Farm Ireland

Sunday 22 April 2018

Prevent nutrient losses to keep soils in good shape

Gerry Bird

The recent weather has forced an end to field operations, and the impact of the extremely low temperatures on crops remains to be seen. Crops of oats, especially Barra, will come under pressure as soil and air temperatures drop to record levels for this time of the year. Forward crops of oilseed rape were being eyed up by hungry hordes of pigeons and will be under early attack. Weed and crop growth will be minimal from now on and there is no pressure to apply herbicides.

Working on some growers' end-of-year records for nitrates compliance, the high-level costs that fertiliser inputs contribute to growing a crop are apparent. The use of organic manures, animal slurries and organic by-products offer good alternative nutrient sources, used in conjunction with chemical fertilisers.

Crop responses from organic manures can be variable due to several factors. A major component is the level of living organisms in the soil. The living organisms comprise the obvious earthworms, beetles and a myriad of insects down to fungi, nematodes and bacteria. In general, nutrients in slurries are in organic forms, not readily water soluble so less are available to a crop and so need to be broken down by soil microbes. A well-balanced population of soil microbes is essential to recycle the nutrients in organic manures and maintain good soil organic matter levels.

Soil organic matter can be divided into four broad groups: all the living organisms, decomposing plant and animal residues (roots, leaves, microbe and insect corpses), residual organic matter (after the decomposition of the above to form humus) and coal.


Organic matter levels are measured by the carbon content of the soil and levels of 2pc are equivalent to an organic matter content of 3.4pc, the threshold target set by the Department of Agriculture.

The level of organic matter in soils is influenced by several factors and represents the balance between the gains from fresh residues and the losses from decomposition. The change in soil organic matter levels -- up or down -- takes place over many years.

Factors such as rainfall, rotation, cultivations, straw incorporation, cover crops and organic manures all influence the levels of soil organic matter -- but only slowly. The influences of soil texture -- where clays bind humus and reduce decomposition, whereas sand does not bind humus and increases decomposition -- is also a factor.

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Wet soils accumulate organic matter -- marshlands and bogs being the ultimate in this. So, heavier clay soils will have higher organic matter levels relative to sandy loams. Aggressive soil cultivations, with multiple passes, introduce air into the soil, which the microbes use to breakdown organic matter. With a loss of carbon dioxide to the atmosphere, this leaves only a small percentage locked into the soil in carbon reserves.

Minimal tillage techniques offer some benefit to organic matter accumulation, however this will only be in the top 20cm -- beneficial to crop production with nutrient availability in the root zone. Straw and residue incorporation also have a role. However, it is a slow process. It would take around 340t of wheat straw per hectare to increase the carbon content by 1pc in the top 20cm. This is the equivalent of 56 years of straw production per hectare.

The application of organic manures all contribute -- but again slowly. The most beneficial are the farmyard manures, poultry manures and old mushroom compost.

The role of rotation can impact on soil organic matter levels. For example, the use of break crops such as oilseed rape, beans, beet and potatoes all add crop residues, which can be broken down. Including grass in the rotation, particularly short leys (1-3 years), is of little benefit to soil organic matter accumulation. Rotation will add slowly to the build up. There is no easy answer to building up soil organic matter, so it is vital to prevent its loss.

Gerry Bird, crop consultant and member of the ITCA. Email:

Irish Independent