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Soil & Tillage Research 79 (2004) 233–238

Prediction of the soil structures produced by tillage

A.R. Dextera,*, M. Birkasb

aInstitute of Soil Science and Plant Cultivation (IUNG), ul. Czartoryskich 8, 24-100 Pulawy, PolandbSzent Istvan University, PaterKaroly u.1, 2130 Godollo, Hungary

Abstract

Data are presented for the amount of clods >50 mm produced when five different soils were tilled at a range of different,

naturally occurring water contents. The optimum water content for soil tillage is defined as that at which the amount of clods

produced is minimum. The amount clods produced at this optimum water content is shown to be linearly and negatively

correlated with the value of Dexter’s index S of soil physical quality. This results in a rational model for soil tillage that enables

predictions to be made for all different soils and conditions. Pedo-transfer functions can be used to estimate the input parameters

for the model for cases, for which measured values are not available. It is concluded that for soils with good physical condition

(i.e. S > 0.035), no clods >50 mm are produced during tillage.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Clods; Pedo-transfer functions; Soil structure; Soil water content; S-theory; Tillage

1. Introduction

It has long been known that there is an optimum

water content for tillage at which the number of large

soil aggregates or clods produced is minimum (e.g.

Sitkei, 1967; Ojeniyi and Dexter, 1979). Tillage when

the soil is wetter or drier than this optimum results in

the production of a greater number of large clods that

must then be broken down in one or more subsequent

tillage operations. For example, according to Sitkei

(1967), with medium-textured Hungarian soil the

optimum water content at ploughing to obtain a nicely

crumbled structure is 21 kg (100 kg)�1. Then, the soil

* Corresponding author. Tel.: +48 81 886 3421;

fax: +48 81 886 4547.

E-mail address: tdexter2@iung.pulawy.pl (A.R. Dexter).

0167-1987/$ – see front matter # 2004 Elsevier B.V. All rights reserved

doi:10.1016/j.still.2004.07.011

can be ploughed with minimum clod production. If the

soil water content is below 16 kg (100 kg)�1, the

medium-textured soil is considered to be dry, and any

cultivation results in clod production. Soil with 18–

24 kg (100 kg)�1 water content is considered to be

moist. In this range of water contents, clod production

during ploughing is minimum and provides the best

conditions for ploughing. In the 24–25 kg (100 kg)�1

water content range, the soil is wet but it is still not

smearing. At water contents above 25 kg (100 kg)�1,

the wet soil can be cultivated only by slicing.

An attempt to predict the soil structures resulting

from tillage was made by Dexter (1979). This involved

an empirical model in which the resulting soil

structural parameters were estimated as the product

of a number of experimentally determined factors

which included soil type, soil water content, tillage

.

A.R. Dexter, M. Birkas / Soil & Tillage Research 79 (2004) 233–238234

implement, previous crop, etc. One conclusion from

this work was that the results of tillage depend much

more on the soil condition than on the type of tillage

implement used.

The extent to which soil breaks down or crumbles

has been shown to depend on the soil friability (Watts

and Dexter, 1998; Dexter and Watts, 2001). Recently,

it has been shown that there is a correlation between

friability and the shape of the water retention curve

because both depend on the soil micro-structure. In

particular, it has been shown that there is a correlation

between the measure of friability, F1, and the slope, S,

of the water retention curve at its inflection point

(when plotted as ln (h) against the gravimetric water

content, u). This empirical relationship may be written

(Dexter, 2004b):

F1 ¼ 15:0S (1)

As shown in Dexter (2004c), S is related to the

sharpness of the pore size distribution which is indi-

cative of the presence of micro-structure. S has been

described by Dexter (2004a–c) as an index of soil

physical quality that can be used in the prediction of

soil friability and break-up of soil during tillage.

The van Genuchten (1980) equation for water reten-

tion is

u ¼ ðusat � uresÞ½1 þ ðahÞn��m þ ures (2)

where usat and ures are the saturated and residual water

contents, respectively; u and h the content and ‘‘suc-

tion’’ (equal to the modulus of the matric water

potential) of the soil water; a the scaling factor for

h; and m and n are parameters that govern the shape of

the fitted curve. In the work presented here, h is in hPa

and u is in kg kg�1.

It is shown in Dexter and Bird (2001) that the

modulus of the water potential at the inflection point,

Table 1

Properties of the soils used in the tillage experiments

Soil Location Clay content

(kg (100 kg)�1)

Silt

(kg

Soil 1 Hatvan 35 42

Soil 2 Hatvan 40 28

Soil 3 Hatvan 50 26

Soil 4 Hatvan 60 33

Soil 5 Szolnok 35 38

a Estimated values (see text).

when this is plotted as ln (h) against u is:

hi ¼1

a

1

m

� �1n

(3)

Substitution back into Eq. (2) gives the water content

at the inflection point as

ui ¼ ðusat � uresÞ 1 þ 1

m

� ��m

þures (4)

As shown in Dexter (2004a), this gives for the

slope of the water retention curve at the inflection

point:

S ¼ �nðusat � uresÞ 1 þ 1

m

� ��ð1þmÞ(5)

For simplicity, we have assumed that m = 1 � 1/n,

according to Mualem (1986).

In this paper, we test and calibrate the relationship

between S and the soil structures resulting from tillage

using field data from a range of Hungarian soils. We

chose to measure the amount of clods >50 mm

because these have no agronomic value and often

create problems for soil management. Therefore, the

minimization of the amount of these clods is important

in practice.

2. Materials and methods

2.1. Soils

Five different Hungarian soils were used in the

tillage experiments. Some key properties of these are

shown in Table 1. All the soils lie in the Carpathian

basin and are Calcic chernozems formed on loess.

Hatvan is located north-east from Budapest on the

northern edge of the basin. Szolnok is on the river

content

(100 kg)�1)

Organic matter

(kg (100 kg)�1)

Bulk density

(Mg m�3)

3.2 1.40

3.4 1.50

3.2a 1.36

3.5a 1.36

3.5a 1.39

A.R. Dexter, M. Birkas / Soil & Tillage Research 79 (2004) 233–238 235

Fig. 1. Amounts of clods >50 mm produced during tillage of the

five experimental soils as functions of gravimetric water content.

Note that the scales on the y-axis for the different soils are different.

Tisza at the centre of the Great Hungarian plain. Soil

bulk density (Mg m�3) was measured using undis-

turbed cores (50 mm diameter and 25, 50 or 100 mm

long depending on the purpose) taken from a depth of

0–0.5 m. Five cores were taken at each depth for each

treatment point.

2.2. Water contents

For each soil, a range of water contents was

obtained as follows (Birkas, 2000). Soil water content

was determined in the 0–50 cm layer by different

methods: (1) sampling from the vertical wall of soil

pits according to Niekrashoff with five replications,

(2) lifting, weighing and analysing of monoliths to the

tillage depth (between 0 and 0.40 m) with five

replications, (3) measuring soil strength and water

content for each 25 mm increment with an electronic

penetrometer (Daroczi and Lelkes, 1999) using 5–10

replications.

The ranges of water contents used in the tillage

experiments were naturally occurring. Artificial

wetting was not used. The differences between tillage

seasons (from June to October), and natural wetting

and drying of soils were utilized in long-term trials.

Water contents of soils were measured twice daily, and

the necessary soil water contents were selected from

the long-term data. Water contents were measured

gravimetrically by oven drying at 105 8C.

2.3. Tillage

The results presented here are from the use of a

mouldboard plough. Tillage was done to depths of 30–

32 cm for soils 1 and 2, and to depths of 22–25 cm for

soils 3–5. The tillage experiments were done in the

following years: soil 1, 2002; soil 2, 1984–1985; and

for soils 3–5, 1989.

2.4. Sieving

The soil structures produced by tillage were

quantified as follows. Samples of the tilled layer of

approximately 16–21 kg were collected and sieved

with a minimum of 6 and a maximum of 10

replications at each water content.

3. Experimental results

Soil structures resulting from tillage are expressed

here as the amount of clods >50 mm as measured by

sieving. These are shown as a function of soil water

content at the time of tillage as shown in Fig. 1. For

each soil, a minimum amount of clods are produced at

a certain water content. This is defined as the optimum

water content for tillage. Tillage of soil that is either

wetter or drier than optimum results in greater clod

production.

In the remainder of this paper, we consider only the

amount of clods produced when tillage of the top-soil

is done at this optimum water content. The minimum

amounts of clods >50 mm were 22.0, 40.7, 30.3, 37.0

and 18.5 kg (100 kg)�1 for soils 1, 2, 3, 4 and 5,

respectively.

Since the water retention characteristics of the soils

were not measured they were estimated using the

pedo-transfer functions of Wosten et al. (1999) using

the appropriate values for these five experimental soils

presented in Table 1. The use of pedo-transfer

functions for this purpose is described and discussed

in Dexter (2004a). The resulting parameters of the van

A.R. Dexter, M. Birkas / Soil & Tillage Research 79 (2004) 233–238236

Fig. 2. Amounts of clods produced during tillage of the five

experimental soils at the optimum water content as a function of S.

Genuchten equation for soil water retention (Eq. (2))

were used to obtain estimates of S using Eq. (5).

The relationship between the amount of clods

>50 mm, C (%), produced by tillage and the value of S

is illustrated in Fig. 2. The line in the figure is the fitted

regression equation

C ¼ 94:5ð�4:3Þ � 2630ð�174Þ S; r2 ¼ 0:99;

P ¼ 0:00063 (6)

It is interesting to note that the value of S that gives

C = 0 is predicted from Eq. (6) to be S = 0.036. This is

not significantly different from the ‘‘critical’’ value of

Table 2

Typical values of S for the 12 FAO/USDA soil texture classes together with t

The values of the parameters usat, a and n of Eq. (2) were calculated using

and bulk density (r) in the pedo-transfer functions of Wosten et al. (199

FAO/USDA texture class Clay % Silt % OM % r (M

cl 60 20 4.47 1.24

sa cl 42 7 3.61 1.33

si cl 47 47 3.85 1.30

cl l 34 34 3.22 1.37

si cl l 34 56 3.22 1.37

sa cl l 27 13 2.89 1.41

l 17 41 2.41 1.47

si l 14 66 2.26 1.49

si 5 87 1.83 1.55

sa l 10 28 2.07 1.51

1 sa 4 13 1.78 1.55

sa 3 3 1.73 1.56

Note: sa: sand, si: silt, l: loam, cl: clay.

S = 0.035 that was suggested by Dexter (2004a–c) to

be the boundary between soils with ‘‘good’’ soil

physical condition and ‘‘poor’’ soil physical condition.

These results, therefore, support the proposed ‘‘criti-

cal’’ value and also show that soils in ‘‘good’’ soil

physical condition are not expected to produce clods

>50 mm when tilled. However, it should be noted that

the ‘‘critical’’ value of S = 0.035 is somewhat arbitrary,

and that no sudden change of soil behaviour occurs at

this point.

The excellent relationship in Eq. (6) is probably

due in part to the fact that we have applied pedo-

transfer functions to a range of soils of similar

pedogenic origin. If our five soils had different

dominant clay minerals, for example, then we would

not expect to obtain such a high correlation coefficient

from the use of pedo-transfer functions. Of course, it

would be best to obtain all values of S from accurately

measured water retention characteristics in order to

avoid the limitations associated with pedo-transfer

functions, but these were not available in the present

study.

4. Predictions using pedo-transfer functions

Given the excellent relationship between S and the

amount of clods, C, produced during tillage, we can

now make some predictions using the pedo-transfer

functions. We do this for each of the 12 soil texture

he parameters used in their calculation as described in Dexter, 2004a.

the values for particle size distribution, organic matter content (OM)

9).

g m�3) usat (kg kg�1) a (hPa)�1 n S

9 0.395 0.0217 1.103 0.0296

4 0.335 0.0616 1.139 0.0317

9 0.362 0.0220 1.104 0.0273

6 0.324 0.0400 1.127 0.0285

6 0.325 0.0226 1.129 0.0290

4 0.299 0.0727 1.169 0.0326

4 0.278 0.0314 1.208 0.0354

2 0.269 0.0134 1.245 0.0385

2 0.243 0.0045 1.392 0.0485

8 0.258 0.0400 1.278 0.0405

9 0.239 0.0534 1.406 0.0488

6 0.226 0.0671 1.581 0.0594

A.R. Dexter, M. Birkas / Soil & Tillage Research 79 (2004) 233–238 237

Fig. 3. Predictions of the amounts of clods >50 mm produced

during tillage of soils of the 12 FAO/USDA soil texture classes

as functions of bulk density.

classes of the FAO/USDA classification system. The

mean values of properties for each of the texture

classes used in the pedo-transfer functions are

presented in Table 2. The values of organic matter

content given in Table 2 were obtained from soils in

the humid, temperate climate of northern Europe.

Values of organic matter content in other climatic

regions may be expected to be different. For example,

in Mediterranean climatic regions, which are warmer

and drier, organic matter contents will be smaller.

The predictions of clod production during tillage at

the optimum water content are shown in Fig. 3.

It must be emphasized that pedo-transfer functions

illustrate only typical values and trends. They may not

be expected to produce accurate predictions for the

physical properties of individual soils. Limitations

associated with the use of pedo-transfer functions have

been discussed by Wosten et al. (2001).

5. Conclusions

A very good correlation has been found between

the amount of soil clods >50 mm produced by tillage

of the five experimental soils at the optimum water

content and the predicted value of the index of soil

physical quality, S. This correlation exists because

soil break-up during tillage and the value of S, as

determined from the soil water retention curve, both

depend on the amount of micro-structure present in

the soil.

It has previously been shown that values of S have

the same physical meaning for all different soil types.

Therefore, different soils may be compared directly in

terms of their S values. Because of this universality of

S values, it has been possible to produce predictions of

the amounts of clods >50 mm produced by tillage of

all 12 different soil texture classes over a wide range of

values of bulk density. Although we have studied the

production of clods >50 mm, we expect that the same

approach could be applied to the study and prediction

of the production of aggregates and clods of other

sizes.

The value of S above which no clods >50 mm are

produced is not significantly different from the critical

value of S = 0.035 that was proposed by Dexter

(2004a–c) as being the boundary between soil of

‘‘poor’’ and ‘‘good’’ soil physical quality.

This work demonstrates that S-theory is consistent

with the results of tillage conducted in the field.

Acknowledgement

This paper presents results of research programs

supported by NKFP-OM-3B/0057/2002 and OTKA-

34.274.

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