Compost Improves Soil Structure and Plant Growth

By Janet Hartin, Environmental Horticulturist - University of California Cooperative Extension

When evaluating compost, look for a dark humus-like consistency with an earthy smell. Putrid smelling compost is not completely cured, and is not ready to use. Always acquire compost from a reputable source. Photos courtesy of Janet Hartin, Environmental Horticulturist. University of California Cooperative Extension

Prehistoric farmers discovered that if they mixed manure with other organic waste, the mixture would change into a fertile soil-like material excellent for growing crops. Today's landscape contractors can benefit from employing this age-old product.

 

Compost is most often applied as a soil amendment and should be evenly and deeply mixed into the soil. The final amended soil should contain at least 40% compost. Compost is not recommended for sites where trees will be planted. This is because tree roots often fail to venture out into poorer quality native soil resulting in root-bound conditions, similar to a root-bound houseplant.

The moisture content of delivered compost should range between 40 to 55 percent. Compost that is too dry is dusty and difficult to work with, while compost that is too wet is heavy and can be hard to apply evenly. Here, compost is used as an inexpensive and effective erosion-control agent.

Using compost can assist the landscape contractor in many time - and money - saving ways. Compost improves low-quality soil by adding stable organic matter, and it contains beneficial soil microbes that may out-compete soil diseases for nutrients. Compost can also be employed as a soil amendment, turf top-dressing, mulch, erosion-control agent, and water-quality enhancer. It also promotes plant establishment, improves moisture retention, and it costs less than adding topsoil.

Compost increases soil's water and nutrient-holding capacity and improves the structure, infiltration and drainage. Compost can also significantly increase the organic material content and biological activity.

To help assure landscapers that they are buying quality compost product, the U.S. Composting Council (USCC) developed a complete nationwide compost testing system. Composts that have been tested using the approved methods can be purchased and used with confidence. Nonetheless, some characteristics of acceptable-quality compost can be evaluated with little or no special equipment.

Landscapers should only obtain compost from a reputable source that guarantees high quality, and provides high-quality product and comprehensive data on the product's origin, processing and characteristics. Due to the diverse nature of feedstock, composting processes and maturation standards, compost quality can vary significantly. Following the composting council's guidelines helps make certain that you choose the appropriate compost for your specific needs. The council's guidelines are online at: www.compostingcouncil.org.

Temperature is a key indicator to gauge how far along the decomposition has progressed. Compost that is ready for use will have sustained a temperature of 131 degrees Fahrenheit for at least three days. This generally takes at least one month to achieve and will occur most quickly when added wood is chipped to one inch or less and the pile is small (3 by 3 by 3 feet). Larger piles take longer to complete the process. Make sure to use a temperature probe that reaches deep into the compost. Leave the probe in place long enough for the reading to stabilize, and then move it to a new location. Take readings in several locations, including at various depths from the top and sides. Compost may have hotter and colder pockets depending on the moisture content and chemical composition of ingredients.

Knowing your Compost

• Carbon-to-nitrogen (C:N) ratio: Nitrogen release rates from composts are difficult to predict with accuracy, but the C:N ratio is a good starting place. At high C:N ratios (approximately 30:1 or greater) nitrogen may be temporarily immobilized by microbes during the decomposition process. Because this deprives plants of needed nitrogen, additional fertilizer is required. Products with C:N ratios below around 15:1 are likely to supply at least some soil nitrogen. However, immobilization is a temporary phenomenon, and immobilized nitrogen eventually is released in plant-available forms.
• Contaminants: Compost materials used for landscape applications should be free of measurable levels of inert contaminants, such as glass, metal and plastic. Contaminants should make up less than one percent by weight of the final product, and should not be detectible with the naked eye.
• Maturity: Maturity refers to the amount of organic substances in the compost that are toxic to plants (phytotoxic). Phytotoxic substances can adversely affect seed germination and plant growth.
• Stability: Stability describes the amount of the compost's decomposition status. Stable compost is well decomposed, consumes little oxygen, and generates little carbon dioxide or heat. Unstable compost heats up significantly if rewetted and stirred, and should not be used.
• Moisture Content: The moisture content of delivered compost should range between 40 to 55 percent.
• Nutrient Content: Composts are not normally considered fertilizers because their nutrient content is often lower than conventional fertilizer, and is highly dependent on the rate of compost applied, as well as its rate of their nutrient release rate. Because feedstock nutrient concentrations and moisture contents vary, nutrient contents cannot be guaranteed with the same precision as fertilizers. Compost derived from manures or bio-solids often have higher levels of nitrogen and other nutrients.
• Organic Matter Content: High-quality organic soil amendments usually contain at least 40 percent organic matter. Concentrations as low as 25 percent are often adequate for mulches. Soil and silica typically make up the remainder of the compost dry matter. In general, a relatively high organic-matter content is preferred for soil amendments and mulches. A moderate amount of inorganic content is desirable as foundation material for compost blankets, filter berms and similar installations.
• Particle Size Distribution: Particle size distribution (gradation) is determined by passing the compost through a set of sieves and determining respective weight fractions retained on each sieve size. Different distributions serve different purposes. For example, at least 90 percent of a compost to be used as a turf or landscape soil amendment should pass through a 5/8-inch screen. Conversely, composts with larger particles serve as excellent mulches.
• pH: The pH scale measures the soil's acidity or alkalinity. The scale ranges from zero to 14. A reading of seven is considered neutral. The most useful composts have a pH between six and eight. When compost is used as a soil amendment, it is generally desirable to have the final soil/compost mixture fall between pH 6.5-7.5.
• Phytotoxicity: Phytotoxicity describes a compound's degree of toxic effect on plant growth; Mature plants are less likely to succumb to phytotoxicity than younger ones. The threat of phytotoxicity is also reduced when compost is allowed to mature in the soil for several weeks before planting. However, it is rare for reputable commercial composts to be phytotoxic.
• Soluble salts (salinity): Many plants are intolerant of high-soluble salts. The salinity (ECe) of a soil after it is amended should be less than 2.5 dS/m. It is important to note that most plants prefer a soil ECe of less than 1.0 dS/m. If a compost's ECe value is measured higher than 1.0 dS/m by a soil test, the ultimate level can be reduced through leaching (a slow, deep application of water to move excess salts beneath the root zone).
• Trace elements/ micronutrients: The presence of trace elements in compost is directly attributable to the production material's origin. Many trace elements, such as copper, zinc, manganese, iron, boron, molybdenum and chlorine are micronutrients required by plants for normal growth. However, many trace elements (particularly heavy metals) often raise concerns when found in high concentrations. Commercial compost producers routinely test for heavy metals as part of their quality-control process, and reputable compost producers will readily provide this data.
• Weeds and disease organisms: Proper composting destroys insects and disease-causing organisms, as well as most weed seeds. Compost generated from green-waste may contain Malva, California burclover, certain palms, and other weeds that are heat-tolerant.

In the next segment, we will explore the specifics of enhancing soil quality by adding compost to a native soil.