how farming affects soil life

The impacts of any particular farming practice on soil life depends on interactions with soil type and other site specific factors. Of course, most of these practices are carried out to effectively use good quality swards to support livestock production. However, it is important to recognise the potential impact that the practices can have on soil life, if we want to maintain a healthy soil.

In general, the following spectrum of effects can be identified from research studies. Click on each practice to read more about the major effects.

TillagePesticidesVetHerbGrazingLimingGrowOrganicDrain

But beware simple conclusions!

Care needs to be taken when comparing data collected in different ways, at different times of year and on different sites. 

Changes in the populations of soil organisms due to weather or season are often not taken into account in studies comparing agricultural practices. These changes may have greater effects on soil organisms than farming practice.

It is difficult to tease out the impact of any particular farm management practice or to distinguish direct and indirect effects of land management practices from data collected in the field.

Tillage

All tillage operations, including aeration and sward lifting, cause direct damage to soil macrofauna and potentially expose them to new predators.  Just think of the birds following the plough.  Reduced numbers of tillage operations and/or increased duration of no-till periods are likely to lead to increased biomass of macrofauna.  This is why grassland soils usually have higher earthworm and insect populations than arable soils.

Increased intensity of tillage is usually linked to disruptions in the habitat space for soil organisms and a decrease in the time the soil is covered by a growing plant (whether crops or weeds).

Reduced depth and intensity of cultivations is considered to be beneficial for soil health.  Find out more  -Reducing tillage intensity

Growing plants – green cover

Plant growth is directly linked to soil organisms – it is the plant that provides energy through root growth and exudation in the soil during growth and the amount and quality of litter or residues returned to the soil.  The action of roots is important for the development and stabilisation of soil structure. Roots tend to compress soil in their vicinity changing pore size distribution in the rhizosphere; in the medium-term rooting increases soil macroporosity through the provision of continuous channels.

The presence of particular host crops is well known to be critical for the survival of certain root-associated species e.g. for rhizobia, AM fungi  and pathogenic species. Using break (non-host) crops or fallow periods can be important to break the pathogen/ host cycle e.g. for cyst nematodes but for AM fungi the inclusion of a non-host species (e.g. brassicaeous species without any weeds!) in a rotation can significantly reduce the AM colonisation of subsequent crops. 

Find out more Increasing plant diversity

Herbicides

At typical field rates of herbicide applied to soil, there is no evidence for any detectable direct effects of herbicides on soil organisms though a few studies have shown negative impacts for beetles and collembola. However, herbicides have a range of target effects on plant cover (restricting weed emergence and/or growth and stimulating crop growth) which are likely to result in a range of indirect effects on soil organisms.   Earthworms seem to benefit more from weedy conditions more than other species groups but, in general, where weeds don’t restrict crop growth or adversely affect forage quality, they should be considered as a positive benefit for soil health.

Pesticides

Pesticides are a diverse group of chemicals used to control insects and other organisms harmful to cultivated plants and animals.  Therefore target effects of pesticides on insect pathogens are the aim.  The highest proportion of negative non-target effects are seen for macrofauna - earthworms, beetles, collembola.  In grassland systems, pesticides are only commonly used for re-seeds (to control insect larvae which can be a major pest). The timing of an application in relation to the life cycle of fauna is also critical in determining the impact on target and non-target species.  However, there is little evidence of long-term harmful effects of the use of typical range of agricultural pesticides, singly or in combination, on the overall activity or population of soil organisms.

 

Grazing

Grazing consumes up to half the annual above ground plant material produced and also affects the plant rooting system.  Other impacts of the livestock in the field, particularly the returns of dung and urine to the soil surface, interact with the direct impact of defoliation within grazing management.  Supplementary feeding of livestock during the grazing period may also increase inputs of C, N, P and other nutrients to the below-ground ecosystem via excreta significantly.  These combined effects therefore mean that grazing affects the organic matter and nutrient inputs to the soil in quite a complex way.

Relationships between livestock density and soil health depend on the typical stocking density of the system and the duration of any intensive grazing period compared with the recovery time. 

  • At very low levels of stocking density (such as seen in very sparsely grazed upland grasslands) an increase in stocking density leads to an increase the activity and number of soil organisms because of increased food sources.
  • High stocking rates such as typically seen in lowland grassland can have a negative impact if not carefully managed. 
  • Overstocking has negative impacts, which probably arise due to increased compaction, poaching, disruption of the sward and an increased proportion of bare ground in overstocked swards.

Veterinary medicines

Veterinary medicines include a variety of nematicides, hormones and anti-microbials, which may impact on soil organisms as a result of deposition in grazing excreta or through application of manures.  Direct application of anti-microbials and nematicides usually used as veterinary medicines to soil has been shown to have a negative impact on soil organisms.  There is also evidence of reduced numbers and activity of dung beetles where veterinary drugs are used regularly, retarded decomposition rates of dung are likely to have an impacts on other soil organisms and reduce the positive impact that dung can have in maintaining soil health.

Liming and mineral fertilisers

Lime and mineral fertilisers are a major input into UK agriculture to provide stable optimum growing conditions, meet plant nutrient demand and maintain a balanced nutrient budget.  In general lower rates of application are used in grassland than in cropping systems; however, intensive dairy pastures may receive high rates of nitrogen fertilisers. In general, balanced fertilisation has a positive impact on soil health. 

Application of some nitrogen and sulphur fertilisers (particularly ammonium sulphate) are known to reduce soil pH.  Many soil organisms show a reduced population with increasing acidity. Where long-term acidification from fertilisation of grassland is not remedied, development of a organic mat on the soil surface will result due to the reduction in mixing and decomposition of plant litter.  Phosphorus fertilisers often contain trace heavy metal contaminants (Cd, Hg, Pb); where P fertilisers have been used regularly long-term chronic toxicity might arise. However, this is more often a problem with contaminated organic amendments.

Organic manures and other organic fertilisers

Organic amendments used in agriculture include a diverse range of materials produced on and off-farm, here defined as including microbial, plant, and animal wastes, including by-products of the food processing industry. The most common wastes used on agricultural land within this category are farmyard manure, slurry and crop residues; but there is an increasing production of green waste composts for use on agricultural land.

These materials provide inputs of organic matter thus providing a source of energy/ food for many of the soil organisms.  Therefore when applied regularly, organic fertiliser inputs generally lead to an increase in the populations of all soil organisms. Composted organic fertilisers, including farmyard manure, have been shown to stimulate soil structure development and improves structural stability in soils. Soils with regular inputs of organic materials therefore have improved structural characteristics with positive benefits for aeration (in clay soils) and water holding capacity (in sandy soils). Decomposition of organic inputs increases cycling of nutrients, hence stimulating plant growth, further stimulating soil organisms through roots, root exudates and plant residues.

Short-term effects of organic inputs are sometimes seen within the soil food web (e.g. changes in predator/prey interactions) caused by the relative availability of energy/nutrients.  Application of organic amendments that are rich in N may reduce soil-borne diseases.  Slurry can contain high concentrations of salts and soluble nutrients which can lead to indirect toxic effects to soil organisms caused by osmotic shock.  Slurry application via tankers is often a high risk farm operation for soil compaction.  Care must be taken in case other contaminants are brought into the system via organic amendments; in the past some applications of sewage sludge  reduced N fixation because of the impact of high metal contents on the N fixing bacteria.

Drainage and irrigation

The main effect of irrigation and drainage on soil organisms and consequently on soil health is indirect. By regulating the seasonal effects of rainfall patterns, irrigation and/or drainage tend to stabilise the soil moisture regime away from extremes. Irrigation in dry seasons reduces the disturbance effect of soil drying and can increase the length of time during which soil organisms are active.   Soil drainage is more common in the UK.  Drained soils tend to be better aerated and have a longer period over which soil organisms are active.

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