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Deep incorporation of organic matter improves wheat growth on a Sodosol with dense subsoil

JS Gill, Peter Sale and Caixian Tang

Department of Agricultural Sciences, La Trobe University, Bundoora, Victoria 3086,


Subsoil constraints are major limiting factors in crop production in many soils of southern Australia. A field study examined effects of deep incorporation of organic and inorganic amendments on soil properties, plant growth and grain yield of wheat (Triticum aestivum) on a Sodosol with dense subsoil with or without lucerne history in a high rainfall region (long-term average annual rainfall 576 mm) of Victoria. Amendments were applied to 30-40 cm at a rate of 10-20 t/ha. Deep-ripping alone did not significantly affect biomass production while application of gypsum increased biomass production by up to 30%. In contrast, application of organic materials doubled biomass production. The improved plant growth with amendments was related to an increase in plant available water in the hostile subsoil and the prolonged supply of nutrients. The trial will run for two more years to confirm the yield benefits from amendments.

Key Words

lucerne, nutrient supply, soil structure, subsoil constraints, water use


Subsoil constraints are a major limitation in achieving the potential yield (Rengasami et al., 2003). Such constraints to root growth and function can be physical, chemical or biological. Chemical constraints can be associated with high concentrations of boron, sodium, aluminum, chloride or carbonate which can restrict root growth, whereas physical constraints due to dense soil may limit oxygen supply and increase soil strength. Lack of meso- and macro-pores leads to very low hydraulic conductivity restricting water redistribution within the profile (Oster and Jaywardane, 1998). The reduction in water storage causes water stress in crops during prolonged dry periods.

In the high rainfall zone of Victoria, most of soils have dense sodic subsoil. In these soils, root growth and water movement are limited to the upper 10 to 40 cm of soil. This means that soil can only store 50 to 90 mm of plant available water. This limits grain filling in crops under dry finish conditions. An increase in root access to an extra 30 cm of subsoil would provide an extra 60 to 80 mm of plant available water to the crop. The paper will report on the effect of deep incorporation of organic amendments on the growth of wheat on a Sodosol with a dense subsoil.


A field study was conducted in 2005 on a Sodosol with dense subsoil (ρb = 1.4-1.6 g/cm3; ESP=6-9%) with or without lucerne history in a high rainfall region of Victoria (long-term average annual rainfall 576 mm). The lucerne paddock had lucerne pasture in 1999-2003 and canola in 2004. Wheat crop (cv. Amroc) was sown at a seeding rate of 100 kg/ha on raised beds on 18 May, 2005. There were nine amendment treatments in four replicates. These treatments were (T1) undisturbed raised bed, (T2) deep ripping, (T3) gypsum, (T4) MAP, (T5) lucerne pellets, (T6) dynamic lifter, (T7) sand, (T8) gypsum plus MAP and (T9) lucerne pellets + MAP + gypsum. The amendments were applied at a rate of 10-20 t/ha at 30 to 40 cm depth, in two bands on each 1.5 m bed. The trial was set up as a completely randomized block design. Plant population, tillers, plant height, shoot biomass and water content in soil profiles were monitored throughout the growing season.


There was no significant difference between plants germinated at the lucerne (124/m2) and non-lucerne (130/m2) sites. Number of tillers produced was about 8/plant, and was similar between the amendment treatments. Plant height at growth stage 34 (22 September) was higher in the treatments with lucerne pellets, dynamic lifter or lucerne pellets + MAP + gypsum, than in the control (with or without deep ripping). This difference remained significant up to growth stage 47. Overall plant height was higher at the lucerne site than the non-lucerne site during this period.

Deep incorporation of dynamic lifter and the mixture (lucerne + MAP + gypsum) had the highest shoot biomass sampled on 22 September, which was 80% greater than the control. Intermediate biomass yields occurred with lucerne, gypsum + MAP, sand, MAP and gypsum. By 4 November, biomass production was highest in the lucerne and dynamic lifter treatments (Data not shown). The effect of treatments on biomass production at maturity followed the same trend but was less (Figure 1). Grain yields were highest in the lucerne and dynamic lifter treatments, followed by treatments T9, T8 and T7. Deep ripping did not have any beneficial effect while gypsum slightly increased the yield (Figure 1). The 5-year lucerne history slightly improved shoot biomass before anthesis but slightly decreased biomass and yields at maturity.

Figure 1. Hand-harvested biomass production at maturity (A) and grain yields (B) of wheat grown in various treatments with or without 5-year lucerne history. Bars represent l.s.d at P=0.05.

Soil water content with deep ripping and amendment treatments was generally lower at the depth of 30-50 cm but higher below 50 cm than that of the control, particularly at earlier stages. Among the measured amendment treatments, T9 (lucerne + MAP + gypsum) had a high water content at 20 cm and tended to have lower water content at 40-60 cm, which became more obvious on 26 October (Figure 2). This indicates that the crop used more water from lower in the profile with this organic amendment than with other treatments.

Figure 2. Volumetric water content in soil profiles of selected treatments with 5-year lucerne history. The measurements were conducted using a neutron moisture probe on 6 September, 6 and 26 October 2005.


Deep incorporation of organic amendments resulted in higher biomass and yields than other treatments. Other measurements of the study suggest that deep incorporation of organic amendments improved physical conditions and this led to increased water storage in soil profile and a stimulation of root growth in deeper layers. Furthermore, organic amendments improved chemical fertility, and prolonged the supply of nutrients, particularly nitrogen, during the later reproductive stage. The study on detailed mechanisms is underway.


The research is supported by the Australian Research Council and Rentiers Machinery Pty Ltd.


Rengasami P, Chittleborough D, and Helyar K 2003 Root zone constraints and plant based solutions for dryland salinity. Plant and Soil 257, 249-260.

Oster JD and Jaywardane NS 1998 Agricultural management of sodic soils. In Sodic Soils: Distribution, Properties, Management and Environmental Consequences. Eds. ME Sumner and R Naidu. pp. 125-147. Oxford University Press, NewYork

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