The Phosphate and Potash in Fibrophos are in both immediately available and sustained forms, providing nutrient throughout the growing season. Extensive trials have shown that Fibrophos is a very effective source of phosphate across a wide range of agricultural soil types throughout the UK.

The phosphate in Fibrophos is more than 80% soluble in 2% citric acid and nearly half is soluble in neutral ammonium citrate providing a long lasting as well as immediate supply to the growing crop. It is 90 to 100% as available as that in triple superphosphate within the first growing season after application.

At high soil pH the phosphate in Fibrophos is as effective as basic slag and superior to rock phosphate.

Trials showed that application of Fibrophos to both acid and alkaline grassland or arable crops significantly increased available soil phosphate within one month and this increase was maintained for at least 12 months. Citric soluble phosphate helps build up soil fertility over time.

Phosphate solubility tests – the way phosphate in fertilisers are declared

There has in the past been confusion about phosphate declarations and in particular concerning the relationship and understanding of the definitions of ‘availability’ and ‘water-solubility’ and the comparisons between different types of solubility.

The confusion has been caused by the different laboratory tests used for testing different fertiliser products. The test for water-solubility involves extracting 5g of fertiliser in 500 ml of water under controlled conditions for 30 minutes. The test was originally developed more than a century ago to identify superphosphates that had been adulterated with rock phosphate so was for quality control rather than for agronomic use.

After application, fertilisers are exposed to conditions unlike those in the laboratory test so water-solubility is not necessarily a good guide to availability.

Materials like basic slag for example have zero watersolubility but are effective sources of phosphate. The small water-insoluble component of superphosphates has been shown to be largely available to plants. The general rule is that water-soluble phosphate is available but water-insoluble phosphate may, or may not, be available.

Other tests were needed to assess the value of inorganic compound fertilisers and of materials containing phosphate with zero water-solubility. Neutral ammonium citrate is used for compound

fertilisers. In the UK, citric acid is used as the extractant for basic slag and Fibrophos and formic acid for rock phosphates. Results of different solubility tests are not comparable. A rock phosphate with 80% solubility in formic acid cannot be said, on that basis alone, to be equally as effective a phosphate source as basic slag with 80% solubility in citric acid. Generally, within a type of material, agronomic effectiveness is related to the solubility measured in the standard test for that material. However, solubility tests do not reveal differences in effectiveness between materials under different agronomic conditions such as soil pH. It’s like the pudding – the real test is in the eating and only the effect on plant growth is a real guide to agronomic performance of many materials.

Only in 'Very Low' P indices (<9ppm) where a newly established crop requires a very quick, ‘rushed’ release of P (e.g. for root development) might using a small or targeted application of water soluble P be justified over Fibrophos. Although, remember up to 80-90% of watersoluble P is lost to lock up or leached away, on certain soils, in such situations it more about timing the Fibrophos application in advance and/or building indices (e.g. consider applying Fibrophos a month before planting or if not practicable apply as maintenance)

Note: The comments and statements made are supported by independent trial results.

Further information available from www.iaea.org and enter phosphate in the search box, then select “Use of Phosphate Rocks for Sustainable Agriculture”. The book was produced jointly by IAEA and FAO. Extract from ‘fertiliser Matters/Ian Richards, April 07

In practice phosphate chemistry is quite complex and it is generally accepted that phosphate is held in ‘Labile Pools’ in the soil. Typically, soils can only hold around 1% of their total P in a water-soluble form in the soil solution. Excess water-soluble P is quickly adsorbed into the labile and then the non-labile pools rendering it less available, whereas P from Fibrophos is held in the more readily available part of the labile pool from where it is transferred back into soil solution as and when required.

This means that any P applied, is providing the optimum amount and helping fuel the growth processes throughout the whole growing season, as well as, adding to the long term reserves and building soil indices, whilst still in a readily available form. This enables annual crop off-take to be balanced with crop in-take or rotational applications to be applied without the possibility of excess P being locked up in the soil non-labile pool or lost to water. Making Fibrophos an economical and sustainable way of applying immediately and long-term available P, whilst not damaging the environment.