Fertilizer International 524 Jan-Feb 2025
22 January 2025
P & K management in soybean/corn rotations
CROP NUTRITION
P & K management in soybean/corn rotations
Major global crops corn and soybean are often grown in rotation. Dr Karl Wyant, Nutrien’s Director of Agronomy, outlines how the phosphorus, potassium and sulphur removed during soybean/corn rotations in Brazil and the United States are best replenished. The importance of these three nutrients for nitrogen fixation in soybean is also described.
Introduction
Corn (maize) and soybean (soya) are major crops grown throughout the world, often in rotation with one another. For the 2023/24 growing season, global corn production totalled 1.2 billion tonnes, while the soybean harvest came in at 395 million tonnes, according to the USDA.
High yielding crops grown at this scale across millions of hectares require careful fertilizer planning. This is necessary to maximise agricultural productivity while at the same time reducing environmental impacts from the loss of excess nutrients.
In this article, we highlight the removal of phosphorus (P), potassium (K) and sulphur (S) as crop nutrients during soybean/corn rotations in Brazil and the United States. The important influence of P, K and S supply on nitrogen (N) fixation in soybean is also outlined.
Major producing countries Brazil 39% Brazil
Brazil and the United States collectively account for approximately 68% of global soybean production and 52% of global corn production (Figures 1 & 2). The climate in Brazil allows growers to plant soybeans in the spring and then follow this with corn planting in the same 12-month period. In contrast, much of the climate across the US corn and soybean belts only permits one crop to be planted per year, resulting in a two-year rotation cycle (Figure 3). In both countries, nutrient management plans need to be adjusted for the local climate to maximise crop yield and quality.
Nutrient removal and fertilizer planning
Estimating how much phosphorus, potassium, and sulphur are leaving the field in the form of a harvested product (e.g., grain) is a crucial tool for nutrient management planning. To maintain long term soil fertility, nutrients exported from the field will ultimately need to be replaced and, consequently, estimates of nutrient removal are used as the basis for fertilizer application rates.
In general, large nutrient removals from the field – as a result of a high yielding crop harvest, for example – will need to be followed by the application of fertilizers or manures at high rates to maintain the productive capacity of farmland (Figure 4).
Assessing the combined quantities of nutrients removed by corn and soybean harvests provides a better holistic understanding of phosphorus, potassium, and sulphur removal from the field across the rotation (Figure 5). The disparities in nutrient removal seen during crop rotations in Brazil and the United States reflect two main factors:
- Differences in the average yields in each country
- Differences is the timeframe for nutrient removal.
The lengthy growing period in Brazil’s tropical climate, for example, allows the soybean and corn rotation to take place during one single year. Whereas the rotation in the more northerly United States, in contrast, typically takes place over the course of two years due to a shorter growing season. These contrasting approaches to soybean and corn growing practice are important, given that a successful nutrient management plan is based on replenishing soils with the right quantities of P, K and S at the right time.
P, K and S for soybean nodulation
One key aspect of soybean/corn rotations is the ability of soybeans to ‘fix’ atmospheric nitrogen into the plant-available form needed for crop growth. Essentially, the grower requires soybean plants to be self-sufficient and produce their own ‘free’ nitrogen fertilizer to drive high yields. Furthermore, soybeans can also enrich the soil and leave nitrogen behind, thereby influencing nitrogen availability and N fertilizer application rates for the corn crop in the next rotation.
Valuably, phosphorus, potassium and sulphur, by positively influencing the ability of the soybean crop to ‘fix’ nitrogen, can boost the generation of this shared nitrogen source for soybeans and corn. The role of P, K and S in maximising the ‘free’ N contribution to the nutrient management plan is reviewed briefly below.
Biological nitrogen fixation functions thanks to a symbiotic relationship between soybeans (and other legumes) and soil dwelling bacteria. In this mutually beneficial relationship, plant roots infected/inoculated by a specific bacterium convert atmospheric nitrogen (N2 ) into a more biologically useful ammonium (NH4 ) form able to fuel plant growth and yield1 . In return for providing this service, bacteria are rewarded by the plant with sugars derived from photosynthesis to fuel their biological activity. In general, the N fixation capacity of legumes has been shown to correlate highly with crop yields2 .
Under low phosphate supply conditions, P deficiency is known to limit both plant root growth and the creation of the energy storing compound adenosine triphosphate (ATP), the biological ‘currency’ used by plants to build sugars. It is important here to remember a critical dictum about the relationship between plants and nitrogen-fixing Rhizobiaceae bacteria:
Research shows that increasing P supply to soybean plants increases both root and nodule weight3 . This, in turn, boosts the above-ground biomass and crop yield, as measured by shoot dry weight.
Plants typically require more potassium than any other nutrient – with the possible exception of nitrogen – due to its vital role in crop growth and development. In general, K has the ability to increase rates of N fixation and overall soybean crop yields via the following mechanisms1 :
- It contributes to good root growth and has been shown to improve the number and size of nodules on roots
- It is a cofactor for the functioning of the enzyme that transport carbohydrates across cell membranes and into the phloem.
Similar to phosphorus, the same dictum – No sugar = no carbohydrates to pay for N fixation – applies to potassium, with research again showing that increasing the K supply to soybean crops increase both nodule number and nodule weight1 . This, subsequently, results in higher above ground yield and seed protein quality (Figure 6).
The role of sulphur nutrition in promoting N fixation in soybeans has been the subject of a recent review4 . This concluded that sulphur deficiency leads to fewer root nodules and limits symbiotic nitrogen fixation in soybeans. In one study, sulphur deprived crops – even when inoculated with Rhizobium bacteria – failed to produce root nodules, significantly reducing nitrogen accumulation and soybean shoot biomass4 . Additionally, a separate study has demonstrated that sulphur nutrient applications significantly increase both the quantity and mass of soybean root nodules5 .
Conclusions
Estimating the quantities of phosphate, potassium, and sulphur that exit the field as harvested products (e.g., grain) can be useful for planning fertilizer applications in soybean and corn rotations. As previously stated, high yielding soybean and corn remove elevated quantities of P, K and S – and, consequently, growers need an effective fertilizer plan to replace these lost nutrients. Furthermore, these three nutrients can be leveraged to boost nitrogen fixation in soybeans and, in doing so, contribute to the ‘free’ nitrogen that supports high-yielding soybean and corn crops across the rotation.
As a final comment, a good set of soil sample test results is useful for determining potential P, K and S deficiencies. Such data, alongside estimated nutrient removal rates, can help refine the nutrient management plan to deliver quality crops at high yields, while simultaneously helping prevent nutrient losses to the environment.
References
