Soybeans and Potassium
If you asked a farmer what his most yield-limiting pest for soybeans was over a decade ago his response would most likely be weeds. With the advent of Roundup Ready® soybeans, that concern has all but disappeared. Today, the soybean pests that take center stage might be aphids, cyst nematodes or Sudden Death Syndrome. Not much is said or written about factors other than pests. However, one of the most yield influencing inputs would have to be fertility and potassium levels in particular.
Potassium (K) is an important nutrient for soybean growth and development. It is involved in the regulation of several plant functions namely water and nutrient movement across cell walls and in maintaining the plant’s defensive mechanisms. A majority of the K uptake by the soybean plant occurs after flowering (early-mid July).
Soybeans are quite responsive to K fertilization and require large amounts of it for optimum yield. A 60 bushel/acre soybean crop removes about 85 lbs of K from the soil, which equates to 1.4 lbs of K removed for every bushel of soybeans. Many research studies have concluded that applications of K can significantly increase soybean seed yield where the soil K test levels were initially low. As an added benefit isoflavone levels in the soybean seed also increase with increasing K fertility (isoflavones have the potential to prevent certain cancers and heart disease in humans). Raising the K levels of the soil takes time and only when fertilization rates exceed the rates of crop removal. Studies show that it generally takes about 4-8 lbs of K fertilizer above crop removal rates to raise the soil test one pound of K per acre. Adding K fertilizer over and above current crop removal rates will pay big yield dividends for successive crops.
In no-till soybean systems, K stratification can occur due to the lack of soil mixing in the top profile. With high surface crop residues, low K mobility in the soil, and broadcast-surface applications of K fertilizer much of the available K is at or near the top of the soil. In years with drought stress, surface root growth is impaired and, thus, unable to take up adequate levels of K. And with lower K levels further down in the soil profile, soybeans can easily show symptoms of K deficiency in a dry summer. Potassium deficiency restricts grain development (reducing seed size and weight) leading to yield reduction. Conservation tillage methods that allows for some soil mixing without too much disturbance of the crop residue may aid in the uptake of K by the plant in years with drought stress. Studies have shown that no-till soybeans yielded more over the old corn row than between the row – again illustrating the low K mobility within a no-till system.
Coarse-textured soils may also exhibit K deficiencies. Coarse (e.g. sandy) soils cannot hold K like clay or loam soils can. Thus annual applications of K are most likely needed in coarse soils to supply adequate K for the developing soybean plant.
Soil test for K periodically and add K based on fertility recommendations. According to the Tri-State Fertilizer Recommendations, the critical soil test level for K for soybeans is around 200-300 lbs/acre based on the soil’s CEC (lower levels for CEC’s below 10). Thus, K soil test levels below this range should be considered K deficient for soybeans.
There are many other factors that can influence the yield potential of a soybean crop; soil drainage and pH, various insect, disease and weed pests, and environmental conditions. However, potassium levels in the soil should not be taken for granted and should be monitored and corrected for optimum soybean yield and profitability. It may be the “best kept secret” in soybean production today!
Have a great weekend!
Dan Childs, Agronomist
