Composition & Grades of Fertilizers

Nitrogen

• Listed first in fertilizer grading, nitrogen is the nutrient that plants require in the largest amount. Although Earth's atmosphere is mostly nitrogen in its gaseous form, that form of nitrogen is not available for plants' growth. Nitrogen in fertilizers is in chemical compound forms that include ammonium nitrate, calcium nitrate, urea and potassium nitrate. Blood meal, with a grading of 12-0-0, is an example of an organic nitrogen fertilizer.

Phosphorus

• Phosphorus, the second of the primary plant nutrients, is critical to plant growth in many ways. One important way is that it tends to stimulate flowering and fruiting. Occurring naturally in the form of what is called rock phosphate, this nutrient is available as an organic fertilizer. Two industrial methods of treating rock phosphate serve to convert it into a form termed orthophosphate, which plants can use even more readily than the native, rock phosphate form.

Potassium

• Mining operations extract potassium from natural earth deposits in the form of potassium salts for the fertilizer industry. Once processed for purification, the fertilizer is available in the forms of the salts potassium sulphate and potassium chloride. Fertilizers that are relatively high in potassium are graded with a high third number on their packaging.

Complete Fertilizers

• Agronomists consider fertilizers that contain all three of the primary nutrients, in one formulation or another, to be complete fertilizers. That term can be a bit misleading in the sense that complete fertilizers may contain only a handful of the 16 essential plant nutrients. They contain, however, the nutrients that plants need in the largest quantities, and the remaining nutrients -- secondary nutrients and micronutrients -- might be available already, to some degree, in the soil or as incidental “contaminants.” Some fertilizer companies may include details of secondary and micronutrients on their fertilizers' labeling.