The fertilizer guaranteed analysis or grade, stated on the bag, refers to how much of an element is in the material (the guaranteed minimum quantity present) based on percentage by weight. All fertilizers are labeled with three numbers which give the percentage by weight of total nitrogen (N), citrate-soluble phosphorus (expressed as P2O5) and water-soluble potassium (expressed as K2O), respectively. Often, to simplify matters, these numbers are said to represent nitrogen, phosphorus and potassium (N, P, K). It should be remembered thai actually it is not N-P-K but N-P2O5-K2O. The first chemists who studied pla nutrition expressed phosphorus and potassium as the oxide form in their This practice continued and was eventually adopted as an industry standard continues to this day.
For example, if we have a 50-pound bag of 10-10-10, there are 5 poi N, 5 pounds of P2O5 and 5 pounds of K2O. To convert the P2O5 to actual elemental P, multiply it by 0.44; to convert the K2O to actual K, multiply it by 0.83. The other 35 pounds of the fertilizer material is filler or the carrier material of the fertilizer ore. The filler or carrier helps to evenly spread the fertilizer and avoids burning plants with too much fertilizer. A 50-pound bag of fertilizer labeled 0-20-20 would have 0 pounds of N, 10 pounds of P2O5, 10 pounds of K2O and 30 pounds of filler or carrier material. Various materials are used as fillers. So common ones are: pelleted biosolids, attapulgite clay, vermiculite, montmo nite, fuller's earth and diatomaceous earth.
The secondary and micronutrient grade of a fertilizer blend is usual on the back of the bag. It is also expressed as a percent of the total mate: weight. Infrequently, it is expressed as a part of the grade designation on the front of the bag. For example, a 10-10-10 blend containing 15 percent sulfu labeled as 10-10-10 +15 S. The type and amount of primary fertilizer materials used in the blend are also usually listed on the back of the fertilizer bag.
Most soil testing laboratories give fertilizer recommendations in terms of the amount of N, P2O5, and K2O needed on a per acre basis. The fertilizer dealer assists the producer in choosing or blending a fertilizer material that best this nutrient recommendation in a cost-effective manner.
State laws require that the manufacturer or blender guarantee what is claimed on the label. In some instances, a fertilizer may contain secondary nutrients or micronutrients not listed on the label because the manufacturer or blender does not want to guarantee their exact amounts. For this reason, some fertilizers (especially organic fertilizers) may have a higher total nutrient content than what is listed on the label. Often, the N, P, K and micronutrients tied up in various organic compounds are not claimed by the manufacturer on the label. One method used to indicate the presence of these organic nutrients has been to use the terms W. I. N. (water-insoluble N) and W. S. N. (water-soluble N) on fertilizer labels. Water-soluble N dissolves readily and is usually in a very simple form such as ammoniacal-N or nitrate-N. Nitrogen that will not dissolve readily may exist in other forms in fertilizer. These are usually the organic forms of N (except urea) that must be broken down into simpler forms before the plant can use it. Water-insoluble N is referred to as a slow-release nitrogen source and delivers nitrogen at different rates according to the amount and kind of material in its composition.
The best fertilizer grade to use depends on the current soil test levels in your field and what nutrients are most needed to ensure that no nutrient will limit production level of the crop to be planted. Sometimes only nitrogen is required, but only a good soil testing program can provide you with that information. Availability of fertilizer materials and costs are also always a consideration in planning your fertility program.
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