Getting the most from muck and slurry
AS fertiliser prices touch four-figures per tonne it has never been more important to get the most from organic farmyard manure (FYM) and slurries.
To maximise soil health and nutrient availability to growing plants it is vital to understand the impact of applying muck, or slurry onto ground. This, then, allows for accurate application of expensive inorganic fertiliser for optimal impact.
The nitrogen (N), which is critical for vigorous plant growth, is available in farmyard manure and is measured in different forms. Firstly, there is Readily Available Nitrogen (RAN) which when measured in a laboratory shows amount of N potentially available to plants as at the point of application.
Then, there is Organic-N which shows the amount of nitrogen contained in organic forms which must be broken down before released which can take anything from a couple of months to a few years to complete. Crop available nitrogen is calculated as it can show the amount of nitrogen available to crops planted in years to come.
Slurries and poultry manures are typically high in RAN, with 35-70% of total nitrogen available to plants straight away, whilst livestock muck is much lower at 10-25%. However, plants don’t always get access to all the nitrogen as losses can occur through ammonia volatilisation, nitrate leaching and de-nitrification.
This can be combated through timing applications to peak growing season for plants from late winter into summer and avoiding spreading on frosty ground. Application in autumn and early winter is often followed by significant rainfall which can wash large proportion of the nitrates out of the soil before the plant get growing and can utilise it.
According to SAC, around 40% of the RAN content of livestock manures is commonly lost via ammonia volatilisation. Ploughing manures and slurries into the ground also helps to reduce nitrogen loss, ideally within six hours of application for slurries and 24 hours for solid manures. Further the use of trailing shoes and trailing hoses can also reduce ammonia emission by 3070%, compared to simple broadcast applications.
Knowing your phosphate levels
Livestock manures are valuable sources of other important plant nutrients, with many released into the soil over several years.
Around half of phosphate (P) in pig and cattle slurries will be used by the growing crop with the remainder available the following year. Only 40% of solid animal manures will become available to the crop in the year of application with 60% available longer term.
However, there are other factors at when calculating plant uptake as opposed to just nutrient availability. For instance, the uptake of phosphate is low in acid soils especially with pH values below 5.5 with lower temperatures further cutting uptake.
Soil reserves of phosphate also play an important part and will decline over the years if only maintenance applications of the nutrient is applied. It is a good idea to take a soil specific approach to P management which considers the relationship between differing soils capacity to regulate P availability for plant uptake.
This can be done through studying the soil phosphorus sorption (PSC) capacity indices from SAC, the farm advisory service, or The James Hutton Institute. This will help work out the nutrient availability to the current and future crops.
In some fields, particularly on intensive livestock farms, it may be recommended to get conditions right for spreading muck and slurry and in some cases to limit manure applications to avoid excessive enrichment of soil P levels.
Phosphorus is important with regards to water quality, because small increases in concentration can cause eutrophication (nutrient enrichment) of fresh waters. The effects of eutrophication include algal blooms, fish death, excessive weed growth, poor water clarity and loss of species diversity.
Potash (K) is also present in farmyard manure and slurry and will become quickly available for crop uptake, estimated around 90% available to the crop in the year of application. The amount of sulphur (SO3) available in an autumn application is 5-10% from livestock manures.
Whilst spring applications have greater SO3 availability around 15% in FYM and 35% in slurry. Magnesium (MgO) behaves in soil more like K than P but moves less freely in soil solution than K, being reduced in colder soil temperatures. Manures can supply useful quantities of 20 to 30 kg/ha MgO which can replace the Mg taken off by the crop.
Practical aspects of using livestock manures
Many farmers will use both organic and in-organic fertilisers when optimising plant growth. This requires calculating the nutrients within the slurry or muck to understand the type and volume of bagged fertiliser to apply.
It is also important to consider the crop which is being gown. Low response crops for phosphate or potash such as cereals and grassland should be treated differently than high response crops such as potatoes or vegetables. Where soil status is at the target level or above, the total phosphate and potash content of the organic fertiliser should be used in planning with the balance applied as bagged P and K.
At high soil P status, take care to ensure that total phosphate inputs do not exceed the amounts removed in crops during the rotation. This will avoid the soil P status reaching an unnecessarily high level.
It is important to manage manure phosphate and potash supply over the crop rotation. Sulphur and magnesium inputs from manures should largely be regarded as contributing to the maintenance of soil reserves.
The official guidance states that surface application rates for solid manures should never exceed 50 t/ha, and liquids including slurries should never exceed 50 m3/ha.
Poultry manures, which are stronger, should not be spread at rates exceeding 15 t/ha. Repeat applications should not be made for a period of at least three weeks which is necessary to allow the crop to utilise the available nutrients.
■ Information supporting this article came from SAC Consulting and farmers can find out more from the Farm Advisory Service.