Subsoil manuring

This subsoil manuring project aims to develop economically viable techniques to ameliorate subsoils with physical constraints. There are three things to consider:

• whether the incorporated organic manures will improve crop performance
• how best to incorporate organic manures 30 to 40 cm below the surface
• what would be the critical rate of incorporation of the manures.

We currently have three sites in the western districts of Victoria where we are evaluating subsoil manuring technology. Annual average rainfall in these districts ranges from 559mm at Wickliffe to 750mm at Penshurst. We also have two sites in Northern Victoria near Dookie, with an annual average rainfall of 560mm.

We collaborate with the Department of Environment and Primary Industries, Southern Farming Systems and other farming groups.

Subsoil and crop productivity

Subsoil constraints that limit crop productivity are considered to be a major issue in southern Australia. A survey done by Richard MacEwan across south west Victoria in 2001 found that clay content varied from 52% at the top of the subsoil, to 60% at 40 to 60 cm depth, and to 67% clay at 80 cm. These sub soils are sodic (high in exchangeable sodium) and so are dispersive. Bulk density values below 40 cm depth range from 1.4 to 1.7 g/cm3. These dense sodic clay sub soils generally have less than 10% macro porosity which is the critical limit for root growth. Hydraulic conductivities in these soils are also very low (around 0.001 cm/hr).

The dense clay subsoil cause two major problems: very low hydraulic conductivity through the clay and restricted root growth. Very low hydraulic conductivity causes surface water logging in winter months which suppress plant growth and, in severe water-logging conditions, plants can die. This can severely affect the crop yields. Runoff losses of rain water also increase.  Restricted root growth into the clay subsoil restricts the 'bucket size' or the volume of plant available water in soil profile for crop production.  When roost are restricted to only the upper 50 cm of soil, then there will be on average only 50 to 70 mm of plant available water for the crop. In case of a dry finish (dry spring) there is not sufficient soil water available for the crop to reach its potential yield. It also reduces water productivity (grain produce per mm of evapotranspiration) as shallow stored water is more prone to surface evaporation compared to the water stored in the deep soil profile. There are millions of hectares of cropping land in the world which have severe sub soil physical constraints. In south west Victoria there were 400-500,000 ha under cropping in 2007 and this area can be expanded to over 1 million ha in coming years.

Improving subsoils

Soil sodicity can be improved by the application of gypsum to reduce the dispersion of clay. Gypsum can check the dispersion of clay and so can improve the micro aggregation of clay which can lead to an improvement in the micro- and meso-porosity of soil. A second approach is to grow deep rooted ‘primer’ crops such as lucerne. Lucerne roots can penetrate hard sub soils and create bio-pores or channels for better infiltration of water in the soil. A third approach is to ameliorate the upper part of the subsoil (around 30 cm below the surface) with organic manure.  It is well known that organic manures can improve soil physical conditions (porosity, strength and water movement). The major issue is how to ameliorate subsoil with manures in an economically-viable way.