanagement, content in the system of metal-P compounds that might be prone todissolution and properties of the substrate solids. The relation between plant-P statusand the rate of root excretion is well established, and usually solution-P concentrationsand the amounts of organic acids excreted exhibited opposing trends.

For example, root excretion significantly decreases as solution-P increases (Gardneret al., 1982, 1983; Lipton et al., 1987; Gerke et al., 1994; Johnson et al. 1994, 1996a,b;Imas et al., 1997; Keerthisinghe et al., 1998). Thus, with the solution-P concentrationsnormally used in soilless culture (0.5–1.5 mM), the enhancement of P availability byorganic acids is expected to be low. In addition, rapid utilization and degradation bymicro-organisms may diminish the content of organic acid in the rhizosphere (Jones,1998). Therefore, even if adsorption by solids reduced the rate of degradation (Jonesand Edwards, 1998), the actual contribution of organic acids to nutrient acquisition byplants grown under proper fertilization regime would be minor (Jones et al., 2003).

The diffusion-coefficients of organic acids in the rhizosphere are very low (Darrah,1991) and therefore the concentration of organic acid is estimated to be heterogeneousin the root zone (Jones et al., 2003). Models can provide insight into the complexprocesses occurring in the rhizosphere, specifically with regard to examination of theeffects of root excretions on nutrient availability under diverse growth conditions asillustrated by Geelhoed et al. (1999) in Figs. 6.17A–D. In particular, solution-citrateconcentrations in the substrate solution increase with citric acids excretion from rootsand decrease through degradation. According to model simulations, the profiles ofcitrate concentration are expected to decrease sharply with increasing distance fromthe root surface over short time of few hours, so that the citrate concentration willfall almost to zero at a distance of few millimetres from the root (Fig. 6.17A). Thisphenomenon is valid as long as: (i) electrostatic adsorption reactions are instantaneous;(ii) no degradation processes are taking place; (iii) only competition between citrateand P on the solid phase surface sites is involved (i.e., no involvement of chelatingeffect of citrate on indigenous metal-P compounds). However, over time and in theabsence of degradation, citrate is diffused to few millimetres from the root surfaces.As citrate accumulates in the boundary layer around the roots, adsorbed phosphateis released from the surfaces of substrate solids into the substrate solution and P ismobilized (Fig. 6.17B). The presence of a microflora population and its utilization ofcitrate significantly decreases citrate quantities in both the solid and the aqueous partsof the substrate (Fig. 6.17C), thus reducing the effect of root-excreted citrate on Ppartitioning (Fig. 6.17D).

By definition, the patterns resulting from this model simulation (Fig. 6.17) are validonly under the assumptions of the model and for the parameters used. The resultsobtained may be affected significantly by the presence of different amounts of P andcitrate in the system or different values for the model’s parameters such as P and


0.20

0.2 days

1 day5 days

0.15

0.10

0.05

00 1 2 3

Distance from root surface /mm

[Citr

ate]

/mM

(A) (B)

(C) (D)

4 5 6

0.2 days1 day

5 days4.0

3.0

2.0

1.0

0

5.0

[P] /

μM

0 1 2 3

Distance from root surface /mm

4 5 6

without degradation

with degradation

solutionadsorbed

solutionadsorbed

4.0

3.0

2.0

1.0

0

5.0

[P] /

μM

Distance from root surface /mm0 1 2 3 4

without degradation

with degradation

solutionadsorbed

solutionadsorbed

0.4

0.3

0.2

0.1

0

0.5

[Citr

ate]

/mM

Citr

ate

adso

rbed

/μm

ol m

–20.6

0.4

0.2

0

0.8

0 1 2 3

Distance from root surface /mm4

FIGURE 6.17 (A) Solution-citrate concentration as a function of distance from root surface in theabsence of degradation processes after 0.2, 1 and 5 days of excretion; (B) Solution-P concentration as afunction of distance from root surface in the absence of degradation processes after 0.2, 1 and 5 daysof excretion; (C) predicted concentration profiles of citrate in solution and adsorbed onto goethite withand without degradation; (D) effect of citrate exudation, with and without degradation, on the predictedP concentration in solution. Reprinted from Geelhoed et al. (1999), with kind permission from BlackwellScience, Ltd.

citrate adsorption rates and the function relating citrate degradation rates to pH. Themodel result may also differ with alternate submodels of related processes such as therate dependence that pH has on the activities of the microflora population. Also, otherfactors used in the model of Geelhoed et al. (1999) such as root density, diffusioncoefficients, tortuosity factor or volumetric water content may significantly affect themodel predictions.

6.4 NUTRIENT RELEASE FROM INORGANIC

AND ORGANIC SUBSTRATES

Reducing the pollution hazards posed to the environment by excess fertilization insoilless production has led to efforts to regulate nutrient availability in the rhizosphereand to develop substrates that might contribute nutrients during the growth period of

6.4 Nutrient Release from Inorganic and Organic Substrates 237

horticultural crops. The goal of reducing excess fertilization may be achievable throughtwo approaches: (i) pre-charging the substrate with nutrients that are subsequentlyreleased during the growth period, that is, the substrate functions as slow-releasefertilizer and (ii) release of indigenous nutrients from the substrate as a result ofchemical/biological processes such as N mineralization in organic media or dissolutionof indigenous nutrients from inorganic components.

Pre-charging of substrates with nutrients requires that the material have high CECor AEC values. The low values of these characteristics of most inorganic substrates,such as stone wool, perlite and pumice, suggest that these widely used horticulturalmaterials may not be suitable for this. Zeolites, however, possess extremely highCEC values of 200–400 cmol kg−1 (Mumpton, 1999). In fact, zeolites are used asnutrient sources in infertile soils and substrates because of their high affinity for K orNH4 ions (Hershey et al., 1980; Chen and Gabelman, 1990; Notario del Pino et al.,1994; Williams and Nelson, 1997; Dwairi, 1998; Mohammad et al., 2004). Althoughthe use of zeolites as a single-component growing substrate is not recommendedbecause of the high bulk density (1.9–2.3 Mg m−3, Ming and Mumpton, 1989), mixedsubstrates which may include organic or inorganic materials as well as zeolites areused worldwide in the production of flowers and vegetables. Theoretically, if all theexchange sites of a zeolite substrate were pre-charged with K ions, the total potentialpool for K would be 80–160 g kg−1. Since the total K acquisition by tomato or pepper(Capsicum annum L.) crops under intensive conditions is 15–20 g per plant (Bar-Yosef,1999), approximately 200 g of zeolite are required to satisfy the total plant demandfor K throughout a growing season. Thus, from the quantitative point of view, thepre-charging approach can be regarded as potentially successful; however, the releaserate may present problems. Basically, the optimal rates of nutrient release must matchthe plant demand, which is obviously low at the beginning of the growing periodand increases gradually during the various growth stages. However, the pre-chargednutrients are released from the surface sites by exchange reactions with competingions (in the case of K+ these are NH4

+, Na+, Ca2+ and Mg2+). Thus, factors suchas concentrations of competing ions in the substrate solution, irrigation doses andplant activity all affect the release rate; therefore, nutrient availability is not actuallyregulated and may be different from the plant’s demands.

A combination of approaches (i) and (ii) to reducing excess nutrient run-off wasimplemented by mixing phosphate rock with zeolite pre-charged with NH4

+ ions(Allen et al., 1995; Williams and Nelson, 1997; Pickering et al., 2002). While theNH4

+ ions served as an N source for plants during the growth period, the main purposeof this process was to induce phosphate rock dissolution, either through acidificationor through exchange with Ca ions (Allen et al., 1993; Notario del Pino et al., 1994)so as to slowly release phosphate over time from the zeolite. Zeolites can also beused as slow-release sources of micronutrients and K (Opena and Williams, 2003;Puschenreiter and Horak, 2003).

The contribution of indigenous nutrients released from inorganic substrates is usu-ally very small and, except in a few cases of utilisation of phosphate rock, the releaseof indigenous nutrients is practically negligible. Organic media, on the other hand, and


especially composts, contain large quantities of valuable N resources and, therefore,the production of composts that may contribute N during the growth period (approach(ii)), especially for organically grown crops, may be of interest. Generally part of theinitial N content of organic constituents is lost during the composting process. In fact,high-quality mature composts are characterized by their low mineralization rate andlow N availability (Hadas and Portnoy, 1994, 1997). It is, however, possible to reducethe N loss during the composting process by using a high C/N ratio, lowering the pHand manipulating the air flow and the composting temperature (Raviv, 2005). Ravivfound that composts enriched with wheat straw or orange peels (imparting a high initialC/N ratio and a low pH, respectively) satisfied the N requirement of tomato plants forseveral months under conditions of minimal leaching fraction (Raviv et al., 2005) butthey were, nevertheless, not able to match the rate of N release to the plant requirement.The rate of N release from a successful N-conservation compost consisting of orangepeels and separated cattle manure (OP-SCM) (Raviv et al., 2005) was very high duringthe first days. The rate far exceeded plant demand for a crop such as, for example,lettuce (Fig. 6.18). However, the release rate declined very rapidly so that after 22days it was lower than the N requirement of the lettuce plants, and the discrepancybetween the two rates increased with time. Nitrogen release from composts is usuallydescribed by first-order kinetic equations (Hadas and Portnoy, 1994, 1997) and isinfluenced by temperature, water content and other environmental factors that affectbiological and chemical processes. If the amount of N required daily by mature plantsgrown under intensive growth condition in greenhouses is taken to be 120–170 mg(Bar-Yosef, 1999), it is not likely that organic composts could contribute sufficientN, even under less favourable conditions for N degradation as presented in Fig. 6.18.

900

800

150OP–SCMN release

LettuceN uptake

100

50

00 10 20

Days

N r

ate

(mg

plan

t –1d–1

)

30 40 50

FIGURE 6.18 Comparison between the rate of N release from compost containing orange-peelamendment to separated cattle manure (OP-SCM) and the N requirements of lettuce. The N release datawere computed from Fig. 6.2 of Raviv et al. (2005), assuming application of 3 l of compost per plant. Thelettuce N uptake data were taken from Silber et al. (2003).

References 239

Nonetheless, inclusion of high-N composts in substrates may allow for reduced Napplication rates during the initial growth period. This is particularly important underorganic agriculture management and in situations where reduction of pollution hazardsis important. This finding may be particularly applicable in short-term crops suchas organic transplant production which usually takes up to 35 days (Raviv et al.,1998).

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