Prevent nutrient losses to keep soils in good shape
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.