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Essential nutrients for plants: P & K
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This week’s column finishes the discussion of the macronutrients known as primary nutrients Phosphorus (P) and Potassium (K). First though, here’s hoping everyone is having a happy Easter Weekend. Second, the moisture situation continues to improve wheat and there is a chance for significant rain for most of the state over the past week and continuing into this week. The only down side is the need to get corn in the ground. And we are fortunate to have warm but not high temperatures. Now on to P and K.
First up is phosphorus, P, which serves two main functions in plants. In plants, and in animals, P equals energy primarily as ATP. The products of respiration, the oxidation of sugar for energy, is captured in energy bonds in ATP. ATP is also involved in photosynthesis so P helps in producing sugar, the product of photosynthesis. P is also involved in DNA and RNA so it important in the genetic code, DNA, and translating that code, RNA. Its concentration is higher in actively growing parts of the plant and structures like seeds. Plants take up the orthophosphate form which is an anion, negatively charged. P deficiencies occur in older tissue first and are common in seedlings and young plants even with adequate soil P. This is typically caused by cool and/or dry conditions and symptoms typically disappear when conditions improve. Deficiencies result in dark green plants, often leaf purpling, and can delay plant growth and maturity. The best organic source of P is manure which is typically much higher in P than nitrogen. Today, except in lawn and landscape applications, straight P is rare. P fertilizers commonly contain nitrogen and may be purchased in liquid or dry formulations. Some P can also be obtained with N and K and is typically used as starter fertilizer. In developing agriculture, P is the limiting nutrient, not N.
Potassium, K, is interesting in several respects; it isn’t a structural component of plants and it isn’t part of compounds like P but its role is equally important. It activates over 60 enzymes throughout the plant, it helps the plant maintain a narrow pH range, and it also helps with disease suppression and decreases lodging. And it regulates the opening and closing of the stomatal guard cells in the leaves, the end of the transpiration stream bringing water and nutrients up from the roots. These openings also allow in needed carbon dioxide and let out oxygen.
K deficiencies occur in older tissues first and often first appear as yellowing along the edge, margin, of the leaf that works its way inward until the leaf dies. Deficiencies also cause the plant stomata to respond more slowly to the environment, further stressing the plant and preventing water conservation. K fertilization is becoming more common here. Manure is an excellent organic K source. Potassium chloride and K-Mag are the common potassium here but it may also be found in low analysis starter formulations.

Dr. Victor L. Martin is the agriculture instructor/coordinator for Barton Community College. He can be reached at 620-792-9207, ext. 207.