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Managing Soil pH and Crop Nutrients
Fabián G. Fernández Department of Crop Sciences fernande@illinois.edu Robert G. Hoeft Department of Crop Sciences rhoeft@illinois.edu

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he inherent complexity of crop production systems requires integrating many factors to ensure maximum crop yields with the least risk to the environment. Assessing present- and reserved-nutrient status of the soil,understanding its nutrient-release and nutrient-holding capacity, and knowing the plant and environmental factors that impact nutrient availability are necessary to guide fertilization rates, sources, and method of application of additional nutrients. The information here is intended to provide fundamental principles to help the reader understand what to do, and why, when making managementdecisions related to phosphorus (P), potassium (K), secondary macronutrients (calcium [Ca], magnesium [Mg], and sulfur [S]), micronutrients (boron [B], chlorine [Cl], copper [Cu], iron [Fe], manganese [Mn], molybdenum [Mo], and zinc [Zn]), and pH.

available. Plants obtain most of their nutrients and water from the soil through their root system. Any factor that restricts root growth and activity hasthe potential to restrict nutrient availability. This is not because nutrients are not plant-available in the soil, but because the ability of the crop to take up those nutrients is restricted. Understanding how these factors can cause nutrient deficiency in crops is important to avoiding excessive concern about the need for additional fertilization when a sound nutrient program is already inplace. Soil compaction can limit or completely restrict root penetration and effectively reduce the volume of soil, including nutrients and water, which can be accessed by the plant. To limit soil compaction, avoid entering fields that are too wet, and minimize the weight per axle by decreasing load weight and/or increasing tire surface area in contact with the soil. Planting when soils are wet cancreate a compacted wall next to the seed that will prevent the seedling from developing an adequate root system. Tilling wet soils will result in clods that become hard and dry out quickly on the surface, preventing roots from accessing resources inside the clod. Soil water content is critical not only to supply the water needs of the crop but also to dissolve nutrients and make them available to theplant. Excess water in the soil, however, depletes oxygen (O2) and builds up carbon dioxide (CO2) levels. While O2 is needed by roots to grow and take up nutrients, high CO2 levels are toxic. Temperature is important in regulating the speed of soil chemical processes that make nutrients available. Under cool soil temperatures, chemical reactions and root activ-

Factors Impacting Plant-NutrientAvailability
Nutrient availability can be impacted by soil chemical and physical properties, including parent material and naturally occurring minerals; amount of organic matter; depth to bedrock, sand, or gravel; and permeability, water holding capacity, and drainage. In addition, environmental conditions and crop characteristics have an important impact on nutrient availability. It is notunusual for crops in fields or portions of fields to show nutrient deficiencies during periods of the growing season, even where an adequate nutrient management plan is followed. The fact that nutrients are applied does not necessarily mean they are

Managing Soil pH and Crop Nutrients

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ity decrease, rendering nutrients less available to the crop. Portions of the plant nutrients are takenup as roots extract soil water to replenish water lost through the leaves. Cool air temperatures can lower evapotranspiration and reduce the convective flow of water and nutrients from the soil to the root. Light intensity is low on cloudy days. Low light intensity reduces photosynthetic rates and nutrient uptake by the crop. Since low light intensity sometimes occurs when soils are waterlogged...