How Does Compost Affect Plant Growth?

Organic wastes -- such as food scraps, coffee grounds, grass clippings and dead leaves -- decompose into materials beneficial for the soil. Gardeners can speed this process along by building simple compost bins in which they intentionally place kitchen and yard wastes to decompose in a controlled manner. With the help of moisture, air circulation, worms, microorganisms and proper temperature control, this debris quickly transforms into the nutrient-rich plant amendment, humus.

A 5-percent increase in organic material -- such as compost -- quadruples a soil's water-holding capacity, according to Washington State University Extension. Excellent water-holding capacity reduces runoff, which in turn keeps more nutrients and water in the soil for plants to use, decreases erosion and reduces gardening chemicals ending up in rivers and streams. Compost also improves soil structure, providing aeration and the proper compaction levels. For instance, it helps sandy soil retain water and nutrients, while loosening compact soil to allow water to drain, roots to spread and air to circulate.

Compost increases nutrient content in the soil and provides conditions for the plants to optimize those nutrients. For instance, compost contains a wide variety of micro and macronutrients not often found in commercial fertilizers and releases them slowly over months or years, as opposed to synthetic versions. Compost, specifically, has high levels of organic carbon. Organic carbon contributes to chemical processes that ultimately increase a soil's fertility. Stable composts also have a pH between 6.5 and 7.5, which helps plants absorb many types of nutrients, encourages beneficial chemical reactions, and supports healthy microbes.

Many organisms and microorganisms share a symbiotic relationship with decaying organic material. Without these organisms, decay would not be possible; without organic wastes, these organisms would not survive. For instance, earthworms feed on organic matter, dig burrows that help aerate and turn the soil and then deposit nutritious waste material. Fungi and bacteria help the decomposition process by feeding on organic wastes in the compost or by converting certain molecules or chemicals into plant-available forms. Some bacteria even take nitrogen from the air and convert it into a form the plants can utilize.

Numerous studies provide evidence that compost can help suppress plant diseases, according to Virginia Tech University. While compost makes plants healthier by providing for good soil composition, nutrition and aeration -- thus increasing their resistance to disease -- it also suppresses pathogens. The organisms and microorganisms in compost compete for the nutrition some disease-causing agents require. Compost also has anti-fungal inhibitors that stave off harmful fungi and act as parasites on some pathogens.