Improvement in plant growth obtained from soil management practices is the result of increased microbial activity in the vicinity of plant roots (the rhizosphere). There is still much that is unknown about the relationship between roots and microbes because of the complexity of this symbiotic relationship.
Vesicular-arbuscular mycorrhizal (VAM) fungi are critically important for virtually all crop plants. Research has shown that a lack of VAM can result in poor plant growth.
Most healthy plants have adequate VAM spores present if humus, compost, or microbial inoculants are added back into the soil. Intensive chemical exposure, such as oil spills and hydrocarbon contamination, can reduce VAM to such low levels that plant health is jeopardized.
The mycorrhizal linkage between plants and soil fungi varies greatly. Some types of plants do not use fungi for nutrient uptake, while other plants have trouble even surviving without mycorrhizae.
Clearly, the evolutionary process over millions of years has led different plants down different paths. It is known that the zone of soil next to plant roots supports a much higher population of microbes than soil even a short distance away from the roots.
The number of microbes near the roots (rhizosphere) is 10 to 100 times greater than in soil just ¼ inch away.
Plants often exude 25% to 50% of their photosynthates to attract and feed these mycorrhizal fungi. Why?
- Microbes digest chemical compounds, helping protect plants from contamination.
- Colonization by VAM fungi can limit root-feeding nematode attacks on the root system.
- Microbes receive nutrition from plants and soil organic matter and convert these materials into humus for storage and future plant use.
- Rhizosphere fungi provide a protective coating on roots that buffers the negative effects of salts and other toxic compounds in the soil.
- The greater the diversity and number of microbes, the higher the fertility of the soil.
Maintaining an environment around plant roots that is favorable for the growth of beneficial microbes will result in increased root-zone health and, ultimately, improved plant health.