Soil Management Practices

How do agricultural practices affect soil animals?

Agriculture generally decreases the number of individuals in the soil and decreases the diversity of soil animals (number of species) compared to native vegetation. Soil animals require certain soil conditions to grow and survive. Agricultural practices alter soil conditions, making them harsher than those of native vegetation:

• soil moisture is decreased and temperature is increased
• wetting/drying cycles are faster
• there is less organic matter in the soil and often only from one or two plants (not a range)
• soil may be disturbed by erosion, vehicle traffic and tillage

The harsher environments mean that some of the original species are not able to survive. This decreases the species diversity and may cause different organisms to dominate.

Intensive tillage practices decrease the diversity of soil animals, especially the larger soil animals, and can interrupt their life cycles. Crop type also has an effect on soil animals, with the numbers in soil decreasing in order of continuous pasture, wheat/lupin rotation and continuous wheat.

How does soil management affect the fungal and bacterial populations in soil?

Fungi and bacteria differ in their responses to changes in agricultural management practices. Fungi are usually more sensitive to these changes. The fungal-to-bacterial ratio is therefore an indicator of environmental changes in the soil. When plant residues are applied as mulch, for example, fungi prosper because their hyphae are able to grow into the litter layer. Tilling, however, destroys large amounts of the fungal hyphae. Incorporation of plant residues into the soil also favours the bacterial population because the contact surface between the substrate and bacteria is increased. This response further depends on the soil type.

Nevertheless, the dominance of either fungi or bacteria also depends on the quality of the plant residue. Substrate structure, C:N ratio and cellulose content are important characteristics of its quality. Fungi are the predominant cellulose decomposers, even though one group of bacteria, the Actinomyces also contribute significantly to its decomposition. Cellulose has a high carbon content and a corresponding high C:N ratio, making it the ideal food source for fungi. Bacteria, which have a smaller C:N ratio than fungi, need food rich in nitrogen (e.g. green manure, legume residues). A fertiliser rich in nitrogen therefore favours the bacterial community in a soil whereas a substrate with a relatively wide C:N ratio enables growth of the fungal population.

The activity of the soil microorganisms also shows strong seasonal variation. Activity increases markedly with increasing temperature and soil moisture. Usually fungi depend on a sufficient amount of water in the soil and are expected to be less active under dry conditions. In many cases a low pH is associated with fungal dominance whereas a higher pH might be related to bacterial dominance.