Why liming is important when nutrient input prices are high

Input costs like fertilizer and limestone continue to be worrisome for farmers. Maximizing the plant availability of soil nutrients is crucial to optimizing crop production and nutrient supply to crops. One way in which to do this is by managing soil pH – actually it is one of the more important management practices. Soil pH is often overlooked when considering nutrient inputs. We have all seen the classic chart from Truog that depicts nutrient availability across a range of soil pH levels (see Figure 1).  

Soil pH is the measure of acidity or alkalinity. The pH is a negative logarithmic expression of hydrogen ions concentration in the soil solution, so values higher than 7.0 indicate alkalinity and lower than 7.0 indicate acidity. Soil pH is important because very low pH value impairs root cell function, reduces nitrogen fixation in legumes, and extreme acidic or alkaline values may reduce the availability of essential crop nutrients and influences nutrient form. The following are optimum pH levels for specific crops in Iowa. 

See Figure 2, for Iowa soil associations with high subsoil pH. This figure along with liming recommendations are found in PM 1688, A General Guide for Crop Nutrient and Limestone Recommendations in Iowa.  Follow liming recommendations from your state if you live outside of Iowa.  

When you receive your soil tests reports, you will see a soil pH value and a buffer pH value.  Soil pH Soil pH relates directly to the soil solution pH, is only a very small part of overall soil acidity, and directly influences nutrient chemical forms and plant availability. Soil buffer pH measures the reserve acidity on the cation exchange of soil clays and organic matter, is the main reserve of acidity in acidic soils (pH below 7.0), and along with soil pH is the number you use to make decisions on the amount of liming suggested. By liming soils, you are replacing hydrogen ions (source of acidity) with calcium and/or magnesium ions, forming water. That reaction removes acidity (hydrogen ions) from the soil system and thus pH increases.  

When preparing to lime soils you should 1) have current soil pH test results, 2) know the buffer pH, 3) know your target pH, 4) know your tillage system (to know the volume of soil to be treated and to what depth to take soil samples for pH), and 5) know your source of lime and it’s effective calcium carbonate equivalent (ECCE). The ECCE (lime quality and thus liming potential) is used to adjust the amount of liming material to be applied. These factors interact to determine if liming is needed, and the amount of lime material to be applied. Monitoring and maintaining soil pH in optimal ranges is the fundamental component of overall soil fertility management, provides the greatest potential for optimal crop response and utilization of fertilizer and manure nutrient inputs, and for crop use of soil nutrient supplies (like phosphorus and potassium) from prior applications. Therefore, soil test for best liming and nutrient application decisions – especially now. 

This article originally appeared in the June edition of Wallaces Farmer and has been adapted to reflect the correct title as it was originally written. Thank you to John Sawyer and Antonio Mallarino, soil fertility emeritus faculty, for their review.