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Managing high soil P indices

Last updated: 29 Apr 2016
The latest report from the Professional Agricultural Analysis Group of UK soil laboratories (PAAG) shows that 46% of samples from arable land and 36% from grassland were above the target index of 2. Soils are not naturally high in available phosphorus so these indices are due to past application of phosphate fertilisers or manures. There is no agronomic reason for indices to be high and there are economic and environmental benefits to be gained from running them down to the target provided this is managed properly.

The simplest way to run down a high index is to stop applying any phosphate, apart from small amounts of starter fertiliser for some crops like maize or vegetables where very rapid establishment is needed. This will do the trick eventually as available phosphorus is lost from the soil but it’s not the whole story and it’s not just a matter of reducing soil available phosphorus by any means possible.

Available phosphorus is lost from the soil in four ways:

Off-take in harvested crops usually is the main loss, typically 50-80 kg P2O5/ha/year. This loss can be estimated fairly well from yields and, when the index is being run down, should be calculated every year.

Adsorption of phosphorus in the soil and reactions with calcium in calcareous soils or with aluminium and iron at low soil pH (lower than around 5.5) reduce the availability of phosphorus over time. This is a difficult one because the rate at which this loss occurs varies among soils and we still can’t predict it reliably. Research was being carried out in the 1970's and there are ongoing projects that might move things forward. It’s quite important because the loss of available phosphorus can be significant – some estimates for particular soils have indicated half of applied phosphate becoming unavailable in less than a year while in other soils, the rate is much lower. Owing to loss of availability through adsorption or reactions, soil P index will tend to decrease even where application always balances crop off-take – another reason why regular soil testing is essential.

Phosphorus moves out of the rooting zone with eroded soil particles or through drains. This gets attention because of the environmental issue. The loss is very small, one or two kg P2O5/ha/year typically but it takes very little phosphorus to enrich surface water (eutrophication), promote algal growth leading to oxygen depletion and cause loss of species. In lowland rivers, concentrations of just 35 µg P/l (0.035 ppm) can cause eutrophication. Most of the loss is due to soil erosion which can be obvious after heavy rainfall but can be steady and imperceptible through the year. There are two ways this loss can be minimised – reduce the amount of soil being eroded and reduce the concentration of phosphorus in the soil particles. Erosion occurs when water moves quickly across the soil surface so anything that slows this movement will help. Cultivating and drilling along contours, breaking up tramlines and avoiding compaction and poaching can all do this. Grass field margins alongside water can then trap soil particles that do erode. Soil particles also can be lost down the profile where soil is cracked so application of fertilisers and manures in these conditions should be avoided. Concentration of phosphorus in eroded soil particles is minimised by bringing soil P index down to the target 2.

Phosphorus can leave a field in surface run-off after application of fertilisers or, especially, organic manures and in the immediate aftermath of outdoor pigs. Rapid incorporation is the answer.

Of these four means by which phosphorus is lost from the soil, only the first, crop off-take, is a desirable way to manage run-down from a high index to the target. At present, not much can be done to influence loss of availability through adsorption or reactions in the soil. However, a lot can be done to minimise loss by erosion and run-off. As in all management, monitoring is the key both to check progress and to avoid overshooting – not much good coming down from index 3 just to find you’re at 1. This means regular soil testing every three to five years and taking action on the results. It’s also easy to calculate the phosphate balance for a field every year (applications minus off-take in the crop) which will show the way things are going.