Do Microorganisms in Soil Affect Plant Growth?

Defining Microorganisms

• Microorganisms are those organisms that can only be seen through a microscope. Bacteria and fungi are two kinds of microbes typically found in soil. Bacteria are single-celled organisms that may be as small as 4/100,000 of an inch wide. According to the Natural Resources Conservation Service (NRCS), a teaspoon of healthy soil contains anywhere from 100 million to 1 billion bacteria. While bacteria exist as single cells, fungi cells merge into long threads call hyphae. These threads range in size from a few cells to many yards long, according to the NRCS.

Beneficial Actions of Microbes

• Bacteria and fungi may be beneficial or detrimental to plant growth. Those microbes that benefit plant growth do so by increasing the nutrient availability within the soil and/or converting those nutrients into a usable form for plant use. Some microbes produce metabolites, substances that stimulate plant metabolisms, resulting in plant growth. Certain beneficial microbes also aid in controlling the growth of detrimental bacteria and fungi.

Harmful Microbes in Soil

• Just as soil is inhabited by millions of beneficial microbes, so too harmful microbes live in soil that may have a negative effect on plant growth. Harmful microbes introduce pathogens into the soil. Pathogens are microbes that carry diseases and further the growth of plant diseases. Seed germination is inhibited and plant growth stunted by the excretions of harmful bacteria or fungi, excretions that may be considered toxic to plant life.

What Microorganisms Need from Soil

• For beneficial microorganisms to maintain control over the soil, they need to feed on organic matter. Finished compost, for example, is derived from the decomposition of organic matter and so contains large populations of microorganisms. The addition of compost to soil increases the microbe count, and these microbes are more likely to benefit plant growth than prove detrimental. Cover crops such as alfalfa and buckwheat, when turned into the soil, act as green manure and further microbial activity, resulting in higher microbe populations, which, in turn, results in healthier plant growth.