Fertilizer International 499 Nov-Dec 2020
30 November 2020
Season-long soybean nutrition maximises yields
AGRONOMY AND PRODUCTS
Season-long soybean nutrition maximises yields
Kyle Lilly, senior product manager at Compass Minerals Plant Nutrition, outlines how a season-long approach to soybean crop nutrition can be created using a variety of tools. When combined together, these can optimise soybean fertilization and deliver impressive yields – as has been shown in both Brazil and the US.
A quick look at the research on soybean fertilization reveals extensive information about the recommended broadcast rates for macronutrients. This article builds on that knowledge by describing additional techniques that have improved soybean crop yields in both North and South America through season-long crop nutrition. The techniques include:
- More even nutrient distribution in fertilizer blends
- Treating seeds with nutrients
- Time specific foliar applications of nutrients.
In 2019, the United States (US) set a new soybean yield record1 of 12.79 tonnes per hectare (t/ha) – nearly four times the average soybean yield2 that year of 3.34 t/ha. Meanwhile, the average soybean yield in Brazil last year3 was an estimated 3.37 t/ha. Brazilian farmers have also shown themselves capable of growing high yielding soybeans, with the 2017 Comitê Estratégico Soja Brasil yield winner4 producing 8.94 t/ha.
These examples from the US and Brazil, the world’s two largest soybean producers, illustrate the ‘yield gap’ – the difference between the average soybean yield and its genetic potential. While weather may be outside of the farmer’s control, crop nutrition is a critical and controllable factor capable of increasing soybean yield and return on investment (ROI).
A season-long approach
Most soybean farmers subscribe to sufficiency fertilization. This involves applying ‘just enough’ fertilizers to the crop, no more, no less. It is unsurprising to find, therefore, that such farmers are nowhere close to achieving the crop’s ultimate genetic potential. However, increasing numbers of farmers are finding that a season-long approach to soybean nutrition provides the ideal framework for maximising both yields and ROI. The 4Rs fertilizer concept – right source, right rate, right time, right place – underpins this approach by helping identify gaps and shortfalls in the farmer’s sufficiency-based fertilization programme.
Fertilizer distribution
Large amounts of macronutrient fertilizers are applied across soybean fields. However, micronutrients are also necessary for a high-yielding crop. Results from over 42,000 tissue samples in the US show that over 75 percent of soybeans are not only deficient in potassium (K) but also in copper (Cu)5 . Copper plays a critical role in photosynthesis. It also improves the structural strength of soybean plants by promoting lignin formation in plant cell walls.
However, evenly distributing micronutrients is a challenge, highlighting the importance of the ‘right place’ part of the 4Rs fertilizer approach. The typical application rate for granular copper sulfate is 6.4 kilograms per hectare (kg/ha). If added to a bulk blend, such a small amount cannot be spread evenly across the field, leaving some areas Cu deficient or, even worse, experiencing Cu toxicity.
Applying micronutrients as dry dispersible powders solve this dilemma, as research from Compass Minerals Plant Nutrition has shown (Figure 1). The dry powder evenly coats macronutrient fertilizer granules due to its small particle size and high surface area. The same effect can be observed by placing your hand inside a bag of flour and noticing how the tiny flour particles stick. Multiple field trials show that dry dispersible powder micronutrient coatings increased zinc (Zn), manganese (Mn), and iron (Fe) uptake in soybeans by 12.4 percent, 15.3 percent and 22.0 percent, respectively, against an untreated control.
Treating seeds with nutrients
Many growers are aware of the importance of early-season fertilization for achieving high soybean yields – and often meet this need by applying a starter fertilizer. However, evidence from recent agronomic trials, together with the higher nutrient demands of new genetic varieties of soybean, suggests that the starter fertilizer may not be supplying enough nutrients or entering the crop early enough in the growth cycle.
There is a fundamental need to get nutrition to the seed as early as possible. This reinforces the importance of the ‘right time’ in the 4R approach. Applying small amounts of nutrients directly to the seed is one method showing great promise – by improving stand count, early crop vigour and nodulation. Applying nutrients to soybean seeds, having become standard practice in Brazil, is now slowly gaining traction in the US too.
Nutrients are commonly applied to seeds as either a dry powder or liquid coating (Figure 2). Liquid coatings are applied with a commercial seed treater. This enables them to be applied at the same time as fungicides, insecticides, inoculants and other crop protection inputs. Dry powders can be applied during the liquid treatment of seeds or, alternatively, at planting when seeds are in the planter box. If applied during the liquid treatment process, seeds can also be treated with a drying agent or a pearlescent pigment alongside the nutrient powder. This simultaneously improves seed flowability and appearance, as well as providing crop nutrients. The planter box application method is a direct replacement for other seed flow aids such as talc or graphite.
Critical nutrients to apply on-seed
Soybean farmers in both Brazil and the US have found success applying molybdenum (Mo), cobalt (Co), and nickel (Ni) – as well as Zn and Mn – directly to the seed, either as a dry powder or as a liquid coating. These nutrients offer specific benefits to early germinating soybean plants.
While nitrogen (N) is king in terms of soybean nutrient demand, its role as a crop nutrients is supported by micronutrients. Soybean crops are, in fact, rarely fertilized with nitrogen as they have the ability to take up 82 kilograms of N per tonne, mostly by ‘fixing’ this from the atmosphere. The establishment of functioning nodules is absolutely critical to this nitrogen fixation process. The micronutrients Mo, Co, and Ni all play important roles in nitrogen metabolism. These nutrients are key to unlocking unusable nitrogen. They do this by converting urea-N into plant-available nitrogen (ammonia-N and nitrate-N) containing amino acids and proteins. This combination of nutrients ultimately builds biomass and yield. Liquid seed application of these nutrients resulted in a 45 percent increase in soybean yields, university trial results from Brazil have shown6 .
As plants begin to grow, Mn and Zn create longer, more developed root systems, and also play a critical role in improving plant resistance to early-season cold and moisture stress. Results from 11 large-scale US soybean field trials showed that a dry seed nutritional application containing Mn and Zn with Fe and Mo delivered a 3.95 percent average yield increase7 . Common benefits of on-seed application of micronutrients include bigger and greater numbers of nodules, thicker stalks, and taller, greener plants (see Figure 3).
Time-specific foliar applications: vegetative stages
As previously mentioned, nitrogen metabolism in soybean is improved by the on-seed application of Mo, Co, and Ni. Foliar applications of these nutrients at the vegetative growth stages have also been shown to improve soybean nutrition and yields. Research from 33 large-scale field trials in the US has shown that foliar application of Mo, Co, and Ni during vegetative growth stages results in a soybean yield improvement of 4.82 percent8 . The author suggests these three nutrients function by delaying ethylene synthesis inside the soybean plant. This allows the nodules in the crop to continue to produce nitrogen and build proteins. However, this is only one hypothesis and further research is needed to conclusively demonstrate this.
Time-specific foliar applications: reproductive stages
Flower retention and pod set in soybeans are critical to yield success. Boron (B) plays a key role during these stages, supporting both pollination and seed production. A foliar application of seaweed extracts containing B, Mo, and Ni was applied to soybeans at the R1 growth stage by researchers in Brazil9 (Figure 4). This treatment delivered a yield increase of up to 15 percent, based on the results of these small plot replicated trials. The treatment also improved the number of pods per plant by up to 18 percent. This again highlights the need for the ‘right time’ in the 4Rs fertilization approach.
“In Brazil, the Compass Minerals nutrition programme has increased soybean crop yields by an average of six percent.”
A season-long approach to soybean crop nutrition
This article outlines how a season-long approach to soybean crop plant nutrition can be created using a variety of tools. These include optimised nutrient applications, even nutrient distribution, seed-applied nutrients and timely foliar nutrient applications. When combined together, this system can optimise soybean nutrition and deliver impressive yields. In Brazil, the Compass Minerals season-long nutrition programme for soybean has increased crop yields by an average of six percent in over 500 field trials. US data for the programme is still being collected currently, but is sure to show similarly positive findings. One highlight for the Brazilian programme was the setting of the national soybean yield record of 8.94 t/ha in 2017 by a participating farmer. We conclude that this new approach to soybean crop nutrition shows great promise by encouraging the adoption of 4R fertilizer principles to improve yields.
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