Soil is more than particles of broken-down rock. Soil is a complex structure that is a home to plant roots and microorganisms. It is a laboratory of chemical and biological processes and the environment that influences plant health and growth.
Microorganisms - bacteria, fungi and protozoa - in soil play a huge part in the processes that determine plant growth. Not only do they secrete acids which make minerals available as plant food but they are also responsible for nitrogen fixation, breakdown of organic matter and nutrient cycling.
In many sustainable agricultural systems, the focus is on feeding the soil microbes, rather than feeding the plant.
What Are Microorganisms?
Microorganisms, as the first form of life on earth, made life possible through the production of oxygen and are essential for many other processes.
Microorganisms are divided into three groups: parasites, saprophytes, and symbiotic microorganisms. Parasites feed on living plants and animals and cause disease and decay.
Saprophytes decompose dead plant material to release nutrients and form humus. Symbiotic microorganisms live in mutual benefit with the plants. An example is the bacteria found in the nodes on legumes such as clovers. These nitrogen-fixing bacteria bind nitrogen from the atmosphere in a form that the plant can absorb.
How Do Microorganisms Work?
Soil microbes work in a complex food chain. A food chain is a sequence of processes where the result of one process is used as a fuel source for the next process. In short, microorganisms are dependent on each other to produce a source of food.
It starts with raw organic matter such as leaves called ‘detritus’ and the primary consumers that feed on it called ‘detrivores’. Detrivores will include woodlice, earthworms and beetles, assisted by fungi and bacteria. Secondary consumers are nematodes, snails, slug, aphids and flies. Algae live in the top layers of the soil where it can access sunlight. Algae are important for their ability to fix atmospheric nitrogen in the soil.
Soil fungi grow masses of long threadlike structures called mycelium. The mycelium absorbs nutrients, protects roots from pests and pathogens and colonises plant roots to help the plant obtain nutrients such as phosphorus from the soil. Fungi are important decomposers of carbon-rich woody residues such as wood chips. Aspergillus and Penicillium are two examples of fermenting fungi. They produce antimicrobial substances and synthesise amino acids, which form the building blocks of proteins.
Yeasts in the soil produce hormones and enzymes needed for cell division and root development. Yeasts’ secretions form the substrate for actinomycetes and lactic acid bacteria.
Actinomycetes are filament-forming bacteria, responsible for decomposing tough woody materials and complex molecules in polluted soils and also help with the formation of humus. They are responsible for the distinctive earthy smell of ploughed soil.
Bacteria in soil often form the greatest mass of life in soil and are responsible for making sulphur and nitrogen available. Lactic acid bacteria produce organic acids that suppress pathogens such as Fusarium and nematodes.
How to Increase Soil Microbes?
Studies have shown higher soil microbe populations in organic rather than conventional farming systems. Soil microbes are, therefore, influenced by farming practices such as no-tillage, the planting of cover crops and crop rotation.
Increase soil organic matter by adding compost, manure, leaves, fruit and vegetable waste or add a mixture of microorganisms to the soil. ‘Effective Microorganisms’, also called EM was developed in Japan in the 1970s as inoculants to increase the microbial diversity of the soil ecosystem and can improve the utilisation of organic matter in the soil. It is a mix of beneficial soil microbes to improve soil quality, growth and yield of crops and are sprayed on the soil, plants and even manure and in areas where livestock are kept.
By Marinda Louw