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Fertilizing Organic Garden

Fertilizing the Organic Garden is part 7 of The Alabama Vegetable Gardener series.

Organic nutrient sources have two distinct advantages: (1) They provide a slow release nitrogen source to crops that won’t burn plants as easily or as severely as some commercial fertilizers, and (2) They improve soil physical conditions by adding organic matter. However, organic nutrient sources also have disadvantages: (1) They are low in nutrients, so large quantities must be applied in order to meet plant needs; (2) They can lead to excess soil phosphorus (P) levels due to the nutrient ratios in many organic-sourced fertilizers (see Excess Phosphorus section). This leads to a potential risk to surface and groundwater contamination if erosion occurs. (3) Weed seed can also be introduced into the garden from some manures.

Phosphorus buildup in soils is almost inevitable with organic-sourced fertilizers unless managed closely. Most manures and composts have a 1:1 ratio of N:P2O5. However, most plants use five times more nitrogen than phosphorus in an annual growth cycle. Excess phosphorus then continues to build up with annual applications. Look for low P organic fertilizers, and plant legume cover crops whenever possible.

The best organic soil fertilizers, animal manures, are abundant in some areas of the state but are hard to find in others. Where available, poultry and cattle manures are excellent sources of nitrogen and phosphorus and help to improve soil tilth.

Moist broiler litter is equivalent to a 3-3-2 fertilizer. Cow, hog, and horse manures usually contain fewer plant nutrients than broiler manure. Following is an average N-P2O5-K2O content of some dry animal manures. These can be compared to 13-13-13 fertilizers, the most frequently used mineral fertilizer in Alabama.

Manures, particularly poultry manure, must also be incorporated into the soil or some of the nitrogen could be lost. If poultry litter has an ammonia smell, valuable nitrogen is being lost as ammonia gas.

Organic fertilizers must first be mineralized by soil microorganisms to the inorganic form before plant roots can take up the nutrients. Once mineralized, these nutrients are subject to the same soil chemical processes as synthetic fertilizers—leaching, fixation, and absorption by soil clay. Therefore, an organic-sourced fertilizer can lead to plant nutritional imbalances just as a synthetic-sourced fertilizer can. Soil testing is just as important in organic gardens as it is in others, so the gardener can keep up with the nutrients present in the soil and avoid potential problems. Following are some other organic and mineral fertilizers for the home gardener wanting to minimize or avoid synthetic fertilizer sources.

N, P2O5, and K2O Organic and Mineral Sources

Nitrogen Organic Sources

  • Leaf compost (1-3%)
  • Chicken manure (3%)
  • Soybean/cottonseed meal (6%)
  • Blood meal (13%)
  • Fish meal (10%)
  • Animal tankage (9%)

Nitrogen Mineral Sources

  • Ammonium sulfate (21%)
  • Calcium nitrate (15%)

Phosphorus Organic Sources

  • Chicken manure (1.2%)
  • Bone meal (23%)
  • Fish meal (2.6%)

Phosphorus Mineral Sources

  • Superphosphate (20%)
  • Concentrated superphosphate (46%)
  • Ammonium phosphate (46%)

Potassium Organic Sources

Wood ashes (6%)

Potassium Mineral Sources

  • Muriate of potash (60%)
  • Potassium sulfate (50%)

Organic Fertilizers Low in Phosphorus

  • Chilean nitrate 16-0-0
  • Feather meal 12-0-0
  • Blood meal 12-0-0
  • Fish meal 10-3-0
  • Cottonseed meal 6-1-1
  • Fish emulsion 5-1-1

 

Some Common Organic Fertilizers

Name of FertilizerAnalysis in Percent of N
(Fresh-Weight Basis)
Analysis in Percent of P2O5
(Fresh-Weight Basis)
Analysis in Percent of K2O
(Fresh-Weight Basis)
Remarks

Blood (dried)

1030A very rapidly available organic fertilizer.
Fish scraps (dried meal)930Do not confuse with fish emulsives that generally are quite low in fertilizer content. Contains 6% calcium.
Guano, bat693Partially decomposed bat manure from caves.
Guano, bird13113Partially decomposed bird manure from islands off coast.
Manure, cattle1.51.51.2
Manure, chicken3.13.02.0
Manure, horse1.70.71.8
Manure, sheep2.01.01.0
Manure, swine2.01.41.8
Manure, mushroom (spent)111
Meal, bone, raw4220Main value is nitrogen since most of the phosphorus is not soluble.
Meal, bone, steamed2200As a result of steaming under pressure, some nitrogen is lost, but more phosphorus is soluble for use by plants. Contains 20% calcium.
Cocoa shell2.513Primarily a conditioner for complete fertilizers.
Cotton seed meal62.52Generally very acid. Useful in alkaline soils.
Hoof and horn1400The steam-treated and ground material is a quickly available source of nitrogen.
Oyster shells0.20.30Because of their alkalinity, these are best used for raising pH rather than as a fertilizer.
Peat (reed or sedge)20.50.7Best used as a soil conditioner rather than as a fertilizer. Breaks down too rapidly.

Rice hulls (ground)

0.50.20.5
Tankage, cocoa41.52
Tankage, garbage331
Tankage, process (leather, hair, wool, felt, feathers, etc.)820
Wood ashes026Quite alkaline. Do not use on high pH soils. Contains 20% calcium.

Read the complete Alabama Vegetable Gardener.

 

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ANR-0479

 

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