How to Determine the Nutritional Value of a Compost Product

A compost product may have an N-P-K (nitrogen, phosphorus, potassium) content label, but these values may not be the actual amounts in the packaged product or bulk supply. Instead, they represent the approximate content for that type of compost product.

These values are given so that the product can be marketed as a fertilizer. In addition, the levels are set so that if the product is analyzed for verification, the minimum will be satisfied, avoiding a financial penalty for testing lower than the label indicates.

Values higher than listed on the label will not be subject to a penalty, but for the user, a higher-than-designated elemental content can lead to a nutrient element insufficiency. Therefore, the actual NPK contents should be provided to the purchaser.

An assessment of the nutrient element value of a compost product means ordering a laboratory analysis to obtain the needed values, which are organic matter content and total concentration of the three major essential nutrient elements: nitrogen (N), phosphorus (P) and potassium (K).

The other major elements—calcium (Ca), magnesium (Mg) and sulfur (S)—as well as the micronutrients might be useful information to have, but not essential for making an assessment of nutritional value for most composts.

There is one possible exception—micronutrients copper (Cu) and zinc (Zn)—because the concentration of these two micronutrients could limit a compost's use, particularly for products that have sewage sludge or an industrial by-product (primarily rubber products) as components.

The determination of the organic matter content is by a technique called loss-on-ignition. An aliquot of compost is oven-dried at 176^oF for 12 hours. After its weight is determined, the compost aliquot is placed in a muffle furnace set at 932^oF. After 24 hours, it is removed from the furnace and allowed to cool. The aliquot is weighed again and the loss in weight designated as the compost's organic matter content.

Knowing the total nitrogen concentration, the crude protein content of the compost can be determined by multiplying the nitrogen percentage by 6.25. Taking the percentage of organic matter content (determined by loss-on-ignition), subtract the crude protein content, which then gives the carbohydrate portion of the organic matter.

The ratio of crude protein versus carbohydrate will give an indication as to the microbial stability of the compost—the larger the ratio (greater carbohydrate content), the greater the stability and the less likely the compost will easily decompose.

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Another potential means of evaluating a compost product's nitrogen-supplying power is based simply on its total percentage of nitrogen concentration. In a recently conducted vermicompost study, obtained plant growth was correlated to the total nitrogen concentration of the vermicompost.

In vermicompost containing less than 1.5% nitrogen, the N-supplying power was minimal, that is, of little value as a nitrogen source. Whether this applies to other types of composts needs to be investigated.

Since generated composts are the end product of microbial decomposition, those substances in the initial source materials (animal manure, silage, yard and kitchen wastes) needed to support microbial activity would have been exhausted. For verification, one can conduct the following tests.

Take a glass jar, such as a pickle jar (1 pint), and put a thin, evenly distributed layer of compost in the bottom of the jar. Moisten the compost with water applied as a fine spray. Tightly screw the lid on the jar and let the jar stand for at least 48 hours at room temperature.

After this time, open the lid carefully and sniff the air in the jar. If there is the smell of ammonia, then there are micro-organisms still functioning in the compost. Another way to make this judgment is to put a lighted wood match into the jar.

If the flame immediately extinguishes (due to the lack of oxygen), then micro-organisms still exist in the compost. However, it has been well established that if the compost is indeed carried to the end product of microbial decomposition, the compost will be essentially sterile.

What about the other two major essential plant nutrient elements, phosphorus and potassium? If these elements are equal in concentration to nitrogen in a compost product, it should be avoided, as using it can lead to excesses and plant insufficiencies if used as a nitrogen source. Both excess and insufficiency are difficult to correct if they occur.

However, the key element is N as it will determine the stability of the compost product when added to a rooting medium, thereby controlling the release of the other two elements into the rooting medium solution for absorption by plant roots.

Much has been written about the value of composts in terms of plant growth and as a potential organic source for essential plant nutrient elements. Following the procedures outlined in this article, one can make an assessment of the ability of an applied compost product to supply a sufficient quantity of an essential plant nutrient element or elements that will ensure normal plant growth free from insufficiency.

Therefore, the investment made in the purchase and use of a compost product can be justified in terms of plant performance.