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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 33, Number 3
Summer 1979

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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 (P2O5) rather than 100 lbs. of single superphosphate. Since single superphosphate is 20% P2O5 the recommended amount of P2O5 - 100 lbs. would be divided by 2 to determine the amount - 500 lbs. - to apply. If treble phosphate 45% P2O5 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 (P2O5). 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.

Table I. Approximate* weight in grams of common
volume measures for several fertilizer materials
  
  Teaspoon
(t)
Tablespoon
(T)
Cup
(C)
% Active
Ingredient
Rate
A.I.A.**
Vol. Measure/
100 square feet
   Nitrogen sources  
   Ammonium nitrate 4.8 14.4 223.0 33N 50 0.50, 2T, 1 t
   Ammonium sulfate 6.0  18.0 270.8 20.5N 50 1.75C
   Calcium nitrate 4.2 12.6 201.0 15N 50 1.5C, 2.5T
   Ferrous ammonium sulfate 5.2  15.6 247.8 7N 50 1.75C, 2T
   Potassium nitrate 6.4 19.2 302.8 13N 50 1.25C, 1t
   Urea 4.1 12.2  186.6 46N 50 0.5C, 1t
   Ureaform 3.2 09.6 147.0 38N 50 0.75 C, 1T, 2t
   10-20-10 5.5 16.4 254.4 10N 50 1.75C, 3T
   10-20-20  5.6 16.8 252.8 10N 50 1.75C, 2T, 2t
   16-20-0 5.3 15.8 249.8 16N 50 1.25C
   Phosphorus sources  
   Raw bone meal 4.1 12.3 195.0 20P2O5 100 2.5C, 1T
   Single superphosphate 6.5 19.4 297.4 20P2O5 100 1.5C, 2.5T
   Treble superphosphate 5.3 15.8 243.6 45P2O5 100  0.75C, 2T
   Potassium sources            
   Potassium chloride 6.3 18.9 285.2 50-62K2O 100  0.5C, 1.5t
   Potassium nitrate 6.4 19.2 302.8 46K2O 100 0.5C, 3T, 1t
   Potassium sulfate 6.8 20.3 325.4 48-52K2O 100  0.5C, 1.5T
   Potassium - magnesium
   sulfate
6.2  18.5 281.6 21-30K2O 100  1C, 2T, 2.5t
   Calcium sources            
   Calcium nitrate 4.2 12.6 201.0 20Ca 62.5 1.5C, 2.5T
   Dolomite 6.9 20.6 292.0 varies 500*** 1.5C, 2T
   Gypsum 7.8 23.4 378.0 23Ca 500*** 1.25C, 1T, 1.5t
   Limestone 5.9 17.7 284.7 varies 500*** 1.75C, 0.5t
   Magnesium sources            
   Dolomite 6.9 20.6 292 varies 500*** 1.5C, 2T
   Epsom salts 4.9 14.7 235.6 10 Mg 50 2C, 2T
   Potassium - magnesium
   sulfate
6.2 18.5 281.6 11 Mg 50 1.5C, 3T, 2.5t
   Trace element sources            
   Borax 6.0 18.0   11.3B 1 1.5t
   Copper sulfate 4.7 14.1   25Cu 2 1.75t
   Ferrous ammonium sulfate 5.2 15.6 247.8 25.6Iron 40  0.5C, 2T
   Ferrous sulfate 3.9 11.8 188.8 33 Iron 40 0.5C, 2T
   F.T.E.503 7.6 22.9   multiple 50*** 2T, 0.5t
   Solubor 2.3 6.9   20B 1 2.25t
   S. T. E. M. 4.6 13.7   multiple 55*** 4T

* 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


Volume 33, Number 3
Summer 1979

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