We know that different nutrients are required at different times for optimum soybean yields. Current soybean biomass production shows a two-fold increase from the 1930s, and yields show a three-fold increase. With these drastic changes in genetics, it only makes sense that we would also see changes in nutrient uptake. In addition to genetics, there is evidence that environmental factors like temperature, moisture, and soil fertility influence nutrient uptake.

Below is the nutrient content found in grain at the end of the season from research conducted in Nutrient Uptake, Partitioning, and Remobilization in Modern Soybean Varieties. Nutrient uptake is slow and steady for the first 40 days after germination. It isn’t until V4 that a steady increase in secondary and micronutrient uptake occurs while macronutrients are taken up in the greatest amounts starting between V7 and R2 (full flowering).

Harvest index is a measure of the percentage of an applied nutrient that is removed in the grain. Several nutrients have consistently shown high harvest index values. In other words, these nutrients are closely correlated to yield. The nutrients on this list: N, P, S, Cu, have all shown a harvest index value greater than 50%. The importance of these nutrients in grain fill emphasizes the need to have adequate amounts of these nutrients during reproductive growth stages.

Nutrient uptake in soybeans closely aligns with dry matter accumulation. Nearly 75% of K uptake occurs prior to seed fill. Then during seed fill the plant moves, or remobilizes, K mostly from the stem and to a lesser degree from the leaves to the grain. The soybean plant remobilizes most of the N and P for seed fill from the stem. The plant steadily accumulates N, P, S, Mg, Ca, Zn, B, and Mn throughout the season and partitions the nutrients to the leaf, stem and petiole. There are increases in plant demand, and therefore uptake, for micronutrients at V4 and for macronutrients between V7 and R2. Because the plants have increased demand at those times in the season, the windows of application found with positive ROI in PFR testing make sense.