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.

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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 (pubs.nmsu.edu/_a/A146/), 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.

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/

For Further Reading

A-114: Test Your Garden Soil
pubs.nmsu.edu/_a/A114/

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

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

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

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

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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.

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Revised November 2020 Las Cruces, NM