Organic materials such as manures and composts are highly desirable for gardens. They supply necessary plant nutrients and improve soil structure, tilth, aeration, and water-holding capacity. They do not change soil texture, however. For clay soils, use long-fibered materials such as straw. For sands, consider a material with more humus such as peat or rotted sawdust.

Nutrients derived from decaying organic materials have the same chemical composition as nutrients from inorganic commercial fertilizers. However, the nutrients in organic materials may not be immediately available to plants because soil microbes must first break them down (decompose them). The nutrients become available over a period of time, sometimes over more than one growing season. Unlike commercial inorganic fertilizers, organic materials offer a valuable source of humus to improve soils.

Organic materials vary widely in their nutrient contents (Table 1). Because their nutrient contents often are low, it can take large amounts of material to supply the needed nutrients.

Because of this wide variability in nutrient content, it is difficult to know the exact amount of nutrients you are applying.

To apply 2 pounds N on 100 square feet (ft²) of garden, you would apply about 120 pounds of dry steer manure (Table 2). The amount you apply will vary depending on the amount of bedding material in the manure. If the manure contains no bedding material, you would apply an amount at the low end of the range given in Table 2. If it contains a great deal of bedding, you would apply an amount at the high end.

It is a good idea to alternate organic with commercial inorganic fertilizers. Some organic materials such as sawdust and straw may require composting before use in the garden.

If you have a fertilizer with an analysis not listed in Table 3, look in the table for the fertilizer with the closest analysis and apply its rate or follow package directions. To convert the rates in Table 3 to smaller areas, first divide the area to be fertilized in square feet by 100 square feet. Then multiply by the rate, in cups, given for 100 square feet.

For example:

1. Area 8 ft by 10 ft, fertilizer analysis 4-10-4, and rate of application 10 cups per 100 ft² block (from Table 3);
   8 ft x 10 ft = 80 ft²
   80 ft²/100 ft² = 0.8
   0.8 x 10 cups (rate of applications) = 8 cups
   Apply 8 cups of a 4-10-4 fertilizer to a 8 ft by 10 ft block.
2. Row 2 ft by 20 ft, fertilizer analysis 6-10-4, and rate of application 6.5 cups per 100 ft² (from Table 3):
   2 ft x 20 ft = 40 ft²
   40 ft²/100 ft² = 0.4
   0.4 x 6.5 cups (rate of applications) = 2.6 cups
   Apply 2.6 cups of a 6-10-4 fertilizer to a 2 ft by 20 ft row.

Fig. 1. Fertilizer placement. Banded fertilizer concentrates in a small zone. Broadcast and incorporated fertilizer distributes evenly to the depth of mixing.

Fig. 2.Side-dressing individual plants and garden rows. When rainwater or irrigation water moves a side-dressing into the soil, its distribution resembles that of banded fertilizer.

Shrubs and trees
Spread fertilizer around the base of the plant inside the perimeter of the top growth. This perimeter is called the dripline. The amount of fertilizer to apply depends on the size of the plant. One-half pound of N per 1 inch trunk diameter (caliper) at chest height is a good rule of thumb for trees. For small shrubs, apply 1 cup of 21-0-0 fertilizer at the dripline. Divide fertilizer applications in two, applying one-half in early spring and one-half in late fall.

Lawns
Keep in mind that established lawns require 0.5 to 1.0 pound of N per 1,000 square feet per month of active growth. Lawn fertilizer should contain N, P, and K in a 3:1:2 ratio. For established grass lawns, N is the principle fertilizer nutrient needed.

Divide the total fertilizer amount into three or four applications. Make the first two applications in fall (September through November), the next one in early spring, and the final one in late May or early June. Avoid fertilizing turf during the heat of summer when cool-season grasses are semidormant.

To avoid severe foliage burns, do not apply fertilizer to wet lawns unless large amounts of irrigation water or rain will immediately follow the application.

For soil applications, apply a chelated Fe at rates shown on the label. For best results, poke 12-inch holes into the soil around the plant's dripline with a soil probe, hollow metal tube, or shovel. Mix the Fe product in water and fill up the holes. Follow package label directions and apply in spring or fall to avoid burning plants.

To apply Fe as a foliar spray, use a solution containing 0.5 percent ferrous sulfate or a chelate at rates shown on the package label. Keep in mind that foliar applications of Fe are only a temporary solution and do not last as long as soil-applied materials. Soil pH may need to be altered for a long-term solution.

In garden soils that are too alkaline (pH greater than 7.5), elemental S can be applied to lower the soil pH (Table 5). Before attempting to alter soil pH, obtain an accurate pH test.

1. Check the fertilizer analysis on the bag or container.
2. Apply the fertilizer at the rates given in this publication or on the fertilizer container.
3. Work fertilizer well into the soil.
4. Don't guess at rates; measure or weigh the fertilizer product.

The Authors -- Robert L. Mahler, professor of soil science, University of Idaho, Moscow; Terry A. Tindall, Extension soil specialist, UI Twin Falls Research and Extension Center; and Susan M. Bell, UI Extension agricultural agent, specializing in horticulture, Ada County, Boise.

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, LeRoy D. Luft, Director of Cooperative Extension System, University of Idaho, Moscow, Idaho 83844. The University of Idaho provides equal opportunity in education and employment on the basis of race, color, religion, national origin, gender, age, disability, or status as a Vietnam-era veteran, as required by state and federal laws.

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Comments to author: karenl@uidaho.edu

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Revised: January 3, 2002

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