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Physical properties of a clay loam soil mixed with sand

Jaikirat S. Gill1, Judy Tisdalll, Sukartono2, I.G.M. Kusnarta2 and Blair M. McKenzie1,3

1Department of Agricultural Sciences, La Trobe University, 3086, Australia. www.latrobe.edu.au.
Email: jaikirat.singh@latrobe.edu.au
2
Department of Soil Science, University of Mataram, Lombok, Indonesia. Email: aciarvertisol@telkom.net
3
Current address: Scottish Crop Research Institute, Invergowrie, Dundee, Scotland, UK, DD25DA.
Email: b.McKenzie@scri.sari.ac.uk

Abstract

Clay soils generally have a greater strength when relatively dry, but are also susceptible to waterlogging, leading to restricted root and shoot growth. The hypotheses tested here were that a) tensile strength is decreased, b) hydrological properties are improved, and c) root and shoot growth are increased, in mixtures of sand and clay loam soil, compared with those in clay loam soil alone. Soil was mixed with coarse (0.2-2 mm diameter) or fine (0.02-0.2 mm) sand, in seven ratios by mass of sand: [ranging from 0:1 (Control) to 1:1]. The tensile strength of the sand:soil mixture was least in the 1:1 sand:soil mix. Reduction in tensile strength by addition of sand was greater when coarse rather than fine sand was used. Total root length (620 cm) and total root volume (1.5 cm3) of three soybean plants (Glycine max) in pots at 3 weeks were each greater in the treatment with a sand:soil mixture of 1:2, than those in other treatments (mean 466 cm and 1.2 cm3, respectively). However, the dry root weight (0.2 g) in the 1:2 coarse sand:soil mixture (but not fine sand:soil) was greater, and dry shoot weight (mean 0.6 g) of 1:2 and 1:1 coarse sand:soil mixtures was greater, than those in other treatments (mean 0.1 g and 0.5 g respectively). Hydraulic conductivities of sand:soil mixture were increased with increased proportions of sand in the mixtures. These data support the hypotheses tested, and suggest that sand:soil mixtures would increase the performance of crops on heavy clay soils compared with those on clay soil alone.

Key Words

Aeration, particle size, root growth, waterlogging .

Introduction

Clay soils have greater strength especially when dry, and are also susceptible to waterlogging due to poor infiltration which results in poor aeration (Mullins et al. 1990). Poor physical properties that result in limited water movement, poor root development and inadequate aeration, have been reported to result in less crop yield (Wallace and Nelson 1986). A clay soil tends to stay wetter for longer as the fine particles hold more water, and more tightly, than does a sand or loam. Critical to the inter-relation of soil type and the potential for structure alteration is a soil's Plastic Limit water content (PL). Soil wetter than the PL is compressed and sheared when loaded, ie the soil is in a “plastic” state and is prone to structure alteration that can create poor soil physical conditions for plant growth. Soil cultivated drier than PL fractures rather than smears. Soil strength is one aspect of soil structure, which is controlled by factors including particle size distribution, which is controlled by several factors including particle size distribution, the amount and type of clay mineral, and particle shape (Dexter 1988; Young and Mullins 1991; Zhang et al. 2001). Soil strength can have both a direct and indirect influence on crop growth (Tisdall and Oades 1979; Young and Mullins 1991; Young et al., 1991).

Heavy clay soils become very hard (> 10 MPa) with less soil water content (<100 g/kg). In agricultural soils, greater soil strength produces greater mechanical impedance, which eventually reduces root growth and proliferation (Goss 1977; Mullins et al. 1990; McKenzie et al. 2001), thus limiting the availability of water and nutrients (Nicou and Chorpart 1979). Furthermore, the greater strength of dry soils can narrow the range of water content suitable for tillage and seedling emergence (Hamblin and Tenant 1979; Mullins et al. 1990). Mixing sand into heavy clay topsoil may alleviate these restrictions. The hypotheses tested were that a) tensile strength is decreased, b) hydrological properties are improved, and c) root and shoot growth are increased, by mixing heavy clay loam soil with sand at less soil water content, compared with the same indices in a heavy clay loam soil alone.

Material and methods

Soil

The soil (Ug 6.2, Northcote 1997) was a clay loam (fine sand 18 %, coarse sand 5 %, silt 21 %, and clay 56 %) collected from 0-150 mm layer at Gnarwarre, Victoria. Soil samples were taken in a diagonal transect of the field and manually mixed into a composite sample, which was then air-dried and passed through a 2-mm sieve. This site is part of an ongoing experiment with a four years cropping system [canola (Brassica napus) – wheat (Triticum aestivum) – pasture (mixture of ryegrass, Lolium perenne and white clover, Trifolium repens) – pasture], of Southern Farming Systems, Gnarwarre near Geelong (380 10’S, 1440 08’E) in south-west Victoria.

Sand:soil mixture

A range of sand:soil mixtures were examined as treatments in a factorial experiment, established in the laboratory in a completely randomised design. Seven ratios by weight of sand:soil (T1 = 0:1; T2 = 1:20; T3 = 1:15; T4 = 1:10; T5 = 1:5; T6 = 1:2; T7 = 1:1) of two particle sizes of sand (fine: 0.02-0.2 mm; coarse: 0.2-2 mm diameter) were studied. Soil and sand was mixed manually and amount was calculated on oven dry basis.

Soil disks

Soil disks were prepared (McKenzie and Dexter 1985) for each treatment with a spatula. Moist soil (at plastic limit, 0.26 kg/kg) was moulded (with spatula) into plastic rings of internal diameter of 26 mm and depth of 10 mm. The disks of soil were air-dried for 24 hour at 25oC, and then oven dried at 105oC for 24 hours. The disks were subsequently allowed to cool over P2O5 in desiccators.

Tensile strength

Seven ratios of sand:soil mixtures (air-dry), and two particle sizes of sand (fine; coarse) were used to study tensile strength. Twenty soil disks were prepared for each treatment. Each disk was weighed and the diameter and thickness were measured with a caliper before the tensile strength was measured. Each soil disk was then crushed across the diameter between flat two parallel plates on a loading frame: Compression Test Machine WF 10020/21. The reading from a reading gauge when each soil disk was crushed was converted to the strength in kilogram force (kgF) and then tensile strength (MPa) was computed.

Hydrological properties

Sand:soil mixtures were loosely (BD 1.1±0.06 on oven dry basis) packed in rings of internal diameter of 32 mm and thickness of 20 mm and then were saturated on ceramic plates. Soil water content of each sample at suctions ranging from 10 kPa to 100 kPa, was determined with pressure cells. Sand:soil mixtures were loosely packed (BD 1.1±0.06 on oven dry basis) in rings of internal diameter of 70 mm and 50 mm deep and saturated hydraulic conductivity (Ks) was measured by constant head method (Klute and Dirkson 1986).

Root and shoot growth

Heavy clay soils in some parts of the world (e.g. Vertisols in eastern Indonesia and India) remain dry (without crops) for some months between rainy seasons. In these areas, soils receive few showers before the rainfall season starts. A study in the laboratory utilising a temperature-controlled growth cabinet, was counducted to evaluate the performance of plants in different sand:soil mixtures treated to simulate these few showers at the break of season. Each sand:soil mixture (100 g) was wetted to a water content of 0.20 kg/kg and placed in a pot (65 mm wide, 70 mm high), and incubated at 21oC for 3 weeks. Pots were sealed with a thin plastic film to prevent evaporation loss. Pots were then incubated at 21oC air-dry for 3 months. Then each mixture was wetted to a water content of 0.15 kg/kg and pots were placed in a temperature controlled growth cabinet (30oC day and 23oC night). Three seeds of soybean were sown in each pot, and water content (0.15 kg/kg) of the soil was maintained by daily addition of water. After 3 weeks, plants were harvested, and root measurements (total root length, total root volume) were taken with a Win Rhizo root scanner. Dry root and shoot (70oC) mass were also measured.

Statistical Analysis

A completely randomized design with three replicates of each treatment was used in the experiments. Analysis of variance was computed with Genstat-5 (release 3.2, Lawes Rothamsted Experimental Station) for tensile strength, hydraulic conductivities, and root and shoot measurements.

Results and Discussion

Tensile strength

The tensile strength tended to decrease as the proportion of fine sand or coarse sand increased; the tensile strength in treatments with at least 1:5 sand:soil was significantly less that in treatments up to 1:10 sand:soil (Figure 1). This result supports the finding of Mullins and Panayiotopoulus (1984) where fine sand:kaolin unsaturated mixtures were stronger than the coarse sand: kaolin unsaturated mixtures.

Figure 1. Tensile soil strength under different mixing ratio of (a) fine or (c) coarse sand:soil (T1 = 0:1; T2 = 1:20; T3 = 1:15; T4 = 1:10; T5 = 1:5; T6 = 1:2; T7 = 1:1).

This phenomenon can be attributed to decreasing the total surface area of particles within aggregates as the proportion of sand increased with less contact between particles and fewer bonds between particles (Utomo and Dexter 1981; Dexter 1988; Oades and Waters 1991). Towner et al (1988) stated that, in soil dominated by sand grains, fine materials such as clay or silt are likely to fill the spaces between sand grains, and (i) become bridges between adjacent grains, or (ii) become attached to the surface of individual grains, but are not involved in bridging.

Soil strength also is closely related to the susceptibility of alteration of soil structure (Cruse and Larson 1977; Bradford and Grossman 1982). Reduction in soil strength can make tilling easier for preparation of seed bed.

Hydrological properties

Soil water content at each suction was significantly (P<0.05) less where the ratio of sand (fine or coarse):soil was 1:5 (T5) or 1:2 (T6) or 1:1 (T7) than that in other treatments (Figure 2). However, soil water content was less at these ratios (T5, T6 or T7) when coarse, rather than fine, sand was used (Figure 2).

Figure 2. Soil water content under different mixing ratios of fine (0.02 – 0.2 mm diameter, a) or coarse (0.2 – 2 mm diameter, b) sand:soil (T1 = 0:1; T2 = 1:20; T3 = 1:15; T4 = 1:10; T5 = 1:5; T6 = 1:2; T7 = 1:1) at different suctions. Error bars = 2 x SEmean (shown when larger than the symbol).

Saturated hydraulic conductivity (Ks) increased with increased proportion of sand in the mixture (Table 1). Saturated hydraulic conductivity was almost four times greater, where the ratio of sand:soil was at least 1:5 (T5), when coarse sand was used instead of fine sand (Table 1). Ks was only significantly increased when sand was mixed with soil at 1:15 (T3) or higher, as mixing sand at 1:20 ratio did not change the Ks of the soil compared with that of control (T1). It shows that 64 tonnes of coarse sand could be mixed in the upper 10 cm layer of a hectare to significantly increase the hydraulic conductivity of a clay loam soil. Economic application of sand to a field would depend on the availability of sand near the field and its transportation cost.

It was noted elsewhere that sand mixed with soil decreased the total plant available water (Taylor and Blake 1979; McCoy 1998). On the other hand, increased hydraulic conductivity due to the mixing of sand in the top soil (0 - 10 cm depth) can increase the penetration of water in the soil profile leading to greater volume of soil water stored in the profile.

Table 1. Saturated hydraulic conductivity (Ks, mm/hr) of different mixing ratio of fine (0.002 – 0.2 mm diameter) or coarse (0.2 –2 mm diameter) sand:soil (T1 = 0:1; T2 = 1:20; T3 = 1:15; T4 = 1:10; T5 = 1:5; T6 = 1:2; T7 = 1:1).

Sand size

     

Treatments

       
 

T1

T2

T3

T4

T5

T6

T7

LSD0.05

Fine

1.02

1.28

1.55

1.72

2.01

3.64

3.63

0.46

Coarse

1.02

1.25

3.66

5.69

7.68

13.28

14.69

0.28

Root and shoot growth

Total root length (Figure 3a) and total root volume (Figure 3b) were each greater in the treatment with a mixture of 1:2 (T6), fine sand:soil, than those in the control (T1, 0:1 mixture). Total root length (Figure 3c) was greater in a mixture of 1:5 (T5) or 1:2 (T6), and total root volume (Figure 3d) was greater in each mixture of 1:5 (T5), 1:2 (T6) or 1:1 (T7), coarse sand:soil, than were those in 0:1 mixture (T1, Control). Dry root weight (mean, 0.11 g) and dry shoot weight (mean, 0.48 g) were each not significantly different between treatments with fine sand (data not shown). However, dry root (Figure 4a) and shoot (Figure 4b) weights of 1:2 (T6) or 1:1 (T7), coarse sand:soil, were each greater than those in 0:1 (T1, Control). It shows that Greater shoot mass produced in pots with a greater proportion of sand:soil (1:1 or 1:2) mixtures, may have been because applied water (0.15 kg/kg) was held at small suction ( about 100 kPa) in those mixtures (Figure 2). Increased air-filled porosity by sand mixing (McCoy 1998) can improve root growth in sand:soil mixtures over soil alone. The results show that growth of roots and shoots of soybean seedlings were greater in sand:soil mixtures with tensile strength less than 200 MPa (Figure 1, Figure 4).

Figure 3. Total root length (a, c) and root volume (b, d) at 3 weeks of soybean grown in mixture of fine (0.002 – 0.2 mm diameter, a, b) or coarse (0.2 – 2 mm diameter, c, d) sand:soil (T1 = 0:1; T2 = 1:20; T3 = 1:15; T4 = 1:10; T5 = 1:5; T6 = 1:2; T7 = 1:1).

Figure 4. Dry root (a) and shoot (b) weight at 3 weeks of soybean grown in mixture of coarse (0.2 – 2 mm diameter) sand:soil (T1 = 0:1; T2 = 1:20; T3 = 1:15; T4 = 1:10; T5 = 1:5; T6 = 1:2; T7 = 1:1).

Conclusion

Our data support the hypotheses that a) tensile strength is decreased, b) hydraulic properties are improved, and c) root and shoot growth are increased in mixture of sand and clay soil than clay soil alone. Coarse sand performed much better in improving the physical condition than did fine sand. Our results show that 1:2 ratio of coarse sand:soil was most appropriate to improve the physical condition of a clay soil. Plant growth was also better in coarse sand:soil (1:2) treated to simulate a few showers on dry soil at the break of the growing season, than were other ratios of sand:soil.

Acknowledgments

We thank the ACIAR (Australian Centre for International Agriculture Research) for funding this study.

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