Determining Amounts of Fertilizer over Small Areas. Fertilizers can promote vigorous growth of plants and good production. However, fertilizing will not correct problems related to soil pH, salinity, or presence of sodium . Successful gardening begins with soil testing in order to get the most out of your dollar for fertilizer. A simple soil pH test can tell you what to do before fertilizing in order to have a successful garden (such as salinity control or managing sodium). Prescription fertilizer recommendations can be made for your specific conditions if your soil is tested.
Proper timing of fertilizer applications will be important once a recommended fertilizer rate is determined from the soil test. Plants vary in their requirements, and soils can contain different amounts of plant-available nutrients based on their history. Fertilizer packaging is required by law to state clearly the percentage nitrogen (N), phosphorus (P2O5), and potash (K2O) by weight. For example, if a container or package reads “16-4-8”, this means that for every hundred kg of this fertilizer there would be 16 kg of nitrogen, 4 kg of P2O5 and 8 kg of K2O. The rest of the weight, all 72 kg worth, is a carrier of the N, P2O5, or K2O. For example, K2O is often present as potassium chloride (KCl) or potassium sulfate (KSO4). The chloride or sulfate helps carry the nutrient of
interest, in this case, potassium (K).
You can also calculate exactly what you need for your specific space with a little practice.
Determine the amount of ammonium sulfate needed by a 1,000 square-foot lawn if the soil test fertilizer recommendation suggests 50 kg of nitrogen per acre.
Lawn: 1,000 square feet
Fertilizer: ammonium sulfate (21-0-0)
Nutrient Rate: 50 kg of nitrogen per acre
Square feet per acre: 43,560
Step 1. Divide kg N per acre by 43,560.
This is the kg needed per square foot.
(50 lb N/Acre) ÷ (1 acre/43,560 sq feet) = 0.00115
Step 2. Multiply by square feet to fertilize. This is the kg of nitrogen needed.
0.00115 x 1,000 = 1.15 lb N
Step 3. Divide the percent N in the fertilizer by 100.
21%N ÷ 100 = 0.21
Step 4. Divide the result in Step 2 by the result in
1.15 ÷ 0.21 = 5.48 kg= 5.8 kg
This is the amount of ammonium sulfate needed over 1,000 square feet to supply an equivalent of 50 lb N per acre. On the other hand, only 2½ kg of urea would be needed since urea contains 46% nitrogen.
From tons to teaspoons
When working with small areas, flower pots, or garden boxes we often switch from using kg to units of volume including pints, cups, tablespoons, and teaspoons. Though it is easy
to over-apply fertilizers in this manner the following tables should help when trying to use fertilizer recommendations based on soil testing. The fertilizer can be mixed with the soil to be put in the pot, or the fertilizer can be dissolved in water and then poured into the pot containing the soil. It is important, however, not to put all the nitrogen or potash material into the pot at one time, especially in liquid form. This can lead to excess salinity in
the pot or loss of nitrogen and potash by leaching.
Slow-release fertilizers should be added in the granular form. Many potting soils are sold with nutrients already mixed into the media, and additional fertilization is often not needed.
Soil test–based fertilizer recommendations are given as weight per unit area. Converting to volume measures means that the fertilizer density needs to be known. Fertilizers do not all have the same density. One cup of urea does not weigh the same as one cup of ammonium sulfate.