This publication is an overview of essential plant nutrients and the ways they interact with one another. Farmers and agronomists can use this guide as a tool in conjunction with a good soil testing program to help diagnose agronomic problems and consider their nutrient management strategy. For detailed information on a specific plant nutrient and its role in plants, refer to publications located at extension.soils.wisc.edu and ipcm.wisc.edu.

Plant Essential Nutrients 

Nutrients that are needed by all plants to complete their vegetative and reproductive life cycles that are irreplaceable by another nutrient are called plant essential nutrients. There are 17 plant essential nutrients, and they are divided into four categories based on the quantity used by the crops: structural nutrients, macronutrients, secondary nutrients, and micronutrients. 

Structural nutrients are crop nutrients obtained from air and water. They include carbon (C), hydrogen (H), and oxygen (O). 

Macronutrients are needed in large quantities for plants and include some of the nutrients commonly applied with commercial fertilizer. They include nitrogen (N), phosphorus (P), and potassium (K). 

Secondary nutrients are needed in medium quantities for plants, in comparison to macronutrients and micronutrients. They include calcium (Ca), magnesium (Mg), and sulfur (S). While calcium deficiencies are rare in Wisconsin, sulfur is often needed in high demanding crops like alfalfa, corn silage, and soybeans, since atmospheric levels of sulfur have decreased in response to clean air efforts. 

Micronutrients or trace elements are needed in very small quantities by the plant. They include iron (Fe), manganese (Mn), boron (B), copper (Cu), zinc (Zn), molybdenum (Mo), nickle (Ni), and chlorine (Cl). 

Understanding Nutrient Availability & Interactions

In addition to soil-derived nutrients, plants utilize nutrients derived from air, water, organic matter, and agricultural inputs like manure. Some nutrients are readily plant-available, while others often need to be assimilated by the plant or broken down by microbes in the soil into plant-available forms. External conditions like the following can affect the availability of nutrients to plants:

• Nutrient availability is restricted for some nutrients at very high or very low pH. Most crop nutrients are readily available at 6.0-7.0 pH.
• Extreme environmental conditions like heavy rainfall or low temperatures that affect factors like soil temperature and oxygen levels can decrease nutrient uptake.
• Variations in soil texture like very low or very high organic matter and sandy soils tend to exhibit more nutrient deficiencies.

In addition to these external factors, sometimes the concentration of certain nutrients will decrease the availability of others, especially if the uptake forms are of similar size, valency, and charge and are competing for the same binding sites in the soil or in the plant. To help consultants and farmers assess nutrient deficiencies and better understand interactions between nutrients when solving agronomic issues, we have developed the following table demonstrating the plant uptake form, ideal soil pH, limiting soil characteristics, and inter-nutrient dynamics for the soil-derived plant essential nutrients.