Acceptable pasture growth and nutritional balance in the forage, begins with proper soil fertility management. All plants have specific requirements for 17 basic nutrients. Of those 17 nutrients, fourteen (including the primary nutrients of nitrogen, phosphorus and potassium) are derived from the soil. No pasture management plan is complete without a plan to manage the important nutritive elements in soils with emphasis on the individual needs of specific forage species (grasses, alfalfa, clovers, vetches and other legumes, etc.), or common mixes of species.
A soil test is the best way to develop a fertilizer program. It will tell you how much fertilizer you need to apply for the amount of forage you estimate will be produced. The local Utah State University Extension Agent can help you with a sample box and the instructions for having a soil analysis done. The routine test includes phosphorus, potassium, pH, salinity, lime and texture. Tests for nitrogen and micro-nutrients are also available. One you have completed a soil test and made the appropriate fertilizer applications, a follow up test every three of four years will indication how your fertility program is progressing.
The three primary plant nutrients contained in commercial fertilizers are (N) nitrogen, (P) phosphorus and (K) potassium. These nutrients are represented by the three numbers on every fertilizer bag. A fertilizer labeled "32-10-10" indicated that 32% of the contents is nitrogen (N), 10% of the contents is available phosphorus (P2O5), 10% of the contents is soluble potassium (K2O) and the other 48% is inert filler.
Example: Suppose the soil test recommended that you apply 100 lbs of nitrogen per acre. Using the 32-10-10 fertilizer, you would need to apply 312 pounds per acre (100 lbs of nitrogen divided by 32% (.32) = 312 lbs of fertilizer).
Most Limiting Factor
The most limiting factor principle states that plant yield is limited by which ever factor is most deficient. This means that if phosphorus is severely deficient in a soil, adding more nitrogen will not increase production. This helps us understand that we must provide for the balanced needs of the pasture for it to reach its full potential.
Fertilizer Effect on Quality
Applying nitrogen fertilizer to pastures not only increased yield, but also improves the quality of the forage produces. The results of a grass pasture trial in Weber County showed that the addition of 150 lbs of nitrogen in the spring increased the protein content of the grass by 36% (from 11.3% to 17.7%).
General Fertilizer Recommendations (for established grass pasture)
Nitrogen (N) - Nitrogen is the plant nutrient required in the largest amount by grass pastures. An application of nitrogen will usually have a profound effect on pasture production. Soil applied nitrogen is effective for about 8 weeks, therefore it needs to be re-applied several times throughout the growing season. A suggested per acre application schedule for a well managed irrigated pasture would be: 100 lbs of N in April; 60 lbs of N in late June; and another 60 lbs of N in late August. Legumes, like alfalfa and clover have the ability to pull nitrogen from the air and make it available to plants. Pastures with a significant legume population require less commercial nitrogen fertilizer. A general rule is to reduce the nitrogen applied by the percentage of legume in the pasture mix.
Phosphorus - Phosphorus is particularly important for seedling growth in a new pasture. Legumes require higher levels of phosphorus than grasses. Work phosphorus in to the soils before seeding a new pasture, or broadcast it over the top of established pastures. Where pasture forages are consumed by grazing and the manure is distributed evenly across the field, phosphorus levels tend to remain constant. Thus, once established, further applications may not be needed for several years.
Potassium (K) - The need for potassium varies widely through Utah. A soil test is very useful in knowing whether to apply this nutrient. Many of our pastures receive sufficient potassium from the irrigation water. Like phosphorus, potassium in the soil remains relatively constant where forages are grazed and manure redeposited on the field.