How Does Compost Help Improve Soil Conditions?

Soil Amendment

Compost confers many chemical, physical, and biological benefits to the soil. First, compost supplies nutrients essential to plant growth. Compost contains small amounts of nitrogen, which is often the limiting soil element for optimum plant growth. Compost nitrogen is in an organic form and must be mineralized (decomposed or oxidized chemically into plant-accessible forms), which occurs slowly in natural systems. Regular compost use can create organic reserves that release nutrients incrementally over many years. Compost supplies many other plant-essential elements such as phosphorus, potassium, calcium, sulfur, and micronutrients. The concentrations of these nutrients depend largely on the feedstocks used to produce the compost.

Plants obtain nutrients from soils in the form of dissolved salts, but high salt concentrations can injure sensitive plants. Salts can occur in native soils, in irrigation water (especially groundwater), and in compost. Manure-based composts tend to have the highest nutrient levels, and also the highest salt levels. Compost made from green materials tends to have lower levels of nutrients and salts. Because plants differ in their sensitivity to salinity, it is best to follow the guidelines for each plant type.

The following are the average nutrient values from more than 1,200 compost samples from the Western United States (data from Soil Control Labs, Watsonville, Calif.):

  • Macronutrients:
    • N=1.6%
    • P=0.6%
    • K=1.1%
  • Secondary Nutrients:
    • Ca=3.3%
    • Mg=0.6%
    • S=0.5% (as SO4)
  • Micronutrients:
    • Fe, Cu, Mn, Zn, B, Na

Water-Holding Capacity and Drought Conservation

Research has demonstrated that compost can allow soil to hold up to 30 percent more water, which can significantly help during periods of drought and provide more efficient water utilization overall. Compost is comprised of 50 percent organic matter, which has relatively large surface areas (in comparison to other soil particles), and strong adhesive forces, which significantly increase the amount of water it can hold. Compost improves soil structure by lowering the bulk density and increasing the permeability and porosity of the soil. Increases in porosity and surface area create more binding spots for water, leading to higher water retention rates when compared to unamended soil. This means more water stays in the root zone, for a longer period of time, where it is needed by the plants the most.

Mulching may be even more important to conserving water. A thick layer of mulch on top of soil reduces the need for watering in the following ways:

  • It reduces soil temperature fluctuations.
  • It reduces the effect of winds on soil moisture.
  • It smothers weeds, which compete for water and nutrients.
  • It protects soil organisms from the elements.

Water availability in California is constrained by agriculture, an increasing population, and drought. A large amount of energy is required to collect, pump, move, store, and treat water. For example, the State Water Project (SWP), which pumps water almost 2000 vertical feet over the Tehachapi Mountains, is the largest single user of energy in the state and accounts for about 2 to 3 percent of all electricity consumed in California. Electricity is the largest contributor to California’s greenhouse gas emissions, at approximately a third of the total.

Erosion Control

Compost can significantly reduce erosion in the immediate short term by increasing infiltration of flowing water and in the long term by supporting the establishment and growth of vegetation. Good soil structure is one of the best defenses against erosion, which can be a considerable hazard in areas where soils are left exposed due to the loss of vegetation (such as from fire). Compost diminishes erosion by run-off because it increases the pore space within the soil, thus improving water infiltration rates (permeability). Compost also introduces microorganisms that produce “cementing agents” (such as gels, gums, and other polysaccharides) that are helpful in binding soil particles into aggregates, which are less prone to detachment and transport by flowing water. When amended with compost, clay soils are less prone to compaction and are more able to retain water and nutrients. A study funded by CalRecycle, Impact of Compost Application on Soil Erosion and Water Quality, demonstrated that compost can significantly reduce soil erosion:

  • All compost treatments substantially reduced storm water runoff compared to the control by an overall average of 7.5 times (1.6 to 23.4 times).
  • Compost use greatly reduced total suspended solids exports from the plots by an overall average of 39 times compared to the control on a mass flux basis.
  • Compost treatments dramatically reduced total sediment losses, with an overall average of 57 times the controls’ flux values.

Mulch can also provide protection against erosion in much the same way as compost. However, in certain situations, mulch can be carried away by high water flow rates, so caution should be used when placing it around water bodies. Mulch use on steep slopes may require retention structures.

For more information, visit Erosion Control.

Soil Organic Matter

Compost is comprised of nearly 50 percent organic matter, and of that, approximately 25 percent is carbon. Although plants generally consume carbon from the air, compost carbon nourishes soil microorganisms. Organic matter is composed of the remains of organisms such as plants and animals and their waste products. The ratio of carbon to nitrogen (C:N) is a common index used for assessing feedstocks and the maturity of any given compost. C:N ratios in finished compost range from 12:1 to 20:1 but are ideally between 14:1 and 18:1

Soil structure—the arrangement of the solid parts of the soil and the pore space between them—is critical to how the soil functions. Organic matter causes soil to clump and form soil aggregates, which improves soil structure. With better soil structure, permeability (infiltration of water through the soil) improves, in turn improving the soil's ability to take up and hold water. When the solid parts—sand, silt and clay particles—cling together as coarse, granular aggregates, the soil has a good balance of solid parts and pore space. Soil organic matter also helps develop stable soil aggregates. Soil microorganisms that are fed with organic matter secrete a gooey protein called glomalin, an effective short-term cementing agent for large aggregates. Organic glues are produced by fungi and bacteria as they decompose plant residues. Water-resistant substances produced by microorganisms, roots, and other organic matter provide long-term aggregate stability from a few months to a few years.

Greenhouse Gas Emissions

According to the California Air Resources Board (ARB), each ton of organic feedstock turned into compost and applied to land reduces greenhouse gas emissions. The range of benefit depends on whether the feedstocks are food materials, green materials, or a mixture of both, with food materials having the highest greenhouse benefits. The factors comprising the emissions reductions are the avoidance of landfill methane emissions, water savings, and the reduction in the use of pesticides and fertilizers that have high embodied energy. ARB publishes a Compost Emissions Reduction Factor that details the GHG benefits from compost production and use. According to ARB, landfills are one of California’s largest methane sources, accounting for around 20 percent of all emissions. In addition to having a climate-forcing potential 25 times higher than CO2, methane has been identified as a short-lived climate pollutant, and reductions in methane emissions are considered critical to blunting the most immediate effects of a warming planet. Elimination of landfill methane emissions is a critical component of ARB’s climate pollutant strategy.

Healthy Soils

Not everyone agrees on the meaning of the term “healthy soils,” but most agree that ensuring soils have adequate soil organic matter or carbon content is a key component. In 2015, Governor Brown created the Healthy Soils Initiative, stating, “As the leading agricultural state in the nation, it is important for California’s soils to be sustainable and resilient to climate change. Increased carbon in soils is responsible for numerous benefits including increased water holding capacity, increased crop yields and decreased sediment erosion. In the upcoming year, the Administration will work on several new initiatives to increase carbon in soil and establish long term goals for carbon levels in all California’s agricultural soils.” In dry climates such as California’s, increased water-holding capacity can reduce the frequency of irrigation needed to support crops. In addition, compost can help a soil decrease the loss of fertilizer, pesticides, and herbicides by erosion, leaching, and runoff.

Soil, especially healthy soil, is full of life. Millions of species and billions of organisms make up a complex and diverse mix of microscopic and macroscopic life that represents the greatest concentration of biomass anywhere on the planet. Bacteria, algae, microscopic insects, earthworms, beetles, ants, mites, and fungi are among them.

The governor’s initiative is being echoed at the federal and international levels. According to the federal Natural Resources Conservation Service, “Consider bacteria, the soil microbes with the highest numbers, for example: you can fit 40 million of them on the end of one pin. In fact, there are more soil microorganisms (microbes for short) in a teaspoonful of soil than there are people on the earth. These microbes, which make up only one-half of one percent of the total soil mass, are the yeasts, algae, protozoa, bacteria, nematodes, and fungi that process soil into rich, dark, stable humus. Like other living creatures, the organisms in the soil also need food and shelter. Some feed on dead organic matter, and some eat other microbes. As a group, they cycle nutrients, build the soil and give it structure.”

Organisms and What They Do

  • Bacteria: Feed on organic matter, store and cycle nitrogen, and decompose pesticides.
  • Fungi: Up to 3,000 species of fungi are in the soil. Some feed on dead organic matter like crop residues that are more difficult to break down. Others are parasites that attack other microbes. Some fan out from the root to get more nutrients and hold more water for the plant, delivering nutrients to the plant in exchange for carbon.
  • Protozoa: Eat bacteria, fungi, and algae. When they eat bacteria, their main food source, they unlock nitrogen that’s released into the soil environment slowly. They convert organic nitrogen to inorganic nitrogen that’s available to plants.
  • Mites: Decompose and shred organic matter as an important part of the nitrogen cycle.
  • Nematodes: These microscopic worms are an important part of the nitrogen cycle. Most are non-pathogenic and don’t cause disease. They eat other organisms in the soil.
  • Earthworms: Expel partially decomposed organic matter, produce nutrient-rich casts, and make lubricated tunnels that aid soil structure and water movement in the soil.

Soil Carbon Sequestration

Soils hold more carbon than the atmosphere or plant and animal life combined. Climate experts say no strategy to reduce climate change is complete without using the vast carbon sinks available in the world’s soils. Soils degraded over the past centuries from human activities have lost a significant portion of their carbon content to the air. Organic materials used to produce compost originated from the process of plant photosynthesis, which uses sunlight to combine carbon dioxide from the air with water and nutrients from the soil to produce plant materials both above and below the ground. If the quantity of plant material is not returned to below the ground, as is the case with urban expansion, deforestation, and agriculture, carbon dioxide is released into the air as a greenhouse gas. About one-third of the surplus CO2 in the atmosphere driving climate change today has come from land management practices. On the other hand, carbon can be stored long-term in the soils, known as soil carbon sequestration, if plant materials that have already removed CO2 from the air are returned to the soil in a stable form, such as by the application of compost to land. A series of experiments done in Northern California shows that one application of compost started a chain reaction of carbon sequestration that will last decades. Therefore, one may infer that more compost and mulch applied to many types of land is the single greatest weapon in the fight to reduce the effects of climate change.