Soil Fertility

Fertilizer Ratios

Fertilizer ratios indicate the % of nitrogen, phosphorus, and potassium, by weight, in a particular fertilizer. Ratios are always given as the % nitrogen (N), phosphorus (P), and potassium (K). For example, a 10-10-20 fertilizer contains 10% nitrogen, 10% phosphorus, and 20% potassium. This means that 100 pounds of this fertilizer will actually contain 10 pounds of nitrogen (10% of 100 pounds), 10 pounds of phosphorus (10% of 100 pounds), and 20 pounds of potassium (20% of 100 pounds). Fertilizer is commonly available in the following ratios: 10-20-20, 15-15-15, 10-10-10, 5-10-5, 5-10-10, 10-20-10, 20-10-10, and 46-0-0.

Nitrogen (N)

Nitrogen is critical for the maximum growth of cool season grasses in pastures. An adequate supply of nitrogen is associated with vigorous vegetative growth and a plant's dark green color. Reduced plant growth and a pale green or yellow color characterize nitrogen deficiency.

Grass pastures containing high yielding forages that are being heavily grazed require approximately 100-150 pounds of nitrogen per year. The nitrogen should be split into three applications of 50 pounds each. Fifty pounds of nitrogen should be applied in spring and fall. An additional summer application of 50 pounds of nitrogen is warranted if summer rain continues to promote the growth of pasture grasses.

Common Sources of Nitrogen

Urea (46-0-0): Urea is an economical source of nitrogen. It is not very stable and must be used when rain is expected within several hours. If it doesn't rain within 24 hours, 66% of the nitrogen will be lost to the atmosphere. Approximately ? inch of rain is sufficient to wash the nitrogen from urea into the soil. Applying 100 pounds of urea to an acre of pasture will supply 46 pounds of nitrogen.

Ammonium nitrate (34-0-0): Ammonium nitrate is a more stable form of nitrogen. It can be applied to the surface of the soil anytime without an appreciable loss of nitrogen. Applying 100 pounds of ammonium nitrate supplies 34 pounds of nitrogen; 150 pounds of ammonium nitrate supplies 50 pounds of nitrogen.


Adequate phosphorus ensures greater crop quality, greater stem strength, increased root growth, and earlier crop maturity. A soil test is needed to determine how much phosphorus is needed.

Common sources of phosphorus

Triple super phosphate (0-46-0): Triple superphosphate can be applied directly to a pasture. Applying 100 pounds of triple superphosphate supplies 46 pounds of phosphorus per acre.

Potassium (K)

Potassium deficiency is characterized by reduced plant growth, poor stalk strength, reduced disease resistance, and reduced winter hardiness. A soil test is needed to determine potassium requirements.

Common sources of potassium

Potassium sulfate (0-0-50): Potassium sulfate can be applied directly to pastures. It has a lower salt index than muriate of potash and also supplies some sulfur as a trace nutrient. Applying 100 pounds of potassium sulfate supplies 50 pounds of potassium per acre.

Muriate of potash (0-0-62): Muriate of potash has a fairly high salt content but can be applied directly to pastures. It is an economical source of potassium. Applying 100 pounds of muriate of potash supplies 62 pounds of potassium per acre.

Applying Fertilizers

You can purchase fertilizer from a dealer or company. It is best to purchase a cone spreader and apply your own fertilizer. This provides you with the advantage of being able to apply the fertilizer at the correct time for optimum plant use. It also allows you to schedule your applications so that you always have some pastures available for grazing. Many fertilizer companies rent large drop spreaders (buggies) that can be filled with bulk fertilizer. The buggy is usually rented for about $25. Most require a 60 horsepower tractor to operate the spreader. Most companies will also spread the fertilizer for you. This option is usually available in summer and fall, after the crop fertilizer work is completed.

Note: Do not allow horses to graze fertilized pastures until rain has thoroughly dissolved the fertilizer into the soil. Ideally fertilizer should be applied after the horses have been removed from the pasture, the pasture has been mowed, and the horses have been moved to another pasture.

For more soil fertility information, follow these links:

  1. Rutgers
  2. Executive Dean of Agriculture and Natural Resources
  3. School of Environmental and Biological Sciences