Plant-availability of catch crop S following spring incorporation

Summary

Catch crops might reduce sulphate leaching and thereby increase the overall S use efficiency in crop rotations. At 2 experimental sites in Denmark (a sandy loam and a coarse sand) S uptake of catch crop species was measured. Furthermore net release of S following incorporation of this material (S contents 0.13-1.03%, C:S ratios of 40-329 and lignin content of 1-10.8%) was investigated in a pot experiment with spring barley in sandy soil. The catch crops showed huge differences in their ability to sequester S. The best catch crops (legumes on sandy loam), sequestered 10-12 kg S ha-1 and the poorest catch crops (ryegrass and sorrel on coarse sand), sequestered less than 3 kg S ha-1. The S mineralisation rates were highest for crucifers (57-85% of total S added) and lowest for legumes (up to 46% of total S added). Differences can partly be explained by the C:S ratio, whereas no significant relationship was found with the lignin content of the incorporated catch crops. The catch crops may constitute a solution that can help avoiding S deficiency and increase synchrony between plant demand and available soil S in a crop rotation. However, the release of S will not fulfil the need of S demanding crops and even for cereals the mineralisation will most often only make a contribution. In the case of legume catch crops it is advisable to use a supplemental S source.