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Phosphate (P) and potash (K) in organic farming


David Michie, Senior Organic farming Consultant, SAC Consulting

Fym store

Soil management

  • Soils must be managed to: prevent damage to soil structure; remediate any soil compaction; maintain a suitable pH; and have adequate soil nutrients to provide fertility for growing crops
  • P and K are essential nutrients for plant growth
  • Routine soil analysis should be regularly undertaken in order to determine P and K status, as well as Mg status and pH

Soil nutrient management – P and K

  • It is important that P and K status are built up to and/or retained at Moderate
  • It is important that the amount of P and K removed from the soil following the harvesting of a crop is returned to the soil
  • The ideal way to return P and K to the soil is through the application of farm yard manure (FYM), slurry, and green waste composts (collectively known as bulky organic manures)
  • Figures for P and K offtake by crops grown in Scotland can be found in SRUC Technical Notes TN633, TN649, and TN652

Bulky organic manures

  • Soil nutrient management on organic farms in Scotland should primarily be based on the use of livestock manures, with the aim of achieving maximum recycling of nutrients with minimum losses
  • Mineral fertilisers must be regarded as a supplement to, and not as a replacement for nutrient recycling using bulky organic manures within the farm
  • Bulky organic manures are very valuable: as well as providing a source of P and K, they also help build organic matter which improves soil structure, water holding capacity, drainage, and cation exchange capacity (CEC)
  • Bulky organic manures are also a source of nitrogen and micronutrients
  • Bulky organic manures are a high value fertiliser and soil amendment; targeted applications can increase yields, and better use of nutrients can also improve the carbon footprint of the farm

The table below give an idea as to the amount of P and K applied to soils with typical muck and slurry application rates (a ‘good mucking’). The figures are taken from SRUC’s Technical Note TN650: Optimising the application of bulky organic manures.

Table 1: P and K amounts in applications of typical manures
Manure Application rate P (kg/ha) K (kg/ha)
Fresh cattle FYM 20 t/ha (8 t/acre) 64 160
Cattle slurry 22 m³/ha (~2,000 gallons/acre) 26 70
Fresh pig FYM 20 t/ha (8 t/acre) 120 160
  • Where soil analysis shows P and K levels as Moderate, you can budget for an availability of 100%, which means you can assume crops will get all the P and K shown in Table 1
  • If soil P and/or K levels are Low, then you should only budget for a crop availability of 50% and 80% respectively

Soil nutrient management planning software

  • PLANET Scotland and MANNER-NPK are nutrient budgeting tools that take bulky organic manures into account
  • You can customise manure nutrient values with your own analysis results to get even more accurate recommendations
  • A good deal of research underpins these calculations, giving useful, realistic recommendations

Sources of P and K

There are many different sources of P and K, and Table 2 provides an overview of many of them. They can be categorised as: permitted, restricted, or prohibited for use in organic farming. Sources that are prohibited have been included so as to provide a comparison with materials used in non-organic farming systems. The figures used for the bulky organic manures are taken from SRUC’s Technical Note TN650: Optimising the application of bulky organic manures.

  • Permitted: Materials permitted for use on organic production, certification body approval may be needed
  • Restricted: Certification body approval required before use
  • Prohibited: Not permitted under any circumstances
Table 2a: Bulky organics
Fertiliser type P content (kg/t) K content (kg/t) Notes
Cattle FYM 3.2 8.0 Nutrient content can be variable – analysis recommended.

Restricted if the material (i.e. livestock manure, green waste, etc.) is from a non-organic source. Livestock manures must be from a free-range system where livestock are fed non-GM feed. Composts must be PAS100 accredited.

Some composts have a useful liming value that should be taken into account.
Cattle slurry 1.2 3.2
Sheep FYM 3.2 8.0
Pig FYM 6.0 8.0
Green waste composts 3.0 5.5
Sewage sludge (thermally hydrolysed) 20.0 0.5 All sewage sludge is prohibited.
Table 2b: Mainly phosphate
Fertiliser type P content (kg/t) K content (kg/t) Notes
Triple super phosphate 460 - Prohibited. This is a highly soluble form of phosphate.
Rock phosphate (Gafsa) 270-300 - Permitted without approval from certification body.
Calcined Al rock phosphate (Redzlaag) 320-340 - Only permitted where pH > 7.5.
Fibrophos 160 160 Prohibited.
Basic slag 160-180 - Restricted. Similar neutralising value to lime, which limits application rate.
Bone meal / meat and bone meals 50-180 - Permission may only be granted for use in protected cropping, and propagating composts. Fish-based fertilisers are higher in phosphate and lower in potash.
Fish meals / solutions variable variable
Table 2c: Mainly potash
Fertiliser type P content (kg/t) K content (kg/t) Notes
Muriate of potash (KCI) - 600 Prohibited.
Sulphate of potash - 500 Permission for use can only be granted where exchangeable K levels are below Moderate status and clay content is >20%. Water soluble.
Sylvinite - 210 Permission for use can only be granted following soil analysis and supported by a written FACTS qualified recommendation.
Kainite - 120
MSL-K (rock potassium) - 80 Restricted. Permitted provided it has a low solubility in water and low chlorine content.
Rock potash (eg Adularian shale) - 90-110
Guano 20 100-120 Not permitted.
Kali Vinasse - 400 Permitted without restriction.
Wood ash variable variable Permitted. Wood must not have been chemically treated. Ash should be added to composts and manures. The liming value may limit the application rate. Potash content will be higher than phosphate content, and an analysis should be carried out to determine their values as well as the Neutralising Value before application.

Further information

  • SRUC Technical Note TN650 provides information on the nutrient content of livestock manures in much greater detail
  • The nutrient content of livestock manures can be quite variable, so it is worth getting an analysis done to determine their actual nutrient value, and improve the accuracy of any nutrient budgeting you are carrying out
  • SRUC Technical Notes TN633, TN649, and TN652 provide much greater detail on the phosphate and potash offtakes of arable crops, fruit, vegetables, and grassland
  • Applying fertilisers that contain nitrogen can contribute to greenhouse gas emissions: there is a Farming For a Better Climate Practical Guide to Applying Nutrients that describes how to minimise these

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