Why are we Fixated on Soil Phosphate Availability?

Despite so much evidence to the contrary there still remain a few agronomists who seem to be fixated on the link between water soluble phosphate and soil phosphate availability.

This should concern us all as they may be misleading British farmers. They are possibly depriving them of the opportunity for a more cost effective renewable phosphate source in favour of more expensive environmentally unfriendly imported alternatives.

The phosphate in Fibrophos PK Fertiliser has been rigorously tested under the EU and UK fertiliser regulations. It has been proven to be available to the plant as required.

Trials have shown this to be the case across a range of soil types and for more than just one season. It has been the sole source of Phosphate and Potash for many UK farmers over the past 25 years, with the added bonus of a wide range of secondary and trace elements at no extra cost.

These facts are strongly supported by a recent report published in Potash News Feb 2019, produced by the very experienced and highly respected agronomist Johnny Johnston. He has been involved in some very long term experiments at Rothamsted and elsewhere and we summarise his main points below:

Phosphate fixation
Johnny believes that the word ‘fixation’ is misleading when describing the reaction of phosphates when applied to soil. It implies ‘fixed’ phosphate is not available to plants – which is then even more “unhelpful and misleading” when it is changed to ‘locked-up’. He suggests a more appropriate word would be ‘retained’.

The implication that fixed phosphate was unavailable for crops appears to have been widely used by some agronomist ‘to persuade farmers’ that it was essential to apply water-soluble phosphate fertiliser for every crop, and ‘not to buy alternative types of inorganic phosphate fertilisers’.

It has been often said that only 10-15% of applied water-soluble phosphate fertiliser is taken up by the crop to which it was applied, and the rest remains fixed in the soil where it is not available for uptake by plant roots. The remaining 90-85% of the phosphorus in the crop has come from available phosphate in soil supplies. However, if these statements were true and that applied phosphate became fixed in the soil, then where has this readily available soil phosphorus come from?

The truth is that most UK soils contain little plant-available “native” phosphate, so the soil phosphorus taken up by crops must come from reserves accumulated from past phosphate applications. Clearly these reserves have not been fixed irreversibly in the soil as shown by the results from an experiment at Rothamsted. Some of the phosphorus applied as single superphosphate, which is water-soluble, to soils between 1856 and 1901 has been retained in the soil and is still being slowly released and taken up by crop roots.

Soil Phosphate Availability diagram

Phosphate Pools
A way of visualising the behaviour of soil phosphorus is to think of it existing in four pools of different availability to crops and with reversible transfer of phosphorus between the pools.

The phosphorus that is immediately available for uptake by plant roots is that in the soil solution (pool 1) and the amount is very small. There is more phosphorus in pool 2, and this phosphorus is readily transferred to pool 1 as the amount of phosphorus there is depleted when taken up by roots. The phosphorus in pools 3 and 4 has low immediate availability and very low availability, respectively. (By analogy to money and its availability, there is cash in the pocket (pool 1), cash in the current account (pool 2), cash in bonds, stocks and shares and least available in the short term, money in land and bricks and mortar (pools 3 and 4 respectively)).

When a phosphate fertiliser, even a water-soluble one, is added to soil only a very small amount stays in the soil solution (pool 1) and the rest transfers to the other pools at varying rates depending on the type of fertiliser and on soil properties. Because there is rapid transfer of phosphorus between pools 1 and 2 the amount of phosphorus in these two pools is often a good indicator of the immediate plant-availability of the phosphate in the soil. Thus, it is this amount that is determined by soil analysis using various reagents that have been shown to correlate well with crop response to soil and applied P. In England, Wales and Northern Ireland, Olsen’s method is widely used. The phosphorus that transfers to pools 3 and 4 is slowly available but is not fixed irreversibly in soil, as shown by the results of the experiment mentioned above.

Target Indices
The general advice today to optimise the efficient use of phosphate in most arable crop production is to raise soils to P Index 2, the critical level, and then maintain them at this level by replacing the phosphorus removed in harvested crops.

For those situations where there is a combination of very low soil indicies and a demanding crop such as potatoes, water- soluble phosphate offers the quickest response. However, these circumstances are quite rare so in most cases it would be commercially better to use a less soluble cheaper phosphate fertiliser, but still NACS soluble, applied at the appropriate rate to maintain a soil at P Index 2.

However, Johnny Johnston offers sound advice that it is essential to sample each field every three or four years to ensure that the phosphate fertiliser or phosphate source is maintaining the appropriate level of plant-available phosphorus. Choosing which source to use will depend on its ability to maintain the required critical level of Olsen P for the crops grown, and the cost and availability of the phosphate to achieve this aim.

As Johnny Johnston has said, it is vital that in order to manage your soil phosphate effectively for a rotation it relies on an understanding of how soil phosphate chemistry works but it is also crucial to understand your specific soil types and crop phosphate needs across your rotation as different crops have different need and modes of accessing the nutrient. Efficiently testing your soils is a good starting point.

Soils: Critically, soils do not lock up phosphates but retain them for later release. The form that they are retained will vary depending on the soil type and the other nutrients within the soil. For example, calcareous soils are more likely to retain the phosphate as calcium phosphate, whereas acidic soils with higher proportions of iron or aluminium are likely to retain phosphate in these forms.

Crops: The form which predominates in soils will have implications for crops trying to access this phosphate, and different crops are able to tap into these resources with differing efficiency. For example, oilseed rape may not be particularly adept at accessing aluminium or iron phosphate, but it does have a greater ability to tap into the reserves of calcium phosphate in soils. This is likely to be due to the acidifying ability of the root exudates close to root tips.

Over 1.5 million tonnes of Fibrophos PK Fertiliser has been applied to UK soils over the past 25 years helping to maintain the nation’s soil P & K indicies.

We think that fact alone is testimony to the efficacy of Fibrophos PK Fertiliser and the commercial judgement of the British farmer.

Phosphate in Fibrophos

Phosphate availability – dispelling the myths

As we move towards a far more environmentally focussed form of farming with future support payments likely to be linked to tighter controlled friendly farming, it is becoming critical that we focus on efficient use of fertilisers – particularly phosphate which is the dominant cause of agricultural water quality failure.

This is the drive behind new ongoing research from AHDB into understanding how phosphate works in our soils. Early results have surprised the traditionalists who have for years peddled the myth about the importance of water soluble phosphate as supplied by TSP. The report has revealed that:

‘The recovery rate of TSP by the first crop is just 4% – the vast majority of the phosphate applied goes into the reserves of fixed P in the soil, which is unavailable to plants and only released very slowly’.

Other forms of traditional phosphate, treated phosphate or fertiliser placement in the trial ‘proved to be little better than TSP’.

These results should come as no surprise to the thousands of farmers who have been using Fibrophos PK Fertilisers across their farms as their sole P&K source for over 20 years. They endorse the evidence that the phosphate in Fibrophos is available to the crop as required and goes on working year after year to help maintain adequate indices for today’s heavy demanding crops.

Chief ADAS scientist Prof Roger Sylvester-Bradley, head of Crop Performance for ADAS, who is conducting the report admits that this information has been a long time coming as trials on phosphate take a very long time.

The report does acknowledge that different soils act in different ways – calcareous soils require P on a more regular basis for instance. It also suggests that in order to protect our environment from excess run off, we need to avoid wasteful over application of expensive phosphate and replace with those more effective sources. This may mean allowing our soils to fall below the recommended index of 2 provided we have the confidence that the levels in the soil phosphate bank is not yield limiting.

This could be achieved in the future as the trials suggest that 80% of the phosphate taken up by the crop comes from the soil rather than applied fertiliser – so further highlighting the need to ‘feed the soil’ with an effective good value fertiliser less damaging to the environment.

With Fibrophos offering a real alternative to traditional thinking, it certainly fits the bill in terms of cost savings – and comes with the added bonus of a wide range of trace elements, Calcium, Magnesium, Sodium and Sulphur. There is also a good range of trace elements – all at no extra cost.

With Fibrophos PK Fertiliser offering so much, why pay more?

More information including extensive trial work supporting our claims can be found in our Handbook.

Peter Hatcher, Fibrophos Ltd 

How is Fibrophos delivered and will it spread evenly?

Fibrophos is a bulk fertiliser available for delivery or collection across the country – typically in bulk 29t loads.

It is ideally tipped on a hard, free draining surface, just prior to time of application. It is weather proof and stands getting wet. Being delivered in bulk there are no bags or packaging to dispose of or recycling costs.

We have taken great care in handling and treatment of the product to ensure it arrives on farm in excellent condition, suitable for accurate and even spreading.

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Find out why incinerated poultry manure is better

I’ve used poultry manure as a fertiliser. Why should I want it burnt to an ash?

Firstly, incineration ensures there are no pathogens. Secondly, poultry manure cannot be applied during the wet winter months due to problems with nitrogen leaching
– the manufacture of Fibrophos takes poultry manure throughout the year thus avoiding leaching issues. Thirdly, by creating a consistent quality ash product it makes it easier to spread accurately.

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Fibrophos is produced from burning the same consistent fuel source all the time. This means the physical quality and nutrient analysis of the product do not vary. It’s the same time after time. Fibrophos Ltd are members of the AIC and all product is sold under FIAS regulations so you may be assured that you get what is declared on the Statutory Statement which accompanies every load. Beware of other fertilisers which are not sold in this way.

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