Determining Amounts of Fertilizer for Small Areas


Guide H-119

Revised by Robert Flynn and Bernd Leinauer

College of Agricultural, Consumer and Environmental Sciences, New Mexico State University


Respectively, Associate Professor/Extension Agronomist and Professor/Extension Turfgrass Specialist, Department of Extension Plant Sciences, New Mexico State University. (Print Friendly PDF)

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.

Photograph of a vegetable garden.

© Hannah Babiak | Dreamstime.com

Timing fertilizer applications will be important once a recommended fertilizer rate is determined from the soil test. Plants vary in their nutrient 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 (K2SO4). The chloride or sulfate helps carry the nutrient of interest, in this case potassium (K).

Soil test interpretations may report fertilizer application rates on a per-acre basis, pounds per 1,000 square feet (sq ft), 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. See NMSU Extension Guide A-146, Appropriate Analyses for New Mexico Soils (https://aces.nmsu.edu/pubs/_a/A146/welcome.html), for appropriate analyses and a list of soil testing labs to consider for evaluating New Mexico soils. Tables 1 through 6 can help estimate how much material is needed for those managing small areas. Several universities provide online tools to help with calculating fertilizer application rates; links are listed at the end of this document.

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

Table 1. Conversion from Pounds Per Acre to Pounds Per 100 or 1,000 sq ft

Rate per acre (lb)

Rate per 100 sq ft

Rate per 1,000 sq ft

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] × (sq ft to be fertilized)

1 acre = 43,560 sq ft, 1 pound = 16 ounces or 454 g

Example

Determine the amount of ammonium sulfate needed by a 1,000 sq ft lawn if the soil test fertilizer recommendation suggests 50 pounds of nitrogen per acre.

Lawn: 1,000 sq ft

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 sq ft.

(50 lb N/acre) ÷ (1 acre/43,560 sq ft) = 0.00115

Step 2. Multiply fertilizer by sq ft. 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 sq ft to supply an equivalent of 50 lb N per acre. On the other hand, only 2.5 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 sq ft.

Photograph of a lawn.

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 must 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 sq ft, 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)

Potassium Magnesium Sulfate (Langbeinite)

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.


Worksheet 1
Don’t see your fertilizer? Here’s how to determine teaspoons of fertilizer for a given sq ft:

1. Determine fertilizer rate (lb/acre) from soil test interpretation

(1)

2. Divide by sq ft per acre (43,560)

(2)

3. Determine area to treat (area = length × width or πr2)

(3)

4. Multiply (2) by (3) to get pounds needed for area

(4)

5. Multiply (4) by 16 (there are 16 oz per lb)

(5)

6. Determine bulk density of fertilizer (oz/pint)

(6)

7. Divide (6) by (5)

(7)

8. Multiply (7) by 96 for teaspoons, or 32 for tablespoons

(8)

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

Flower Pots*

Flower boxes

4-inch
teaspoons

6-inch
tablespoons

8-inch
teaspoons

1 sq ft
tablespoons

4 sq ft
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 (lb)

Equivalent weight to apply to

Volume per 3-foot row spacing

100 sq ft (ounces)

1,000 sq ft (lb)

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, and 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 sq ft, or pounds per 100 sq ft.

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

Rate per

Distance between rows

Acre

1,000 sq ft

100 sq ft

One foot

Two feet

Three feet

lb

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 they 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

lb/acre

Fertilizer volume for specified volumes of soil

Per cubic yard

Per cubic foot
teaspoon

5 gallons
teaspoon

tablespoon

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)


Worksheet 2

How to make your own volume calculations specific to your needs:

1. Determine effective rooting depth (8 inches for most plants) or pot depth

(1a)

inches

To convert to feet, divide (1a) by 12

(1b)

feet

To convert to yards, divide (1a) by 36

(1c)

yards

2. Determine the surface area (sq inches) of the pot or planter box

Square/rectangle: length × width

(2a)

inches2

Circle: 3.14 × r2 = π × r × r

(2a)

inches2

To convert to sq ft, divide (2a) by 144

(2b)

feet2

To convert to square yards, divide (2a) by 1,296

(2c)

yards2

3. Multiply (1a) by (2a) to get cubic inches of potting volume

(3a)

inches3

Multiply (1b) by (2b) to get cubic feet of potting volume.

(3b)

feet3

Multiply (1c) by (2c) to get cubic yards of potting volume.

(3c)

yards3

4. Convert to gallons, if needed

Divide (3a) by 231

(4a)

gallons

Multiply (3b) by 7.48

(4b)

gallons

Divide (3c) by 0.00495

(4c)

gallons

5. Obtain fertilizer rate from Table 5 or the soil test report

Pounds per acre

(5a)

Pounds per 1,000 sq ft

(5b)

Pounds per 100 sq ft

(5c)

6. Determine fertilizer weight needed for area specified

Pounds for square inches: Divide (5a) by 6,272,640, then multiply by (2a)

(6a)

pounds

Pounds for sq ft: Divide (5b) by 43.56, then multiply by (2b)

(6b)

pounds

Pounds for sq ft: Divide (5c) by 4.356, then multiply by (2b)

(6c)

pounds

7. Convert to ounces

Multiply (6a) by 16 (ounces per pound)

(7a)

ounces

Multiply (6b) or (6c) by 16 (ounces per pound)

(7b)

ounces

8. Obtain fertilizer density (refer to Table 2)

(8a)

ounces/cup

Multiply (8a) by 2 to get oz per pint

(8b)

ounces/pint

9. Determine cups of fertilizer needed

Divide (7a) by (8a)

(9a)

cups

Divide (7b) by (8a)

(9b)

cups

10. Convert to tablespoons of fertilizer

Multiply (9a) by 16

(10a)

tablespoons

Multiply (9b) by 16

(10b)

tablespoons

11. Convert to teaspoons of fertilizer

Multiply (10a) or (10b) by 3


teaspoons

Online Tools to Determine Amount of Fertilizer

For Further Reading

A-114: Test Your Garden Soil
https://aces.nmsu.edu/pubs/_a/A114/welcome.html

A-114: Haga un análisis de suelo a su jardín
https://aces.nmsu.edu/pubs/_a/A114sp/welcome.html

H-158: How to Collect and Send Plant Specimens for Disease Diagnosis
https://aces.nmsu.edu/pubs/_h/H158/welcome.html

CR-457: Home Vegetable Gardening in New Mexico
https://aces.nmsu.edu/pubs/_circulars/CR457/welcome.html


Original author: Esteban Herrera, Extension Horticulturalist. Subsequently revised by Robert Flynn, Extension Agronomist.


Photo of Robert Flynn.

Robert Flynn is an Associate Professor of Agronomy and Soils and an Extension Agronomist at New Mexico State University. He earned his Ph.D. at Auburn University. His research and Extension efforts aim to improve grower options that lead to sustainable production through improved soil quality, water use efficiency, and crop performance.


To find more resources for your business, home, or family, visit the College of Agricultural, Consumer and Environmental Sciences on the World Wide Web at aces.nmsu.edu/pubs.

Contents of publications may be freely reproduced, with an appropriate citation, for educational purposes. All other rights reserved. For permission to use publications for other purposes, contact pubs@nmsu.edu or the authors listed on the publication.

New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department of Agriculture cooperating.

Revised November 2020 Las Cruces, NM