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Organic Fertilizers

Organic or 'natural' materials such as manures and composts are more cumbersome and possibly more expensive than synthetic fertilizers, but nothing quite takes their place. In addition to providing some nutrient value, they improve soil structure and increase the ablility of the soil to hold air, nutrients and water.

Plants take up nutrients in specific forms regardless of whether the source is organic or synthetic. It is possible to supply all the nutrients needed by your plants by using only organic materials, but care and effort are needed to assure that sufficient amounts of nitrogen, phosphorus and potassium (the nutrients needed in greatest quantities by plants) are available to the plants throughout the season. Because the nutrients in organic materials are tied up in more complex molecules, it often takes longer for them to become available to the plants. This delay can result in temporary nutrient deficiencies, especially in the spring.

Organic materials which have not broken down sufficiently (fresh manures, manures with lots of wood shavings, fresh leaf piles, etc.) should be turned into the garden in the fall or a few weeks before planting or applied only in small quantities. An alternate method is to set them aside for the season, allowing them to decompose. Fresh manures can 'burn' plants; woody materials (wood chips, sawdust, leaf piles, etc.) can cause a temporary nitrogen deficiency until they are sufficiently decomposed. This deficiency can be counteracted somewhat by applying about 1 lb. nitrogen for every 100 lbs. of organic matter in the spring. A rule of thumb is that when the material starts to resemble soil, it is ready for the garden. (Note: manures and composts do not provide sufficient phosphorus for good yields of crops.)

One possible 'recipe' for a complete fertilizer is the following applied per 1,000 square feet:

  1. 1,000 lbs. of compost or rotted, not dehydrated, manure (25 bushels) source of Nitrogen.
  2. 20 lbs. of bone meal (10 quarts) - source of Phosphorus.
  3. 30 lbs. of wood ash (15 quarts) - source of Potassium.

Work the materials into the top four inches of soil in early spring. This amount should supply your plants with sufficient nutrients throughout the growing season.

Common natural fertilizers and their nutritive value

Materials Nitrogen Phosphorus Potassium Availability Comments
Dried Blood 12 0 0 high  
Bone Meal (steamed) 0 9 0 medium  
Rock Phosphate 0 13 0 low must be ground to 200 mesh powder
Fish Emulsion 4 0.1 0 high may attract pests
Fish Meal 10 4 0 high may attract pests
Leaf Mold 1 0.4 1 medium  
Seaweed 1.5 0.7 5 medium  
Cottonseed Meal 7 2.5 2 high may contain pesticide residues
Wood Ashes 0 2 5 high hardwood preferred, liming effect.
Garden Compost 1 0.3 1 medium varies with ingredients
Cow Manure 0.5 0.2 0.5 medium rotted not dehydrated
Horse Manure 0.7 0.5 0.6 medium rotted not dehydrated
Rabbit Manure 4 3 1 medium rotted not dehydrated
Hen Manure 1.8 1 0.5 medium rotted not dehydrated
Hog Manure 0.3 0.3 0.4 medium rotted not dehydrated
Sheep Manure 1 0.35 0.5 medium rotted not dehydrated
Sludge 4 2.5 0 medium may contain toxic metals
Granite Dust 0 0 6   virtually insoluble
Limestone - - -   used to raise pH
Dolomite Limestone - - -   corrects magnesium deficiency and raises pH
Peat Moss - - -   improves soil structure

NOTE: Materials with medium availability - double amounts; materials with low availability - use four times amount.

*Human, cat and dog feces should not be used due to the danger of disease.

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This page was last updated on November 16, 2002