Know your soils for better management decisions

A 'rain-out' shelter to allow measurement of a dry soil profile in a mature wheat crop.

A SOIL benchmarking project run by Bill Manning and Ian Daniells of NSW Agriculture is measuring the yield capabilities of those soils identified by growers and agronomists as being important for dry land cropping in north-western NSW.

The project is measuring the water-holding characteristics and soil chemistry of soils to highlight potential productivity and limitations. A knowledge of a soil's capabilities helps farmers make appropriate decisions about crop and pasture rotations, for maximum productivity and sustainability.

Results and recommendations from the project will appear in a book for growers and agronomists, called District Guidelines for Managing Soils in North West NSW, to be published later this year. The project is supported by growers and the Federal Government through the GRDC and by the Natural Heritage Trust.

The soil types include grey and brown cracking clays (varying from soft, self-mulching to the harder, crusting types) of the north-west flood plains, black earths of the undulating country around Warialda, and sandy loams over clay in the Pilliga district and in the Coonamble and Coonabarabran Shires.

The researchers say major results of the project will be measures of plant-available water capacity (PAWC) in the various soil types. In dryland cropping, PAWC determines the maximum amount of rain that can be stored during a fallow, and hence much of the crop yield potential of a soil.

Knowledge of a soil's PAWC allows more effective management of rain. Water that is not used to grow plants can contribute to rising watertables and run-off, increasing the risk of salinity and erosion.

The researchers measured PAWC as the difference between a fully wet soil profile and a dry soil profile at wheat harvest. The project has measured PAWC under 'ideal' wetting conditions: a grid of drippers beneath a sheet of black plastic. Thus, the wet soil profiles were as wet as they could be. The dry soil profiles were measured in a mature crop under a 'rain-out' shelter.

A drum feeding water through drippers under black plastic allows measurement of a fully wet soil profile.

Contributing factors

The soil surface can have a large influence upon infiltration and therefore how quickly the soil profile refills with rain during a fallow. Stubble retention can reduce water wasted as run-off, and enhance infiltration. Good weed control is another important fallow management tool. (Northern readers, see related Stubble and infiltration research pI-Ed.)

PAWC depends not only upon soil texture (clays hold more water than sands), but also upon the ease with which crop roots can extract stored water. Subsoil conditions may constrain root depth and reduce the amount of stored water available to the crop.

The project has identified subsoil salinity and sodicity as constraints in some grey and brown cracking clays. In some black earths, insufficient soil depth above rock can be a constraint to PAWC. In contrast, some of the lighter soils can be quite deep and hence have high PAWC, provided that the subsoil is not saline, sodic, acid or compacted.

In assessing the value of a parcel of land, growers should pay attention to soil depth and subsoil conditions as well as judging the topsoil.

Program 4 Contact: Mr 8i11 Manning or Mr Ian Daniells 02 6763 1100 email or

Warms in soil