Farming News - International studies look at phosphorous use

International studies look at phosphorous use

20 Dec 2012
Arable / Machinery

Two studies investigating the use of phosphorous fertiliser have been published in the United States. Researchers from the United States and Canada, who found earlier in the week that green revolution technologies are losing their effect across a third of the world's croplands, have called for changes to agricultural research and development in line with their findings.


Soybean harvesting in Mato Grosso

The researchers, from the University of Minnesota's Institute of the Environment, also said phospohorous, a key non-renewable input, is being wasted by overuse and improper use in the country, either being retained in soil, lost post-harvest or washed away. The researchers estimate that, with the inclusion of non-food crops, such as biofuels and cotton, and arable crops produced for feed, only 8 percent of phosphorous applied goes to produce food effectively.


The international research team also examined the distribution and use of phosphate fertilisers. Graham MacDonald, one of the lead researchers, said, "Given these dimensions of phosphorus – as a key fertilizer, a potential pollutant and a non-renewable resource that is being redistributed by humans – we felt that it was important to consider how phosphorus moves through the food system."


MacDonald and his colleagues made a series suggestions aimed at preserving the vital resource. They called for further efforts to focus on increased recycling of animal waste for use as fertiliser, more R&D in the area of precision fertiliser application to reduce pollution, lessening phosphorus used in livestock production and production of a wider range of agricultural commodities.


Looking at current patterns of consumption, the amount of phosphorus used in the US could be halved by addressing farm management and consumer waste alone, the researchers said. This would protect non-renewable phosphate reserves and reduce pollution around the world.


Deepak Ray, another of the study's authors, recommended focusing efforts to increase yields on the 61 percent of croplands where yield is still climbing. He said, "Previous research suggests that many factors work together to limit yield growth, from cultivation practices to pests to a need for improved seeds. [Thanks to our work,] policy makers can identify regions where yield growth has stagnated or reversed, figure out what limiting factors are at play, then work to turn that trend around."


Illustrating the international IofE reserchers' point, scientists from Brown University, Rhode Island who compared soils in Brazil to those in the United States, said soil type dictates the effect of phosphorous and outlined how developments in global agriculture have wrought changes in what were once diverse regions.

Brown study examines role of soil in phosphorous use


The Brown study focused on the relationship between soils and phosphorous; researchers elaborated, "Typically in short supply, particularly in tropical soils, phosphorus is unique among fertilizer requirements. It is finite, irreplaceable and mined in just a few places around the world."


The scientists looked at soybean cultivation in the United States, where production is well established, Argentina and Brazil, where the soils were once deemed unsuitable, and areas of tropical forest have been cleared for growing. However, with the addition of phosphorous, South American soils now produce soybeans at a rate comparable to the US.


The researchers said, "Brazil's soybean yields have become competitive with those of the United States and Argentina, but the soil demands a lot of phosphorous, which is not renewable. In the United States, meanwhile, historical applications of the fertilizer have polluted waterways."


Based on his examination of the different growing regions, biology professor and Brown study co-author Stephen Porder said, "Having a one-size-fits-all approach to our understanding of interaction between people and their environment via agriculture is going to lead us to some erroneous concerns and conclusions, if we don't take the regional biophysical setting into account.


"If you are concerned about the global phosphorous supply, Brazil is your problem — they are using a ton of it. If you are concerned about lakes and rivers being filled with algae, then Iowa is your problem, and learning how to mitigate even very small amounts of loss after decades of overfertilization is a real challenge."


Soils in the soybean fields of Brazil's Mato Grosso state, which were once rain forest land, absorb phosphorous but retain it; successful soybean cultivation there since the 1990s has only been possible with improved soybean varieties, and large inputs of lime (to raise the soil pH) and phosphorus (to overcome the soils native deficiency). These large inputs mean the soils in Mato Grosso are gaining about 31 kg phosphorus per hectare per year, according to the researchers’ measurements.


However, Brazil's deep soils have kept such a tight grip on their phosphorous that almost none of the fertiliser has polluted nearby waterways. Conversely, in Iowa, where the relatively fertile soils release phosphorous much more readily, and soybean farming goes back to the 1920s, excess phosphorus used decades ago is still causing widespread pollution.


Although Iowan farmers have used much less phosphorous since 1995, local lakes and rivers are still being polluted because the soil is still releasing all that phosphorous applied long ago. Half of the waterways in Iowa assessed under the Clean Water Act are listed as impaired as a result of such pollution.


Professor Porder explained the complexity of the problem facing policy makers, "[In addition to] scarcity, which is a concern, the overuse of phosphorus can also pose another problem, causing harmful algal blooms in waterways. It's a bit of a Goldilocks problem — too much and our waterways are choked with algae, too little and we cannot produce enough food."


The researchers acknowledged that producing adequate amounts of food with "a minimum of environmental costs, will be one of the great environmental challenges of the 21st century." Porder's colleague Shelby Riskin said "Phosphorus in soybeans is just one example, but illustrates a broader point: We have to produce a lot while being aware of local and global environmental costs. What this means for management is different in different places."