Martiens du Plessis, a soil expert at NWK Agricultural Management Services, recently studied how winter cover crops impact soil health in South Africa, where growers have started planting winter covers to counter the disadvantages of bare fields. 

He conducted an independent scientific study, using a calibrated neutron moisture meter to gather data. He used twelve moisture measurement pipes, six in a soybean field and six in a maize field, on a farm in the Viljoenskroon district between December 2020 and April 2021. The maize plot was on well-drained soils and the soybean plot on a shallow water table.

According to a report in Farmers Review Africa, there were three big takeaways from the research. 

The first finding was that winter cover crops are difficult to establish in the western region’s dry topsoil at the end of summer. From a seed mix consisting of grasses, tubers and legumes, only rye emerged and became established.

The second finding was that less moisture than expected is extracted during winter, but that moisture use increases sharply in spring.

“Cover crops must, therefore, be terminated in late August, early September (early spring in South Africa) before they can deplete the soil moisture,” du Plessis says.

Thirdly, it was found that the water costs of a winter cover crop are considerably lower on a shallow water table than on a drained profile. Growers must carefully consider the fields on which cover crops are planted, as well as how wet the soil profile is at the end of summer. “Winter cover crops must preferably be limited to wet seasons,” says du Plessis. “In a dry season they can significantly impair the follow-up crops.”

The study showed that circumstances should determine the decision regarding winter cover crops. In wet seasons winter cover crops are an excellent way to improve biological diversity and soil health, whereas in dry seasons the risk to the next season’s summer crop yields could be too high.