How To: Calculate Fertilizer Rates

It is difficult to generalize about fertilizer rates on turf. Different species require different amounts of Nitrogen. Climate and other environmental factors also vary nutrient demand, and intensity of use and level of maintenance are issues as well. Many turf sites, such as home lawns, can perform well with a range of fertilizer rates, depending on the level of quality the owners desire. Thus, no single formula exists for determining how much fertilizer to apply, and no perfect fertility program exists for any turf.

Fertilizer rates usually are given in terms of pounds of Nitrogen per 1,000 square feet. Fertilizer products often have a nutrient ratio of 3 to 1 to 2 for Nitrogen (N), Phosphorus (P) and Potassium (K). This reflects the needs of turf for these nutrients. By keeping with the desired ratio, varying the level of N you apply also results in proportional variation of the other nutrients present in the formulation, keeping them properly balanced for turf's needs.

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When using fertilizers, you should make several important calculations to determine how much to buy, which is the best buy and how much to apply to supply the recommended nutrient amounts. Such calculations will ensure cost savings and enable you to apply the recommended amounts. (See “Best Buy,” right.)

As a general rule, 1 pound of quickly available N should be the maximum you apply at any one time to turf. In hot summer conditions, reduce this maximum to 0.5 pounds of N for cool-season turfgrasses. You can safely apply up to 3 pounds of slow-release N at once, but more than this is risky. Close-cut turf such as golf greens should receive no more than 0.5 pounds of N from a quick-release source in one application.

Ideally, N levels should be kept as even as possible. This limits ups and downs in frequency of mowing and reduces deficiency symptoms between fertilization. That is why slow-release products are useful and, in the case of quick-release sources, why it is better to apply less fertilizer more often. However, this must be balanced against the cost of labor, which will be lower with less frequent (presumably heavier) applications. Applying 1 pound of N at a time is a middle ground that seems to work for many turf managers.

A reasonable range for golf greens is 0.75 to 1.5 pounds of N every 2 to 6 weeks during the growing season. This is a large range, and the actual rate depends on climate, rainfall and length of growing season. For example, warm, rainy, tropical climates result in nutrient leaching, rapid growth and a long (sometimes continuous) growing season. N demand in this type of situation could be close to 2 pounds of N per month all year long. You should base P and K (and other nutrient) applications on soil tests, but you can expect demand for these nutrients also to be high on golf greens. Temperate climates result in considerably less demand.

Fairway rates vary according to region and species, but 2 to 3 pounds of N annually is a reasonable average figure. The need will be higher in tropical climates, perhaps more than double. The objective is to apply the minimum necessary to maintain a dense, vigorous turf without creating undue mowing requirements.

Depending on many factors, 2 to 6 pounds of N annually are required by lawn turf. Low-N turf such as buffalograss may need no more than 1 pound each to look its best.

Athletic fields require up to 10 pounds of N annually depending on the level of play. However, if fertilizations result in succulent turf, you should adjust rates or timing; succulent turf is more tender and suffers more from traffic. Conversely, inadequate levels will prevent turf from recovering from damage.

Because so many factors are site specific, turf managers must find what works for their particular situation.

BEST BUY

  1. Compile the necessary information:
    Product A
    Analysis: 16-8-8
    Cost: $19.95 per 50-pound bag
    Product B
    Analysis: 10-5-5
    Cost: $9.50 per 20-pound bag

  2. Calculate:
    The amount of N per bag = bag weight × % N
    Product A
    50 pounds × 0.16 = 8 pounds N
    Product B
    20 pounds × 0.10 = 2 pounds N

  3. Calculate the cost per pound N:
    Cost per bag/pounds of N per bag = cost per pound of NProduct A
    $19.95/8 pounds N = $2.50 per pound N
    Product B
    $9.50/2 pounds N = $4.75 per pound N

Fertilizer A is the best buy.

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