Hollard Insure and Farmingportal.co.za and Agri News Net - Young Agri Writers competition- Rest of the winners ( In no specific order)
Soybeans (Glycine max) form part of the legume family - which is unique in that it has the ability to utilize residual soil fertility well.
This legume has the ability to fix nitrogen gas (N2) from the atmosphere through a symbiotic relationship with Rhizobium bacteria (bud bacteria). Because of this unique ability, the assumption is often made that soybeans do not respond well to fertilization.
Soybeans are mostly known as a crop which yields good profits when produced. But just like with any other crop, low cost production inputs, produce low yields.
Nutrient extraction
According to the Fertilizer Association of Southern Africa (Fertasa), a total extraction rate of approximately 104 kg N, 9 kg P, 38 kg K and 3 kg S per ton of grain is estimated. Nearly 80% of the total amount of phosphorus (P), as well as 60% of the potassium (K) and sulphur (S) absorbed by the plant end up in the grain (AGTAG, 2019).
How to tackle fertilization
Soybeans thrive on fertile, deep and well-drained soils. The ideal pH (H2O) varies between 5.8 and 7.5 and liming is often recommended on acidic soils (Ceronio, 2018). Liming is extremely important to ensure that the necessary plant nutrients are available for root uptake and to create a favourable pH for bud bacteria. Micro-elements (zinc, iron, manganese, copper, boron and molybdenum) should also receive attention, as any form of deficiency will impair yield.
According to Prof. Fred Below and Dr. Ross Bender of the University of Illinois, research shows that nutrient consumption reaches a peak between the R5 and R6 growth stages. Little response to fertilization will occur if the soil can supply the nutrient demand during these stages and vice versa (Grain SA, 2019).
Nitrogen fertilization
Dr. André Nel (previously from the ARC Grain Crops Institute, retired) states that soybeans do not necessarily transfer nitrogen (N) to the follow-up crop, but that the increase in yield of maize after soybean cultivation can rather be attributed to an improved root system (16% more roots than maize on maize) (Grain SA, 2019).
On sandy soils, relatively low residual nitrogen is retained and a small amount of nitrogen applied can be beneficial. This gives the plant a good start, while the symbiotic action between the soybean plant and the bacteria only takes effect later. Successfully grafted soybeans (sufficient nitrogen-fixing buds on the roots, which show a pink colour when cut through) provide enough nitrogen for the production of 4 tons of grain per hectare. It is therefore not necessary to fertilize soybean fields with nitrogen (AGTAG, 2019).
Figure 1 Nitrogen-fixing nodules on the roots (Marken, 2020)
If the residual nitrogen from a previous maize crop is high, due to over-fertilization or drought, the follow up soybean crop will obtain sufficient nitrogen from the soil and will form relatively few buds. This plant then usually grows lushly with long internodes, until the residual nitrogen runs out later and a deficiency occurs during pod filling, which results in a lower yield (Grain SA, 2019).
Phosphate and Potassium Fertilization
Fertilizing soybeans is part of the production process that is probably most neglected. Due to soybeans sustaining their own nitrogen needs and South Africa's soils mostly having sufficient available potassium, soybean cultivation mainly focuses on phosphate fertilization.
Due to the extensive root system and the soybean’s sensitivity to high concentrations of mineral salts, this crop responds more favourable to general soil fertility than to direct fertilization. It is recommended to apply phosphate and potassium broadly and to incorporate it into the soil, so that the crop can absorb these nutrients later during the growing season (AGTAG, 2019). Soils with a phosphorus status of approximately 25 to 30 mg kg-1 (Bray 1) can be considered acceptable for soybean cultivation.
Potassium deficiencies occur mostly on sandy soils. Approximately 100 mg kg-1 can be set as a general guideline in a potassium fertilization program. Where the fertility status of a soil is acceptable, a lower application than removal rate becomes an economical fertilizer recommendation.
Fertilization of soybeans should in principle be dealt with like any other crop. What has been removed should be restored to the soil. If this is not the case the farmer ends up mining the the soil – a practice that is not sustainable. A balanced fertilization program will produce the best yields in the long run.
Rachel Rossouw
