Fertilizer & Nutrition

Determining the Fertilizer Value of Worm Castings

Worm castings are marketed as a suitable organic source for plant essential nutrient elements, primarily the major elements, nitrogen (N), phosphorus (P) and potassium (K), also known as the “fertilizer elements.” Worm castings are the “end-product of the breakdown of organic matter by earth worms” (Wikipedia.org). Another word for designating worm castings is vermicast which is also known as either worm humus or worm manure. The process of composting using various types of worms is known as “vermicomposting.”  In the process of vermicomposting, it is believed that the contaminate content of the composted material is reduced, while the nutrient element concentration is increased, thereby generating a product that would have value as an organic fertilizer source for essential plant nutrient elements.

The elemental content of a vermicast will reflect that of the material that was decomposed by worms, the extent of concentration being determined by the degree of decomposition. It is believed that the elements may exist in various forms in the vermicast, although there is no significant literature source that would verify this to be true. The fact that the elemental content of a vermicast would reflect the elemental composition of the composted material means that there would not be a consistent elemental content. Therefore, for the user of a vermicast material, it would be advisable to know its elemental content to determine how much vermicast would be needed to provide, either all, or a portion of a plant’s elemental requirement when used as a fertilizer source. This also assumes that there would occur sufficient decomposition to release the element(s) into the rooting medium solution in that form suitable for root absorption. Since a vermicompost is the product of decomposition, it may be resistant to further decomposition, a factor that may also be determined by the physical and chemical properties of the rooting medium.

One can find vermicast elemental content values on various internet web sites, but they are not adequately identifiable in terms of product characteristics or method of determination. One set of content data were for a vermicompost using 2 species of worms and farmyard manure as the source material, the range in elemental percent content for the 3 fertilizer elements being:

Nitrogen (N)            0.56 – 0.66%

Phosphate (P2O5)     0.75 – 1.93%

Potassium (K)         0.40 – 2.30%

Such a range in elemental content would pose a problem for the user in determining what application rate would be appropriate for its designated use.

In another set of content data for an unidentified vermicast product, the 3 fertilizer element percent contents were:

Nitrogen (N)            2.64%

Phosphate (P2O5)     3.06%

Potassium (K2O)      1.08%

Two different batches of a vermicast obtained by vermicomposting dairy manure solids from a dairy lagoon for the 3 fertilizer elements were:

Nitrogen (N)           2.92 and 3.21%

Phosphate (P)         1.10 and 1.10%

Potassium (K)         1.50%

Based on these limited data, there exists a substantial range in element content for vermicast, therefore, an elemental profile can not be established as each vermicast lot of material will have its own profile. In addition, the method of expression for the elements is not always consistent for the elements P and K as they can be given as either their element or oxide form (P2O5 and K2O).

I obtained 5 vermicast materials for determination of their total elemental contents. The range in N (1.25 to 3.70%), P (0.05b to 0.77%) and K (0.07 to 3.36%) were considerable with most of the 5 vermicast materials having less percentages of N, P and K than that reported on internet websites. Another question that needs to be determined is “at what elemental content level would a vermicast material be unsuitable as a nutrient element source.”

Other essential plant nutrient elements were determined, calcium (Ca, 076 to 3.36%), magnesium (Mg – 0.21 to 1.20%), copper (Cu – 21 to 455 ppm), manganese (Mn – 154 to 496 ppm), and zinc (Zn – 14 to 750 ppm), elements that varied widely in their concentration. For the element sulfur (S – 0.21 to 0.64%), its variation among the 5 materials in concentration was considerably less.

One formulator of soilless organic mixes uses a vermicast material as a N source, applying at that amount to give 10 lbs N per cubic yard of mix. That has been found to be sufficient to supply the initial N requirement for those plants that have a relatively low N-requirement. For the 5 vermicast materials assayed, the application rates would range between 2.5 to 10 lbs per cubic yard of soilless mix. For those plants with a high N requirement, the use of a vermicast material may not provide that quantity of N sufficient to support good plant growth.

In the preparation of the 5 vermicast materials for their determination of pH and water-extractable elements, it was obvious that there were differences in the physical characteristics, 2 being more soil-like in their consistency, while the remaining 3 had the consistency of a thick paste when wet. The equilibrium pH among the 5 vermicast materials ranged from 3.9 to 7.4, while the water-soluble element concentrations generally reflected their totals. The color of the water extracts differed from colorless to highly colored as the longer the vermicomposting process is allowed to continue, the greater degree of decomposition and with it, the greater the generation of colloidal material. What effect of degree of decomposition on nutrient element availability needs to be determined.

What is needed is a plant growth test, using a verimcast material as either the rooting medium, or as an amendment to a low-nutrient element-content rooting medium so that the growing plant would be drawing its nutrient element requirements from the applied vermicast material. To establish a standard method for assessing a “nutrient element value,” characteristics of the rooting medium and indicator plant selected need to be established. The assessment of plant response could be either based on plant growth and/or elemental content at the termination of growth. With such a determination, the “fertilizer,” or “nutrient element value” could be established to provide the user with information essential for proper use of a vermicast material. It seems at this point, the element of value is N, the concentration in the vermicast material determining the rate to be applied to a rooting medium. It could be that the other elements in the vermicast material could also become influencing factors; therefore, a complete elemental analysis would be essential to avoid either a deficiency or toxicity for occurring.

J. Benton Jones, Jr. has a PhD in Agronomy and is the author of several books including Hydropopnic Handbook: How Growing Systems Work available on Amazon.

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