the shape of the water release characteristic as affected by tillage, compaction and soil type

Soil & Tillage Research, 25 ( 1993 ) 339-349 339 Elsevier Science Publishers B.V., Amsterdam

The shape of the water release characteristic as affected by tillage, compaction and soil type

M.F. O'Sullivan and B.C. Ball Scottish Centre of Agricultural Engineering, Bush Estate, Penicuik, EH26 OPH, UK

(Accepted l l May 1992)

ABSTRACT

O'Sullivan, M.F. and Ball, B.C., 1993. The shape of the water release characteristic as affected by tillage, compaction and soil type. Soil Tillage Res., 25: 339-349.

The shape of the water release characteristic was described by deriving two independent parameters. The first is the pore median diameter, which corresponds to the potential at which half of the effective pore volume is drained. The second is the water release index, which is the slope of the central portion of the water release characteristic; this is the maximum volume of water released for a ten-fold decrease in water potential. Water release characteristics were measured from saturation to - 1500 kPa on undisturbed cores taken from the topsoils of seven field experiments on tillage and traffic in southeast Scotland.

Both pore median diameter and water release index increased with particle median diameter (soil type effect ) and with total porosity (compaction or tillage effect ). On a fine-grained soil, compaction by conventional field traffic reduced pore median diameter from 48 to 4.5/lm and water release index from I 15 to 85 mm m - 1. On a coarse-grained soil, severe compaction was required to give such large decreases in parameter values. Soils with pore median diameters of less than 6/ lm were considered likely to be at risk from restricted soil aeration. These shape parameters appeared to be more sensitive indicators of changes in soil structure associated with soil management than individual water contents such as field capacity or wilting point. They are also less susceptible to errors in individual values because they are derived by effectively smoothing the complete characteristic.

I N T R O D U C T I O N

The water release characteristic is a fundamental soil property, influenced by texture, structure, clay mineralogy (Williams et al., 1983 ), organic matter and bulk density (Hall et al., 1977; Bathe et al., 1981 ).

Soil management may change the water release characteristic by altering soil bulk density and structure. This influences the response of cropping sys- tems to wheel traffic (Campbell et al., 1986) and to min imum tillage (Ball and O'Sullivan, 1987a). Compaction may increase or reduce available water

Correspondence to." O'Sullivan, Scottish Centre of Agricultural Engineering, Bush Estate, Peni- cuik EH26 0PH, UK.

© 1993 Elsevier Science Publishers B.V. All rights reserved 0167-1987/93/$06.00

340 M.F. O'SULLIVAN AND B.C. BALL

capacity, depending on texture (Hall et al., 1977 ). Compaction decreases air capacity (air-filled porosity at field capacity ). Air capacity and available water capacity were derived by making simplifying assumptions about soil water dynamics and choosing values for field capacity and permanent wilting point (Hillel, 1971 ). The shape of the characteristic may change with compaction, e.g. compact soil may retain more water at low potentials and less water at high potentials than uncompact soil and tillage may produce a bimodal pore size distribution (Klute, 1982). Such changes in shape cannot be described adequately by the use of air capacity and available water capacity alone. A non-linear model of the water release characteristic, such as that proposed by Van Genuchten (1980), gives a better description of the shape of the curve. It allows the complete curve to be described by a small number of parameters, which facilitates statistical analysis and mathematical modelling of water flow. However, the parameters of Van Genuchten's (1980) model are interrelated and are difficult to interpret in isolation.

The objectives of this work were to derive suitable parameters to represent the effects of tillage and compaction on the water release characteristic and to summarise, thereby, the effects of such treatments on typical soil types under cereals in southeast Scotland.

M A T E R I A L S A N D M E T H O D S

Soils and sites

Samples were taken at a range of topsoil depths from within seven field experiments made at different locations in southeast Scotland (Table 1 ). The experiments compared the effects of tillage (ploughing, tine cultivation and direct drilling) and traffic or compaction on the growth and yield of barley. Ploughing was to a depth of 200-250 mm, unless otherwise stated. A chisel plough was used at the Glencorse traffic site and a mouldboard plough at all other sites. Direct drilling was with triple-disc coulters which caused little disturbance below a depth of 50 mm. 'Broadcast' refers to the spreading of seed on to stubble with a fertiliser spreader. This seed was incorporated by tillage, causing soil disturbance to a depth of approximately 100 mm. Where rele- vant, traffic treatments were applied uniformly across each plot width before sowing and secondary cultivation. Plots received no further traffic until next season. Samples were taken within 4 weeks of sowing. Further details of the treatments and sites may be found in the references in Table 1. Soils were generally in a friable condition at the time of tillage. Although weather conditions before sampling varied between sites, the soils were generally slightly drier than field capacity at sampling.

The soil particle size distributions and organic carbon contents are shown in Table 2. The textures ranged between loamy sand and clay loam (Hodgson,

SHAPE OF THE WATER RELEASE CHARACTERISTIC 341

TABLEI

Site details. Soil series descriptions may be found in Ragg and Furry ( 1967 )

Site Experiment Treatments ~ Soil series FAO Reference type classification

Gullane Tillage P, D Luffness Luvic Ball et al. (1980) Arenosol

Rosewell Tillage P 10, D Darvel Eutric Ball et al. ( 1985 ) Cambisol

Bush Compaction ILR, 4HR, 4T Darvel Eutric Reid (1983) Cambisol

Carberry Tillage P, D, B Macmerry Gleyic O'Sullivan and Ball Cambisol ( 1982 )

South Road Tillage P, D Macmerry Gleyic Ball et al. (1989) ( Macmerry ) Cambisol South Road Tillage P, D Winton-Macmerry Gleysol Ball et al. (1989) (Winton) complex Glencorse 2 Tillage and P0, P 1 R, P 1, Winton Gleysol Campbell et al.

traffic DO, D1R, DI (1986) Glencorse Tillage P, D, B Winton Gleysol Ball and O'Sullivan

(1987b)

~P, PI0, ploughed to a depth of 200-250 mm and 100 mm, respectively; D, direct drilled; B, broadcast seed and shallow cultivation; 1LR, one pass, light roller; 4HR, four passes, heavy roller; 4T, four passes, tractor; 0, zero traffic; 1R, one tractor pass, tyres at half recommended pressures; 1, one tractor pass, tyres at recommended pressures. 2Treatments at this site were combinations of tiUage and traffic, e.g. P0, ploughed and zero traffic.

TABLE2

Soil particle size distributions and organic carbon contents

Site Particle size distribution (% w/w) Particle median Organic diameter (am) carbon (%)t

2000-200 #m 200-20/~m 20-2 #m < 2 #m

Gullane 60 25 8 7 270 1.6 Rosewell 51 33 8 8 210 3.8 Bush 25 60 5 10 110 3.3 Carberry 24 45 13 18 78 4.0 South Road Macmerry 18 50 16 16 61 4.6 Winton 19 47 16 18 56 3.7 Glencorse Traffic 15 38 25 22 25 3.3 Tillage 15 36 25 24 24 4.7

~Hydrogen peroxide oxidisable carbon.

1974). Although the organic matter contents were high for arable soil of these textures, the structural stability was probably only low to moderate compared with soil of equivalent organic matter status in warmer, drier regions (Stengel et al., 1984). Kaolinite was the dominant clay mineral in the two series con-


taining most clay, namely Macmerry and Winton (Ragg and Futty, 1967). Average annual rainfall ranged from 646 mm at Gullane to 845 mm at Glen- corse. Long-term average summer soil water deficits varied between 75 and 100 mm.

Methods and data analysis

Intact samples were taken in stainless steel rings 74 m m in diameter and 50 m m high. The samples were saturated at atmospheric pressure and equili- brated on tension tables and pressure plates as described by Ball and Hunter ( 1988 ). Total porosity was estimated from measured particle density and bulk density. This was taken as an estimate of water content at saturation. Field capacity was taken as - 6 kPa (Duncan, 1979) and permanent wilting point as - 1 5 0 0 kPa. Thus, air capacity is the air-filled porosity at - 6 kPa and available water capacity is the difference in water content (expressed as millimetres of water per metre of soil) between - 6 kPa and - 1500 kPa. Readily available water is defined here as the water released between field capacity ( - 6 kPa) and - 100 kPa.

Field capacity and permanent wilting point are static values which take no account of the dynamics of water flow in the field and so they are only crude indicators of field behaviour (Hillel, 1971 ). An alternative to their use is to describe the characteristics using parameters derived directly from the shapes of the curves. Water release characteristics tend to be S-shaped when plotted with potential on a log scale (Van Genuchten, 1980). The parameters which we found most useful to describe such a curve were the total porosity, the residual water content, corresponding to the lower arm of the curve, the slope of the curve at the mid-point between these two limits and the potential corresponding to this mid-point. The residual water content was estimated by extrapolation and is an estimate of the volume of water unavailable for plant use. The water available for plant use is assumed to be contained in the effective pores. The larger effective pores are also available for fluid transport. The median diameter of these effective pores is calculated from the potential at the mid-point of the characteristic. This calculation assumes a simple capil- lary model, in which pore diameter is inversely proportional to potential. The slope at the mid-point is inversely related to the uniformity of the pore size distribution and is a measure of the max imum volume of water released by the soil for a ten-fold decrease in potential. It is here called the water-release index.

The pore median diameter and the water-release index can be estimated graphically or by fitting a suitable equation to the data. We used the equation proposed by Van Genuchten (1980)

0s-0r O=Or-I [l+(P/pi)u] [1-(l/u)] (1)


where 0 is volumetric water content (m 3 m - 3 ), p is the matric potential (kPa) , and 0s is the saturated water content. The residual water content, Or and the in terdependent constants Pi and u were fitted to the characteristic by least squares, non-linear regression. We preferred this equation to the simpler and widely-used power curve (Buchan and Grewal, 1990) because it fits the data more closely, especially close to saturation and in fine-grained soil.

The water release index is defined as the slope of a semi-log plot of 0 against J P I, i.e. d0/dlog ( J e l ), at (0s + 0r)/2, expressed as millimetres of water per metre of soil. Pore size is related to potential, according to the conventional assumptions (Hillel, 1971 ), as follows

diameter (#m) = - 300 /P (kPa) (2)

Therefore, the pore median diameter was defined as - 3 0 0 divided by the potential at (0s + 0r)/2. These definitions of the parameters are illustrated in Fig. 1.

We assessed a total of 61 water release characteristics, each one being the mean of between four and eight replicates. Equation 1 fitted the data within experimental errors. Root mean square deviations of water contents from the fitted lines were between 0.01 and 0.02 ( v / v ) for most of the curves and exceeded 0.03 for only 10% of the characteristics considered.

In addition, air capacity and available water capacity were estimated from the original data, i.e. before curve fitting. Water content at - 100 kPa was not

0.6

v

C

\

0.5

0.4 Air _ \ apacity

0.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

\ Jod., available Available

0.2 \ ~ water water \~ ~ I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . .

0.1 - _~_-Z_-_--~-]_-_-_-~:_-~::]----_-_-~I . . . . . . . . . . . . . . . . . -- ~- Residual ~ ~ wafer

0.0 'J J content i i I I ~ l i i l i i , i i l l l i i i i l l l l l i r i i i i i I I i i

-0.1 -1 -10 -100 -1000

Mafric potential (kPa)

Fig. l. A plot o f eqn. 1 representing a typical water release characteristic. The arrowed circle indicates the point at which water release index and pore median diameter are derived. The slope of the dashed line is the water release index.


measured on all samples , so, for these samples , readily available water was calculated from the e s t imated water content at - 100 kPa.

RESULTS

The changes in both water release index and pore median diameter with total porosity are shown for all the samples in Fig. 2. The trend lines were fitted by regression. For a given soil type, both parameters tended to increase

250

A E200 E

150

=o 1

.= 100

50

30

o Gullons, Rosewell o and Bush

• Corberry South Rood both soils o o

# Glsncorse ° ~ o both experiments o ~ o ~

o • o

~. ~ ~ . ~ . . .

3'5 ,o ,'5 5'o 5'5 0'o Porosity (g v /v )

100

.5 L o @

o E 10 -o

E

30

o . ~ ~ o

o o •

o &~ ~ # # o / : .

$

i ,o i i ° ~ ~o Porosity (~ v/v)

Fig. 2. The variation of water release index and pore median diameter with total porosity. The trend lines were fitted by regression.

SHAPE OF THE WATER RELEASE CHARACTERISTIC

TABLE 3

Examples of the parameters of the water release characteristic at each site

345

Site Treat- Depth Porosity Water Pore Air ment ~ (mm) (%) release median capacity

index diameter (%) ( m m m -1) (am)

Available water capacity ( m m m -1 )

Readily available water ( m m m -~ )

Gullane P 100-150 50.6 137 66.6 22.7 135 112 D 47.4 194 64.3 18.2 144 124

Rosewell PI0 50-100 54.1 188 58.0 22.4 178 143 D 52.8 177 54.1 21.2 180 149

Bush ILR 50-100 52.2 191 77.1 24.5 161 112 4HR 42.3 142 29.4 16.7 184 134 4LT 34.8 83 3.0 6.8 199 107

Carberry P 0-50 48.1 137 57.4 17.1 130 96 D 43.5 89 51.1 13.3 118 94 B 46.2 86 37.2 13.8 133 74

South Road Macmerry

Winton

Glencorse Traffic

Tillage

P 0-50 54.4 120 16.2 13.2 245 151 D 52.1 101 10.4 11.3 210 131 P 0-50 53.9 113 28.7 17.7 190 111 D 49.0 77 5.5 8.9 219 98

P0 0-50 60.1 120 55.9 23.3 185 86 P1R 56.1 115 15.9 14.5 231 89 P1 56.2 111 14.5 14.9 218 96 DO 59.9 115 48.4 23.0 164 90 DIR 50.4 76 3.5 8.4 211 88 DI 52.0 85 4.5 9.7 224 88 P 50-100 55.6 86 4.8 11.3 192 96 D 54.2 68 1.7 7.6 182 86 B 52.0 68 1.7 7.6 178 86

1See Table 1 for notation details.

with total porosity as influenced by soil management. The increase in pore median diameter with porosity was greater in the fine-grained than in the coarse-grained soils. Examples of the parameter values of cores taken from near the soil surface in specific tillage and compaction treatments are given in Table 3. Derived residual water contents were between about 0.02 and 0.12 ( v / v ) and showed no clear trends with porosity or particle diameter. For a given porosity, both water release index and pore median diameter tended to be greater in the coarser soils (Fig. 2 ).

Available water capacity tended to be greater in fine-grained than in coarse- grained soil for a given porosity. It tended to increase with decreasing porosity in the sandy soil, e.g. Gullane and Bush in Table 3. In the fine-grained soil, however, available water capacity tended to increase with porosity up to some turning point and to decrease with increasing porosity above this point. An example of this behaviour can be seen in the results for the Glencorse traffic

346

25

20

15

L

o

# # o o o

° ° •

O 0

.# . . , ~ 0 e' '~ o _Gullane, # ~ = ~ ^ # Rosewoll.

South Road both soils

O # Glencorse both exper iments

I 10 100

Pore m e d i a n d i a m e t e r 0 z m )

M . F . O ' S U L L I V A N A N D B.C. B A L L

160

"E 140

E

~ 1 20

= ~ 100

~ 8 0

60

0

o o

6 o

o o

A ~ ~ ~o o

l l I I 50 100 150 200 250

Water re lease index ( m m / m ' ;

Fig. 3. Relationships between air capacity and pore median diameter and between readily available water and water release index. The trend lines were fitted by regression.

experiment in Table 3, where the porosity at the turning point was around 54%. Most of the treatment differences were in the readily available water range ( - 6 to - 100 kPa).

Air capacity tended to decrease with particle size. Air capacity was greater than 10% in the coarse-grained soils unless they had been subjected to severe compaction, whereas moderately-compacted, fine-grained soils tended to have air capacities less than 10%. Relationships between the conventional param-


eters of air capacity and readily available water and the new parameters are shown in Fig. 3.

DISCUSSION

The influence of soil management on water release parameters

The fine-textured soils (particle median diameter less than about 60/ tm) tended to have low pore median diameters when compacted. For the coarse- grained soils, pore median diameter was decreased markedly only by severe compaction. A pore median diameter of less than approximately 6/tm seemed to correspond to soils where aeration problems were likely to occur. Such low values occurred in the Glencorse traffic experiment (Table 3 ). In this experiment, Campbell et al. (1986) attributed the decrease in crop yields which they found under compaction and direct drilling to problems of soil aeration.

The uncompacted, coarse-textured soils (particle median diameter greater than about 100/tm ) tended to be very porous, so that pore median diameters and water-release indices were high. Compaction tended to increase water availability by reducing the water-release index. However, this was unlikely to cause problems of soil aeration because the accompanying decreases in pore median diameter were small. At the Bush site, for example, pore median diameter was reduced to less than 6/~m by severe compaction only. From the results of the field experiments, we deduced that values of pore median diameter greater than about 60/tm indicated that the soil was undercompacted. Taking into account other experimental conditions, moderate compaction was considered to increase crop yield, and direct drilling tended to be successful on these soils (e.g. Ball et al., 1985).

Interrelationships among water release parameters

Readily available water was around 75% of the available water capacity in coarse-grained soil (particle median diameter greater than about 100/~m), but approximately 50% of the available water capacity in fine-grained soil (particle median diameter less than about 60 #m).

In fine-grained soils, both readily available water and available water capacity tended to increase with water release index because the characteristics of these soils are almost linear, on a semi-log scale, between - 6 and - 1500 kPa. In fine-grained soil only, available water capacity was approximately 2.2 times the water release index. In coarse-grained soil, by contrast, water-release index was not closely related to available water capacity because the characteristics were strongly curved within the available water range of potentials. However, water-release index was related to readily available water.


Over all the soils, readily available water was approximately equal to 80 mm m - l plus one third of the water-release index (Fig. 3 ).

Air capacities of less than 10% were found in some of the compacted and direct drilled soils. This value of air capacity is often taken as an indicator of possible aeration problems and corresponded with a pore median diameter of about 6/~m (Fig. 3 ). In comparison with air capacity, pore median diameter differed more between soils and was more influenced by soil compaction.

CONCLUSIONS

The proposed water release index and pore median diameter describe the shape of the water release characteristic. Both parameters were sensitive indicators of the structural changes influencing water availability and soil aeration status. They showed larger effects of compaction and tillage than were revealed by total porosity. As the entire characteristic is used in the derivation of these parameters, errors associated with estimating individual moisture contents on undisturbed soil cores are considerably reduced.

REFERENCES

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Ball, B.C. and Hunter, R., 1988. The determination of water release characteristics of soil cores at low suctions. Geoderma, 43:195-212.

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Ball, B.C. and O'Sullivan, M.F., 1987b. Cultivation and nitrogen requirements for drilled and broadcast winter barley on a surface water gley (gleysol). Soil Tillage Res., 9: 103-122.

Ball, B.C., Lang, R.W., O'Sullivan, M.F. and Holmes, J.C., 1980. Soil and spring barley re- sponses to conventional and direct drills on a blowing sand in East Lothian, 1979. Depart- mental Note SIN/313. Scottish Institute of Agricultural Engineering, Penicuik, UK, 39 pp.

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Buchan, G.D. and Grewal, K.S., 1990. The power-function model for the soil moisture characteristic. J. Soil Sci., 41:111-117.

Campbell, D.J., Dickson, J.W., Ball, B.C. and Hunter, R., 1986. Controlled seedbed traffic after ploughing or direct drilling under winter barley in Scotland, 1980-84. Soil Tillage Res., 8: 3- 28.

Duncan, N.A., 1979. The moisture regimes of six soil series of the central lowlands of Scotland. J. Soil Sci., 30:215-223.


Hall, D.G.M., Reeve, M.J., Thomasson, A.J. and Wright, V.F., 1977. Water retention, porosity and density of field soils. Technical Monograph 9, Soil Survey, Harpenden, UK, 75 pp.

Hillel, D., 1971. Soil and Water: Physical Principles and Processes. Academic Press, New York. 288 pp.

Hodgson, J.M., 1974. Soil survey field handbook. Soil Survey, Harpenden, UK, 100 pp. Klute, A., 1982. Tillage effects on the hydraulic properties of soil: a review. In: P.W. Unger and

D.M. van Doren (Editors), Predicting Tillage Effects on Soil Physical Properties and Pro- cesses. American Society of Agronomy, Madison, WI, pp. 29-43.

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Stengel, P., Douglas, J.T., Guerif, J., Goss, M.J., Monnier, G. and Cannell, R.Q., 1984. Factors influencing the variation of some properties of soils in relation to their suitability for direct drilling. Soil Tillage Res., 4: 35-53.

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the shape of the water release characteristic as affected by tillage, compaction and soil type

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