Monday, February 20, 2017


WHAT FERTILIZER SHOULD I USE? What type of fertilizer should you buy and apply this spring. The answer to that question is not an easy one. Economics is important, but quality, rather than cost per unit of nutrient, should be the deciding factor in fertilizer section. The cost and availability of fertilizer took an unprecedented roller coaster ride in the past few years leaving many farmers and home owners wondering what this year is going to be like. Fertilizer quality cannot be defined in rigid terms. If you know the composition and properties of fertilizers and their behavior in the soil, you have a guide for choosing them for specific purposes. Making the right choice has real economic consequences. Characteristics that should be appraised before a fertilizer is selected are: solubility, effect upon soil pH, form of nitrogen, the salt index, and cost per unit of available nutrient. Solubility: Fertilizer compounds differ greatly in their solubility in water. These differences are usually unimportant for application in the solid form. High solubility, however, is one of the major considerations for the grower who purchases solid fertilizer for dissolving in irrigation water, or for application in foliar sprays. Let's compare the solubility of a few fertilizers. When looking at this chart think of the 100 gal of water as being a pickup and the amount listed under it as the number of pounds the pickup can carry or each type of fertilizer. Solubility of Some Fertilizers Pounds Soluble Compound in 100 Gallons of Water __________________________________________________________________ ammonium nitrate 1,617 urea 902 ammonium sulfate 623 diammonium phosphate 574 monoammonium phosphate 312 muriate of potash 283 potassium nitrate 263 sulfate of potash 92 __________________________________________________________________ Fertilizers that have low initial solubility will release nutrients to the soil and plant gradually over an extended period of time. Fertilizer nutrients may be divided into three classes according to their influences on the soil reaction (pH): (1) acidic -- pH below 7.0 (2) neutral -- pH of 7.0 and (3) basic -- pH above 7.0. Use of anhydrous ammonia and ammonia compounds will eventually have an acidifying effect on the soil. This soil acidifying action tends to bring into solution or make available some minor elements such as iron, manganese, copper, zinc, and molybdenum. This does not mean that the entire soil becomes acidic, just the area around each fertilizer particle or concentration of particles. This acidifying effect is usually of short duration particularly in Eddy county soil. Phosphates are generally neutral in their effect on soil pH, and potassium carriers are basic. The form of nitrogen applied influences directly the nutrition and growth of the plant. In general, plant roots assimilate two common forms of nitrogen-nitrate (NO3) and ammonia (NH3). Most plants seem to prefer the nitrate form. Nitrate and ammonium are readily absorbed and utilized by plant roots. Under favorable conditions, ammonium nitrogen is converted to the nitrate form by nitrifying bacteria in the soil. Therefore, the ammonium forms are not available as rapidly as the nitrate forms. Under adverse soil conditions (temperature below 50 degrees F, too wet or too dry, high salt concentrations, etc.), the conversion of ammonium to nitrate is retarded or halted, and the soil accumulates high levels of ammonium. These high levels may become toxic to plants. Urea is quite soluble and is rapidly broken down to a usable form of nitrogen. Besides uptake by plants, nitrogen is also removed from the soil by leaching and volatilization. Leaching means the nitrogen is washed or flushed out of the soil by irrigation or rain water. Volatilization means the nitrogen escapes as gas. Nitrate nitrogen is sometimes leached from the soil, and ammonium is sometimes volatilized if it is not converted to the nitrate form. Ammonium is rarely leached because it is usually adsorbed to the clay particles of the soil. Nitrate does not become attached. Stabilized ammonium fertilizer is slowly converted to the nitrate form. Since that nitrate is available to the plants over a longer time, it is less subject to leaching or conversion to nitrogen gas. Next week we will cover fertilizer salt index. Subscribe to Eddy County Ag news at: Eddy County Extension Service, New Mexico State University is an equal opportunity/affirmative action employer and educator. All programs are available to everyone regardless of race, color, religion, sex, age, handicap, or national origin. New Mexico State University, U.S. Department of Agriculture, and the Eddy County Government Cooperating.

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