Nutritional summary

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.

The only N management has been limited to the rhizobium inoculation of fields which are new for soybean cultivation. As yields increase, 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. It normally increases from the plant emerging until the point of maximum accumulation, which is around 75 days. After this period, there is a decrease on nutrients accumulation, as a function of nutrients movement from vegetative parts to the grains.

The highest uptake occurs at 45 days after emergence, which is the inflection point, 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 attacks and diseases may dramatically reduce the yield. At that time, the nutrient uptake is 52% of the maximum accumulated amount.

In a 30-day period, which is the difference between the inflection point and the maximum accumulation point, the plant take up around 50% of its nitrogen needs, while the other 50% is absorbed from emergence until the inflection point at 44 days.

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).

For soybean cultivation, it is usual to perform corrective fertilization in order to increase soil nutrients availability. After that, the maintenance fertilization becomes the usual, in which nutrient demand and grain export needs to be analyzed. 

Below is the nutrient removals for 1 ton of soybean.

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