Agronomy: Planting tips for wet soils


By Marleen Smith | 7 December 2017
wet
Photo: Johan Norval

Repeating wet seasons can leave fields leached and water profiles filled to the brim. Marleen Smith asked some experts about the measures grain farmers need to keep in mind when preparing for planting in wet soils.

Root development and fertilisation are two factors that play a crucial role in the success of grain crops, says Kobus van Zyl, agronomist from Omnia, who works mostly in areas with sandy soils and water tables.

“Amid the wettest conditions in years, and with much of the air in the soil replaced by water, it will be important to aerate soils as effectively as possible before planting. This should be the primary aim of pre-planting cultivation.”

He adds that cultivation should be deep enough to ensure deep root penetration. “If cultivation is too shallow, it may lead to a situation where roots develop only in a narrow upper layer, and plants suffer drought damage even though sufficient moisture is still available in the deeper layers.”

Farmers should provide enough stimulation so that plants can develop early and be strong by the time the first summer rains fall when waterlogging and flood damage could occur once again.

“By planting early, you can avoid the heavier rainfall during the early developmental stage. The risk of waterlogging will probably be greater this coming year than the risk of midsummer drought,” he warns.

FERTILISATION

Van Zyl’s advice to farmers who are unsure of the type of cultivation needed is to have an advisor look at the soil with them before deciding. He adds that a special fertilisation strategy will also be necessary to use all the available moisture for optimal yields.

Unusually wet and cold soil conditions hamper microbe activity, slowing both the mineralisation process and plant growth. In addition to this, above-average leaching takes place. Nitrogen that did not leach, denitrified and was released from the soil in gas form.

Van Zyl stresses that the choice of nitrogen source is most important. Urea might be the wrong choice, given impeded soil microbial activity and slowed reaction. An ammonia-nitrate base that is readily accessible to plants should rather be used.

He advises farmers to spread out 3 nitrogen applications over the course of the season. He also advises them to analyse the soil and monitor the plants regularly during the season for possible shortages of macro- and micro-elements – especially given the wet and cold conditions slowing nutrient uptake.

Prof. Alan Bennie, a retired soil scientist from Free State University and a grain producer, agrees that proper aeration is one of the most important strategies for a very wet planting season.

Sandy soils with water tables should be ripped 40 cm deep, and those without a water table, between 20 cm and 30 cm. “Microbes usually derive oxygen from organic material in the soil, but under poorly aerated conditions they convert nitrates into nitrites that are lost as gas, while they feed on the resultant oxygen,” he explains.

Bennie adds that these high moisture conditions aren’t all bad news, because they come with the promise of relatively certain high yields the next year. However, farmers need to adjust fertilisation rates accordingly. Spending more on nitrogen in particular may be well worth the extra cost.

WEED CONTROL

Dr. André Nel with the South African Agricultural Research Council’s Summer Grain Crops Institute in Potchefstroom, believes grain producers’ main challenge is the control of winter and spring weeds to prevent unnecessary soil profile moisture losses for the coming season.

The earlier weeds are controlled, the more effectively they can be exterminated. Special care should be given to controlling water-hungry species and to prevent weeds from enlarging their seed bank in the soil, by taking controlling measures well before they seed.

Nel adds that crop residue in the soil is an effective way of preventing moisture losses, as long as the residue is spread horizontally.