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Building up soil indices

Last updated: 13 Apr 2016

The Professional Agricultural Analysis Group (PAAG) laboratories report for 2014/15 shows that the P index was lower than target (2) in 25% of samples from UK arable land and in 34% of samples from grassland. K index was lower than target (2-) in 30% of samples from arable land and in 40% of samples from grassland. Both P and K indices were lower than target in 13% of samples. These percentages haven't changed greatly since the first PAAG report in 2009.

There's plenty of evidence that crop yields are at risk at index 1 and reduced at index 0 so the PAAG figures indicate a fairly widespread need to build soil indices. Questions that then get asked are 'How much phosphate or potash must be applied to raise an index?', 'How quickly can an index be raised?' and 'What is best applied to raise an index?' The 'How much?' and 'How quickly?' questions are linked because you could try small applications over many years or larger applications for a quicker effect. The Fertiliser Manual (RB209) takes the first of these alternatives, recommending for most crops additional build-up applications of 60kg P2O5 or K2O/ha at index 0 and 30kg P2O5 or K2)/ha at index 1. Getting to target index could be a drawn-out affair possibly interrupted by retirement. On the other hand, doing things more quickly can mean a large investment in the short term.

The PDA Nutrient Calculator allows different strategies to be compared. Inputs are current crop species and yield, soil type, current and target indices and the number of years allows for raising the index. The calculator shows offtake of P2O5 and K20 and amounts needed to achieve the change in index. For example, for wheat with a yield of 10 t/ha on a medium soil, soil index could be raised from 0 to 2 over five years by applying 128 kg P2O5/ha above maintenance every year (640 kg P2O5/ha in total). In this situation raising K index from 0 to 2- would involve applying 192 kg K2O5/ha above maintenance every year (960 kg K2O/ha in total). The amount recommended in RB209 (60kg/ha above maintenance) would take 11 years to raise the P index and 16 years to raise the K index from 0 to targets.

Where they are available, livestock manures are good for raising soil indices. They should be the first choice thought he usual care is needed with nitrogen in NVZ's. Failing that, most forms of fertiliser are suitable. TSP, DAP and MAP, the phosphate sources in most compound fertilisers, are all effective. For raising K index, muriate of potash, again the source in nearly all compounds, is the obvious choice. Water-solubility is a common concern with phosphate fertilisers but it's not a good guide to availability. Some materials with zero water solubility are good sources of phosphate - remember basic slag? All that can be said is that high P water-solubility indicates good immediate availability but low or zero water-solubility can indicate good availability depending on the product and soil conditions. Rock phosphates can be effective where soil pH is lower than around 6.5 (as in a lot of grassland) and ash-based products can be useful at any pH.

Having said all this, there are some soils where it can be very difficult if not impossible to raise the P or K index to target. Recent research by AHDB has shown the difficulty of raising P index to 2 on some shallow soils over chalk or limestone - probably due to reactions with calcium forming insoluble compounds. Leaching of potassium from very light soils can prevent K index reaching 2- or staying at that level. In these situations, it's best to accept 1 as a target but to ensure sufficient phosphate or potash is applied every year to prevent deficiency.