Del Voight – Penn State Extension
As yields in soybeans continue to increase the demand for the soil to produce the required amount of fertility are becoming more important. A typical removal of the plant available form of Phosphorus(P205) and Potassium(K20) for soybeans is 1lb of P205 and 1.4 lbs of K20. This equates to a normal yield of 60 bu/acre removing a total of 60lbs of P205 and 84lbs of K20. I was talking to a grower last week whom had whole field averages of 87bu/acre and I commonly hear of yields in the low 80 bu/acre range. If a maintains his soil fertility based on a 60 bu/acre harvest yet commonly harvests 80 bu/acre in time the soil will become depleted of fertility. In this example the difference of 60 vs 80 bu/acre would about 20lbs of P205 and 28lbs of K20 for each year of missed calculations. I commonly do not run into any issues with P205 however I have been running into numerous issues with K20 on all crops from alfalfa to corn and soybeans. This link Soybean K is to a photo gallery of a case study where the producer had deficiency and I took soil tests from both the good and bad areas. Later in the day the producer spun on 0-0-60 at 200lbs per acre and the week later photo illustrates the rapid response by the soybeans to the fresh supply of K20.
To begin, it is important to know the goals within the soil when viewing soil tests. Maintain P levels in the optimum range from 60 to 100ppm range and K levels also in the optimum. I tend to commonly use Dr Beegles recommendation of K levels maintained in the 2.5 to 5% saturation on the CEC.
Where I see foliar fertilizer fit is in optimum levels. In past years with growers topping 90 bu/acre in many cases a foliar material is utilized and I have seen this first hand. I have also documented through tissue testing the however slight bump in K levels within the leaves. For these extreme yields all stresses must be taken away. These products are not designed to remedy soil deficiency issues. Again to supply 84lbs of K20 with 0-0-60 it would take 200/bs per acre of dry product while to meet the same 84lbs of K20 one would have to apply from 84 to 160 gallons per acre of foliar fertilizer depending on the formulation.
It is time to make the adjustment to increase rates of K20 applications to manage soil fertility. As a source muriate of Potash(-0-0-60) is a great source for to correct deficiencies in the soil as well as in season deficiencies. There are times when soils are adequate however the roots might not get to the nutrient either from compaction and or other drought times. In these cases either dry and or liquid foliar applications might be useful. However, if a deficiency is caused by low soil level K then foliar liquids are not economical to correct the soil issues however they will bring the plant out of the deficiency for a short time. This season we had deficiency at a On Farm site and used 1 gallon per acre of a foliar K product. It really greened up the plants however in as little as 2 months the plants displayed symptoms again. This further supports two prongs correct the soil K levels and tissue to test to begin correcting any hidden deficiency.
If a grower decides to apply foliar fertilizer products to the field it is best to apply as a separate application and not combined with fungicide or insecticide materials. In fact it is best utilized by taking a tissue test when the soybeans are in the R2 stage and select a foliar fertilizer that meets the needs of the plant. Depending on the weather(hot(86 plus F), humid (60% plus), foliar burn (phyto toxicity) may result. In our On Farm Network a few days afer treatments we observed burn in a few locations. We put a short video together to discuss this seasons plots. On Farm Foliar Fertilizer Phytoxicity
It might be too late for this season however to avoid this situation next season growers need to test the soil this fall, plan on meeting the need of the crop at a minimum of 84 lbs of K20 (80bu/acre)(140lbs/acre of 0-0-60)
