Measuring plant available phosphorus to increase crop yields and minimise nutrient leaching

Phosphorus is a key nutrient for plants and needs to be added as fertiliser to facilitate efficient and economic production of agricultural crops. There is no general soil test which can reliably predict how much fertiliser should be added for optimal crop growth. If too much is added it is costly and phosphate can leak into lakes and rivers, causing severe pollution problems known as eutrophication. If too little is added crop yields are poor and uneconomic. The situation is critical because world reserves of usable phosphorus are limited. Liming soils, which generally benefits acidic soils by improving nutrient availability to plants may further complicate the phosphate problem by locking up part of the phosphate so that it cannot be used by plants.
A new analytical technique called DGT, which mimics the way that phosphorus in the soil interacts with the plant, provides a good prediction of how much phosphate fertiliser should be added for optimal plant growth. The phosphorus accumulated by DGT during the placement of the DGT device in the soil has until now been measured in the laboratory using conventional procedures, but a new handheld XRF device that can provide an instant readout has become available.
This project will first establish that DGT with analysis by portable XRF can used to measure the phosphorus in soils that is available to plants. It will compare the new measurement with established chemical tests and investigate how well it can be used to predict fertiliser requirements for two crop types (a brassica crop and a legume) grown in a range of soils. In a set of experiments where soils are limed, the new method will be used to advance understanding of how liming affects phosphorus availability to plants and of how liming is likely to affect the release of phosphorus to water bodies (due to leaching from the crop rootzone). This work will inform strategies for applying phosphate fertiliser and lime and provide a basis for providing farmers with a fairly rapid and, more importantly, reliable tool for assessing fertiliser requirements. The outcome of the work will provide clear economic benefits to farmers. Environmental gains stemming from reduced loss of phosphorus from agricultural soils into watercourses will benefit the general public and government agencies concerned with the environment, and in some cases water companies by limiting the costs of removing P from drinking water.

Phosphorus (P) is an essential nutrient needed for plant growth, and P deficiency can often limit crop yields and thus economic profitability for farmers (the key stakeholder group whose needs that this research programme addresses). Thus farmers usually apply some P fertilisers annually. However, the use of rock phosphate reserves is increasingly geopolitically sensitive, with the mineral deposits under the control of a handful of countries such as China, the USA and Morocco. China has recently imposed a 135% export tariff to ensure domestic supply. In addition, the import of P rock from Morocco is sensitive, as it currently occupies the Western Sahara, controlling its P reserves. Hence, trading with these regions is condemned by the United Nations. Further to these politically unstable sources, the USA is estimated to have less than 30 years of high quality rock phosphate reserves remaining. Hence, for countries such as the UK with no natural rock phosphate mineral reserves, political, legal and economic challenges related to primary P use are likely to become increasingly important issues as global supply declines. It is therefore imperative that the best use is made of available P fertilizers, to ensure they are applied when and where needed, to efficiently use this valuable, although diminishing, global resource.

To inform farmers whether their fertiliser applications have been adequate, soil P analyses are usually performed every 2-3 years. Plants are best able to take up P from the soil when the soil pH is within a narrow range (pH 6-6.5). Since soil acidification is a natural process (but hastened by the application of nitrogen-rich fertilisers to boost crop yields), soil pH analyses are also usually performed every 2-3 years to inform whether lime should be applied. Although there is usually a delay between (winter) lime application and (spring) cropping, our recent research indicates that in some soils this may be insufficient to prevent liming actually decreasing P uptake by the plant. However, our conventional soil P analyses in this work suggested sufficient P in the soil, indicating that current soil P analytical techniques may not be fit for purpose.

This project proposes a new analytical technique (DGT-pXRF) to provide a rapid, sensitive measurement of plant-available P, to better inform P fertiliser recommendations. The new technique will be compared against conventional soil P analyses, and plant responses including crop yields, in field trials (on growers holdings) that vary P fertiliser rates and liming applications in a range of soil types. This will not only provide a more accurate measurement of plant-available P, but speed the time taken from soil sampling to provision of analytical data to end users.

By conducting field trials at sites that allow analysis of drainage water from agricultural fields, it will be possible to measure P export to watercourses, thereby assessing possible environmental consequences of different agronomic techniques. Paradoxically, although liming may "lock up" P in the soil making it unavailable to the plant, it may concurrently allow greater P retention within soil, minimising export of P. Currently, diffuse water pollution from agriculture is a major concern within sensitive catchments, and water companies have invested heavily in technologies to remove P from our water supplies. Ultimately, addressing the problem at source within agricultural soils is the most sustainable solution.

In summary, this research programme will better inform P fertiliser management decisions (of value to farmers), which will limit P export from agricultural fields (of value to water companies and a broader range of stakeholders), through developing a viable DGT device coupled with analysis by pXRF for commercial application.