Fertilizer Requirements for Soybeans

Soybeans, like all legumes, have somewhat different nutritional needs than non-legume vegetables. The principle difference is that soybeans and other legumes are able to fix nitrogen, which means that they can extract it from the air and therefore do not require any traces of the element in the soil. However, soybeans do require other nutrients, such as phosphorus and potassium, and can derive these from fertilizer.

Soybean plants respond well to fertile soil with ample amounts of phosphorus. On average, a single crop of soybeans will take in about 60 percent of the available phosphorus from the soil. Depending on your local soil makeup, fertilizer may or may not be necessary. As with other nutrients, soybeans tend to respond best to fertilizers that have been applied prior to planting. If you're hoping to plant soybeans but you have poorly fertilized soil, you might work phosphorus-rich fertilizer in this growing season and wait until next year to sow any soy. The amount of phosphorus in soil is described by its soil test level, which ranges anywhere from 0, with no phosphorus, to 21 and up, an extremely high level of phosphorus. As a rule of thumb, adding 20 lbs. of phosphorus pentoxide per acre will increase the soil test level by one point.

Potassium is another important nutrient for soybeans. As for phosphorus, it's best to attend to your soil's potassium levels well before you plant. Ideally, you should prepare your soil the season before you plant your soybeans. By the time they are ready for harvest, mature soybean plants consume about 50 percent of the available potassium in soil, on average. To raise the soil test level for potassium, it takes just 10 lbs. of potassium oxide per acre. The soil test levels have a different scale for potassium than for phosphorus, as a particularly high potassium content would have a soil test level of 161 or higher.

After nitrogen, phosphorus and potassium, zinc is probably the next most important nutrient for healthy soybean plants. A soy plant with zinc deficiency is usually growing in sandy soil or in soil with a high calcium carbonate level. Zinc deficiencies also happen in areas where the topsoil has been removed. You can spot a zinc deficiency by looking at the soybean plant's leaves. The older leaves tend to have light green coloration between their veins and potentially bronze coloring. New leaves develop very small and may develop spots. If your soybean plants are growing in zinc deficient soil, you can safely increase the level by adding 3 to 5 lbs. of zinc per acre. Common forms of zinc include zinc sulfate, which has a 36 percent zinc content.