Cut Out the Phosphorus

      Horticulture is a science. It is not based on intuition, feelings, grandpa or great grandma. When I started college and my career, horticulture professors often would say that 25 percent of what we know is based on science, 25 on hearsay and 50 percent on experience,

        Today the saying goes: 50 percent of what we know is based on science, 25 percent on experience and 25 percent on hearsay. We have come a long way since 1956.

        The Bay Gardener is glad to see that readers are beginning to appreciate the science of gardening. This spring I have received a number of soil test results from Bay Weekly readers. Some of the soil test results were from lawns, but the majority were from vegetable gardens.

      From the consistency of the results, I see that most of the gardeners have been brainwashed into making repeated applications of 10-10-10 fertilizers year after year. The 10s represent the three primary nutrients: nitrogen (N), phosphorus (P) and potassium (K). A bag of 10-10-10 fertilizer contains 10 percent nitrogen, 10 percent phosphate and 10 percent potash.

      Readers’ soil test results indicate levels of phosphorus high enough to cause problems.   

       The old school taught that phosphorus is not very soluble in the soil, especially in the spring when soils are cool. Thus fertilizer containing phosphorus was applied every year. 

       That antiquated thinking was true when rock phosphate was the primary source of phosphorus back in the 1950s. But today, no one should be using rock phosphate now that we know it is naturally contaminated with the heavy metal cadmium. 

      The sources of phosphorus in today’s fertilizers are super phosphate, treble phosphate, DI-ammonium phosphate, ammonia-ted poly-phosphate, etc. These sources are more soluble and only contain trace levels of cadmium.

      Since plants use a relatively small portion of phosphorus as compared to potassium and nitrogen, what is not used by the roots binds to clay colloids and with essential plant nutrients such as iron, zinc, copper and calcium. 

      The level of phosphorus in soil test results submitted by Bay Weekly readers is most likely reducing yields and causing unusual symptoms on the plants. Some of the symptoms described lead me to suspect iron deficiency, which should not occur because most of our soils are rich in iron. Holes in beet leaves are an indication of zinc deficiency. Another effect of high phosphorus level is the reduction in the quantity of tomatoes being harvested despite irrigation during drought. Beneficial fungi in soils are also adversely affected by excess levels of phosphorus.

        Maryland law now requires farmers and commercial operators to show proof that soils are in need of phosphorus to purchase fertilizers containing the element. Perhaps the law should be applied to home gardeners as well.