Quality Compost: A Sustainable Soil Amendment
Optimum soil fertility is realized with a combination of balanced soil physical characteristics, well-tuned soil chemistry and thriving soil biological communities. A grower makes annual inputs on his farm to gain his desired goal with a mix of products that would include nutrients and fertilizers, biological inoculants, amendments to raise or lower soil pH and organic (carbon) amendments that impact soil structure and sustain microbial populations. Let’s focus on a common organic amendment – quality compost – and its use in agriculture.
Compost is the end product of the “managed microbial decomposition” of organic waste. When properly made, it is a sanitized and stable product that bears little resemblance to the material from which it was derived. It is often mislabeled as “humus,” and while mature compost contains a very small fraction of actual humus, finished compost is primarily stable organic matter. It is typically applied to fields to reduce soil compaction and increase soil porosity in clay soils and to increase water holding capacity and nutrient retention in sandy soils.
The US Composting Council (USCC) is a national organization dedicated to establishing standards for the composting industry. There are many products marketed as “compost” or “humus”; unfortunately, few meet industry standards and might be more accurately described as aged manure, wood fines or biochar. Often, these waste products are sold as “compost” while never actually going through the composting process. The waste processing stream employed by commercial composting facilities is heavily regulated by the State of California through CAL EPA. For a commercial composter to market their product as compost, it’s required that established processes are followed, temperature and turning records kept and the end product completely analyzed by approved labs using standard methodology.
Quality compost can provide some nutrients for plants, especially phosphorus and potassium. In fact, most growers and consultants tend to look first (and only) at the nutrient content of compost as a measure of quality. If organic sources of nutrients are all that the grower is looking for, raw manure would offer more. Unfortunately, there can be a big down side with raw animal waste, with food safety issues being the main one.
Most ag labs can provide an analysis of the nutrients in compost, but it requires a compost lab or an environmental lab to adequately assess finished compost for two of the most important quality factors: maturity and stability. One such lab is Control Labs in Watsonville. Their analysis is expensive yet comprehensive and issued with a report that will interpret the terms and numbers.
With compost, two terms are used to describe quality of the end product: stability and maturity.
- Stability in compost refers to resistance of the finished compost to further decomposition by microbes, and it’s measured by the rate of release of CO2-C / grams compost/ day. Compost is stable if the sample releases less than 8 mg of CO2-C/ day. As microbial decomposition of carbon components is completed, the rate of release of CO2 gas is reduced. After a month of intensive management at the composting facility, with the compost windrow regularly turned and watered, compost is usually stable. In stable compost, the bulk of the carbon compounds have been broken down, the decomposers are largely inactive and the carbon: nitrogen ratio is fixed. When applied to the soil, stable compost will not continue to significantly decompose in the field, and it will not cause immobilization of nitrogen in the soil.
- Maturity refers to the fitness of the end product and is assessed by germinating cucumber seeds in a 50/50 blend of finished compost and vermiculite. Seeds are germinated under controlled laboratory conditions, while carefully monitoring for moisture and temperature. Maturity is measured by percent of seeds germinated – greater than 80% germination indicates that the compost product is sufficiently matured, or aged and would be appropriate for most uses. The highest quality compost is achieved when the stable pile is left to age for several months. If that process is not followed, the compost could contain toxic compounds (such as ammonia gas) that could be harmful when applied to certain crops.
Amending soils with a stable organic amendment like compost has several benefits, and other than cost, all with a positive upside. Generally, western soils have an organic matter content of less than 1%, or less than 20,000 lbs SOM/ acre. Most agronomists would agree that for a fertile, sustainable, biologically active soil system to perform well requires something greater than 2-3% soil organic matter. Annual applications of five to ten tons of stable, mature compost would be one way to get there.
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Source: Compost Utilization in Horticultural Cropping Systems. 2000. Stoffella and Kahn.
- Don Wolf, CCA, PCA, Agronomist