Nutritional Summary of Soybean Crops

Soybean is able to fix most of the N the plant needs through its symbiotic relationship with rhizobium bacteria. But it can also take up residual and mineralized N from the soil. Traditionally, soybean has been grown successfully without any addition of N fertilizer.

Nitrogen management has been limited to the rhizobium inoculation of fields which are new for soybean cultivation. As yields increase though, fixed N may not provide sufficient N to cover needs and therefore N deficiency may result. As higher soybean yields become more common due to improvements in genetics and management practices, addition of N may be needed to maximize potential yields.

The measurement of soybean nutrients uptake is based on the accumulated amount on plant leaves and stems. Uptake normally increases from emergence until the point of maximum accumulation, which is around 75 days. After this period, there is a decrease in nutrient accumulation, as a function of nutrients movement from vegetative parts to the grains.

The highest uptake occurs at 45 days after emergence, which is at the beginning of soybean flowering (R1). This 30-day period, between the inflection point and the maximum accumulation point (between flowering -R1 and pod filling – R5), is the critical period of the crop, where several factors such as drought, nutritional deficiency, pest disease pressure may dramatically reduce yield. At this crucial stage, the nutrient uptake is 52% of the maximum accumulated amount.

The critical phase for nutrient supply starts at 40 days after emergence, extending to the point of maximum accumulation. Therefore, when there is need for nutrient application, it must be performed before the inflection point, which represents the start of flowering (R1).

After that, maintenance fertilization becomes the usual, in which nutrient demand and yield potential need to be taken into account. 

Below are the nutrient removals for 1 ton of soybean.

Table: Average amount of absorbed and exported nutrients by soybean crop Source: Embrapa, Brazil 2008