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4.1 Why Is Life in the Soil Important?

Biota sustain plant growth

Soil is alive with billions of microbes, microscopic animals and larger animals, such as termites and earthworms.  Without this teeming population the soil is dead and unable to sustain plant growth.

Changing the balance

Before the land was cleared, soil biota lived in harmony with the stable vegetative cover.  As soon as the trees and scrub were cleared, the land ploughed and pastures or crops planted, a new biological balance had to develop.

Two way relationship with farming

If the farming is exploitative with burning, ploughing, no liming and low fertilizer inputs, then the soil microbes and animals suffer and their numbers decline (see Graph 1).  When this happens, the soil processes which are essential for agriculture, such as organic matter breakdown, nitrogen fixation, nitrogen cycling and soil structure maintenance, all decline.  This is why we have to do everything possible to maintain an active living soil population. 


  • An active soil biota is essential to improve and sustain agricultural production.

  • We can ensure a healthy soil by increasing the food supply for the microbes and soil animals.

  • A healthy food supply can be achieved through increasing residue return, increasing pasture production and maintaining soil pH above 5.5.

Three major factors affect life in the soil:

  1. Input of Plant Residues.  Organic matter is the fuel which drives the soil biota.  The higher the input of plant residues, the higher the biological activity and the greater the improvement in soil fertility.

  2. Soil pH.  Soil acidity develops after land is cleared and pastures or crops grown.  This acidity comes from processes such as:

  • nitrogen fixation by legumes

  • growing of shallow rooted crop and pasture plants

  • leaching of mineral nitrogen such as nitrate. 

Soil acidity is not directly caused by the use of superphosphate, but rather by the increased plant production following the use of fertilizers.


As soils become more acidic, microbial activity slows down and earthworms disappear.  This results in organic matter breakdown, nitrogen fixation and productivity being reduced.


Research I did some years ago showed that microbial numbers and soil respiration more than doubled when a pasture soil was limed from pH4.5 to 6.5.  Liming of agricultural soils is a common practice overseas, but here in Australia, liming has not been widely adopted, much to the detriment of farm productivity.

  1. Cultivation.  Every time a soil is cultivated there is a flush in microbial activity with a loss of valuable organic matter and this leads to a breakdown in soil structure.  So wherever possible, we need to minimize cultivation.

Figure 1: 
Soil biota carry out 5 key functions which interact with production


1-1 fig 1 Soil Biota Diagram.gif (70266 bytes)
Click for larger image

Where do soil biota live?

The majority of soil organisms are found in the top 10cm of soil.  Agricultural practices which change the environment in the top 10cm of soil will impact on the type and number of soil organisms present.  


Many soil organisms are smaller than soil particles.  If top soil is being lost by erosion, so are soil organisms.


4.1 Why Is Life in the Soil Important?

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