It is not currently possible to provide general guidelines for fallowing in the Mallee, because of the variability of the soils and environments. In the core research sites there have been no yield benefits from long fallow and no significant impact of tillage practice. In fact, in seasons following high summer rainfall, yields have been reduced after fallow due to leaching of N (up to 50 kg/ha).
However, there is well-documented evidence that up to 20 mm of extra soil water can be accumulated under fallows in some soils which can potentially lead to an extra 0.4 t/ha. Anecdotal evidence also suggests that in some soils cultivation, especially cultivation after February, removes an unknown soil constraint which would otherwise limit cereal yields.
The variable responses to fallowing and cultivation are most probably due to differences in soil type (clay content), subsoil constraints (soil depth), soil nutrition, root disease and seasonal rainfall patterns.
These variable soil factors also affect environmental issues. Assessments of focus paddocks indicate that soils with at least 20 per cent clay through the profile and no significant subsoil problems do not exhibit any deep drainage, even with long fallow rotations. Other soils with lower clay contents and/or major subsoil problems can suffer significant deep drainage even in continuous cropping.
The MSFP is currently working towards clearly defining these key soil factors and the impact of management practices to allow farmers to make informed decisions on a paddock-by-paddock basis.
Contact: Dr David Roget 08 8303 8528 email David.Roget@adl.clw.csiro.au