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Everyone knows what phosphorus (P) deficiency looks like in corn—purplish color leaves. So it would be easy to expect similar symptoms for soybeans. But it turns out to not be the same. Phosphorus deficiency isn’t as easily identifiable as in corn or as potassium deficiency is in soybean tissue. As an agronomist I must admit that I never thought much about P deficiency symptoms below.

In early July I decided to look for an image online and see if a P deficiency in beans was similar to corn. I quickly learned that it isn’t and that there are really no images posted online. What I found was a general description that said that plants were smaller, spindly and had a dark green to blue appearance. Most online postings had the same description and most seemed virtual copies of one another. Dr. Antonio Mallarino, soil fertility expert at Iowa State University, responded to my email query and stated “No Dan, I have never seen a clear soybean P deficiency symptom in my life and don’t think there is any, and don’t trust any. Just reduced growth.” That is pretty much what I deduced after my Google search.

Leaf tissue testing is the only method to accurately diagnose P deficiency. Mallarino also posted an extension article online about tissue testing for phosphorus and potassium. He reported the critical concentration range for plants was 0.32 to 0.42% P. The full article is online here. Penn State University has a chart that includes four categories: low 0.16%, normal 0.26%, high 0.51% and excessive 0.81%. The full article is online here. A bit more online sleuthing seems to confirm that the general critical level is around 0.35%.

Bobby Golden, Agronomist, Delta REC, Mississippi State University, published the image above of P-deficient soybean plants indicating they are short, spindly and off color. 

Soybeans require a lot of phosphorus and some recent research by Fred Below at the University of Illinois strongly suggests that P is more limiting than K. However, the fact that we can see and recognize K deficiencies but not P deficiencies has led many of us to believe that K is probably the most limiting nutrient—while it may be P in the end.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.