Suitable Phosphate fertilizers for organic farming system


Livestock Farmer
I was wondering if anyone can help (sorry my first post too so no idea what I'm doing).

I would like some potash fertilizer suggestions for a Organic grassland system - Silage fields. P index is low so will need additional K onto of what's available through FYM.

Any suggestions would be appreciated.

New Puritan

Arable Farmer
East Sussex
@CKatz - have you had the land tested?

With no particular endorsement, you could try any of these companies:

All of the above do some form of potash fertiliser suitable for organic systems. I'm sure there are other suppliers out there too, but the above might give you a start as to what's out there and prices etc. Mostly it'll just be rock potash but some put other stuff in too. Haulage in terms of distance as well as the number of bags you want will make a difference in the overall price.


Calcined means roasted or baked which is claimed to make the phosphorus more available. It is not citric acid soluble as such but in the UK at least is citrate soluble as defined by statute, and is assumed to be all available to plants:

Calcined phosphateProduct obtained by heat treatment of ground rock phosphate with alkaline compounds and silicic acid, with alkaline calcium phosphate and calcium silicate as essential ingredients, and containing not less than 25% phosphorus pentoxide (P2O5) soluble in alkaline ammonium citrate (Petermann). Not less than 75% of the material should be able to pass through a sieve with a mesh of 0.160 mm and not less than 96% through a sieve with a mesh of 0.630 mm

Only powerful mineral acids will liberate 'hard phosphorus' from rock phosphate in a short time in the lab, but formic acid dissolves 'soft phosphorus' which is more available than hard phosphorus, though not as available as citrate soluble, so comparison of the two gives a measure of real life availability.

Soft ground rock phosphateProduct obtained by grinding soft mineral phosphates with tricalcium phosphate and calcium carbonate as essential ingredients and containing not less than 25% total phosphorus pentoxide (P2O5) (soluble in mineral acids), at least 55% of the declared total phosphorus pentoxide being soluble in 2% formic acid. Not less than 90% of the material should be able to pass through a seive with a mesh of 0.063 mm and not less than 99% through a sieve with a mesh of 0.125 mm.

Citric acid extraction is officially used only in basic slag analysis I believe.

Be aware that P can mean elemental phosphorus or used slackly can mean phosphates - it is the elemental phosphorus content that is important. Laws appear somewhat confusing on this point and you should check what they actually mean. Yara do a handy converter:

This fella in Scotland has some some sound words on organic P and K:

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
ManureApplication rateP (kg/ha)K (kg/ha)
Fresh cattle FYM20 t/ha(8 t/acre)64160
Cattle slurry22 m³/ha(~2,000 gallons/acre)2670
Fresh pig FYM20 t/ha(8 t/acre)120160
  • 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 typeP content (kg/t)K content (kg/t)Notes
Cattle FYM3.28.0Nutrient 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 slurry1.23.2
Sheep FYM3.28.0
Pig FYM6.08.0
Green waste composts3.05.5
Sewage sludge (thermally hydrolysed)20.00.5All sewage sludge is prohibited.
Table 2b: Mainly phosphate
Fertiliser typeP content (kg/t)K content (kg/t)Notes
Triple super phosphate460-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.
Basic slag160-180-Restricted. Similar neutralising value to lime, which limits application rate.
Bone meal / meat and bone meals50-180-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 / solutionsvariablevariable
Table 2c: Mainly potash
Fertiliser typeP content (kg/t)K content (kg/t)Notes
Muriate of potash (KCI)-600Prohibited.
Sulphate of potash-500Permission for use can only be granted where exchangeable K levels are below Moderate status and clay content is >20%. Water soluble.
Sylvinite-210Permission for use can only be granted following soil analysis and supported by a written FACTS qualified recommendation.
MSL-K (rock potassium)-80Restricted. Permitted provided it has a low solubility in water and low chlorine content.
Rock potash (eg Adularian shale)-90-110
Guano20100-120Not permitted.
Kali Vinasse-400Permitted without restriction.
Wood ashvariablevariablePermitted. 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


Thanks, yes I read the SAC report. To a poor yokel - am I therefore doing any good by buying in a product like GAFSA or is the calcined product worth the (presumably) extra cost? A pity I cant use sewage sludge which I can on my own place

The calcined product should offer relatively readily available phosphorus and would be advisable if indices are below recommendations, but the Gafsa rock phosphate will take a considerable time to break down especially if the soil is not acid. It might be more appropriate where you want to remedy phosphorus off take in future crops in advance. The trick is to apply a thundering dose of triple before converting to organic but I suppose that ship has sailed.

Update on the Sustainable Farming Incentive pilot

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