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Here are instructions for calculating fertilizer rates yourself. Or, click to see an automatic rate calculator.
It would be nice to find a fertilizer with nearly the same ratio of ingredients as the recommendation for your crop, but often you will need more than one kind. For the sake of the environment, and also because it is usually the most expensive ingredient, you should try to avoid applying excess phosphorus. Often a low phosphate analysis like 28-4-12 or 18-9-18 match recommendations better than 12-12-12 or 15-10-10.
Starting with the pounds of Nitrogen recommended per acre, and a type of fertilizer you are considering to meet that need, (for more about types of fertilizers, click Selecting fertilizers) calculate the pounds of that fertilizer needed as follows:
Divide recommended pounds by percent in the bag, then multiply by 100.
Example: to apply 1 lb Nitrogen per 1000 sq ft using 27-3-3,
1lb ÷ 27 x 100 = 3.7 (pounds of 27-3-3 to apply)
This application will meet the Nitrogen recommendation, but perhaps more Potash (Potassium) is needed.
Example; if 4 pounds of potash per 1000 sq ft is recommended for the lawn above, the 3.7 lbs of 27-3-3 supplies only 3.7 x 3%, or .1 lb potash. One tenth lb. is negligible. You could use plain (muriate of) Potash fertilizer, 0-0-60, and calculate as above
4 (lbs needed) ÷60(%) x100 = 6 (pounds of potash to apply to 1000 sq ft. in addition to the 3.7 lb. of 27-3-3) The potash can be applied once per year. After 3-4 years, retest the soil to see if the same rate is needed.
For a lawn, you never apply more than a pound or so of nitrogen at one time, and often make more than one application to reach the total recommended. The same rationale applies to flower and vegetable gardens, but in those, usually at least half the recommended nitrogen is worked into the soil before planting, small amounts of soluble fertilizer are used around the roots of transplants when they are set out, and the remaining nitrogen is applied one or more times as sidedressing during the growing season. ( See vegetable gardens) All of the Phosphate and Potash may be applied at once, ideally worked into the soil since they do not wash into the root zone with rain or irrigation as Nitrogen does.
One way to look at the various combinations is to use a worksheet like the one below: You can print a copy if you wish. print a worksheet .pdf file.
Fertilizer Worksheet: All rates are per 1000 sq.ft.
N P2O5 K2O
Recommended nutrients (write in amount): ___1___ __0____ __4____
Fertilizer # 1 analysis _27_- _3__- _3__
pounds calculated __3.7__ supplies ___1___ _.1___ __.1__
Subtract to show remaining nutrients needed: ___0___ __0____ ___3.9__
Fertilizer # 2analysis _0__- _0___- _60___
pounds calculated __6____ supplies ____0_ ____0__ ___4___
Subtract to show remaining nutrients needed: ___0___ ____0___ __-.1__
Fertilizer # 3 analysis ____- ____- ____ (that is close enough)
pounds calculated ____ supplies ________ _______ ______
Subtract to show remaining nutrients needed: ________ _______ ______
The final step is to take these amounts of fertilizer which are lbs required per 1000 square feet, and adjust to the area of your garden or lawn. Divide your area by 1000 and multiply by the pounds of fertilizer you calculated above.
example 1: 12000 sq ft lawn/ 1000 = 12 , 12 x 3.7 = 44.4 lbs of 27-3-3 to purchase and apply for the whole lawn (45 is close enough).
example 2: 120 sq ft garden/1000 = .12 , .12 x 3.7 = .4 lbs of 27-3-3 to purchase and apply for the garden 4/10 lb or about 1/2 lb.