Most Western Cape wheat farmers who also plant canola, might be able to cut back on the amount of nitrogen fertilizer applied to canola. This saving is a positive result of the decades' worth of applying the principles of conservation agriculture. It has improved soil health and has led to soils being naturally richer in nitrogen and beneficial microbes. So says Dr Izané Crous, who receives her PhD in Agronomy from the Faculty of AgriSciences at Stellenbosch University (SU) in December.

Her study is the first to evaluate canola's reaction to different rates of nitrogen fertiliser, and to determine the nitrogen needs of canola grown in the Western Cape's canola producing regions.

Dr Crous works as a barley expert at ABInBev's research farm near Caledon in the Overberg. Between 2016 and 2019 she ran experiments in five of the major regions in the Western Cape where canola is being produced: Darling, Porterville, Riversdale, Moorreesburg and Riviersonderend.

She thanked all farmers who had in one way or another helped her with her research: “They were my true mentors."

The research was conducted under supervision of Dr Johan Labuschagne of the Western Cape Department of Agriculture and Dr Pieter Swanepoel of the SU Department of Agronomy in the SU Faculty of AgriSciences. The field trials were funded by the SASOL Agricultural Trust and the Western Cape Department of Agriculture, and she received bursaries from the Protein Research Foundation and Southern Oil Ltd.

“To be able to reduce the amount of nitrogen fertilizer is a positive outcome from many decades that producers have already been practising the basic principles of conservation agriculture on their farms. In the process the organic carbon content of the soil has increased, as well as the microbial activity in the soil. This has resulted in more nitrogen being converted to plant-available forms of nitrogen in soil. As a result, canola nowadays probably needs less fertilizer-applied nitrogen to ensure a particular yield," Dr Labuschagne explained.

Canola in the Western Cape

Canola (Brassica napus L.) was first planted in the Western Cape in 1993. This distant relative of cabbage is among others used to produce oil and animal feed. According to the 2020 SOILL Yield Survey, production in the province has increased from 400 hectares in the 1990s to around 75 000 in 2020. Dr Crous says around 95 000 hectares were planted in 2021.

Wheat farmers use canola as a rotation crop, among others to help prevent diseases and to help curb weeds and in so doing to improve yield potential. Once the seeds have been harvested, all other plant material is left in the fields or baled. Over the years the planting of canola has therefore ensured significant improvement of soil health in grain producing areas.

Practising conservation agriculture has gone hand in hand with higher nitrogen levels in the soil, says Dr Crous. Therefore, producers should nowadays be careful not to overfertilize with nitrogen.

Suggestions Nitrogen is one of the most important nutrients needed to ensure a good yield – but it can cost a pretty penny. Applying too much is not always beneficial, either. With canola it can inhibit plant growth, and the production of seeds and oil. Too much nitrogen in the soil does not equal a higher yield, Dr Crous warns, and can cause damage to the environment and unnecessarily eat into producers' profits.

Dr Crous says decisions on how much nitrogen fertilizer is to be applied, as well as the source (be it nitrate, ammonium or urea with or without a urease inhibitor) being used must be made based on the climate of a particular region and the condition of the soil itself.

“It is especially important not to add too much nitrogen in dry years," she points out.

Her study is the first to specifically consider the nitrogen requirements of canola being produced under conservation agricultural conditions in South Africa. So far local producers have been using adopted guidelines set for the wheat industry or ones that had their origin in Australia. According to these guidelines, 40 kilograms of nitrogen is needed to ensure a yield of 1 ton of canola seed.

“Australia's soil and climate differs from ours," Dr Crous notes.

• Dr Crous is a barley expert and works at ABInBev's research farm in Caledon. She is responsible for pro-active research and for the implementation of initiatives that will ensure that raw materials used in the production of beer, such as barley, are produced optimally, sustainably and with as small risk as possible.