# Understanding Fertilizer

J Cheston Stevens  |  3/10/2011 11:19:58 PM

Fertilizer comes in different strengths and blends. The three numbers on the fertilizer bag show the percent by weight of the three major nutrients. The first number is always the percent nitrogen (N). The second is always the percent phosphate ( P2O5). The third number is always the percent potash (K2O).

The higher the number, the higher the content of that nutrient in the fertilizer material. You could apply more of a weaker fertilizer to get the amount needed or less of a stronger fertilizer.

Blended fertilizers have more than one nutrient like a 0-20-20 or 8-24-24. A complete fertilizer contains some of all three of the major nutrients like 8-24-24. Muriate of potash is 0-0-60; triple super phosphate is 0-46-0. Nitrogen sources include the fertilizer materials like ammonium nitrate (34-0-0), urea (46-0-0), or ammonium sulfate (21-0-0). Other fertilizer materials include potassium sulfate (0-0-52), diammonium phosphate -- DAP (18-46-0), sulfur-coated urea -- SCU (36-0-0), ureaform -- UF (38-0-0), bone meal (2-20-0) or cottonseed meal (6-3-2). About seven pounds of cow manure can substitute for one pound of 8-8-8.

Dividing the percent of a fertilizer nutrient into 100 gives the pounds of that fertilizer material needed to supply one pound of that nutrient. Take 8-8-8 as an example: 100 divided by 8 = 12.5. Therefore, 12.5 pounds of 8-8-8 provide one pound each of nitrogen (N), phosphate (P2O5) and potash (K2O).

The ratio of a fertilizer refers to the comparison of nutrients to each other. Different crops and soils may need different ratios. For example, 8-8-8 has a 1:1:1 ratio, while a 5-10-15 fertilizer has a 1:2:3 ratio, showing a generally low N, moderate phosphate and higher potash.

Application rates given in pounds per acre can be converted to pounds per 1,000 square feet by dividing by 44. For example: 264 pounds per acre divided by 44 = 6 pounds per 1,000 square feet.