influence of antecedent soil moisture suction on saturated hydraulic conductivity of soils1
TRANSCRIPT
DIVISION S-6—-SOIL AND WATER MANAGEMENTAND CONSERVATION
Influence of Antecedent Soil Moisture Suction on Saturated HydraulicConductivity of Soils1
C. J. GERARD2
ABSTRACT
Studies were conducted to evaluate influences of antecedentsoil moisture suction on saturated hydraulic conductivities ofsoils. These studies revealed that saturated hydraulic conduc-tivity of a soil is a dynamic property which is markedly in-fluenced by antecedent soil moisture suction. Antecedent soilmoisture of < 0.33 bar suction, such as is often practiced withtrickle irrigation, caused marked reduction in the ability ofsoil to conduct water. Cases produced by microorganisms atlow suction, their subsequent entrapment, and their influenceon soil macrovoids are largely responsible for reduction inability of soil to conduct water. Reduction of effective soilmacrovoids by microbial growth, flowing water, and/or soilmoisture suction can also reduce soil permeability. These find-ings indicate that soil moisture suction and management such astrickle irrigation can influence the ability of soil to conductwater.
Additional Index Words: trickle irrigation, CO2 production,microorganism.
R ECENTLY trickle or daily flow irrigation has been pro-posed as a more efficient method of applying water to
growing plants. One of the advantages cited for trickle irri-gation is the maintenance of low soil moisture suction inthe effective root zone. It was observed that soil in a citrusgrove which was irrigated with 0.5 cm/day for 40 days haddrastically lower soil permeability. Research by Allison
(1), Christiansen (2), and Poulovassilis (5) establishedthat hydraulic conductivity of saturated soils undergochanges with time. These investigators attributed large re-duction in hydraulic conductivity of saturated soil to dete-rioration of soil aggregation, soil microorganisms, andchanges in volume in entrapped air. The influences of ante-cedent soil moisture greater than zero suction on saturatedhydraulic conductivity as evaluated according to proceduresby Klute (4) have not been determined. It was the purposeof this investigation to determine the influences of antece-dent soil moisture suction on saturated hydraulic conductiv-ity of soils and the causes for reduction in saturated hydrau-lic conductivity of soils. Results of these investigations andtheir implications are discussed.
MATERIALS AND METHODS
Ten-year-old citrus trees grown on Willacy fine sandy loamsoil were continuously irrigated at a rate of 0.5 cm/day for a40-day period in June and July, 1971. When water began topond on the soil surface during an irrigation, undisturbed 7.5 by7.5 cm cores were taken at soil depths of 0 to 7.5 and 7.5 to 15cm in the wetted zone near emitters and out of the wetted zone.Wetting of the cores was effected from the bottom and hydrau-lic conductivities of cores were determined by the method de-
GERARD: ANTECEDENT SOIL MOISTURE SUCTION ON SATURATED HYDRAULIC CONDUCTIVITY 507
Table 1—Characteristic textural and structural properties ofsurface 30 cm of Willacy fine sandy loam
Table 2—Average bulk density and saturated hydraulic con-ductivity of surface 15 cm of Willacy fine sandy loam
wetted and nonwetted by trickle irrigationemitters in citrus in 1971
SandSiltClayOrganic matterWater stable aggregate (>0. 125 mm)
77.613.78.71. 12.8
scribed by Klute (4). Water used in the hydraulic conductivitymeasurements had an ECe of 1.25 and a sodium absorption ra-tio of about 2.0. Bulk densities of cores were also determined.After the 40-day irrigation period, three pits, 60 cm wide and60 cm deep were excavated adjacent to the emitters. Emitterswere about midway between edge of tree canopy and tree trunk,120 cm from the latter. The sides of pits were divided into 100cm2 and numbers of roots in each square were counted. Thenumbers of roots in each square were expressed as a percentageof the total numbers of roots.
The saturated hydraulic conductivity and root data promptedresearch to evaluate influences of different antecedent soil suc-tions at 25C ± 2C on saturated hydraulic conductivities ofsoils. Willacy fine sandy loam soil was ground to pass through a0.5-mm sieve and wetted to 8 to 10% moisture using a spraygun. Properties of Willacy fine sandy loam are described inTable 1. Sufficient soil to prepare 7.5 by 7.5 cm cores of 1.4and 1.5 g/cm3 was weighed and compressed with a laboratorypress. Uniformity of compaction procedures was evaluated bymaking saturated hydraulic conductivity measurements. Ini-tially, cores with density of 1.4 g/cm3 had an average saturatedhydraulic conductivity of 7.0 cm/hour ± 0.8; cores with a den-sity of 1.5 g/cm3 had an average saturated hydraulic conductiv-ity of 3.0 cm/hour ± 0.4. The latter soil cores were studied ini-tially because the saturated hydraulic conductivity of this sou"under field conditions is often about 0.5 to 3.0 cm/hour (3).However, as shown in Table 2 in the field this soil can exhibitmuch higher saturated conductivity characteristics.
Cores were wetted and placed on a pressure plate and equili-brated at the desired soil moisture suction levels. After equili-bration, wetting of cores was effected from the bottom thenhydraulic conductivity was determined. This procedure was re-peated a minimum of three times. Saturated hydraulic conduc-tivities of soils were determined for 0.04, 0.1, 0.33, 0.67, 1.0,and 3.0 bar antecedent suction treatments. A minimum of 16cores were evaluated for each soil moisture suction.
Related studies were conducted to estimate the direct and in-direct role of microorganisms on soil permeability. After equili-bration to 0.1 and 1.0 bar suction, CO2 production for 10 daysby 2,500 g of Willacy fine sandy loam with approximate den-sity of 1.3 g/cm3 was determined by gas chromatographic tech-nique. The soil was rewetted and equilibrated to 0.1 and 1.0 barsuction and CO2 production determined for a second 10-dayperiod. Microbial growth on the soil surface maintained at 0.1bar suction for 5 to 20 days was observed and photographed.
Willacy fine sandy loam was heated to 600C. This causeschanges in properties, but as shown in Table 1, this soil has veryweak structure having only 2.8% water stable aggregates. Struc-tural changes due to heating would be small and possibly evennegligible. Furnace treated soil would be largely free of micro-bial activity, and therefore, the influences of gaseous produc-tion and entrapment on soil permeability would be negligible.Saturated hydraulic conductivity measurements and gaseousproduction by this soil indicated that this assumption was justi-fied. This soil was used to evaluate the importance of flowingwater and soil moisture suction on saturated hydraulic conduc-tivity of soils.
Cores with a range of bulk densities from 1.4 to 1.6 g/cm3
were made as previously described. Six cores for each densitytreatment were wetted from the bottom, and saturated hydraulicconductivities were determined. Each density treatment wasbrought to equilibrium with 0.33 bar pressure, saturated, andevaluated as to hydraulic conductivity. This procedure was re-peated a minimum of three times. Production of CO2 by 2,500
Wetted zoneSoil depth
cm0 - 7 . 57.5- 15.0
Bulk densityg/cm3
1.361.56
Kcm/ hour
0.590.28
Nonwetted zoneBulk density
g/cm3
1.241.65
Kcm/ hour
8.530.33
g of soil heated to 600C was also determined by gas chromato-graphic technique.
Air entrapment during the wetting of cores from the bottomwas evaluated on selected cores of untreated and furnacetreated soils. Cores with bulk densities of 1.4, 1.5, and 1.6 g/cm3
were made with soil wetted to 11.5 to 12.7% moisture. Afterweighing, water was added from the bottom of the cores. Dis-placed air during the wetting process was quantitatively deter-mined. The air entrapped was calculated by subtracting soilmoisture plus displaced air from total pore space. Air entrap-ment of cores was determined initially and after equilibrium to0.33 and 1.0 bar soil moisture suction. This procedure wasrepeated three times.
RESULTS AND DISCUSSION
Average saturated hydraulic conductivities and bulk den-sities of soil in citrus and wetted and nonwetted by trickleirrigation are reported in Table 2. The hydraulic conductiv-ity of surface soil after irrigation for 40 days was about 7 %of the conductivity of soil not wetted by trickle irrigation.The layer of reduced permeability at 7.5 to 15.0 cm soildepth may have contributed to the reduced conductivity ofsurface soil. Earlier observations indicated that root con-centrations near emitters were high. However, in July, soilin the vicinity of emitters was almost devoid of roots (Fig.1). The zone of high root concentration was located 40 to60 cm from emitters and 30 cm in depth.
The influences of 0.1 and 0.67 bar suction and dryingcycles on saturated hydraulic conductivities of soil areshown in Fig. 2. (Drying cycle is defined as the change inwater content from saturation until equilibration at desiredsoil moisture suction.) The saturated hydraulic conductivi-ties of soil approached equilibrium after one to three wet-ting and drying cycles. Conductivities of soils after threedrying cycles as influenced by soil moisture suction treat-
DISTANCE FROM EMITTER-CM
Fig. 1—Percentages of citrus roots with respect to depth anddistance from emitters in 1971.
508 SOIL SCI. SOC. AMER. PROC., VOL. 38, 1974
75 2D
1.0S
o 0.10 bar• 0.67 bar
I 0 I Z 3Drying Cyctot
Fig. 2—Influence of 0.1 and 0.67 bar soil moisture suction anddrying cycles on saturated hydraulic conductivity of Willacyfine sandy loam soil.
ments and initial saturated hydraulic conductivities ofabout 3 and 7 cm/hour are reported in Fig. 3. Maintenanceof low soil moisture suction, <0.33 bar, reduced the abilityof the soil to conduct water (Fig. 3). In the case of themore permeable cores, antecedent soil moisture <0.67 barsuction caused substantial reduction in saturated hydraulicconductivities of soil (Fig. 3). Possibly soil conditions oflower density cores were more favorable for gaseous pro-duction. The primary cause of reduced hydraulic conduc-tivity of saturated soil samples given by Poulovassilis (5)was gases produced by microorganisms.
In these investigations, as in those of Poulovassilis (5),swelling probably due to colloids and air entrapment, wasobserved. Gases produced in close proximity to macrovoidsin unsaturated soils probably diffuse to atmosphere withlittle influence on soil permeability. However, gases pro-duced and entrapped in capillary pores in unsaturated soilcan cause movement of particles, swelling and reduction inmean pore space, and hence a reduction in soil permeabil-ity. Poulovassilis (5) showed that partially degased satu-rated soil samples partially regained conductivity. However,the procedure of degasing soils does not ascertain whetherthe gases decrease saturated hydraulic conductivity byblockage or reduction in large pores. Gases removed fromsoil during the degasing process could cause substantial in-creases in mean pore space and permeability. Applyingvacuum to wet soil prior to saturation disturbed the coresespecially those of low density and high permeability.Christiansen (2) also pointed out that wetting of coresunder vacuum can cause soil structural changes and spuri-ous results.
The measured production of CO2 by soil after equilibra-tion at 0.1 and 1.0 bar suction is reported in Fig. 4. Soil
Hydraulic CcnductivHycm/hr
02 0.4 06 OB IX)Soil Water Suction - Bars
3.0
Fig. 3—Influence of antecedent soil moisture suction on satu-rated hydraulic conductivities after three drying cycles ofWillacy fine sandy loam soil with initial conductivities of 3and 7 cm/hour.
equilibrated at 0.1 bar suction produced 1 to 2.5 times moreCO2 than soil at 1.0 bar soil moisture suction. The initiallevel of microbial activity influences the amount of CO2production because the production of CO2 after rewettingand equilibrium at indicated suction levels (second cycle)was about twice the production of first cycle.
A photograph of undisturbed surface soil equilibrated to0.1 bar suction and an incubation period of 10 days isshown in Fig. 5. Mycelial strands can be observed on thesoil surface and especially in the large pores. Mycelialstrands appear to reduce the size of effective macrovoidsand probably reduce the permeability. Products of micro-bial growth may also cause reduction in soil permeability.
Reduction in saturated hydraulic conductivity of un-treated and furnace treated soils would be due to changes involume or in effective volume of macrovoids. Carbon diox-ide production of furnace treated soil was only 2 to 5%of untreated soil. Because gaseous production of furnacetreated soil was small, factors other than gaseous productionand entrapment would have to augment volume and size ofmacrovoids. It is postulated that flowing water and soil
300
5 250CO•5 200
* 150<su•8
100
50
0
SOL MOISTURE SUCTION—— O.I BAR
- 0.1 BAR
I 2 3 9 \Q4 5 6 7DAYS
Fig. 4—Influence of 0.1 and 1.0 bar soil moisture suction,time, and drying cycles on CO2 production by Willacy finesandy loam soil.
Fig. 5—Mycelial strands after 10 days on soil surface of Wil-lacy fine sandy loam maintained at 0.1 bar suction (30x )•
GERARD: ANTECEDENT SOIL MOISTURE SUCTION ON SATURATED HYDRAULIC CONDUCTIVITY 509
Table 3—Air entrapment during wetting from the bottom ofsoil cores at 0.33 bar suction. Cores were wetted and
brought to equilibrium with 0.33 bar pressurethree times or cycles
Table 4—Air entrapment during wetting from the bottom ofsoil cores at 1.0 bar suction. Cores were wetted and
brought to equilibrium with 1.0 bar pressurethree times or cycles
Bulk densityg/cms
1.41.51.6
1.41.51.6
0 cycle*
18.724.922.3
Furn16.218.417.9
1st cycle———————— %t
Wlllacy fine i23.324.524.1
2nd cycle
sandy loam23.323.924.1
ace treated Willacy fine sandy17.117.117.1
17.116.917.1
3rd cycle
23.022.021.9
loam17.116.715.7
Sf>content
«t19.520.921.8
9.911.512.2
* Soil was wetted to 12. 7% moisture prior to making cores.f Refers to volumetric percent of voids In soil.j Refers to volumetric water content at 0. 33 bar suction.
moisture suction could cause changes in pore geometry andcontribute to lower conductivity of soil to water. In hisstudies, Poulovassilis (5) estimated that changes in poregeometry caused a decrease of 15% in the initial hydraulicconductivity of saturated soil.
The reduction in hydraulic conductivities of furnacetreated soil was a function of initial saturated hydraulicconductivity as shown in Fig. 6. For soils with high densityor low hydraulic conductivity, the influence of flowingwater and soil moisture suction was small but of importancein soils of low density or high permeability. For example,this study would suggest that soil with hydraulic conductivi-ties of less than 4 cm/ hour was not markedly influenced byflowing water and soil moisture suction. However, waterflow and/or soil moisture suction on soil with initial satu-rated hydraulic conductivities of about 7 and 12 cm/hourreduced saturated conductivities by about 15 and 35%, re-spectively.
Air entrapment as influenced by drying cycles, soils andsoil moisture suction is reported in Tables 3 and 4. Air en-trapment when wetting the untreated soil was significantlygreater than air entrapment when wetting the furnacetreated soil. The differences in air entrapment between 0(initial) and one to three cycles were small and greatest for1.4 g/cm3 cores. It is surprising that air entrapment of soilfor 0 and one to three cycles and for 0.33 and 1 bar suctionwas not greatly different as shown in Tables 3 and 4. Thedifferences between initial saturated hydraulic conductivitiesof soil and after being equilibrated for one to three dryingcycles to soil moisture suction of 0.33 and 1.0 bar depictedin Fig. 3 do not appear to be due to volume of air entrap-
30S!
II 20
10
4 6 8 10Hydraulic Conductivity
(cm/hr)
12
Bulk densityg/cm3
1.41.51.6
0 cycle* 1st cycle 2nd cycle 3rd cycle
Wlllacy fine sandy loam17.520.922.1
23.723.621.6
21.523.022.4
22.723.322.6
HjOcontent»
17.318.719.6
Furnace treated Wlllacy fine sandy loam1.41.51.6
12.514.118.5
16.414.918.1
15.614.620.2
15.614.620.2
. 5.26.05.8
Fig. 6—Percent reduction in saturated hydraulic conductivityafter three drying cycles to 0.33 bar suction of furnacetreated Willacy fine sandy loam as a function of initialsaturated hydraulic conductivity.
Soil wetted to 11. 5 to 11. 9% moisture prior to making cores,t Refers to volumetric percent of voids In soil,t Refers to volumetric water content at 1. 0 bar suction.
ment. It is possible that the distribution of the entrapped airin the soil for 0 and one to three cycles and for 0.33 and 1.0bar suction are markedly different. Regardless, it is likelythat gases produced by microbial growth cause soil particlemovement which can cause significant reduction in effec-tive pore space and soil permeability. Increased gaseousproduction and the influence of gases on mean pore spaceat low soil moisture suction may be responsible for signifi-cant reduction in soil permeability. Soil particle mobilityand therefore soil structural changes would be greater inwet than in dry soil.
These findings show that soil moisture and managementsuch as trickle irrigation can influence the ability of soilsto conduct water. Factors such as soil moisture, residue andsoil temperature markedly influence microbial activity andtherefore can markedly influence gaseous production whichdirectly or indirectly can cause significant reduction in soilpermeability. Time dependent differences in saturated hy-draulic conductivities and water intake characteristics offield soils may largely be an expression of microbial activ-ity.
Trickle or daily flow irrigation will often maintain lowsoil moisture suction in a portion of the effective root zone.Root growth possibly can augment the influences of low soilmoisture suction and maintain more favorable soil waterintake characteristics in and around the emitters. However,as was found in studies with citrus, the reduction in the abil-ity of soil to conduct water can be serious enough to createsaturated soil conditions and significant loss of effectiveroots. Occasional drying in and around the emitters prob-ably can help maintain more favorable soil conditions forwater intake and root growth.