By James Hoorman, Hoorman Soil Health Services
A basic understanding of soil fertility is important for high crop production. All crops require seventeen essential nutrients for proper growth and development, the specific amount of each nutrient depends upon the crop. The atmosphere provides hydrogen and oxygen and carbon (most comes from the soil first). The rest must come from the soil and the amount available for a plant depends upon many factors such as the soil type, organic matter, pH, drainage, microbes, temperature, and rainfall. Soil nutrients are absorbed by water being pulled through the plant through transpiration and by roots intercepting the nutrient molecules.
Some nutrients are required in large amounts compared to other nutrients and are called primary or macro nutrients. Primary nutrients include nitrogen (N), phosphorus (P), or potassium (K). Nitrogen is used to form amino acids and proteins in the plant and most plants need 3-5.5% of their plant tissue biomass as N. Phosphorus is used in plants as the genetic backbone for DNA, plant cell well formation, and for energy transfer. Plants generally need 0.2 to 0.5% of their plant tissue biomass as P. Potassium is used in the plant to provide plant turgor, to move sugars and starches, to increase photosynthetic production, and to activate enzymes and proteins. Increasing plant K increases protein content or N content. Optimal K levels are 2.5 to 4.0 percent in the whole plant tissue of corn and 4% in soybeans. In most cases the soil cannot provide enough primary nutrients at a critical time to optimize crop production so farmers add N-P-K fertilizer to improve grain yields. Providing for these three elements provide 85-90% of the typical crop yield.
Calcium, magnesium and sulfur are also needed in relatively large amounts but much less than N-P-K and are collectively called secondary nutrients. Organic matter in our soils generally provides adequate levels of sulfur, which limits the need for secondary nutrient fertilizers. However, sulfur may be lost from the soil much like nitrogen. Sandy soil are the most susceptible to low sulfur levels Over the years, most sulfur came from impurities in other fertilizers and from atmospheric deposition (provided by the heavy industry and coal burning facilities). These depositions have gradually decreased in recent years as improvements have been made in air quality. There is now a concern that crops may not be able to obtain adequate levels of sulfur without supplementing with fertilizer.
Plant tissue calcium should be in the 0.25 to 0.8 percent range and magnesium in the 0.15 to 0.6 percent range for corn and soybeans while small grains require slightly higher levels. Calcium in plants is used in cation exchange and transport while magnesium is the critical core molecule for chlorophyll used in photosynthesis. Sulfur is tied to protein production and levels range from 0.2 to 0.8 for most crops depending upon the age of the plant. For most plant nutrients, the nutrient concentrations are higher in young plant tissue and during reproduction.
The remaining eight nutrients are required in very small amounts by plant tissue and are called micronutrients or trace elements. These plant nutrients are used in parts per million (ppm). The micronutrients include boron (20-100 ppm), copper (5 to 25 ppm), iron (40 to 300 ppm), manganese (25 to 160 ppm), zinc (20 to 80 ppm), molybdenum (0.2 to 2 ppm), nickel (1-10 ppm) and chlorine. Historically, our soils have adequate levels of these micronutrients. Soil test and tissue test will identify if a micronutrient is deficient. Often the micro nutrient is found in the soil but it may not be in the right form to make it plant available. Increasing soil organic matter and improving soil health though increased soil microbial activity makes all nutrients more available. Manure applied at proper rates is a good source of many macro and micro nutrients and promotes soil health.
Farmers often apply micro nutrients to increase crop yields. Some plant food companies promote products based on testimonials and not scientific research, so be careful of what, why, and if you need to buy a particular product. Information for this article was obtained from the Ohio Agronomy Guide and Iowa State University Extension “Plant Analysis and Micronutrients for Crops” bulletin. Written by Ed Lentz, Hancock County OSU Extension and Jim Hoorman.