Soil Biology

Hello :) In this post we'll take a look at the factors which contribute to a healthy soil biota and how this biota keeps soil healthy. We'll also discuss the relationships between the major species in the food web in soils and the function of each species in maintaining soil health.

Soil is alive as it contains billions of microbes, as well as microscopic animals and larger animals such as termites and earthworms. Without this life, normal soil processes will fail and plant growth cannot be sustained. There are three main drivers which ensure that this life continues. This includes:

1. Organic Matter From Plant Residues: this is the fuel for the biota (the animal life present in the soil). The more fuel there is, the more biological activity occurs.
2. Cultivation: In the short term this results in much microbial activity. However, this results in a reduction in organic matter and this ultimately lowers biological activity. 
3. Soil pH: Microbial activity slows down as soils become more acidic. Animals such as earthworms also prefer less acidic environments. The optimal pH is above 5.5.

The Food Web

There are four categories of life which are present in soils, these are microflora, microfauna, mesofauna and macrofauna. 

Microflora (μm): 

This includes bacteria, fungi and mycorrhizae (fungi which form a symbiotic relationship with the roots of a plant).

Fungi: 

These are plant-like cells which grow in thread-like structures (called hyphae) which makes up a mass called a mycellium. There are three groups of fungi:

1. Decomposers (Saprophytic fungi): they convert dead organic matter into fungal biomass, carbon dioxide and organic acids and are capable of degrading cellulose, proteins and lignin. They convert these substances into a material that is more accessible to other organisms.
2. Mutualists: these have a symbiotic relationship with plants as they colonise plant roots and help the plant to obtain important nutrients. The mass that they form also hides the plant's roots from pests and pathogens. The best known fungal mutualist is Mycorrhizal fungi.
3. Pathogens: These fungi penetrate plants and decompose their living tissue and this weakens or kills the plant.

Bacteria:

These small organisms exhibit a rapid response to changing environmental conditions and require moisture, warmth and a carbon substrate. Different species of bacteria have different roles, such as:

• Decomposers: these break down organic matter, especially in the early stages of decomposition when moisture levels are high.
• Sulphur Oxidisers: these convert sulfides (which can't be used by plants) into sulfates (which are used by plants).
• Aerobes (need oxygen) and Anerobes (don't need oxygen). These bacteria may produce harmful toxins when the soil is saturated with water.
• Actinobacteria: these slowly break down humates (organic residues of decaying organic matter) in soils.
•  Nitrogen fixers: eg. rhizobium. These are able to extract nitrogen from the air and convert it to plant-usable nitrogen.
• Disease Suppressors: a variety of bacteria have been used commercially to suppress diseases.

Microfauna (μm): 

This includes bacterial and fungal feeding protozoa as well as bacterial and fungal feeding nematodes. Nematodes are small non-segmented worms which are between 50μm and 1mm long. They play have three important functions in soils:

1. Nutrient cycling: for example ammonia stored in the bodies of bacteria and fungi. 
2. Dispersal of microbes: the bacteria and fungi move around the soil by 'hitching a ride' on the nematodes. 
3. Disease and pest control: beneficial nematodes are able to kill several pests such as borers, grubs, thrips and beetles.

A Thrip
source

Furthermore, there are three functional groups of soil nematodes, this includes:

• Saprophytic: these are decomposers as they break down organic matter. They are the most abundant type of nematode in soils and their presence improves the structure of the soil.
• Predacious: these feed on other nematodes as well as bacteria, fungi and protozoa.
• Parasitic: These are problematic as they feed on plant roots and slow down their plant growth.

Protozoa are single celled organisms which are very common in soils and their main food source is bacteria and fungi. They play an important role in regulating the populations of soil microbes and their activity may release nutrients which are available for use by plants. They may also prey on some pathogenic bacteria and fungi which is beneficial for agriculture.  

Mesofauna (mm): 

This includes microarthropods (Collembola and mites). Collembola are small organisms which are generally only a few millimetres long. They are also known as springtales. These organisms consume organic materials that are partially decomposed and thus feed on bacteria and fungi as well as speed up the decomposition process. 

Mites are also very abundant in soils and there are two categories of these creatures:

• Mesostigmata: these are predators and can be successfully used as biological control agents.
• Oribatida: which feed on decomposing material, bacteria and fungi. When these feed, they shred the organic material into smaller pieces and this increases the surface area available to bacteria and fungi during decomposition.

Macrofauna (cm): 

This includes enchytraeids, macroarthropods and earthworms. Earthworms improve the soil by improving:

• nutrient availability: they feed on plant debris and make this material more available to plants
• drainage: burrowing by earthworms loosens and aerates the soil. This also dramatically improves water infiltration.
• soil structure: earthworm casts cement the soil particles together and this forms water-stable aggregates. 

An Earthworm

 

That's it for this post, see you next time :)