Native unfertilised pasture Fertilised pasture

Unfertilised pasture (left) dominated by native Australian grasses. Pasture on right is fertilised with super phosphate and sown to exotic grasses and legumes (e.g. white clover).

Fertilisers are used to increase agricultural productivity and to replace nutrients that are either removed in agricultural products (meat, wool, crops) or eroded by water and wind.

Agricultural fertilisers fall into several groups. Inorganic chemical fertilisers include phosphate, nitrogen and sulphur fertilisers while organic fertilisers include manures and compost. Lime is used mainly as a soil ameliorant to reduce acidity.

Inorganic Fertilisers

Mostly, inorganic fertilisers increase soil biological activity because they increase plant production leaving organic residues of high quality as food for soil biota. There may be some short-term adverse effects, possibly due to pH change but, in the long-term, effects of inorganic fertilisers on overall biological activity are mostly stimulatory. However, species types and diversity of soil biota may change with fertiliser use.

Microarthropods (springtails and mites) from native unfertilised pasture (left) and fertilised pastures (right). Note larger numbers collected in same volume of fertilised pasture. Note greater diversity of types of animals in the native unfertilised pasture.

Agar plates growing microbes from native unfertilised pasture (left) and fertilised pasture (right). Note larger numbers of microbes on plates from fertilised pastures in same volume of soil.

P and S Fertilisers.

Superphosphate (a source of P and S) is used widely on both pastures and on cropping soils and is not considered to be toxic to soil biota in the long term. The increased secondary production that follows from their use, along with the increased quality of organic residues that form on those farming systems, increases biological activity.

Effects of superphosphate on soil biota include:

  • Increase in earthworm numbers by between 2 to 5 times
  • Increase in springtail, mite and nematode numbers by up to 3-4 times
  • Increase of microbial abundance in soil by 2 times
  • Species diversity of some groups decline (e.g. springtails)

Effects of elemental S fertilisers on soil biota include:

  • Decrease in some types of Protozoa
  • Decrease in fungal biomass
  • Increase in abundance of sulphur -oxidising bacteria

N Fertilisers

Nitrogen content of soil can be increased with the use of legumes in pastures or green manure rotations in cropping soils and this can increase soil biota abundance. Nitrogen can also be applied as inorganic fertilisers. There are different forms of nitrogen-containing fertiliser which are commonly used and they have variable effects on soil biota. As a general rule, if soil pH declines after application of the fertiliser, a reduction occurs in abundance of some soil biota. Urea is generally considered to be less harmful than the anhydrous ammonia as the pH change is less. These deleterious effects may be only short-term. Long-term effects of nitrogen fertilisation can be stimulatory to soil biota with increased plant production providing better quality residues However, soil microbes decline if soil becomes acidic with prolonged use of anhydrous ammonia.

Effects of urea fertilisers on soil biota:

  • Increase in free-living nematode abundance
  • Species diversity of nematodes can change: root and bacterial feeders can increase with N fertiliser use while fungal feeders and omnivores decrease

Effects of anhydrous ammonia on soil biota:

  • Decrease in soil microbes in immediate vicinity of fertiliser (short term) or after prolonged use of this fertiliser
  • Decrease in earthworm abundance in immediate vicinity of fertiliser


Effects of lime on soil biota include:

  • Change in earthworm species from acid to alkaline tolerant. Overall numbers may remain the same
  • Increase in bacterial to fungal ratio. Fungi more acid tolerant than bacteria.

Organic Fertilisers

Organic amendments in the form of composts or manures, generally increase biological activity in soil through an increased supply of energy (food for soil biota) and nutrients to the detrital food web.

Manure spreading on cropping soil.

Effects of organic fertilisers on soil biota:

  • Increase in earthworm numbers
  • Increase in nematode, springtail and mite abundance
  • Increase in microbial biomass