To determine whether adjustments need to be made to a range of decision support systems (DSSs) to quantitatively account for the effect of gravel on inputs such as fertilisers, lime, pesticides and herbicides.

The gravel content of a soil certainly affects soil physics and chemistry as well as crop growth processes.

In terms of nutrient management, currently increased gv% is assumed to reduce the amount of nutrients available to the crops, but not to concentrating the effect of fertiliser inputs. Adjustments for gv% are made in systems which use soil test calibration curves to produce recommendations when in fact those adjustments are probably not justified. All of the adjustments which are made, lead to higher fertiliser recommendations.  Even so, no attempt is made to better sample for or estimate gv%.

Whether it is worth investing further resources to confirm or refute the importance of the questions raised in this study is open to question. The complexity of the effects of gv% in real farming systems is such that there may well be no way to improve the predictive power of the models (both regression and mechanistic) to any great extent.

If resources are found to fund future work then it would be best to start with direct measures of whether gravel is inert or not – particularly with respect to water.  Experimental work used in parallel with ROOTMAP model runs to study the effect of gravel on nutrient uptake could also be worthwhile.