Agricultural Development and Advisory Service (ADAS) has recently compiled a series of guidelines, advising farmers on how they can make best use of organic manures and slurries.
Accurate information on the nutrient content is essential for making the best use of organic materials.
Average figures are available, however, the nutrient content of organic materials will vary depending, on a number of factors.
These include: livestock feed composition; variable use of bedding; storage period, and specifically where slurries are concerned, dilution by rainwater.
If possible, farmers should get a laboratory analysis of organic material. A laboratory analysis should include dry matter total nitrogen (N), phosphorus (P), potassium (K), sulphur (S), magnesium (Mg) and ammonium-N.
For poultry manures, the analysis should also include uric acid-N.
Minimising N losses
Wherever possible, applications of organic materials should be managed to minimise N losses to the environment.
The risk of N loss is greatest, following the application of high readily available N materials – such as livestock slurries, poultry manures and digestate.
Farmers should act to minimise the risk of nitrate leaching losses by applying high readily available N materials in the spring/summer rather than the autumn/winter period.
If applying in the autumn, spread to crops with an autumn N requirement, such as oilseed rape and grass.
Ammonia volatilisation losses can be minimised by applying slurries or digestate with a band spreading or shallow injection equipment.
Application rates
To calculate the application rate, farmers and contractors need to know the capacity of the spreader and number of ‘loads’ spread over the field area.
Alternately for slurries, it’s a case of using a pump with a known output.
For solid materials, the best way to assess the capacity of the spreader is to weigh it empty and then full.
All organic materials should be spread accurately and evenly.
Application rates and evenness of spread should be checked at least annually.
However, data from the UK indicates that only 35% of farms that spread manures using their own equipment adequately calibrate spreaders.
In addition, half of farms that spread manures never calibrate their manure spreaders.
Phosphate and potash
In the majority of cases, the value of available phosphate and potash within organic manures is worth more than the N value.
A typical 50 m3/ha spring application of cattle slurry will supply 46 kg/ha crop available N, 60 kg/ha P2O5 and 125 kg/ha K2O.
In order to make the most of the phosphate and potash in manures, farmers are encouraged to target applications to fields where the soil is low in phosphate and potash, and as a replacement for inorganic phosphate and potash fertiliser.