QBARS - v33n3 Applying Fertilizers on a Small Scale

Applying Fertilizers On A Small Scale
R. L. Ticknor, Aurora, OR

The amount of fertilizer to apply is usually given in pounds per acre or in the case of lime in tons per acre. Translating these quantities into the amount to use when fertilizing a small area can be bewildering.

To make it easier to accomplish this, the weights in grams of common volume measures - teaspoons (t), tablespoons (T), and cups (C) of many fertilizer materials were determined and a table prepared. Each material was weighed three times and the average is shown in the table. Unfortunately different sets of measuring spoons vary in weight for a given size of spoon. Some sets do not have the normal 1:3 ratio between tea and tablespoons or the 1:16 ratio between one tablespoon and one cup. This variation is not large enough to cause problems with most fertilizers with the possible exception of boron fertilizers. Boron is needed but twice as much is definitely not better and could be harmful.

To use Table I, three factors have to be known: 1) the area to be fertilized; 2) the amount of fertilizer to be applied; and 3) the percentage of active ingredient has to be known if a recommendation is for 100 pounds per acre of an element such as phosphorous (P ₂ O ₅ ) rather than 100 lbs. of single superphosphate. Since single superphosphate is 20% P ₂ O ₅ the recommended amount of P ₂ O ₅ - 100 lbs. would be divided by 2 to determine the amount - 500 lbs. - to apply. If treble phosphate 45% P ₂ O ₅ was used instead, it would take 100 lbs. divided by .45 or 222.2 lbs.

Since one gram per 100 square feet is approximately one pound per acre, it would take 500 grams of single superphosphate per 100 square feet to equal 100 lbs. per acre of phosphorous (P ₂ O ₅ ). According to Table I, 1.5C, 2T, and 2t of single superphosphate would equal 497.7 grams or very close to the desired 500 grams.

The bulk of most fertilizers supplying major elements is large enough that spreading is not a problem. The quantities used to supply trace elements are much smaller but most of them are soluble so can be applied with a watering can or sprayer. An exception would be fritted trace elements such as F.T.E. 503 which is not soluble but could be mixed with another material which is applied in larger quantities to make spreading easier.

Good luck and may the batteries of your calculator never fail.

* Different sets of measuring spoons will have slightly different volumes. Even the same set might not have the normal 1:3 ratio between teaspoon (t) and one tablespoon (T) or 1:16 between one tablespoon and one cup.
** A.I.A. = Active ingredient per acre (in pounds)
*** Amount of product per acre