By Dr. Glen Harris
As a corn grower, you’ve probably heard it a million times: Soil test and apply the recommended lime and fertilizer so that soil fertility will not be your limiting factor for making high yields. But do you believe it? I do. But maybe you just heard the following so many times “lime to the proper pH, apply P and K according to soil test, choose the correct N rate according to yield goal” that it sounds old and out of date. Surely there must be something new in the fertilizer world that can get us to the next level.
Well, there are plenty of new products out there as you probably know. Most are fertilizer additives aimed at making the plant more efficient at utilizing the fertilizer nutrients you apply. There are urease and nitrification inhibitors for nitrogen, complexing-type materials for phosphorous and chelating agents for micronutrients. Not to mention what seems to be the latest popular trend of adding humates or humic acid to just about anything to make it work better.
Do these new products really work? Maybe. Maybe not. A lot of it depends on the situation – soil type, fertility levels, weather, rate and timing, etc. In theory, many of these products make perfect sense. The question becomes: Can we make them work in practice or in the field and are they economical?
What About Humates?
Coastal Plain soils in the southeastern United States are notoriously low in organic matter. Typical levels in a south Georgia corn field would be around one percent. Since organic matter helps hold water and nutrients, and since humates are essentially breakdown products of organic matter, it makes sense that adding humates to these soils would be extremely beneficial. However, in practice, it appears that you cannot apply enough humate, economically, to produce the desired effect. Maybe a better strategy would be better residue management, including growing winter cover crops. Basically: “Build it and they will come.” In this case, build up the soil organic matter by utilizing all crop residues, and the humates and humic acids will hopefully come along in amounts that are beneficial.
But do products like humates make fertilizer work better? Good question. Most fertilizer materials work pretty well on their own. Adding this type of product may help some. The better question is: Does it increase the efficiency enough to cover the added cost or allow me to reduce my fertilizer rate? Now don’t get me wrong. I believe we should continue to look at new products, field test them and determine which ones work and under what conditions. But, I also believe that sticking to the basics, or going back to the basics, is something we need to do.
Two Different Theories
Take, for example, pH and liming. Coastal Plain soils, in addition to being low in organic matter, are naturally acidic and need to be limed to produce good yields. A common application rate to raise the soil pH to the recommended target of 6.0 or 6.5 would be around one ton of lime per acre. Recently, there has been a theory unleashed in the Southeast that suggests growers should apply 10 to 20 tons of lime per acre!
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New Products And Theories Raise Questions
• Do humates increase efficiency enough to cover the added cost or allow fertilizer rates to be reduced?
• What are the arguments for and against growers applying 10 to 20 tons of lime per acre instead of one ton per acre?
In theory, this saturates the cation exchange or base saturation of the soil with calcium, which somehow improves the efficiency of fertilizer uptake. However, the alternative theory is that instead of improving fertilizer efficiency, it can cause potassium and micronutrient (such as manganese and zinc) deficiencies. Not to mention that at approximately $25 per ton of lime, this high rate would not be cost effective. I must confess that I have not tested this new practice in the field. I plan to. If it works, I’ll let you know.
Dr. Glen Harris is an Extension Agronomist,
University of Georgia.