Looking through a popular farm magazine recently, I noticed an article which focused on soybean fertilization and the use of tissue testing (plant analysis) to direct the fertilization program. I was especially interested in the comments about manganese (Mn). The author stated that “Manganese, which tends to be deficient in a large number of Minnesota fields helps increase the efficiency of photosynthesis.” Then, I remembered that there had been studies conducted in Western Minnesota to determine if the use of Mn as a fertilizer was really necessary for soybean production. So, it was time to look at the yield data and the concentrations of Mn in leaf tissue that were associated with these yields.
I’m going to summarize the results of two studies in Western Minnesota in which iron was applied both with and without Mn. Iron was applied because Iron Deficiency Chlorosis (IDC) is frequently a problem with soybean production in Western Minnesota. In the first study, the iron was coated on the seed and the Mn was applied as a foliar treatment. With iron, but no Mn, the yield was 56 bu./acre and the concentration of Mn in leaf tissue at early bloom was 76.8 ppm. When foliar manganese was applied with the seed coated with iron the yield was 53.0 bu./acre. The concentration of Mn in the leaf tissue increased to 86.7 ppm. The addition of the Mn did not increase yield when there was an adequate supply. Therefore, the concentration of Mn in the leaf tissue was not important. A range of 21 ppm to 100 ppm Mn in leaf tissue of soybeans tissue has been considered to be adequate for optimum yield of soybeans for a long time. The leaf tissue concentration in this study falls within the adequate range. A higher concentration of Mn in leaf tissue produced by the foliar application of Mn suggests that the Mn entered the leaf tissue. The fact that this increased concentration was not associated with higher yields is good evidence that Mn was not deficient in soil at this site.
In the second study, both iron and Mn were coated on the soybean seed before seeding. There was no foliar application of the two micronutrients. When the seeds were coated at a rate to supply 1.0 lb. iron/acre without Mn, soybean yield was 53.8 bu./acre. If seed coating was used to supply 1.0 lb. iron and 1.0 lb. Mn /acre, yield was 53.1 bushels per acre. It’s obvious that use of Mn in a fertilizer program did not increase yields. Without Mn coated on the soybean seed, concentration of Mn in the soybean leaf tissue was 56.3 ppm. This concentration increased to 61.6 ppm when the Mn was used. Both of the values fall into the adequate range.
As at the first study site, application of Mn did not increase soybean yield. Again, the soil was supplying adequate Mn for soybean production in Western Minnesota. The higher concentration of Mn in plant tissue produced by coating the seed with Mn is simply evidence that the applied Mn entered the plant but had no impact on yield.
There are two important conclusions that emerge from trials conducted at these two sites. First, the statement that “Manganese, which tends to be deficient in a large number of Minnesota soybean fields —— “is an exaggerated statement. Further, if Mn helps “increase the efficiency of photosynthesis” is true for Minnesota, Mn in a fertilizer program would increase as well. That was not the observation for the two sites described in the previous paragraphs.
There have been arguments for several years that the nutrient status of various crops can be determined by the use of tissue testing (plant analysis). There have been arguments that rate of application for some nutrients can be determined via the use of tissue testing. It is true that tissue testing can be used as a tool in the diagnosis of potential problems. However, there is no independent research to show that rate of nutrient application can be linked, with confidence, to results of tissue testing. The data needed to establish these relationships simply does not exist.
If there was a solid link between yield and nutrient concentration in plant tissue, the listed ranges for adequate concentration would be more than the presented values. The “adequate” range for Mn is a good example. A range of 21 ppm to 100 ppm Mn in soybean tissue is considered to be adequate for optimum soybean yield. This wide range indicates that there can be many concentrations of Mn associated with optimum yield.
It’s nice to dream of the time when analysis of plant tissue collected early in the growing season could be used to predict the rate of any nutrient during the growing season. However, that concept would require a very extensive data set. That data base does not exist at this time.
There have many criticisms of collection and analysis of soil samples when designing a fertilizer program. However, there is no equal or better substitute.