Sustainable approaches to plant nutrition

Continuing our series on plant nutrition, John Hammond and Philip White consider how phosphorus can be reused or recycled, and how the industry's reliance on phosphate fertilisers can be reduced.

Phosphate fertilisers, which are made from mined phosphate rocks, are essentials for modern agricultural production but are a non-renewable resource - the global phosphorous cycle takes a thousand years to come full circle. To ensure that this resource remains available for as long as possible we must therefore develop ways to reduce our use of phosphorus and recycle it locally before it is returned to the global cycle.

The geological phosphorus cycle begins with the weathering (exposure to the weather) of sedimentary rocks containing phosphorus. The gradual weathering of these rocks releases phosphate into the environment. Phosphate is taken up by plants, which are eaten by animals that return it to the ground. This phosphorus can be recycled through the food chain many times. Eventually, however, it will become part of an aquatic plant or animal that, when it dies, falls to the lake or sea bed, where the slow process of making sedimentary phosphate rocks once more occurs.

This geological process is too slow to replenish the phosphate used by intensively grown agricultural crops. Consequently, humans have developed ways to speed up parts of the phosphorus cycle to meet the needs of their crops and the burgeoning human population that depends on them.

The acceleration of the natural cycle starts with the mining and processing of phosphate rocks. About 80 per cent of mined phosphate rocks go into the production of fertilisers, with the rest being used in detergents and animal feed supplements. Most of the phosphate rock is treated with acids to produce more soluble phosphate fertilisers. These fertilisers are then applied to crops - most of which are eaten - so much of their phosphorus ends up in human and animal wastes. The phosphorus then continues through its cycle, via sewage treatment works in the case of human waste, into the rivers and seas, where it will eventually become sedimentary rocks.

Humans cannot accelerate the formation of these rocks, so we can only rely on those already formed to make our phosphate fertilisers. Estimates vary on how much phosphate rock is available, with most recent estimates suggesting that there remains about 46,000 million tonnes, located mainly in the US, China, Morocco and Western Sahara. Although this sounds like a lot, much of this is not economically recoverable at present. Furthermore, at current rates of consumption (170 million tonnes a year) we will have used all of this in less the 300 years.

More alarming is the fact that, if we only consider that which is economically recoverable, we could be running out of phosphate rocks in about 60 years. We must, therefore, start to use our phosphorus more wisely by reducing its use, reusing it or recycling it.

Reducing phosphate fertiliser use

There are several ways to reduce our use of phosphate fertilisers, many of which have the added environmental benefit of reducing the amount of phosphorus reaching watercourses and contributing to eutrophication (when water is over-enriched with nutrients).

Integrated management plans based on soil testing and use of recommendation systems such as Defra's Fertiliser Recommendations for Agricultural and Horticultural Crops (RB209) or computer models can help. They will optimise fertiliser applications to match the phosphorus removed by the crop and lost to the environment, while maintaining crop yield.

Precision agriculture is also offering new ways of reducing fertiliser applications. Targeting the application of fertilisers to the rooting zone of crops, either by injection of liquid fertilisers or placement of solid fertilisers, can also reduce the amount of phosphate fertiliser required. This is particularly effective for crops grown in widely-spaced rows, such as field vegetables and potatoes, where broadcast applications of fertiliser may not be fully utilised by the crop.

By using this technique it may be possible to halve the amount of fertiliser applied without adversely affecting crop yield or quality. Moreover, in some instances it may even improve yield and quality by promoting early crop development and crop uniformity. Future developments in real-time detection of soil phosphate concentrations could also enable the development of variable-rate phosphate fertiliser applications, allowing applications to areas of the field that need it most.

Finally, growing crop varieties that require less phosphate to achieve commercial yields could reduce phosphate fertiliser use.

Reusing and recycling phosphorus

Minerals have been reused and recycled for use in agriculture for centuries, through the use of animal manures and slurries. These tend to recycle phosphorus within the farm system, delaying its return to the sea. Manures and slurries are still widely used in many parts of the world as the sole fertiliser for crops.

But for intensive agriculture, animal manures and slurries do not contain sufficient phosphate on their own to meet the needs of the crop. They can, however, act to offset the use of inorganic fertilisers, provided growers have access to them. Since the phosphate content of manures is variable - depending on the animal, type of feed, collection method and length of storage - applying precise quantities can be difficult.

Similarly, sewage sludge can act as a phosphate source for some crops, but because it can also contain heavy metals, the process is regulated. One solution to the low phosphate content and variability in manures and sewage sludge may be a product called struvite.

Pure manure

Struvite is a natural waste product, which in addition to phosphorus generally contains nitrogen and magnesium (N:P2O5:K2O:MgO = 5:31:0:16). It is a fine, white crystalline powder, and is a more concentrated source of phosphate than the sewage or waste product from which it is derived. In contrast to sewage sludge, it also appears to have lower levels of heavy metals and pathogens.

Struvite can naturally precipitate out of sewerage sludge and animal waste, and can build up in sewers and in treatment works. This causes operational difficulties and decreased efficiency.

The physical removal and disposal to landfill of crystallised struvite is expensive and can cause pollution at the site of disposal. However, researchers are currently developing ways to manufacture struvite from sewage effluent, so it can be recovered economically and used as a fertiliser.

It has recently been estimated that sewage treatment works release approximately 44,000 tonnes of phosphorus into the surface waters of Great Britain annually. If all this phosphorus was captured as struvite at the sewage treatment works, struvite could potentially supply 44 per cent of Britian's fertiliser requirements. This would also represent a local source of phosphate fertiliser, and could result in a reduction in our reliance on imported phosphate fertilisers. By capturing the phosphorus in the form of struvite, before it is returned to the water, we can slow down the global phosphorus cycle and also reduce the impacts of excessive phosphate on water quality.

For all these reasons, the use of struvite as an alternative phosphate fertiliser is an attractive proposition.

None of these solutions can solve the problem of declining reserves of phosphate rocks, but in combination they could help to extend the lifetime of the phosphate rocks we have. Introducing these practices now may allow us to maintain a secure phosphorus source for agriculture for longer.

- Dr John Hammond is a senior research fellow at Warwick HRI and Professor Philip White is a crop mineral research expert at SCRI.

STRUVITE TRIALS

Recent trials have established that struvite is a suitable source of phosphate for potato production. Struvite was compared to triple super-phosphate (TSP) as a source of phosphorus for the potato crop in a three-year field trial at Warwick HRI (Defra-funded project HH3504SPO).

When harvested, there was no significant difference between the commercial yields of plants supplied with struvite and the plants supplied with TSP as their sole source of phosphate. However, additional work is required to make struvite a realistic source of phosphate for crop production.

Research is needed to: work on the efficiency and scale with which struvite is recovered from sewage; develop efficient methods for the application of struvite to agricultural land; and develop appropriate formulation and application rates to different crops under different soil and climatic conditions.


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