Understanding Organic Fertility

All crops, turfgrass, or ornamental plants need both macro and micronutrients at sufficient levels to maintain proper plant health and vitality. Fertility can be achieved in different ways and by using several methods. Regardless of the method, plants need certain amounts of nutrients. Getting these nutrients to plants can be achieved with conventional or organic inputs.

Before fertilizing, a soil sample should be taken to ensure the appropriate amounts of nutrients are applied. Ideally, this should be done well in advance of planting (at least three months) to allow any required lime time to make the necessary pH adjustments in the soil. Because applying lime and fertilizer to a garden will change the nutrient content and pH of the soil, a new soil sample should be taken each year. For information on taking a soil sample and interpreting the results, see HGIC 1652, Soil Testing.

Organic Fertility

Organic fertilizers include materials such as bone meal, blood meal, feather meal, and fish meal. These fertilizers are often made from previously living organisms, and their nutrients are contained in complex amino acid chains known as proteins. These proteins must be broken down into amino acids by microbes in the soil. Once the proteins are broken down, the nutrients are available to the plant for uptake. The length of this process can vary in time due to environmental factors such as temperature, soil moisture, and pH, as well as the number of microbes present in the soil and the type and amount of fertilizer used. Most organic fertilizers take between one and four months to become available to the plant. In contrast, most synthetic fertilizers are available immediately. The organic fertilizer, Chilean nitrate or sodium nitrate, is an exception and is available to the plant immediately after it is applied. Gardeners must plan accordingly when using organically sourced fertilizer to achieve maximum availability and uptake by plants.

A bag of mined Chilean nitrate is an approved organic fertilizer.

A bag of mined Chilean nitrate is an approved organic fertilizer.
Zack Snipes, ©2015, Clemson University

Table 1a and 1b. A comparison of Synthetic and Organic Fertilizers Synthetic

Pros Cons
Typically, more nutrients per pound of material Can burn plants due to high salt levels
Cheaper fertility source per pound High nutrient loading in soil
Quicker release and nutrient availability to the plant Leaching and volatilization of nutrients can occur
Comes in a wide range of nutrient ratios
Widely available


Pros Cons
Slower release of nutrients More expensive per unit of nutrient
Promotes soil microbial health Takes time for nutrients to become available to plant

Table 2. Common Organic Fertilizers

N P2 O5 K2 O Release Time
Plant By-Products
Alfalfa Meal or Pellets 2.0 1.0 2.0 1 to 4 Months
Corn Gluten Meal 9.0 0.0 0.0 1 to 4 Months
Cottonseed Meal 6.0 0.4 to 3.0 1.5 1 to 4 Months
Soybean Meal 7.0 1.2 to 2.0 1.5 to 7.0 1 to 4 Months
Kelp Powder 1.0 0.0 4.0 Immediate to 1 month
Animal By-Products
Bat Guano (high N) 10.0 3.0 1.0 4 plus months
Bat Guano (high P) 3.0 10.0 1.0 4 plus months
Blood Meal 12.0 to 14.0 2.0 1.0 1 to 4 Months
Bone Meal (raw) 3.0 22.0 0.0 1 to 4 Months
Bone Meal (steamed) 1.0 to 2.0 11.0 to 15.0 0.0 1 to 4 Months
Feather Meal 7.0 to 12.0 0.0 0.0 4 plus months
Fish Emulsion 5.0 2.0 2.0 1 to 4 Months
Fish Powder 12.0 0.3 1.0 Immediate to 1 month
Enzymatically Digested Hydrolyzed Liquid Fish 4.0 2.0 2.0 1 to 4 Months
Fish Meal 10.0 6.0 2.0 1 to 4 Months
Worm Castings 2.0 1.5 1.5 1 to 4 Months
Natural Minerals
“Soft” Rock Phosphate 0.0 14 to 16 0.0 Very slow (years)
Greensand 0.0 0.0 3.0 Very slow
Table Source: 2023 Vegetable Crop Handbook for Southern United States

Table 3. Nutrient Values for Manure and Cover Crop Residues.

N P2 O5 K2 0
Pounds Per Ton
Cattle manure 5-201 18-10 14-19
Poultry litter & manure 25-651 20-60 10-55
Swine manure 5-401 2-17 10-55
Horse manure 6-121 3 6
Liquid Poultry Manure

(5 – 15% solids)

7-151 5-10 5-10
Alfalfa sod 50-1002 0 0
Hairy vetch 50-100 0 0
Ladino clover sod 60 0 0
Crimson clover sod 50 0 0
Birdsfoot trelfoil 40 0 0
Lespedeza 20 0 0
Top and Roots 40 0 0
Grain harvest residue 15 0 0
1Manures are highly variable. Consult a state or private lab for a nutrient analysis prior to application.

275% stand = 100 – 0 – 0, 50% stand = 75– 0 – 0, and 25% stand = 50 – 0 – 0.

Table Source: 2023 Vegetable Crop Handbook for Southern United States

A beautiful Austrian winter pea cover crop is turned into the soil to provide nutrients for the next season’s crop.

A beautiful Austrian winter pea cover crop is turned into the soil to provide nutrients for the next season’s crop.
Zack Snipes, ©2018, Clemson Extension

Calculating Nutrient Needs

The nutrient content of fertilizer is represented by three numbers in the following format: XX-XX-XX. The first number represents nitrogen content, the second represents phosphorus, and the third is potassium. The numbers represent the percentage of each nutrient contained in the fertilizer. For example, a bag of 10-2-8 contains 10% nitrogen, 2% phosphorous, and 8% potassium, in that order. If that bag of 10-2-8 weighed fifty pounds, then five pounds would be nitrogen, one pound would be phosphorous, and four pounds would be potassium. A soil test recommendation may say: “Broadcast twenty-five to fifty pounds of nitrogen per acre.” To achieve twenty-five pounds of nitrogen per acre, one would have to use five bags of the 10-2-8 or 250 pounds of the product.

  • 25 pounds of nitrogen divided by (.10 or 10 % nitrogen) = 250 pounds of the 10-2-8 product needed

Many organic fertilizers will need to be mixed with other organic fertilizers to achieve the correct amount of nutrients for plants. Refer to soil test results for proper nutrient requirements. Some fertilizers are high in nitrogen but will provide little to no phosphorous or potassium (ex., feather meal), while some are low in nitrogen but provide high phosphorous (ex., bone meal). When using organic fertilizers, properly mixing them to get the correct ratio for plants is a necessary step. Another consideration when using organic fertilizers is how quickly the nutrients are available to the plant. One may need to mix Chilean nitrate (quick release) with bone meal (one to four month release time) to achieve proper fertility throughout the season.

Getting the Most Out of a Fertilizer Purchase

Not all fertilizers are created equal and vary in cost. Just because a particular fertilizer is more expensive does not mean it is a bad deal. Remember that plants need a certain amount of nutrients to achieve their maximum growth. From an economic standpoint, figure out how much each nutrient costs. For example, a bag of blood meal (12-2-1) can be six times the cost of a bag of worm castings (2-1.5-1.5) and still give the gardener the same amount of nitrogen.

If this document didn’t answer your questions, please contact HGIC at or 1-888-656-9988.

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