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29 Jun 2021

Liberate phosphate from soil lock-up

As featured in Arable Farming Magazine

Liberate phosphate from soil lock-up

by Arable Farming

With prices of phosphate having jumped, can growers do more to release what is present? Martin Rickatson reports.

Phosphate fertiliser prices have taken a big leap of late, with DAP and TSP at their highest for a decade, making many growers reconsider future fertiliser purchase plans and look more closely at ways to release the P already present in their soils.

Considering the updates to P advice in 2020s revision of the AHDB Nutrient Management Guide (RB209) could result in significant reduction of the economic and environmental impacts of fertiliser wastage.

That is according to researchers from both the independent and industry sectors of agronomy advice and research.

AHDB suggests last years RB209 update introduced some of the most significant updates to P advice in recent decades.

Sajjad Awan, former resource management scientist at AHDB and co-author of the guide, points out that over the past 35-40 years there has been little change in P recommendations.

He says: However, analysis of 10 years of trials data showed that the offtake of P by winter wheat grain was lower than that previously indicated by RB209 advice.

Where previously it had been calculated that this was 7.

8kg/tonne of freshweight grain, our research indicated the figure was 6.

5kg/t, so the RB209 recommendations were reduced correspondingly.

The reduction doesnt perhaps seem much at first sight, but in a 10t/hectare crop over many hectares, over-application of P can cause significant environmental issues in watercourses.

Not only did we subsequently reduce P recommendations for winter wheat, but in response to demand from FACTS advisers we also aligned the nitrogen and phosphate recommendations for cereal crops.

Prof Roger Sylvester-Bradley, ADAS head of crop performance, led the decade of research behind the revised recommendations, which included input from organisations such as Rothamsted Research and NIAB and was supported by the wider industry.

Challenge He says: Finding uniform sites where P was not plentiful at index 1 was a challenge, but we found 11 to test different strategies and quantities, using TSP as a control.

We found the crops grown immediately after application were utilising only around 4% of the quantity applied, so this is a very inefficient way of feeding crops.

Soil analysis isnt brilliant at predicting where crop P supplies need increasing and it is very surprising that grain analysis, which now forms part of the YEN programme, isnt more widely used globally for this and for the other 11 key nutrients to provide an overall picture of nutrient capture.

The cost of analysis is a fraction of fertiliser input costs and without it, its difficult to know whether crops are under- or over-supplied.

UK grain analysis shows offtake is not as high as previously thought, hence the RB209 recommendation reduction.

We then looked at analysing whether crops had suffered a yield loss because they couldnt find sufficient P and this proved a good method of diagnosis.

A figure of 0.32% in grain dry matter is a threshold below which growers should think about improving the P supply to crops.

Taking a couple of handfuls from each trailer from a field as it is tipped at harvest can provide an overall picture, when analysed, of a fields P status.

In terms of soil analysis, samples should be taken from the same depth at the same time of year and the same place in the rotation to compare like with like.

And calculations should take into account whether you have been taking off more than you have applied.

He adds: Theres much potential to improve the efficiency of fertiliser use.

Trials comparing TSP with Struvite, a product of sewage treatment thats a slow release source of P, doubled the efficiency of use compared with water-soluble P, showing the benefit of a P source that isnt fixed so quickly but is mineralised slowly.

This underlines the value of similar slow-release products such as manures.

Chris Martin, head of soil at Agrovista, provides a reminder that phosphorus is the key energy driver of crop growth, stimulating photosynthesis and respiration and is in major demand during both autumn and spring growth phases of winter crops.

He says: However, its easily locked-up in soil yet, conversely, easily lost through run-off when supplied directly if timings and quantities are miscalculated.

The primary key to accessibility of soil P is pH.

In acidic soils P can form complex insoluble compounds with elements such as iron and aluminium, limiting its availability to the crop.

Conversely, the calcareous soils across much of the UK wheat area tend to be of high pH, which again limits accessibility to the crop.

Therefore, while most UK soils have potentially hundreds of years of phosphate within them, across the vast majority P exists in huge immobile pools and while release will naturally occur, it will take place gradually over a long period.

Indices are not that great a guide to providing the full picture of availability and crop requirements and are often produced as a basic tool to help sell P and K alongside a pH test to sell lime.

A full soil test will indicate what youre working with, while a tissue test will show how well youre utilising it.

And finally, a grain test at harvest will provide a post-mortem of the strategy youve used to correct any issue.

More judicious use Mined phosphate is, of course, a non-renewable resource, while watercourse pollution by P is a significant environmental issue.

This all adds up to mean more judicious use of applied P is required.

Ensure you use the right product at the right time for the right field, according to soil type and analysis.

TSP is probably the most common P source, and does a good job, but in high pH calcareous soils the P will very quickly be attracted to calcium ions.

This makes it essential to time applications to exactly when the crop needs P, matching nutrition to need.

Dont put it all in the seedbed, as it can easily be locked-up by March, before peak demand in the following months.

While increasing numbers of farms have adopted variable rate nutrient application and many have spent a lot variably applying P, Mr Martin points out this is likely to have been wasted if there is a soil pH imbalance, so a pH test should precede any decision to try to correct P availability.

Low pH can, of course, be addressed through liming, but high pH is more difficult and less economically viable to address properly.

Ammonium-based phosphate products, such as DAP, are more likely to be consistent here, as their generally higher pH means the P wont be locked-up so quickly.

The crops assimilation of ammonium causes its excretion of hydrogen ions, resulting in localised acidity and reduced fixation around the roots.

Alternatively, organic forms of phosphate where the P is ‘recycled such as farmyard manure, compost, Fibrophos can offer more cost-efficient methods than mined P of addressing deficiencies.

Your soil test is a good guide to the form of phosphate boost best suited to the situation.

P is very immobile and doesnt leach readily.

The bigger problem in high rainfall periods is its attraction to the cations on clay particles and losses in subsequent soil/water run-off.

To retain both soil and phosphorus in situ, a number of practical measures should be adopted, including avoiding over-cultivation and following field contours.

Using buffer strips on headlands where possible can also help, as can retaining plant cover on soils, using measures such as direct drilling, cover cropping and companion plants.

Cover crops in particular can be a good way of retaining and releasing locked-up phosphate ahead of spring crops.

Species with expansive rooting structures such as black oat work especially well, combined with the cluster roots of something like buckwheat.

Trials Mr Martin has been involved with extensive Agrovista trials on phosphate release products and suggests that the more imbalanced the soil the better the results they produce.

Weve seen the best response from products based on carboxylic acid technology that dissolves calcium phosphate, chiefly Plater Groups Phosphorus Liberator.

Its effect in releasing legacy phosphate significantly reduces the need for applied product.

Used in the seedbed and in early spring, weve also seen some farmers on high calcium soils eliminate applied phosphate altogether.

It has wide tank-mix potential, so can be easily put in the tank and applied concurrently with another job.

Alternatives include Phosta, a complexing agent from De Sangosse.

This works by binding up calcium ions and making soil phosphate more readily available by stopping the calcium from reacting with it.

He adds: There are also a number of sprayed-applied mycorrhizal fungi products that, although more expensive, have proven relatively successful.

But our trials show the carboxylic acid products to be most consistent and these will produce better results than sustained high applications of phosphate, particularly if you have soils with a high pH and high calcium content.

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