1Respectively, Associate Professor/Extension Agronomist and Professor/Extension Turfgrass
Specialist, Department of Extension Plant Sciences, New Mexico State University.
COLLEGE OF AGRICULTURAL, CONSUMER AND ENVIRONMENTAL SCIENCES
aces.nmsu.edu/pubs • Cooperative Extension Service • Guide H-119
New Mexico State University
aces.nmsu.edu
The College of
Agricultural,
Consumer and
Environmental
Sciences is an
engine for economic and community
development in New Mexico, improving the lives of New Mexicans through academic, research, and Extension
programs.
Determining Amounts of
Fertilizer for Small Areas
Revised by Robert Flynn and Bernd Leinauer1
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 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.
© HannahBabiak | Dreamstime.comGuide H-119 • Page 2
pdf), 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.
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.
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.
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.
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.Guide H-119 • Page 3
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.
ONLINE TOOLS TO DETERMINE
AMOUNT OF FERTILIZER
•
University of Missouri Extension:
http://agebb.missouri.edu/fertcalc/
•
University of Kentucky Cooperative
Extension Service:
http://soils.rs.uky.edu/calculators/mult_fert.asp
•
University of Georgia Extension:
http://aesl.ces.uga.edu/soil/fertcalc/
•
Texas A&M Agrilife Extension:
http://soiltesting.tamu.edu/calc/UNCalclist.htm
•
Purdue University:
https://turf.purdue.edu/fertilizer-calculator/
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.
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 gGuide H-119 • Page 4
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.
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.Guide H-119 • Page 5
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
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 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)
Guide H-119 • Page 6
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
Guide H-119 • Page 7
WORKSHEET 2 (CONTINUED)
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
Original author: Esteban Herrera, Extension Horticulturalist. Subsequently revised by
Robert Flynn, Extension Agronomist.
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.Guide H-119 • Page 8
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