NMSU: Determining Amounts of Fertilizer for Small Areas
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Author: Extension Agronomist, Agricultural Science Center at Artesia, Department of Extension Plant Sciences, New Mexico State University.


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Fertilizers can promote vigorous growth of plants and good production. However, fertilizing will not correct problems with pH, salinity, or sodium in soils. Successful gardening begins with soil testing in order to get the most out of your dollar for fertilizer. Soil testing 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.

Timing 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 pounds of this fertilizer there would be 16 pounds of nitrogen, 4 pounds of P2O5 and 8 pounds of K2O. The rest of the weight, all 72 pounds 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).

NMSU soil test interpretations report fertilizer application rates on a per-acre basis, pounds per 1,000 square feet, or the pounds needed for the size of the garden or lawn specified on the form submitted with the sample. Other labs may provide recommendations in pounds of fertilizer per acre. Tables 1 through 6 can help estimate how much material is needed for those managing small areas.

Table 1. Conversion from Pounds Per Acre to Pounds Per 100 or 1,000 Square Feet

Rate per acre
lb
Rate per 100 sq. feet Rate per 1,000 sq. feet
lb + ounces
lb ounces
100 0.23 4 2 , 5
200 0.46 7 4 , 9
400 0.92 15 9 , 3
500 1.15 18 11 , 8
600 1.38 22 13 , 12
700 1.61 26 16 , 1
800 1.84 29 18 , 6
1,000 2.30 37 22 , 15
2,000 4.59 73 45 , 15
General Formula: Desired pounds = [(pounds per acre) ÷ 43,560] × (square
feet to be fertilized)
1 acre = 43,560 square feet, 1 pound = 16 ounces, 454 g = 1 pound.

You can also calculate exactly what you need for your specific space with a little practice.

Example:
Determine the amount of ammonium sulfate needed by a 1,000 square-foot lawn if the soil test fertilizer recommendation suggests 50 pounds of nitrogen per acre.

Lawn: 1,000 square feet
Fertilizer: ammonium sulfate (21-0-0)
Nutrient Rate: 50 pound of nitrogen per acre
Square feet per acre: 43,560

Step 1. Divide pounds N per acre by 43,560.
This is the pounds 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 pounds of nitrogen needed.

    0.00115 × 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 Step 3.

    1.15 ÷ 0.21 = 5.48 pounds = 5 pounds 8 ounces

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 2 1/2 pounds of urea would be needed since urea contains 46% nitrogen.

The NMSU fertilizer recommendation would present pounds of ammonium sulfate needed for the specified area of interest. It would state that 239 pounds per acre of ammonium sulfate would be needed, or 5.48 pounds per 1,000 square feet.

From Tons to Teaspoons

When working with small areas, flower pots, or garden boxes we often switch from using pounds and ounces 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. Blended fertilizers vary in density based on what is used to arrive at the “grade” or percent N, P2O5, and K2O. Density refers to weight per unit volume. Table 2 lists the approximate weight per level cup of different fertilizers. Worksheet 1 shows how to calculate teaspoons of fertilizer for a given square footage, for any fertilizer.

Table 2. Fertilizer Densities (ounces per cup) Used to Make Volume Calculations for Fertilizer Rates (densities are for dry, loose, not packed or tamped, unless otherwise noted; blended fertilizers vary in density based on what products are used to make the blend)

Nitrogen Sources
46-0-0 (Urea) Ammonium Sulfate (21-0-0-24S)
Prilled Granular
(tamped)
Loose Tamped
6.0 6.8 7.6 7.9
Phosphorus Sources
18-46-0 (DAP†) 11-52-0 (MAP‡) 0-46-0 (TSP¶) 16-20-0-13S§
7.5 8.3 9.1 8.1
Potassium Sources
0-0-60 (Muriate of Potash) (Langbeinite) Potassium
Magnesium Sulfate
Loose Tamped Loose Tamped
10.0 11.0 11.1 12.6
Elemental Sulfur Sources
90% Soil
Acidifier
Granular (loose) Granular
(tamped)
Flake (tamped)
9.4 10.2 11.8 11.4
† Di-ammonium Phosphate
‡Mono-ammonium phosphate
¶ Triple superphosphate
§ Ammonium phosphate sulfate
If you are using a blended fertilizer it is best to tare, or zero out, a 1-cup measure on a scale, fill the cup with the fertilizer level with the top, and weigh.

Keep in mind that Tables 3 and 4 are for conversion purposes only and are not to be used as recommendations. Recommendations come from actual soil testing.

Table 3. Conversions for Flower Pots or Flower Boxes†

Pounds per acre Volumes of average blended fertilizer (with a bulk density of 16 ounces per pint) to meet pound-per-acre recommendation from soil test interpretation.
4-inch
Teaspoons
6-inch
Tablespoons

8-inch
Teaspoons

Flower boxes
1 square foot
Tablespoons
4 square foot
Teaspoons
250 2 1/2 1 1/4 1 1/2 1/2 2
500 5 2 1/3 3 1 4 1/2
750 7 3 1/2 4 2/3 1 2/3 6 2/3
† Volume relationships: One pint = 2 cups = 32 tablespoons = 96 teaspoons
*If the volume of fertilizer to add seems impossible to mix with the soil, consider dissolving a portion of the fertilizer (1 teaspoon, for example) in a pint of water and then adding this liquid mixture to the pot. Do this in increments during the growing season to reach the recommended rate given in the soil test and meet the plants' demand for nutrients. Remember, too much fertilizer at one time can cause damage to plants.

Table 4. Conversion for Fertilizer Products of Given Density for Small Areas

Weight per pint† Rate per acre
From soil test
Pounds
Equivalent weight to apply to: Volume per 3 feet row spacing
100 sq ft
Ounces
1,000 sq ft
Pounds
10 feet of row
Tablespoons
100 feet of row
Pints
13 oz/pint 100
300
500
3.7
11.0
18.4
2.3
6.9
11.5
2.7
8.1
13.6
7/8
2 1/2
4 1/4
16 oz/pint 100
300
500
3.7
11.0
18.4
2.3
6.9
11.5
2.2
6.6
11
3/5
2
3 1/2
18 oz/pint 100
300
500
3.7
11.0
18.4
2.3
6.9
11.5
2.0
5.9
9.8
3/5
1 3/4
3
22 oz/pint 100
300
500
3.7
11.0
18.4
2.3
6.9
11.5
1.6
4.8
8.0
1/2
1 1/2
2 1/2
Conversions are approximate. Fertilizers vary in density. Weigh one pint of your fertilizer to determine which density is closest to your product.
†Determined from product specification sheets or Material Safety Data Sheets. Blends depend on what products are used to make grades (percent N, P2O5, K2O).
To do your own volume calculation: Pints per unit area = [(lb/ of a product) × (1 acre ÷ 43,560 sq ft) × (16 oz/lb) × (linear feet × row spacing)] ÷ product density.
Convert to cups by multiplying by 2.

Sometimes only one, two, or three rows in a garden need to be fertilized. Table 5 helps estimate how many ounces of fertilizer would be needed for differently spaced rows when given pounds per acre, pounds per 1,000 square feet, or pounds per 100 square feet.

Table 5. Conversion From Pounds Per Acre of Average Mixed Fertilizers to Ounces Per 10 Feet of Row at Three Different Row Spacings

acre Rate per
1,000 sq ft
100 sq ft Distance between rows
One Foot Two Feet Three Feet
Pounds oz/10 feet oz/10 feet oz/10 feet
100 2.3 0.23 1/3 3/4 1 1/8
200 4.6 0.46 3/4 1 1/2 2 1/4
400 6.9 0.69 1 1/2 3 4 1/2
500 11.5 1.15 1 3/4 3 2/3 5 1/2

Some gardeners are familiar with the quantity of soil needed to fill a raised bed or practice container gardening. Table 6 converts fertilizer products of various densities when rates are given in pounds per acre to volumes of fertilizer for a given volume of soil. Worksheet 2 shows you how to make volume calculations specific to your needs.

Table 6. Approximate Volume of Fertilizer for Specified Volume of Soil for an 8-inch Rooting Depth (values in parentheses are nearest tenth of a teaspoon, tablespoon, or cup)

Fertilizer Pounds
per acre
Fertilizer volume for specified volumes of soil
Per cubic yard Per cubic
foot tsp
5 gallons
tsp
Tbsp Cups
13 oz/pint 100
300
500
3 1/3 (3.7)
11 (11.0)
(18 1/3) (18.3)
1/4 (0.2)
2/3 (0.7)
1 1/8 (1.1)
1/2 (0.4)
1 1/4 (1.2)
2 (2.0)

1/4 (0.3)
3/4 (0.8)
1 1/3 (1.4)

16 oz/pint 100
300
500
3 (3.0)
9 (8.9)
14 7/8 (14.9)
1/8 (0.2)
1/2 (0.6)
1 (0.9)
1/3 (0.3)
1 (1.0)
1 2/3 (1.7)
1/4 (0.2)
2/3 (0.7)
1 (1.1)
18 oz/pint 100
300
500
2 2/3 (2.6)
8 (7.9)
13 1/4 (13.2)
1/8 (0.2)
1/2 (0.5)
7/8 (0.8)
1/4 (0.3)
7/8 (0.9)
1 1/2 (1.5)
1/4 (0.2)
5/8 (0.6)
1 (1.0)
22 oz/pint 100
300
500
2 1/8 (2.2)
6 1/2 (6.5)
10 3/4(10.8)
1/3 (0.1)
2/5 (0.4)
5/8 (0.7)
1/4 (0.2)
3/4 (0.7)
1 1/4(1.2)

1/5 (0.2)
1/2 (0.5)
3/4 (0.8)


Worksheet1


Worksheet2


Original author: Esteban Herrera, former Extension horticulturalist


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Revised and electronicaly distributed March 2009, Las Cruces, NM.