Influencing soil life

Soil organisms can make available soil biodiversity more functional. This works through three pillars: the cropping plan, soil tillage and fertilisation.

Cropping plan
An intensive cropping plan increases the risk of soil-borne diseases and pests (including nematodes). Grubbing crops (potatoes, sugar beet) degrade the soil structure, whereas mowing crops (cereals, grasses) allow the soil to rest and recuperate. Crops provide soil life with nutrition, through the sugars they excrete and the residues they leave behind. Incorporated straw promotes soil life including earthworms, bacteria and fungi. German research shows that earthworms are able to digest 6 tonnes of straw per ha.

Maximum use of green manures ensures the supply of fresh organic matter as nutrition for soil life. The below-ground biomass of green manures is often even greater than the above-ground biomass. Cereals and grasses, in particular, make a major contribution to below-ground biomass through intensive rooting. Sowing in August or early September provides sufficient ground cover for most green manures. It is important when choosing green manures to be aware of the risk of propagating harmful nematodes. To control plant parasitic nematodes, a growth period of at least 3 months is required and sowing should take place before 1 August.

Green manure crops by soil type

Soil tillage
Riding a tractor and tilling the soil degrade soil life. The deeper the tilling operation, the greater the disturbance. Harvesting in wet conditions leads to soil compaction and a lack of oxygen for soil life. Using tyres at low pressure (<1 bar) limits axle and wheel loads and prevents compaction.
Inversion tillage (ploughing) is more damaging to most earthworms than non-inversion tillage. In particular, non-inversion tillage causes less damage to litter dwellers. Soil dwellers often still manage to survive reasonably well. Non-inversion tillage generally retains more organic matter in the top 10 cm. As a result, the density of bacteria, fungi and other soil life tends to be higher.

Fertilisation – general
Fertilisers, in particular those with a high content of effective organic matter, such as animal (solid) manure and compost, provide food for soil life. Mineral manure can also stimulate soil life by promoting crop growth and increasing crop residues.
Soil acidifies every year, mainly due to depletion by crops, leaching and possibly the acidifying effect of some mineral fertilisers and manure. Ammonium sulphate, urea and urean are acidifying, calcium ammonium nitrate not so much, whereas calcium nitrate is alkaline. In addition, the way the choice of fertiliser affects soil pH depends on the soil type: an acidifying fertiliser has no effect on pH in calcareous soils, but in non-calcareous soils it decreases pH, which can result in a shift in soil life composition and activity. It is therefore important to correct the pH, if necessary by liming the soil.
Recent research on dairy farms shows that soil life adapts to the type of manure used on a farm. This means that on a farm that always uses slurry, soil life takes longer to break down solid manure as compared to a farm that has always used solid manure.

Fertilisation – effect on salinity

Fertilisers increase soil salinity immediately after application. This applies to both mineral and animal fertilisers. The increase in salinity depends on a large number of factors, such as the amount of fertiliser applied, the fertiliser used, soil type, moisture content, soil buffering capacity and the time elapsed after application. Immediately after application, certain microbial conversion processes in the soil may be inhibited but this effect is usually temporary. The best-known example is the temporary inhibition of the microbial nitrification process when ammonium-based fertilisers are applied topically, such as row fertilisation with ammonium sulphate. Because nitrification is temporarily inhibited, nitrogen remains in the ammonium form for longer, making it less likely to leach out as nitrate. Inhibition lasts a few weeks at most.

The scientific literature shows that, in general, the use of mineral fertilisers has a positive effect on the number of earthworms and potworms by stimulating crop growth and therefore the amount of organic matter that soil life feeds on.

Fertilisation – effect on acidification or pH increase

Acidifying fertilisers can have an inhibiting effect on soil life and its activity, as the optimum pH for soil life activity is often around 6-7. Any inhibition depends on the acidifying effect of the fertiliser in question, the current pH of the soil and the buffering capacity of the soil. Ammonium-based fertilisers are generally acidifying and will bring about a significant drop in pH if used for long periods, especially on lime-poor sandy soils with limited buffering. This can, for example, lead to inhibition of the nitrification process.

The use of urea temporarily increases pH, which causes ammonia volatilisation, with the pH subsequently dropping again. The level of ammonia volatilisation depends on the type of fertiliser (granules or liquid), granule size, soil pH buffering, precipitation and temperature. On average, the N loss due to ammonia volatilisation when urea is used is about 8-15%, whereas it is at most 2% when calcium ammonium nitrate is used.

Fertilisation – specific inhibitors (nitrification and urease inhibitors)

Nitrification and urease inhibitors are examples of specific inhibitors. Nitrification inhibitors specifically inhibit the conversion of ammonium to nitrate by means of nitrifying bacteria of the genus Nitrosomonas, Nitrosolobus and/or Nitrobacter by inhibiting ammonium monooxygenase, an enzyme. The best-known nitrification inhibitors are DCD and DMPP. Because the conversion of ammonium to nitrate is inhibited for several weeks after application, nitrogen leaching can be prevented during a wet and cold period following fertiliser application.

Fertilisation – urease inhibitors

Urease inhibitors delay the conversion of urea by the enzyme urease into ammonium, which can reduce ammonia volatilisation. The length of the delay mainly depends on the product used and can be a few days to several weeks.

Sources:
Eekeren, N. van, J. Bokhorst, J. Deru en J. de Wit (2014). Regenwormen op het melkveebedrijf. Handreiking voor . herkennen, benutten en managenPublicatie nummer 2014-004 LbD. Louis Bolk Instituut. 38p.
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