• 4R Nutrient Stewardship- right source, right rate at the right time and in the right place is compromised if field pH is not properly balanced.
• Soil nutrient cations have different levels of attraction and retention to the CEC with H⁺ having the greatest.
• Acidifying soils results in nutrient availability challenges and movement of nutrients deeper in the soil profile.
• Many of these nutrients are very important to yield but deficiencies often go unnoticed.
• Grid sampling and the creation of zones of application is the best way to adjust field pH and insure that there is not over or under application of lime.

The tenets of 4R Nutrient Stewardship involve choosing the right source, applying the right rate, at the right time, in the right place for efficient crop use. Subscribing to these tenets carries with it a concern for the environment, the economic well-being of all both in agriculture and the public beyond. Maintaining proper pH is essential to maximizing both nutrient retention by the soil CEC and efficient crop use of applied fertility.As soil pH decreases, so does the soils ability to hold nutrient ions such as ammonium (NH₄⁺) and potassium (K⁺).When referencing most land grant university literature regarding soils, soil testing and/or crop nutrient amendments they begin their discussion with soil pH. The reason being that by maintaining proper soil pH we impact the very foundation of good soil fertility.

Understanding soil chemistry dynamics helps explain what happens underneath the soil surface as pH drops or rises outside the proper range for best nutrient availability.Cations in soils have different affinities or strengths attraction and retention to cation exchange sites.This is due to a combination of net electrical charge and hydrated radius size.Without getting into a bunch of chemistry, an easy way to envision this phenomenon is to view cations as simply being magnets of different strengths.The chemical order of affinity that exists is “H⁺ > Al⁺³> Ca⁺² > Mg⁺² > K⁺ = NH₄⁺ > Na⁺”.While affinities have an impact on soil chemistry they are not hard unchangeable rules, otherwise liming soils with carbonate based compound would not work to neutralize pH.That is because by liming we create a chemical reaction.However first, how do soils become acidic in the first place?When crops are fertilized with nitrogen fertilizers containing ammonium, H⁺ ions are released into the soil as nitrification takes place. As plants absorb base cation nutrients (calcium, magnesium and potassium) H⁺ ions take the place of those nutrients on the CEC.In the process of absorption by the plant hydrogen ions are released by the plant at the root surface contributing further to soil acidification.Hydrogen ions occupying the cation exchange sites are firmly attached due to their greater affinity.Further addition of potassium and ammonium fertilizer for crop use leaves these ions few cation exchange sites to bind to, often leading ions to move deeper in the soil profile before attaching.This sometimes results in fertility moving outside of the active root zone.Additionally, as the soil acidifies phosphate ions begin to combine with aluminum (Al⁺³) and iron (Fe⁺³) to form compounds unavailable to plants.The phosphorus sweet spot for pH falls around 6.7-6.8.Also as soil pH drops below 6.5, molybdenum (Mo) availability begins to decreases.Mo is responsible for nitrate conversion in non-legume plants resulting in protein building. In non-legume plants Mo is needed for nodule formation by rhizobia effecting nitrogen fixation in a protein rich crop. Both calcium and magnesium availability begins to be effected below pH 6.5.Calcium deficiency leads to weakness in cell walls that can result in a weaker stalk strength and loss of plant fluids.Magnesium is important to chlorophyll production.True nutrient deficiency symptoms typically go unnoticed until chiseling away at yield potential behind the scene. These are only a few of the nutrient dynamics that change in the acid soils when soil pH falls outside the optimum range of 6.0-7.2. Acid soils have additional detrimental effects on microorganism populations and their effect on nutrient mineralization and other plant processes.

As we all know the correction for low pH is through the application of lime materials containing calcium carbonate that have the ability to chemically overcome the H⁺ ions affinity to the CEC.When lime is applied excess H⁺ ions both in solution and on the CEC first form carbonic acid that in the end dissociates to water and carbon dioxide.Calcium ions released in the process attach to the CEC and pH increases.The state you operate in will determine the information needed to calculate a lime recommendation.While most states require samples to have buffer pH done to calculate lime needs, Illinois requires only water pH and then groups soils based on CEC.With states using buffer pH, along with water pH lime recommendations are closer tailored to actual lime needs, however the end buffer pH or soil CEC groupings results are very close.

Proper soil pH is the foundation to good soil fertility.In acid soils important plant nutrient such as N, P, K, Ca, Mg, and Mo to name a few become less available affecting important plant processes, the end result being unrealized yield loss.Broadcasting additional fertilizer cannot cure the problem and could, lead to nutrient loss or movement further into the soil profile outside the reach of the greatest portion of the root-mass.Grid sampling every four years and VRT application of lime more than pays for the cost of sampling by preserving unrealized yield loss.