Availability and Application of pkup
pkup is currently only available as a coarse powder and is best spread using contractors' moving belt machines. There is a comprehensive network of distributors and merchants throughout the UK who have access to suitable machines. It is ideally tipped on a hard, free draining surface, just prior to time of application. It is not easily affected by normal rainfall while standing.

Arable Land
pkup is an effective phosphate fertiliser and can be used as a substitute for conventional fertilisers in nearly all arable situations.

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 fertilisers. The test for water-solubility involves extracting 5g of fertiliser in 500ml 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 water solubility 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 and formic acid are used as extractants for some phosphates. Results of different solubility tests are not comparable. A rock phosphate with 40% solubility in formic acid cannot be said, on that basis alone, to be equally as effective a phosphate source as basic slag with 40% 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 pkup. Although, remember up to 80-90% of water-soluble P is lost to lock up or leached away on certain soils. In such situations it more about timing the pkup application in advance and/or building indices (e.g. consider applying pkup a month before planting or if not practicable apply as maintenance) 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, 2007 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 absorbed into the labile and then the non-labile pools rendering it less available, whereas P from pkup 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 pkup an economical and sustainable way of applying immediately and long-term available P, whilst not damaging the environment.

Farmed animals must not be allowed access to the land for at least 21 days following application to land. This is precautionary advice as pkup is not itself harmful to livestock if ingested in small quantities.