David M. Kopec, Extension Turfgrass Specialist
Definitions and Terms
When shopping for lawn fertilizers or for landscape plants, there are some general terms that you should be aware of. These terms help you to be a "smarter shopper" and it also makes it easier for you to plan your fertilizer needs.
Listed below are some definitions that help you in your fertilizer knowledge.
Fertilizer analysis or fertilizer grade:
This is the amount of fertilizer amount as percent of the product by WEIGHT. A 10-6-4 fertilizer is 10% by weight nitrogen, 6% by weight phosphorous, and 4% by weight potassium. (Phosphorous is expressed as P205 equivalent and -K- is K20). A 1 lb. box, a 50 lb. bag, or a 1 ton dump truck load of 10-6-4 all have 10% -N- by weight. The grade or analysis is always the same!
To figure out how much of the element is present, simply multiply the percent of the element by the weight of product. For example, a 50 lb. bag of 10-6-4 fertilizer has 10% (5 lbs.) of -N- contained in the bag. Multiply 0.10 x 50 lbs.= 5 lbs.
When the analysis amount is less than 10%, it is important to remember that you have to place a zero before the value. Otherwise, the value -P- amount will be ten times greater than it actually is. For example, in our 10-6-4 analysis fertilizer, there is 6% phosphorous. To figure out how much -P- is in the 50 lb. bag, multiply 0.06 x 50 lb. = 3 lbs. of -P-.
You must do the same for -K-, noting that the -K- is 4% of the total weight of the product.
Example, 0.04 x 50 lbs. = 2 lbs. -K-.
If you did not put the "0" before the right of the decimal, you would erroneously calculate 0.4 x 50 lbs. = 20 lbs. of -K-, which is 20 times greater than the proper amount of 2 lbs. of -K- in the 50 lb. bag.
A fertilizer that has some value other than zero for all three elements of N-P-K is termed a "complete fertilizer." It says nothing about the fertilizer analysis, fertilizer grade or the relative amounts of each element to each other. Examples of complete fertilizers are 10-6-4, 21-7-14, 30-10-6, and 40-10-8. Note that both a 2-2-2 fertilizer and a 30-30-30 are both complete fertilizers.
Anytime a fertilizer has any -N- or any -P- or any -K-, the fertilizer bag must list the amount of all three elements, even if they do not exist the bag. Examples of these fertilizers include 34-0-0, 16-24-0, 21-0-0, 15-0-34 and 0-0-50. Some of these have one element, others have two elements, but the bag has to list amounts for all three even if any value is zero.
The "fertilizer ratio" is defined as the relative amount of N-P-K. To find out the fertilizer ratio, take the lowest number in the grade analysis and divide it into the other remaining value (s). For a 21-7-14 fertilizer, divide the "7" into the "21", the "7" into itself, and the "7" into "14." The result is a fertilizer with a 3-1-2 fertilizer ratio.
Note that the lowest quantity element -K- is 7%. The nitrogen is three times greater and the phosphorous is two times greater than that of -K-. Thus the 3-1-2 ratio. Note that the following fertilizers all have the same 3-1-2 ratio, but have different fertilizer grades. 6-2-4, 15-5-10, 30-10-20 and 9,3,6. Obviously the 6-2-4 fertilizer has the lowest grade (actual fertilizer contact) and the 30-10-20 has the most. They all have a 3-1-2 ratio. Note that a 10-0-50 fertilizer has a fertility ratio of (1-0-5).
Fertilizer ratios recommended for turf have typically been those of 4-1-2, 3-1-2 or 2-1-2 ratios. The ratios have been determined to keep N-P-K balanced for both soil storage and plant uptake requirements. On sandy soils, it is a wise bet to use complete fertilizers that have 3-1-2 ratio at each fertilizer application. On heavier textured soils (loams or heavier soils with clay and silt) you can often just apply nitrogen on 75% of the fertilizer application dates. On the other 25% you can apply a complete fertilizer.
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona. The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.