Normally we speak about compost in terms of its value as a landscape mulch or for its contribution to the physio-chemical properties when mixed into a garden soil. Some home gardeners are avid composters, collecting yard-generated vegetative materials and putting them into a pile for composting. Depending on the characteristics of these materials and the method of composting, composts are generated with widely varying physical and chemical properties.
I have a family member who composts all her kitchen wastes, adding to an existing compost pile that consists of some yard gathered tree leaves and grass cuttings. The kitchen waste consists primarily of citrus, banana and melon rinds as well as other typically removed non-eatable portions of vegetable and fruit products. Also included in this kitchen waste are egg shells.
The compost pile is confined within a 4 foot x 4 foot pallet-sided enclosure with the pile itself being about 4 feet deep. Almost daily, kitchen wastes are added to the pile into an opening in the top made by the use of a pitchfork. The pile itself is never turned nor mixed. There are times when the pile becomes anaerobic with ammonia given off. The compost pile is about 8 years old. Periodically, one side of the pile is opened by the removing of one of the pallets and the composted material removed for use mainly as landscape mulch. Periodically, additional yard waste is added to keep the pile at a 4 foot depth.
I recently took a sample from the center of the compost pile. The gathered sample was oven dried at 80oC and then passed through a Wiley mill fitted with a 40-mesh screen. The prepared sample was assayed for its total element content. The elemental contents of the compost are given in Table 1. The elemental contents are as would be expected, but for calcium (Ca) that is high since egg shells are included in the kitchen waste materials. The compost would not be considered a significant source for any of essential plant nutrient elements, except for possibly Ca, therefore, not to be considered as a potential source of the essential plant nutrient elements if added to a garden soil.
I have a colleague who is also a composter who adds kitchen waste to his compost pile, although not as a significant portion since most of the materials for composting are gathered yard wastes, such as leaves, vegetative plant and grass clippings. I asked him to send me a sample of his compost for elemental analysis. The assay results are given in Table 2. As I expected, there are no significant differences in element content between the 2 composts, except possibly for the elements, magnesium (Mg) and sulfur (S), which reflect what exists in the source material. However, the content levels are not sufficient to be a significant source for either element in terms of affecting the nutrition of a plant when added to a rooting medium.
There has been much interest in what percent of a compost is organic. The organic content was of these 2 composts was determined by loss-on-ignition (Jones, 2001). For my family member’s compost (Table 1), the organic matter content is 61%, while that my colleague’s (Table 2) only 30%. Why the difference? There are two possible reasons, a higher degree of decomposition that would reduce the organic portion as well as the source of the composted materials. If most of the composted materials are yard wastes, primarily tree leaves and plant vegetative clippings, the lower will be the organic matter content. The question is what constitutes the inorganic potion – probably most silicon – what would be considered as soil material.
Those who believe that composts can be a primary source of essential plant nutrient elements, the elemental content data given for these 2 composts would indicate that such is not the case. In addition, the relatively high inorganic content, 39 and 70%, Table 1 and 2, respectively, would also suggest that these 2 composts will do little to alter the organic matter content when added to a garden soil. It should be remembered that composts are the end product of biological decomposition, so their addition to a garden soil will not stimulate biological activity since all of the “energy” compounds have been consumed in the composting process. If you wonder what is the elemental and organic matter contents in your generated compost, I can assist in having it analyzed (firstname.lastname@example.org).
Element Total Content
– – – % – – –
– – ppm – –
*61 % organic matter content
Element Total Content
– – – % – – – – – –
– – ppm – –
*30.0% organic matter content
Jones, Jr., J. Benton. 2013. Instructions for Growing Tomatoes in the HomeGarden and Greenhouse. GroSystems, Inc., Anderson, SC (available in soft cover and e-book format for Kindle at Amazon.com)
Jones, Jr., J. Benton. 2001. Laboratory Guide for Conducting Soil Tests and Plant Analysis. CRC Press, Boca Raton, FL.