Ideal and saturated soil fertility as bench marks in nutrient management: II. Interpretation of chemical soil tests in relation to ideal and saturated soil fertility

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  • Ideal and saturated soil fertility as bench marks in

    nutrient management

    II. Interpretation of chemical soil tests in relation

    to ideal and saturated soil fertility

    Bert H. Janssen a,*, Peter de Willigen b

    aWageningen University, Department of Soil Quality, The NetherlandsbAlterra, Green World Research, Soil Science Centre, P.O. Box 47, 6700 AA Wageningen, The Netherlands

    Available online 18 April 2006

    www.elsevier.com/locate/agee

    Agriculture, Ecosystems and Environment 116 (2006) 147155Abstract

    In a previous paper (Part I), the ideal soil fertility and the saturated soil fertility were expressed on a relative scale, called soil fertility

    grade (SFG). In the current paper (Part II), the relation between SFG and soil test values is discussed. The required uptake of nutrients

    from the soil is translated into soil organic carbon, P-Olsen, exchangeable K, and pH (H2O) using relationships developed for a model on

    Quantitative Evaluation of the Fertility of Tropical Soils (QUEFTS). Target soil test values were calculated for target yields between 2

    and 10 Mg ha1 season1. The required uptake of soil nitrogen is a function of target yield, and it is linearly related to soil organic carbon.Results of the calculations indicate that when target yields are less than 78 Mg ha1, stover must be incorporated to maintain soil organiccarbon above the critical level of 6 g kg1. When yields are below 2 Mg ha1, also organic sources from outside the field have to bebrought in.

    The interpretation of chemical soil test values according to the ISF-SSF framework may be rather difficult in practice, as is demonstrated

    with eight African soils. The major reason is that the soil supplies of N, P and K seldom are in the same proportions as in ISF-SSF. For none of

    the used African soils replacement input or a neutral nutrient budget would be the best management option. Replacement input will often lead

    to inefficient use and even waste of nutrients. Optimum soil test values depend on target yield, but the ratios of soil test values do not depend on

    target yield. Therefore key values were established for the ratio of soil organic carbon to P-Olsen and for the ratio of soil organic carbon to the

    Abbreviations: CEC, cation exchange capacity (mmolc kg1); Ex-K, soil exchangeable K (mmol kg1); HSUgsK, maximum K uptake from soil

    (kg ha1 season1); HSUgsN, maximum N uptake from soil (kg ha1 season1); HSUgsP, maximum P uptake from soil (kg ha

    1 season1); IK, input ofK (kg ha1); IN, input of N (kg ha1); IP, input of P (kg ha1); IUgsK, K derived from input, present in grain and stover (kg ha

    1); IUgsN, N derived from input,present in grain and stover (kg ha1); IUgsP, P derived from input, present in grain and stover (kg ha

    1); ISF, ideal soil fertility, fertility at which the soil incombination with replacement nutrient input does exactly satisfy the nutrient demand of a maximally producing crop, provided no nutrients get lost; PhE,

    physiological efficiency (or internal utilization efficiency), ratio of grain yield (Y) to uptake in grain and stover (Ugs) (kg kg1); PhEN, physiological efficiencyof nitrogen, ratio of grain yield (Y) to uptake of nitrogen in grain and stover (UgsN) (kg kg1); PhEP, physiological efficiency of phosphorus, ratio of grain yield(Y) to uptake of phosphorus in grain and stover (UgsK) (kg kg1); pH (H2O), soil pH measured in a 1:2.5 extract of soil: water; P-Olsen, soil P extracted with0.5 M NaHCO3 (mg kg

    1); QUEFTS, Quantitative Evaluation of the Fertility of Tropical Soils; RAS, required amount of stover (Mg ha1); RF, recoveryfraction, fraction of applied nutrients that is absorbed by the crop in grain and stover (IUgs (I)1, kg kg1, subscripts c, a, refer to crop, accumulation; RFK,recovery fraction of applied K (kg kg1); RFN, recovery fraction of applied N (kg kg1); RFP, recovery fraction of applied P (kg kg1); Sav, soil availablenutrients (kg kg1); SFG, soil fertility grade, fraction of SSF; SSF, saturated soil fertility, fertility at which the soil by itself does exactly satisfy the nutrientdemand of a maximally producing crop; SOC, soil organic carbon (g kg1); SOM, soil organic matter (g kg1); SUgs, nutrients, derived from soil, present ingrain and stover (kg kg1); SUgsK, potassium, derived from soil, present in grain and stover (kg kg

    1); SUgsN, nitrogen, derived from soil, present in grain andstover (kg kg1); SUgsP, phosphorus, derived from soil, present in grain and stover (kg kg

    1); TEx-K, target soil exchangeable K (mmol kg1); TP-Olsen, targetsoil P extracted with 0.5 M NaHCO3 (mg kg

    1); TSOC, target soil organic carbon (g kg1); TUgs, nutrients present in grains and stover at target yield (kg ha1);

    TY, target yield (Mg ha1); UgsN, nitrogen present in grains and stover (kg ha1); UgsP, phosphorus present in grains and stover (kg ha

    1)* Corresponding author at: Department of Plant Sciences, Wageningen University, P.O. Box 430, 6700 AK, Wageningen, The Netherlands.

    Tel.: +31 317 482141; fax: +31 317 484892.

    E-mail address: Bert.Janssen@wur.nl (B.H. Janssen).

    0167-8809/$ see front matter # 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.agee.2006.03.015

  • class

    and

    ry fr

    relationships between nutrient uptake and chemical soil

    data and their use in the ISF-SSF framework. Section 3

    equations have been adopted to that pH. At ideal soil fertility

    pH must be optimum. A value of 6 for pH (H2O) is

    cosystems and Environment 116 (2006) 147155chemical soil test values and nutrient inputs in relation

    to target yield. Section 3 also discusses the interpretation of

    soil test values for N, P and K in practice. Section 4

    discusses the applicability of the framework of ideal and

    saturated soil fertility in practice, the implications of N:P:K

    proportions and the validity of neutral nutrient budgets and

    following equations are used to calculate the highest

    possible uptake (in grain and stover) from soil (HSUgsN,

    HSUgsP, HSUgsK, in kg ha1 season1):

    HSUgsN 5SOC g kg1 (1)1 1deals with maintenance of organic matter, required considered as optimum. For soils with pH (H2O) of 6, thesquare root of exchangeable K. Based on these key values, a new

    scheme has six classes for N and P ratios, and seven classes for N

    # 2006 Elsevier B.V. All rights reserved.

    Keywords: Nutrient input; Nutrient ratios; Nutrient use efficiency; Recove

    Target yield

    1. Introduction

    In a previous paper (Janssen and de Willigen, 2006),

    concepts from plant physiology, soil chemistry and

    agronomy were integrated into the framework of ideal soil

    fertility (ISF) and saturated soil fertility (SSF). Fertility itself

    was considered in a restricted sense as the capacity of the

    soil to supply nutrients to the crop. We alleged that the

    resulting coherent and transparent framework would enable

    the setup of nutrient management advisory frameworks, on

    the basis of chemical soil test values, for any crop at any

    place. In routine soil testing, chemical soil characteristics are

    assessed with the objective of obtaining a value that will

    help to predict the amount of nutrients needed to supplement

    the supply in the soil (Tisdale et al., 1985). The required

    nutrient supplement depends on the economics of fertilizer

    use. The ISF-SSF framework, however, takes sustainability,

    environmental protection and balanced plant nutrition as

    starting points.

    ISF and SSF were expressed in a relative scale, called

    soil fertility grade (SFG). The value of SFG was set at 1

    for saturated soil fertility (SSF). At the ideal soil fertility

    (ISF), SFG is a fraction (1 RF) of SSF, where RF standsfor the recovery fraction of input nutrients. ISF is

    precisely the steady-state soil fertility level that is

    obtained when nutrient input is equal to nutrient output

    in harvested products and no nutrients get lost. In the

    present paper, the relation of ISF and SSF with soil test

    values is discussed. Although a soil test is seen as a

    chemical method for estimating the nutrient-supplying

    power of a soil (Tisdale et al., 1985), direct relations

    between data from chemical soil analysis and nutrient

    supply by the soil are seldom presented. We apply

    equations developed for that purpose by Janssen et al.

    (1990) in the model on Quantitative Evaluation of the

    Fertility of Tropical Soils (QUEFTS).

    In this paper, Section 2 describes the QUEFTS

    B.H. Janssen, P. de Willigen / Agriculture, E148replacement input.ification scheme with recommended input ratios is presented. The

    K ratios.

    action; Replacement input; Soil tests; Soil fertility; Stover incorporation;

    2. Relations between soil fertility indices and

    nutrient uptake from soil

    2.1. Relations developed for the model QUEFTS

    The model QUEFTS (Janssen et al., 1990) uses relations

    between data from chemical soil analysis and nutrient

    uptake by maize. They are based on experimentally

    established regression equations derived from fertilizer

    field trials in Suriname and Kenya, described in earlier

    reports (Janssen, 1973, 1975; Guiking et al., 1983; Smaling

    and Janssen, 1987; Boxman and Janssen, 1990) and in

    unpublished student theses and reports of the Centre of

    Agricultural Research in Suriname (CELOS). For the sake

    of simplicity, we apply here the equations of the original

    QUEFTS paper (Janssen et al., 1990). Somewhat different

    equations proved more appropriate in specific areas

    (Smaling and Janssen, 1993; Samake, 2003), but they

    require more input data than those of the original version. In

    fertilizer trials designed to find relations between chemical

    soil analysis and nutrient uptake, it is essential that the crop

    grows under excellent conditions and that the nutrient under

    study is the major growth limiting factor. Only if soil fertility

    is less than saturated soil fertility (SFG < 1) for theparticular nutrient, and the other nutrients, water and

    sunshine are amply available, the uptake of the particular

    nutrient can be the highest possible uptake from soil.

    Between 10 and 20 soil properties have been investigated

    for the development of the original QUEFTS model, of

    which four proved best serving the purposes: soil organic

    carbon (SOC), available P according to the method of

    Olsen, exchangeable K (Ex-K), and pH (H2O) (Janssen

    et al., 1990). The relationships of the uptake of N, P and K

    with SOC, P-Olsen and Ex-K are affected by pH (H2O), for

    which different pH correction factors are applied in

    QUEFTS. Using one fixed value of pH, the pH correction

    factors can be left out provided the coefficients in theHSUgsP 0:35SOC g kg 0:5P-Olsen mg kg (2)

  • this 10 g kg is the critical SOM content of soils with

    200 g kg1 of (clay + silt), i.e. loamy sands to sandy loams.

    B.H. Janssen, P. de Willigen / Agriculture, Ecosysmust be worked into the soil is 2778 kg. The requiredHSUgsK 250Ex-K mmol kg1 2 0:9SOC g kg11: (3)

    In this paper a modified version of Eq. (3) is used to

    facilitate the calculation of the ratio of soil nutrients in

    Section 3.4:

    HSUgsK 250Ex-K mmol kg1SOC g kg11: (4)At a value of 20 g kg1 for SOC, Eqs. (3) and (4) are

    identical.

    Although the equations were found by regression

    analysis they be interpreted in terms of soil chemistry. It is

    assumed that the mass of the topsoil (020 cm) is

    2500 Mg ha1, and that C:N is 10; so, 1 g kg1 of SOCrepresents 2500 and 250 kg ha1 of organic carbon and N,respectively. Eq. (1) calculates that HSUgsN is 5 kg ha

    1

    per growing season per g kg1 of SOC, which correspondsto 2% of organic N in the topsoil. Because the ratio Ugs(Ugsri)

    1 is 0.8 for N (Table 2 in Part I), the total turnoverof N (Ugsri) is 1/0.8 or 1.25 as high as Ugs, and

    corresponds to 2.5% of topsoil organic N per season.

    Eq. (2) indicates that HSUgsP is related to inorganic P and

    to organic carbon and hence to organic P. Exchangeable K

    regulates HSUgsK. The inverse relationship between

    HSUgsK and SOC in Eqs. (3) and (4) takes into account

    that with increasing SOC the cation exchange capacity

    (CEC) increases, and hence the relative K saturation

    decreases for a given value of exchangeable K. Flaig et al.

    (1963) found that CEC of organic matter varies from 2.5

    to 4 mmolc per gram organic matter, which comes down to

    47 mmolc, say 5.5 mmolc per gram SOC. Neglecting the

    contribution of clay to CEC, CEC is estimated at 44 and

    220 mmolc per kg soil with 8 and 40 g kg1 SOC, the

    values of SOC found for ISF and SSF, respectively, at a

    target yield of 10 Mg ha1 (Table 1). Relative K saturationis then 5 and 11% at ISF and SSF, respectively. So, K

    saturation at ISF is 0.47 times K saturation at SSF, while

    Ex-K at ISF is around 0.1 times Ex-K at SSF. In reality, K

    saturation will be a little lower, depending on the

    contribution of clay to CEC. These are realistic values

    for K saturation.

    2.2. Calculation of soil fertility indices at ISF and SSF

    For the calculation of soil fertility indices as a function of

    SUgs, the QUEFTS equations are applied in the opposite

    way, again assuming an optimum pH (H2O) of 6. Using T for

    target, TSOC, TP-Olsen, and TEx-K are calculated with

    Eqs. (5)(7):

    TSOC 0:2SUgsN (5)

    TP-Olsen 2SUgsP 0:7TSOC (6)

    TEx-K 0:004 SUgsK TSOC: (7)Given a turnover rate of 2.5% per season (Section 2.1),

    375 kg C is converted into CO2 per ha per season, and hence

    a same amount of 375 kg C has to be applied with effective

    organic matter. Assuming the humification coefficient

    (Annex 2 in Part I) is 0.3, the required production of root

    C is 375/0.3 or 1250 kg C per ha, and the required

    production of root biomass is 1250/0.45 or 2778 kg ha1.The corresponding grain yield is 7.5 Mg ha1, because rootbiomass is 0.37 times grain biomass, as follows from Table 2

    in Part 1. At lower yields, roots and stubble alone cannot

    maintain SOC at the critical level of 6 g kg1. Other carbonsources are required. The source easiest at hand is stover.

    Assuming that stover has the same values as roots for

    humification coefficient (0.3) and C mass fraction

    (450 g kg1), the sum of roots and stover dry matter thatThe values of SUgs are in kg ha1, those of TSOC in

    g kg1, while TP-Olsen is in mg kg1, and TEx-K inmmol kg1. TP-Olsen and TEx-K in Eqs. (6) and (7) canonly be found after TSOC has been calculated with Eq. (5).

    In Table 4 of Part I (Janssen and de Willigen, 2006), an

    example was given of the procedure for the calculation of

    SUgs. The objective was to calculate the minimally required

    values of SUgs at ISF, and therefore maximum values of RF

    were applied, being 0.8, 0.4 and 0.6 for N, P and K,

    respectively (Part I). For a target yield of 10 Mg ha1, therequired SUgs values of N, P and K were 40, 17.1 and

    62 kg ha1, respectively. With Eq. (5) was calculated thatTSOC is 8 g kg1. The values for TP-Olsen and TEx-Kwere found with Eqs. (6) and (7), and with TSOC is

    8 g kg1.

    3. Some complications and implications

    3.1. Maintenance of critical SOM levels

    From Eq. (5) it follows that target soil organic carbon

    (TSOC) is linearly related to the uptake of soil N (SUgs) and

    hence, as shown in Section 2.2 of Part 1, to target uptake

    (TUgs) and target yield (TY). At low TY, the calculated

    TSOC may be below values considered as critical from a soil

    physical point of view. According to Pieri (1989), soils are

    physically degraded if the ratio of SOM to (clay + silt) is

    below 0.05. Janssen (1993) considered SOM contents of 15

    and 35 g kg1 desired for sandy loams and clay soils,respectively. For sandy loams containing 100 g kg1 of clayand 200 g kg1 of silt, and for clayey soils containing400 g kg1 of clay and 300 g kg1 of silt, the SOMrequirements set by Pieri and Janssen coincide. In the

    present paper we consider a SOM content of 10 g kg1 or25,000 kg ha1 (in a topsoil of 2.5 million kg ha1) as theabsolute minimum, which comes down to a SOC content of

    6 g kg1 or 15,000 kg carbon per ha. Following Pieri (1989),1

    tems and Environment 116 (2006) 147155 149

  • cosys

    rain y

    iven camount of stover (RAS in Mg ha1) is related to grain yieldvia:

    RAS 2:778 0:37TY: (8)The minimum required TY following from Eq. (8) is

    2.027 Mg ha1, rounded to 2 Mg ha1. If yields are lower,organic sources from outside the field or from agroforestry

    trees have to be brought in. The required stover incorpora-

    tion to maintain SOC at the critical level is given in Table 1.

    The incorporation of stover has consequences for the

    quantity of nutrients leaving the field, and hence for the

    B.H. Janssen, P. de Willigen / Agriculture, E150

    Table 1

    Soil data at SSF and ISF and inputs at ISF in relation to target maize yield

    Unit Target g

    10

    Soil data at saturated soil fertility (SSF)

    SOC g kg1 40.0P-Olsen mg kg1 29.0Ex-K mmol kg1 24.8

    Soil data at ideal soil fertility (ISF)

    SOC g kg1 8.0P-Olsen mg kg1 28.6Ex-K mmol kg1 2.0

    Inputsa at ideal soil fertility (ISF)

    Incorporated stoverb kg ha1 0Fractionc

    N kg ha1 200P kg ha1 28.5K kg ha1 155

    a Required input of N, P and K to reach target yield on soils with the gb Required incorporation of stover to maintain SOC at 6 g kg1.c Incorporated stover as fraction of total stover production.required nutrient inputs and the demanded levels of P-Olsen

    and Ex-K. Per ton of stover incorporated, the external input

    can be reduced by 5 kg N, 0.45 kg P and 12.5 kg K (see

    Table 2 in Part I). Because of the higher SOC content, P-

    Olsen can be lower than when no stover is incorporated

    Eq. (6), but Ex-K has to be higher Eq. (7).

    3.2. Nutrient management in relation to soil fertility

    Table 1 lists target soil fertility indices at SSF and ISF,

    nutrient applications at ISF, and rate of stover incorporation,

    all in relation to target yield. The minimum target yield is set

    at 2 Mg ha1, because at lower yields roots plus stover arenot sufficient to maintain SOC at 6 g kg1. It is assumed thatno losses occur. SOC and P-Olsen at SSF and ISF are

    linearly related to target yield. Exchangeable K decreases

    more than proportionally with decreasing target yields. The

    effect of stover incorporation on required nutrient input is

    stronger for K than for N and P, because straw is relatively

    high in K.

    In Table 1 SOC and Ex-K are much greater at SSF than at

    ISF. In contrast, P-Olsen is hardly higher at SSF than at ISF,

    especially when stover is incorporated. Because the recoveryfraction (RF) of N is set at 0.8, it follows from Eq. (9a) in Part I

    that soil fertility grade (SFG) for N is only 0.2 at ISF, and

    hence SOC at ISF is only 0.2 times SOC at SSF. For P and K,

    RF is set at 0.4 and 0.6, and hence SFG at ISF is 0.6 and 0.4 for

    P and K, respectively. However, P-Olsen at ISF must be much

    greater than 0.6 times P-Olsen at SSF, because SOC

    contributes to P supply (Eq. (2)) and SOC at ISF is only

    0.2 times SOC at SSF. On the other hand, TEx-K is at ISF

    much smaller than 0.4 times Ex-K at SSF, because TEx-K is

    linearly related to SOC (Eq. (7)). The data in Table 1

    underscore that P-Olsen alone and Ex-K alone are insufficient

    tems and Environment 116 (2006) 147155

    ield (Mg ha1)

    8 6 4 2

    32.0 24.0 16.0 8.0

    23.2 17.4 11.6 5.8

    15.9 8.9 4.0 1.0

    6.4 6.0 6.0 6.0

    22.9 16.5 9.9 3.4

    1.3 1.0 0.8 0.7

    0 555 1296 2000

    0.09 0.32 1.00

    160 117 74 30

    22.8 16.9 10.8 4.8

    124 86 46 6

    haracteristics for ideal soil fertility.to characterize soil P and soil K, respectively, as their

    interpretation is greatly affected by soil organic carbon.

    At ISF, nutrients are used in the most efficient way, but

    when soil fertility is between ISF and SSF, nutrients can still

    be used efficiently, provided nutrients are applied according

    to I = (1 SFG)TUgs(RF)1 (Eq. (6) in Part 1). Above SSF,the potential loss of nutrients is at least the quantity that

    forms the difference between available soil nutrients (Sav)

    and the total turnover (TUgsri) of nutrients of the crop:

    Sav TUgsri. The ratio SUgsri (Sav)1, an indicator fornutrient uptake efficiency, is then less than 1. Such soils

    should not receive any input but must be mined.

    High fertility usually is found in soils of volcanic, marine

    and fluvial origin, in areas with high livestock density and

    around villages. The SSF values of SOC 40 g kg1, P-Olsen29 mg kg1, and Ex-K 24 mmol kg1 for yields of10 Mg ha1 (Table 1) are close to those found nearhomesteads on loamy to clayey soils in Tanzania (Baijukya

    and De Steenhuijsen Piters, 1998), and to the data of Kenya,

    Field 11 in Table 2 referring to soils derived from volcanic

    ash in Kenya (Van der Eijk, 1997).

    On low fertility soils, mass fractions of N and P in roots

    and stover may be very low, resulting in immobilization of N

  • cosys

    ut, an

    Ma

    MZ

    5.6

    9.3

    10.3

    3.7

    47

    8.4

    99

    0.23

    0.29

    0.64

    192

    50.2

    93

    5.5

    1.0B.H. Janssen, P. de Willigen / Agriculture, E

    Table 2

    Soil data, uptake of soil nutrients, soil fertility grade, required nutrient inp

    Referencesa 1 2 3

    Country Kenya Tanzania Malawi

    Codeb Kwale Tumbi MZ 18

    Soil test values

    pH (H2O)c 6 6 5.8

    SOC (g kg1) 6.5 6 4.2P-Olsen (mg kg1) 0.5 7.5 12.5Ex-K (mmol kg1) 1.6 2 2.6

    Calculated uptake of soil nutrients (HSUgs), kg ha1d

    N 33 30 21

    P 2.5 5.9 7.7

    K 62 83 155

    Soil fertility grade (SFG)d

    N 0.16 0.15 0.11

    P 0.09 0.21 0.27

    K 0.40 0.54 1.00

    Required inpute

    N 209 213 224

    P 64.9 56.6 52.0

    K 156 119 0

    Proportions of uptake of soil N, P and Kf

    N 12.9 5.1 2.7

    P 1.0 1.0 1.0and P and reducing the uptake fraction of both applied and

    native soil N and P. If not sufficient N or P is available for

    immobilization, the formed SOM is of poor quality with

    high C:N and C:P ratios and low N and P mineralization.

    Because of the low nutrient supply, crops are forced to invest

    in roots at the expense of grain and straw, resulting in low

    physiological nutrient use efficiency (PhE). Sandy and

    loamy soils with lower SOC levels than the set minimum

    level of 6 g kg1 have low water-holding and nutrientretention capacities, so that nutrients are leached easily,

    further lowering the nutrient uptake efficiency. Such soils

    have a poor physical stability and are prone to slaking, run-

    off and erosion. The often resulting low plant density

    increases erosion risks and give weeds the opportunity to

    germinate. Under such conditions QUEFTS equations

    should not be applied. It would be better not to use these

    low-fertility soils for production of annual food crops.

    3.3. Interpretation of soil test values

    In principle, soil test values can be interpreted by

    interpolation or extrapolation of the data in Table 1. In

    practice, however, interpolation may prove quite difficult,

    because the proportions among soil test values often differ

    K 24.4 14.2 20.0 11.8

    a References: 1, Smaling and Janssen (1993); 2, Nyadzi (2004); 3, Makumba (

    (1997).b Code refers to the code of the particular field in the cited publication.c Some African soils were selected with pH (H2O) around 6.d Calculated uptake of soil nutrients (HSUgs) and corresponding soil fertilitye Required nutrient input for a target yield of 10 Mg ha1.f For the calculation of the proportions of uptake of soil N, P and K, HSUgsPtems and Environment 116 (2006) 147155 151

    d proportions of soil N, P and K

    4 5 6

    lawi Ivory Coast Tanzania Kenya Kenya

    21 Site VII S2 Field 13 Field 11

    6.1 5.8 5.8 6.2

    12.9 18 20 30

    3 6 3.4 59.6

    2.5 5 10 20

    65 90 100 150

    6.0 9.3 8.7 40.3

    48 69 125 167

    0.32 0.45 0.50 0.75

    0.21 0.33 0.31 1.41

    0.31 0.45 0.81 1.08

    169 138 125 63

    56.2 48.0 49.5 29.5178 143 50 19

    10.7 9.7 11.5 3.7

    1.0 1.0 1.0 1.0from those at ISF and SSF as given in Table 1. Another

    difficulty in practice is that many soils have higher or lower

    pH (H2O) than the optimum of 6, for which Table 1 has been

    set up. An example of the difficulties encountered during soil

    fertility evaluation is given in Table 2. The table presents soil

    test values of a number of African soils. These soils were

    selected as their pH was close to 6. Soils with other pH could

    have been used as well, but it would make the procedure

    more complicated because then pH correction factors

    (Janssen et al., 1990) must be incorporated in the equations

    relating soil test values to uptake of soil nutrients.

    The interpretation of the soil test data in Table 2 started

    with the calculation of the uptake of soil N, P and K (HSUgs),

    applying Eqs. (1), (2) and (4). The further evaluation was

    done for target maize yields of 10 Mg ha1.The target uptakes (TUgs) and replacement inputs for a

    target yield of 10 Mg ha1 are 200, 28.5 and 155 kg ha1 ofN, P and K, respectively (Table 1). Soil fertility grade (SFG)

    was calculated as the ratio of HSUgs to TUgs. In Part 1 of this

    paper, it is shown that at ISF, SFG for N, P, and K is 0.2, 0.6

    and 0.4, respectively. Only one of the calculated SFGs,

    namely for K in Kenya, Kwale, corresponded to ISF. The

    actual SFG was less than SFG at ISF (Table 3) in three of the

    eight soils for N, in seven soils for P, and only in one soil for

    8.1 7.5 14.4 4.1

    2003); 4, Van Reuler and Janssen (1996); 5, Mowo (2000); 6, Van der Eijk

    grade (SFG) are explained in the text.

    was set at 1.

  • cosystems and Environment 116 (2006) 147155

    kg1,ptake1. Re

    lsen)K. The required input was calculated as (TUgs HSUgs)(RF)1, where the recovery fractions (RF) were set at thevalues derived in Part 1: 0.8, 0.4 and 0.6 for N, P and K,

    respectively. The required inputs were of course higher than

    replacement input where actual SFG was less than SFG at

    ISF. Field 11 from Kenya was so rich in P-Olsen and

    exchangeable K, that the calculated required inputs were

    negative.

    In none of these soils the soil supplies of N, P and K were

    B.H. Janssen, P. de Willigen / Agriculture, E152

    Table 3

    Key valuesa of SOC (P-Olsen)1 and SOC (Ex-K

    p)1

    SOC (P-Olsen)1b SUgsN (SUgsP)1c

    1.37 >7

    SOC (Ex-K

    p)1 SUgsN (SUgsK)

    1

    3.33

    a For explanation see text.b SOC, P-Olsen and Ex-K are expressed in g kg1, mg kg1 and mmolc SUgs stands for uptake (in maize grain and stover) of soil nutrients. Ud Recommended ratios of inputs (I) of N, P and K, expressed in kg kge State of soil nutrients in relation to the given key valuesa of SOC (P-Oin the proportions 2.3:1:3.7 which are found at ISF (Table 4

    in Part I). As a consequence for none of these soils

    replacement input, or more general a neutral nutrient budget,

    would be the best management option. It is concluded that in

    practice the simple recommendation to keep nutrient inputs

    equal to nutrient outputs seldom is correct and often will

    result in inefficient use and even waste of nutrients. Nutrient

    management cannot do without chemical analysis of at least

    soil N, P, K and pH, and should be tailored to target yields.

    3.4. Ratios of soil test values as a tool in nutrient

    management

    From Table 2 it was inferred that soils usually do not

    supply nutrients in appropriate ratios. Which ratios are

    appropriate depends on soil fertility level. At SSF, N, P and

    K are taken up from the soil in the optimum proportions for

    maize (7:1:5.5, Table 4 in Part I). As shown in Section 3.1 in

    Part I, neither soil nutrients nor input nutrients are taken up

    in optimum proportions at ISF, but the sums of N, P and K

    taken up from both soil and input are in the optimum

    proportions of 7:1:5.5. Although the required soil test values

    depend on target yield level, the ratios of soil test values do

    not change with target yield. Hence, these ratios may be usedfor the interpretation of soil nutrient states and for advice on

    the relative amounts of N, P and K to be added.

    In Table 3, four key values of the ratios of the uptakes of

    soil N, P and K are presented. One value concerns ISF.

    Another one is the balanced situation, where the proportions

    of uptake of soil N, P and K are 7:1:5.5. The other two key

    values refer to nutrient ratios extremely out of balance with

    one nutrient maximally diluted (maximum (PhE)), and the

    other maximally accumulated in the crop (minimum (PhE)).

    IN (IP)1d State of soil N and Pe

    Only N Extreme N deficiency

    >7 Severe N deficiency

    7 N deficiency, as at ISF

    3.57 Moderate N deficiency

    3.5 Balanced soil N/P, as at SSF

  • cosysand

    5SOC 1:29 250 Ex-K SOC1; orSOC2 Ex-K1 is 64:5:It is more convenient to use SOC

    Ex-K

    p 1instead of

    SOC2 (Ex-K)1.At balanced N/K, SOC

    Ex-K

    p 1is

    64:5

    pwhich is

    8.03. At lower values of SOC (P-Olsen)1 andSOC

    Ex-K

    p 1, N is limiting, and at higher values P

    and K are limiting. Once more it is stressed that these

    relationships are valid for soils with a pH (H2O) around 6.

    When the uptakes of soil nutrients are balanced, also the

    uptakes of input nutrients must be balanced. The appropriate

    ratios of the inputs (IN, IP, IK) can be calculated with the

    help the recovery fractions of N, P and K (RFN, RFP and

    RFK), which have been set at 0.8, 0.4 and 0.6, respectively:

    IN : IP IUgsN RFN1 : IUgsP RFP1 7=0:8 : 1=0:4 3:5 : 1

    IN : IK IUgsN RFN1 : IUgsK RFK1 1:29=0:8 : 1=0:6 0:97 : 1; rounded 1 : 1:

    The values of 3.5 and 1.0 are found in Column 3 in

    Table 3 for balanced soil N/P and balanced soil N/K,

    respectively, in Column 4.

    At ISF, the ratios of the inputs are by definition equal to

    the ratios of target uptakes. So the ratio of the inputs of N and

    P (IN/IP) is 7, and the ratio of the inputs of N and K (IN/IK)

    is 1.29. The uptake of soil nutrients (HSUgs) at ISF is also a

    function of the recovery fraction: (HSUgs) = (1 RF) TUgs(see Part 1). It follows (see also Table 3):

    HSUgsN HSUgsP1 7 1 RFN 1 RFP1

    70:2=0:6 2:33;and

    HSUgsN HSUgsK1 1:29 1 RFN1 RFK1

    1:290:2=0:4 0:65:The values of 2.33 and 0.65 are found in Column 2 of

    Table 3 for N deficiency, as at ISF in Column 4. The

    corresponding SOC (P-Olsen)1 and SOCEx-K

    p 1(Column 1) were again derived from Eqs. (1), (2) and (4).

    In a similar way, the values of SOC (P-Olsen)1 andSOC

    Ex-K

    p 1, and the corresponding IN/IP and IN/IK

    for the other key uptake ratios of soil nutrients in Table 3

    were calculated. At ISF, the ratios SOC (P-Olsen)1 andSOC

    Ex-K

    p 1are considerably lower than for soils with

    balanced nutrient supplies. At lower ratios than at ISF, more

    N, and at higher ratios more P and K must be applied than

    corresponds with replacement input. At the extreme

    ratios, only the deficient nutrient should be applied. At

    extreme P deficiency, SUgsN (SUgsP)1 must be 20. When P-1

    B.H. Janssen, P. de Willigen / Agriculture, EOlsen is 0, the ratio SOC (P-Olsen) is 14.3 (=5/0.35).4.2. Nutrient proportions

    The proportions of soil nutrients often differ from theTherefore SOC (P-Olsen)1 for extreme P deficiency couldnot be assessed.

    4. Discussion

    4.1. Application of the ISF-SSF framework

    The second part of this paper mainly deals with the

    relation between the uptake (in grains and stover) nutrients

    derived from the soil (SUgs) and chemical soil test values.

    This relation is the most vulnerable component of the ISF-

    SSF framework, because not many field trials have been

    designed especially for the establishment of the relation

    between soil test values and uptake of soil nutrients. We have

    used the relationships as applied in the model QUEFTS. Soil

    pH (H2O) was set at the optimum value of 6. Studies are on

    the way to establish similar soilcrop relationships as used in

    QUEFTS, for a wide range of soils and ecological zones,

    also for other crops than maize.

    Given the fact that additions of nutrients are practically

    always needed, farmers ask for advice about application

    rates. Recommendations are sometimes presented in

    models, but mostly in booklets with tables, and usually

    based on soil analytical data. In many tropical environments,

    such tables are not available, and extension officers and even

    agronomy researchers may feel uncertain about the

    interpretation of the soil data (e.g. Drechsel et al., 1996;

    Struif Bontkes and Wopereis, 2003; Bruulsema, 2004).

    Moreover, recommendations should take environmental

    risks of nutrient emissions into account.

    As said in Part I (Janssen and de Willigen, 2006), a major

    advantage of the proposed framework with ideal and

    saturated soil fertility as bench marks is that it can be used as

    an building set and be applied to all practical situations, even

    if no local experimental results are known. Moreover, the

    reasoning and line of thinking underlying the ISF-SSF

    framework are helpful in designing effective field trials. It is

    possible to calculate the required soil test values for SSF and

    ISF, and the corresponding optimum inputs for each nutrient.

    In principle, recommendations for soils with other chemical

    data can be formulated by interpolation of the ISF-SSF

    framework. Five soil fertility classes can be distinguished:

    (i) over-saturated, above SSF; (ii) SSF; (iii) satisfactory soil

    fertility, between SSF and ISF; (iv) ISF; (v) low soil fertility,

    below ISF. The corresponding recommendations for nutrient

    inputs are: no input for soil fertility classes (i) and (ii), less

    than replacement input for (iii), replacement input for (iv),

    and more than replacement input for (v). Eq. (6) in Part I

    forms the guideline for input (I) recommendations:

    I = (1 SFG)TUgs(RF)1.

    tems and Environment 116 (2006) 147155 153proportions in replacement input. As a consequence, the

  • or

    in

    th

    recommendations according to the ISF-SSF than for

    cosysteminput of one nutrient must be relatively high and that of

    another nutrient relatively low. Replacement input may be

    advised without knowledge of soil analytical data. It will,

    however, be an exception rather than a rule that replacement

    input is the best thing to do. ISF is the only soil fertility level

    for which replacement input is recommendable. Never-

    theless, it is obvious that also at ISF nutrient input must be

    higher than the nutrient output in the crop if nutrient losses

    occur. It was also shown in Part I that when nutrient input is

    equal to the output in crops and losses, ISF is still the only

    level of soil fertility that is in steady-state, at least for

    nutrients that do not accumulate in the soil.

    Recommendations on nutrient inputs should be based on

    soil analysis, and be tailored to target yields. At soil fertility

    levels lower or higher than ISF, nutrient input must be higher

    or lower than replacement input. Inappropriate ratios among

    soil nutrients are a major reason why replacement input is

    seldom recommendable. Because nutrient ratios have a

    strong impact on nutrient use efficiency, it was tried to

    establish key ratios of chemical soil test values. They were

    derived from the ratios of nutrients in crops, recovery

    fractions of input nutrients and the equations used for the

    translation of uptake of soil nutrients into chemical soil test

    values. Three key ratios were presented for ratios SOC (P-

    Olsen)1 and four for SOCEx-K

    p 1, resulting in six

    classes of recommended input ratios of N and P, and seven

    classes of recommended input ratios of N and K.

    4.3. Soil organic matter

    If target yields are low, the minimum requirement for soil

    organic matter is not related to soil N supply, but to soil

    structure stability. In sandy loams, minimum soil organic

    carbon (SOC) to prevent collapse of soil structure was set at

    6 g kg1 (as derived from literature). It was shown that SOCcannot be maintained at this critical level by roots and

    stubble alone when maize grain yield is below 78 Mg ha1,and not by roots plus incorporated stover when grain yield is

    below 2 Mg ha1. Soils should better not be planted toannual food crops, when yields of maize receiving

    replacement nutrient input are below 2 Mg ha1. Theyshould be left to nature, or used for grazing or for perennials

    (tree crops). Sometimes it is worthwhile to grow highly

    valuable horticultural crops on these soils, because they pay

    the investments of fertilizers, irrigation and biocides.

    On the other hand, there is little wisdom in trying to get

    SOC contents far above the minimum requirement,

    because that demands huge applications of stover and

    organic manure and locks up large quantities of N and P in

    the soil. Moreover, unnecessarily high SOC contents may

    cause low availability of K, and stimulate leaching of N

    and P. The latter happens especially in ecological zones,

    such as in Western Europe, where during the winter period

    nutrient uptake by crops mostly is absent while miner-

    alization may continue but slowly, and rainfall exceeds

    B.H. Janssen, P. de Willigen / Agriculture, E154evapotranspiration.recommendations according to systems where profit

    maximization is strived at. Does this mean that yields are

    lower when the ISF-SSF framework is followed? Not

    necessarily. By taking balanced nutrient proportions as a

    leading guideline, the part of yield response curves

    representing diminishing returns to a single nutrient is

    shortened. As has been stated by De Wit (1992), inputs lose

    their variable character when other growing conditions are

    optimized. Balanced nutrient proportions are part of those

    optimized growing conditions. Important is that the overall

    use efficiency increases when all resources are considered at

    the same time.

    Another question is in what way the ISF-SSF framework

    differs from the model QUEFTS. First of all, the ISF-SSF

    framework explicitly takes sustainability, environmental

    protection and balanced plant nutrition as starting points and

    not the economics of fertilizer use. Further the ISF-SSF

    framework is simpler than QUEFTS, mainly because the

    physiological efficiency (PhE) is set at a default optimum

    value while QUEFTS takes into account that PhE can vary

    between the minimum and maximum values that are shown

    in Table 3 of Part I (Janssen and de Willigen, 2006). There

    is also a great resemblance in that both systems make use of

    the same equations for quantitative interpretation of soil

    data.

    The ISF-SSF framework deals in the first place with

    tropical agroecosystems, but applies equally well to affluent

    countries coping with nutrient surpluses, eutrophication of

    the environment and degradation of natural ecosystems.

    Finally, did we learn something from the development of

    ISF-SSF framework? Yes, a lot, or better we were reminded

    to many simple facts in soilcropenvironment relations. To

    mention some items:

    ideal soil fertility is different for N, P and K; soil N may below while soil P must be high,

    from an environmental as well as from an agronomicpoint of view it is the question whether neutral nutrient

    budgets or replacement input of nutrients form a well

    thought-out concept,

    soil organic matter is the most important soil fertilitycharacteristic, but this does not imply that it should be

    maintained at very high levels,

    soil test values should be considered in mutual connec-ofrive at other nutrient management recommendations than

    e existing systems. The answer is that the application rates

    fertilizers in general will be somewhat lower forarF framework. It is not a new methodology of soil analysis

    field research. It may be seen as a new methodology of

    terpretation of soil test values.

    The next question is then: does ISF-SSF frameworkSS4. Is the ISF-SSF framework different?

    The question has been raised what new is about the ISF-4.s and Environment 116 (2006) 147155tion.

  • It is our hope that the ISF-SSF framework can find its way

    as a building set, especially in those areas where soil testing

    still has to take off.

    from the proportions required for ISF. Because replacement

    input is only advisable at the ratios of soil test values found

    recovery fractions of input nutrients. For replacement input,1

    Zanderij Soils in Suriname. Wageningen Agricultural University Papers

    90-5. Agricultural University, Wageningen, pp. 7399.

    Bruulsema, T.W., 2004. Understanding the science behind fertilizer recom-

    mendations. Better Crops 88 (3), 1619.

    De Wit, C.T., 1992. Resource use efficiency in agriculture. Agric. Syst. 40,

    125151.

    B.H. Janssen, P. de Willigen / Agriculture, Ecosystems and Environment 116 (2006) 147155 155the appropriate ratio of SOC (in g kg ) to P-Olsen (in

    mg kg1) was calculated to be 0.28, and the appropriate ratioof SOC to the square root of Ex-K (in mmol kg1) was foundto be 5.7, in soils with a pH (H2O) of 6.

    It was calculated that SOC cannot be maintained at the

    critical level of 6 g kg1 by roots and stubble alone whenmaize grain yield is below 78 Mg ha1, and not by rootsplus incorporated stover when grain yield is below

    2 Mg ha1. Hence, soils should better not be planted toannual food crops, when yields of maize receiving

    replacement nutrient input are below 2 Mg ha1.Although soil organic matter is the most important soil

    fertility characteristic, it is not efficient to try to keep it at

    very high levels.

    Acknowledgement

    The authors gratefully acknowledge the critical com-

    ments by Dr. Ellis Hoffland and Prof. Oene Oenema on a

    previous version of the manuscript.

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    Ideal and saturated soil fertility as bench marks in nutrient managementIntroductionRelations between soil fertility indices and nutrient uptake from soilRelations developed for the model QUEFTSCalculation of soil fertility indices at ISF and SSF

    Some complications and implicationsMaintenance of critical SOM levelsNutrient management in relation to soil fertilityInterpretation of soil test valuesRatios of soil test values as a tool in nutrient management

    DiscussionApplication of the ISF-SSF frameworkNutrient proportionsSoil organic matterIs the ISF-SSF framework different?

    ConclusionsAcknowledgementReferences

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