This week’s column starts tackling the macronutrients we fertilize for – Nitrogen (N), Phosphorus (P), and Potassium (K). These are also termed Primary Nutrients as they are needed in larger amounts. Nitrogen is found in the soil as nitrate, a negatively charged ion, and ammonium, a positively ion. Phosphorus is found as a negatively charged ion and Potassium as a cation. N is the only one found in both a positively and negatively charged for. These nutrients are found in the soil solution and organic matter. Ammonium and potassium as cations may also be found on cation exchange sites. Nitrate and phosphorus being negatively charged are not.
Nitrogen is used by the plant in a number of key ways. It is part of the chlorophyll molecule necessary for photosynthesis. N also serves as the backbone for amino acids and amino acids are necessary to make proteins, RNA, and DNA. The plant can take up wither form but nitrate is much more common in the soil. N as nitrate is easily lost through leaching and with good precipitation moves below the root zone. Soil N testing, along with sulfur must be done to a two-foot depth because of the ease of movement. N comes from the decomposition of organic matter and through fertilizer additions. The most common forms as anhydrous ammonia, urea, and ammonium nitrate. Manures may serve as an N source, however, you must test the manure for N level and P concentration. Manure supplies much more P than N so you will not be able to supply all you N needs with manure or you would over apply P. Liquid, solution, nitrogen is a combination of urea and ammonium nitrate. Another important source of is rhizobium bacteria species in plants like alfalfa, peanut, soybean, clovers, and locust trees. These bacteria form a symbiotic, mutually beneficial, relationship by invading plant roots. They can take atmospheric N and convert it to a useable form by the plants. In exchange, the plant supplies it with a home and nutrition.
Of all the nutrients besides carbon, hydrogen, and oxygen, N is needed in the largest amount. An N deficient plant will exhibit symptoms on lower leaves first as N is robbed from older plant tissue and moved to developing structures such as leaves and seed. It exhibits as a V shape moving along the midrib-back to the base of the leaf. Deficiencies may be caused by several factors including inadequate N fertilizer, saturated soils, dry soils, and factors that inhibit root growth and function such as root diseases and nematodes. Extended periods of cloudy and or cool weather also cause deficiency symptoms.
Ammonium-N and urea and rapidly converted in the soil to nitrate which is easily leachable by bacteria. This can be inhibited by inhibitors that designed to keep urea as urea or ammonium as ammonium but only for a short period of time. Since N can be easily lost by several processes, it is normally advisable to not apply all your N fertilizer at once but to split the application over two applications or even spoon feed it through irrigation water. In developed agriculture, N is typically the yield limiting nutrient.
Next week P and K.
Dr. Victor L. Martin is the agriculture instructor/coordinator for Barton Community College. He can be reached at 620-792-9207, ext. 207.