Fungi, actinomycetes, and algae are microscopic and play a critical role in maintaining soil fertility, structure, drainage, and aeration.
They also break down plant and animal tissues, releasing stored nutrients and converting them into forms usable by plants.
This article is dedicated to the classification and types of soil fungi, actinomycetes, and algae and their activities in the soil.
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Characteristics of Soil Fungi in Agriculture
Fungi have well-developed organelles, including nuclei and mitochondria, and are more developed than bacteria. The most important characteristic of fungi is the possession of a filamentous body consisting of strands of hyphae.
The mycelium can be subdivided into cross-walls called septa; however, there are many non-septate fungi. They are about 5 µm in diameter, with populations ranging between 0.1–1 million propagules per gram of soil.
Almost all fungi are heterotrophic in nature and all are aerobic, thus they do not occur in as diverse environments as bacteria.
Importance of Fungi in Soil Processes
Fungi are important in the decomposition of organic residues in soil. They are especially critical in decomposing woody material, which many bacteria cannot decompose. They are significant in processes leading to humus formation.
They play key roles in the formation of stable aggregates in soil. Some soil fungi cause plant and animal diseases. Some fungi form symbiotic associations with the roots of higher plants.
Characteristics of Actinomycetes in Agricultural Soils
Structurally, actinomycetes lie between bacteria and fungi; they bear similarity to bacteria in terms of cell size and structure characteristics and are filamentous organisms like fungi.
They are the next most populous in soil after bacteria, with numbers ranging from 10^5 to 4×10^6 cells per gram of soil. The organisms prefer moist and well-aerated soil. They are sensitive to acidic conditions, with an optimum pH ranging from 6–7.5.
Importance of Actinomycetes in Soil Fertility
Actinomycetes are important in the decomposition of organic matter, especially cellulose, chitin, and phospholipids. Some actinomycetes produce antibiotics, e.g., Streptomyces sp. Some actinomycetes cause plant diseases, e.g., potato scab disease.
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Characteristics of Soil Algae in Agricultural Systems
Algae are subdivided into two groups: Green algae (True algae) and Blue-green algae (Cyanobacteria). Morphologically, true algae have a nucleus, with cell walls composed mainly of cellulose and chloroplasts distributed within various organelles.
Blue-green algae do not possess a nucleus, and their cell walls are composed of a substance called muramic acid. They have a blue pigment called phycocyanin distributed throughout the cytoplasm. Nutritionally, algae are autotrophic.
Importance of Algae in Soil Fertility and Crop Production
Some algae are capable of nitrogen fixation, which can be especially important in certain ecological conditions, e.g., rice paddies. They form symbiotic associations with fungi (lichens) and freshwater ferns (Azolla).
Lichens are important in the early stages of pedogenesis, while Azolla is significant in the fertility management of rice paddies.
Fungi are more developed than bacteria and have hyphae. Almost all fungi are heterotrophic in nature and all are aerobic, thus they do not occur in as diverse environments as bacteria.
Actinomycetes have similarities with bacteria in terms of cell size and structure characteristics and are filamentous organisms like fungi. The major divisions of algae are: (1) Green algae (True algae) and (2) Blue-green algae (Cyanobacteria).
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