Sally A. Flis, Ph.D. – Feed and Crop Support Specialist, Dairy One
The next nutrient in the series is Potassium (K). Potassium is a key nutrient for plants and animals, but excess supply in some animals can lead to health complications, such as dry cows and post calving health.
Soil – Total soil K is usually many times greater than the amount taken up by the crop during the growing season, but most of the K in the soil in in unavailable forms. Soil soluble or available K is generally less than 2% of the total soil K. Potassium is absorbed by plant roots as K+ and cycles in the soil through plant uptake and leaching. The amount of leaching of K from the soil is dependent on soil type and pH.
Crop Use – Potassium is absorbed by plants in larger amounts than any other nutrient except N. Potassium is involved in enzyme activation, water uptake by providing the osmotic pull that draws water into plant roots, and in the production of ATP. In plants that have a K deficiency, N uptake and protein synthesis are also reduced due to the need for ATP for both processes.
Feed Analysis – Potassium in feeds is measured as a percent of dry matter (K, % DM). This is measured by a mineral digestion. The K ranges are highly variable depending on the forage or grain type. Alfalfa hays and silages tend to be the highest >2.5 % DM. Grasses will vary depending on K soil availability and application.
Animal Nutrition – Animals have a requirement for K. Potassium in animals, like in plants is used for water balance and osmotic pressure regulation. Additionally, K is used for acid-base regulation, nerve impulse transmission, muscle contraction, oxygen and carbon dioxide transport, and in enzyme activation and protein synthesis. The body does not store K, so it must be consumed daily. Almost all K consumed is used, 95%.
Nutrient Management – Potassium is not a problem pollutant. However, when K is applied in excess, often because manure is applied to meet N or P crop needs, it can lead to high K levels in forages that can be a problem for some species and for dry cows. There is no information on what the upper limit of K soil concentration or application. However, high soil K levels can result in depressed uptake of magnesium (Mg) by plants.
There is a lot more detail in each part of the system, but the important point to remember is that the K we are applying to the crop is creating water balance and energy for growth in the plant, that is then used by the animal for water balance and multiple other body functions, and is excreted largely in urine and applied to fields.