Beneficial and Harmful Effects of Soil Micro-Organisms

After reading this article you will learn about the beneficial and harmful effects of soil micro-organisms.

Beneficial Effects of Soil Micro-Organisms:

Plant and animal residues are broken down by the soil fauna and flora which prevent unwanted accumulation of fresh organic material. Secondary complex organic forms of nutrients are converted to the simple inorganic forms.

The stability of soil aggregates are increased both by the slimy intermediate products of decay and by the humus. When sulphur is added to alkaline soils, it gets oxidized by the activity of sulphur oxidising bacteria to sulphuric acid that neutralizes the alkalinity. Soil alkalinity is quickly neutralized if sulphur is added along with bulky organic manures.

Proteins are decomposed to ammonium compounds by the soil micro­organisms, which further oxidize the ammonium compounds to nitrites and nitrates. In well drained soils, divalent ferrous ions are oxidized to trivalent ferric ions and divalent manganous ion to trivalent and tetravalent manganic ions. These higher valent forms are insoluble.

Hence the greater part of iron and manganese are kept in the insoluble non-toxic form by the soil micro-organisms. Molecular nitrogen is assimilated by the symbiotic and non-symbiotic bacteria and also by certain Blue Green Algae.

Insoluble phosphorus compounds are dissolved in soil by organic acids produced by certain genera of soil micro­organisms to release the phosphorus contained in them in the soluble form for crop growth. Hence seeds of crops are inoculated with cultures or preparations of Specific micro-organisms to increase the amount of water soluble nitrogen and phosphorus in soil for crop growth.

Harmful Effects of Soil Organisms to Higher Plants:

Some soil fauna such as snails and slugs damage and destroy crops. Ants transfer aphids to certain crops. Some crop roots are infested with nematodes sometimes seriously enough to affect the growth of these crops adversely.

Bacteria, fungi and actinomycetes cause plant diseases. However, fungi are responsible for most of the common plant diseases. For example, species of Pythium, Fusarium and Rhizoctonia are responsible for the “damping off” of seedlings.

Injurious organisms live in the soil for variable periods, soil. Some disappear within a few years if their host plants are not grown, but some can survive on other hosts. Once a soil is infested, it is likely to remain so for a long time. Organisms may be carried from infested fields to other fields on implements, plants or debris of any kind.

Soil micro-organisms may compete with crops for available nutrients, especially when their supply is limited. Micro-organisms assimilate appreciable amounts of nitrogen, phosphorus, potassium and calcium and growth of crops would suffer due to their non-availability. Competition for trace elements may be even more serious. Soil micro-organisms take their quota of soil nutrients first and crops survive on the remaining soluble nutrients.

If the drainage of the soil is somewhat restricted, the aerobic micro­organisms may deplete the limited oxygen supply of the soil. This may adversely affect growth in two ways: The minimum amounts of oxygen necessary for the respiration of the root would not be available.

Crops need respiratory energy for absorbing nutrients from the soil. Nitrates and sulphates would be reduced to nitrous and nitric oxides and nitrogen gas, and hydrogen sulphide respectively and lost to the atmosphere. The reduced forms of iron and manganese are excessively soluble and become toxic for crop growth. Thus micro-organisms can induce the deficiencies of some nutrients as well as toxicities of some others.