Practical use of potash on the farm

Potash (K) is an important nutrient for crop growth. Any potash removed by a crop should be replaced, so that soils do not get run-down.  It is important to have a regular programme of soil analysis in place in order to monitor the nutrient status of your soils: if a field is low in potash then you will know to apply more to it.  In addition, if potash (and/or phosphate) levels are low, then crops will not get the full benefit of the nitrogen from a ploughed ley, and excess nitrates may run-off into a watercourse.  This means an economic loss in terms of potential crop yield, the nitrogen you have purchased, as well as a potential source of diffuse pollution.

Different soil textures have different reserves of potash, with heavy soils having more than lighter soils. In Scotland the types of clay we have do not release much potash, and so should not be counted on as a potash source.  Sandy soils do not hold onto potash very well, so an application policy of ‘little and often’ is best for these soils.

The removal of straw at harvest, and cutting of grass for silage or hay will remove a lot of potash, and livestock manure applications should be targeted at these fields if you are not reliant on purchased fertiliser (i.e. have an organic farming system).

Livestock manures are an excellent source of potash, particularly farmyard manure (FYM) from well-bedded cattle courts. Green waste compost also contains a decent amount of potash.  If your farm is near a compost producer, this source of nutrients and organic matter is worth considering, after the cost of haulage has been taken into account.  The table below gives you an idea of the amount of potash you can get from a ‘good mucking’.  Livestock manure nutrient contents can be quite variable, with different feeding and bedding at different farms, so it is worth getting an analysis done to see exactly what nutrients you have in your muck.

List: Potash (K) content of different manures (information from SRUC Technical Note TN650: optimising the application of bulky organic fertilisers)

Manure Type:

Cattle FYM

K content: 8.0 kg/t

Application rate: 20t/ha or 8t/acre

K application: 160kg K/ha or 128 units/acre

Pig FYM

K content: 8.0 kg/t

Application rate: 20t/ha or 8t/acre

K application: 160kg K/ha or 128 units/acre

Broiler litter

K content: 18 kg/t

Application rate: 10t/ha or 4t/acre

K application: 180kg K/ha or 144units/acre

Cattle slurry

K content: 3.2 kg/m³

Application rate: 40 m³ /ha or  3,500 gal/acre

K application: 128kg K/ha or 102 units/acre

Pig Slurry

K content: 2.4 kg/m³

Application rate: 40 m³ /ha or  3,500 gal/acre

K application: 96kg K/ha or 77 units/acre

Green Compost

K content: 5.5kg/t

Application rate: 50 t /ha* or  20 t/acre

K application: 275kg K/ha or 220 units/acre

Muriate of potash

K content: 600 kg /t

Application rate: 125 kg/ha or  1 cwt/acre

K application: 75kg K/ha or 60 units/acre

* within an NVZ a higher application of compost is permitted within a 24 month period

Remember that livestock manures must not be applied within 10 m of a watercourse, or within 50 m of a spring, well, or borehole, or on waterlogged or snow covered ground. Additional restrictions on manures apply within NVZs.

 

David Michie, Agricultural Consultant, SAC Consulting