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

    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: (B.H. Janssen).

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

  • class


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    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


    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


    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...