# 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...

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