FarmTalk #16 – Dealing with subsoil compaction in Mallee soils

Fact Sheet #16 – July 2004

Víctor Sadras, David Roget, Garry O’Leary, Bill Davoren – CSIRO Land and Water, Adelaide

The Issue
In recent cropping seasons, a group of Mallee farmers have observed poor crop growth on sand hills. A range of factors, including shortage of nutrients and diseases were investigated and discarded as major causes of poor crop growth. Eventually soil compaction was identified as a key, previously unnoticed, constraint in this type of soil.

To further investigate the extent to which soil compaction could be impacting on grain yields, deep ripping trials were initiated at Caliph and Loxton, SA. Treatments included ripped and un-ripped strips in large paddocks including both sand hills and sandy loam flats.

What we know about compaction in Mallee soils
Compaction in sandy Mallee soils is NOT a major impediment for downward movement of water through the profile. Subsoil compaction thus operates as a one-way valve
– water can move down, but water uptake by plants is restricted.

Compacted soil layers impose a severe barrier for root penetration and water uptake from deep in the soil profile. The accumulation of water below the compacted zone can increase the risk of deep drainage.