Fall is the Best Time for Lime Application

• Soil nutrient availability and fertilizer use efficiency is pH sensitive
• Grid sampling provides the best method for measuring pH differences
• Field variance of soil pH is driven by yields, ammonium fertilizer and manure applications since the last pH correction

Soil pH is the foundation to obtaining the most from soil fertility and maximizing yield.  Extreme soil acidity or alkalinity can affect both nutrient (Diagram 1) and fertilizer availability.  Additionally, soil pH outside the range of 6.0-7.5 can create an unfavorable environment for soil microbial life, adversely affecting soil health.  Phosphorus is one of the most pH sensitive crop nutrients with a pH requirement around 6.7-6.8.  At low soil pH levels phosphorus begins to form plant unavailable compounds with iron and aluminum, and with calcium when pH is high.   Even major nutrients like nitrogen become less available at pH 6.0 or below. The ideal soil pH range for a corn-soybean rotation is 6.0-6.5. When alfalfa is in the rotation, a pH of 6.8 is recommended.  Ideal pH ranges are partially based on each essential nutrient having an optimum range of availability that is represented by the green-yellow range of each bar in the image below (Diagram 1).  While these areas of optimum availability don’t all align perfectly, macronutrient availability of nitrogen, phosphorus and potassium lies in the 6.0-6.5 pH range.  Macronutrients are those needed by plants in the greatest quantity.  While this pH range places calcium and magnesium at a slight availability disadvantage, these two secondary nutrients are found in the higher volumes in mineral soils, being continually added with lime across time. Still, micronutrient availability is best at this pH range (6.0-6.5), except for molybdenum (Mo).  Mo ideal pH range is extremely high and unsuitable for all other nutrients.   

Diagram 1. Soil pH effect on nutrient availability.

Soils are acidified by the use of ammonium containing fertilizer, manure application, mineralized organic matter, and plant roots as they take up nutrients.  Once applied to the soil, ammonium ions (NH4+) go through a process called nitrification, where they become nitrate (NO3-). Through this process, H+ ions are released from NH4+ and added to the soil, acidifying it.  These H+ ions are needed to detach calcium, magnesium, potassium, manganese, zinc, iron, copper and ammonium nitrogen from the cation exchange capacity (CEC), making them available for plant uptake. As H+ ions increase and the CEC pH drops, nutrients previously immobile in the soil are now available for crop uptake or movement deeper in the soil profile. Thus, the release of hydrogen ions from the root while necessary for crop growth, can be detrimental to soil nutrient retention when not corrected. It is only through the addition of carbonate containing materials such as lime, that calcium and/or magnesium ions displace H+, form water and release carbon dioxide. Once applied to the soil, this chemical reaction can continue for up to four years with a well sized ag-lime. 

What’s the best manner to soil test to get an accurate measure of soil pH? Grid sampling is the best way to systematically and geospatially get a measure of soil pH differences within a field.  Soil pH variation can be extremely dynamic. While grid soil sampling can be the most expensive, it provides the best view of soil pH within a field.  Many University studies across time back this statement.  In addition, the same research provides good evidence that the variable rate application of lime makes up for the grid soil sampling cost.  While zone sampling is less costly and some may feel the results of yield differences in the field, zone sizes are often too large (not resolute enough), and not an unbiased approach to determining pH variance in a field.  Grid samples provide a random method to determine pH and nutrient distribution and the resulting values can be interpolated for VRT application of lime or crop nutrients.  Considering that nutrient availability and mobility is driven by soil pH, this can be extremely important.  For both corn and soybeans, 80% of the crop’s root mass is located in the top 12 inches of the soil.   Shouldn’t the majority of the crop nutrients be present and most available in this area?

Choosing the best lime depends a lot on the length of time the field will be farmed.  While pelleted limes, fine powdered limes and liquid limes react quickly with the soil to adjust pH to a desired level, they require more frequent applications to maintain this pH level.  Use of these lime products best fits short-term leases.  Under longer term leases, a good quality ag-lime of various mesh-size (particle size) is a better choice.  The different particle sizes provide ample time to adjust pH to the proper initial level while maintaining soil pH at a desirable level for up to four years, sometimes more, optimizing nutrient availability across multiple seasons.