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NC State Extension

Chemical Soil Health

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Overview

Chemical soil health refers  to the capacity of soil to provide crops with macro and micronutrients in sufficient quantities required for development and growth. Excess nutrients can cause nutrient toxicity as well as leaching, leading to chemical pollution. While nutrient deficiencies can lead to reduced crop yields. 

Soil chemical measurements related to soil fertility and nutrient availability include  pH, salinity, sodium adsorption ratio, electrical conductivity, cation exchange capacity, extractable macronutrients, secondary nutrients, and micronutrients. Soil pH is based on the concentration of hydrogen ions and ranges from highly acidic (pH 4) to neutral (pH 7) to highly basic (pH 10). Soil pH determines the  potential availability of both beneficial nutrients and toxic elements. Soil electrical conductivity measures the soluble salt concentration and indicates soil salinity. Clay and organic matter content determines the cation exchange capacity (CEC), which refers to the ability of soil to absorb, store, and exchange positively charged nutrient ions like hydrogen, calcium, magnesium, sodium, and potassium. The sodium adsorption ratio provides an estimate of the sodicity, or the extent of sodium saturation. Extractable macro (nitrogen, phosphorous, potassium), micro (boron, chlorine, copper, iron, manganese, nickel, molybdenum, and zinc), and secondary nutrients (calcium, magnesium, sulfur) indicate availability of  plant-essential nutrients in the soil for crop growth. 

North Carolina soils are mainly formed under heavy rainfall, which can cause heavy leaching of positively charged nutrients leading to highly acidic, low pH soils. Beneficial nutrients like nitrate (nitrogen), phosphorus, calcium, magnesium, boron, and molybdenum tend to be deficient in low pH soil, whereas aluminum and manganese can be present at toxic levels. Low pH can alter microbial activity resulting in lower organic matter mineralization, and can also reduce calcium content which is essential for soil aggregate formation. In contrast, alkaline soils (high pH) tend to be deficient in nutrients like phosphorus, iron, copper, zinc, and boron.

The soil clay minerals formed in North Carolina (e.g., Kaolinite) tend to have low cation exchange capacity, or the ability of soil to retain positively charged cations, which impacts  nutrient availability. Piedmont soils (fine-textured, high clay content) can have higher CEC than Coastal soils (sandy, coarse-textured) due to their clay mineral content. In contrast, Tidewater wetland soils have high CEC due to high organic matter. While clay mineral composition is an inherent and unchangeable feature acquired  during the soil formation, soil organic matter content and nutrient storage can be optimized through management practices to increase crop productivity.

Management Recommendations

Conservation strategies and 4R nutrient management can be implemented to maintain chemical soil health. The 4R nutrient management framework outlines recommendations for applying fertilizers  based on four principles: 

1) Right source, 

2) Right rate, 

3) Right time, and 

4) Right place. 

Following these principles ensures the uptake of nutrients in suitable form and concentration, at the appropriate time, and through effective placement. Conservation management practices like cover cropping can provide soil cover, minimize wind and water erosion of chemicals, reduce nitrate leaching, and improve soil nitrogen quantities (nitrogen fixation by symbiotic microbes living in the roots of legumes). Crop rotation can also diversify nutrient inputs, thereby reducing the need for fertilizers and pesticides. Liming to regulate pH should consider organic matter levels, targeting an optimum of  pH 5 for crops in organic soils and pH 6 in mineral soils. 

Please see Tillage Management and Managing Cover Crops sections in the Soil Health and Management extension portal and the Soil Fertility extension portal for additional information on management practices to maintain soil chemical health. 

Assessment Tools

The USDA-NRCS has developed a soil health and management assessment framework (SMAF), which measures primary indicators of soil health. Other analyses  developed by the Cornell Soil Health Laboratory (Comprehensive Assessment of Soil Health or CASH) evaluate additional  chemical properties. In addition, the North Carolina Department of Agriculture & Consumer Services offers free soil testing (April-November), and Soil Test Reports include pH and nutrient analysis. New and emerging technologies, like artificial intelligence, are also used to conduct soil health assessments. NCSU researchers are leading the development and adoption of these tools.