Africa’s potash problem

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This article is published in collaboration with MIT News.

A silent crisis is brewing in Africa where current farming methods strip potassium out of soils faster than it is being replaced with fertilizer. University of Guelph Professor Peter Van Straaten estimates for Uganda there is a nutrient deficit of about 77 pounds per acre. “This is a silent crisis, and it continues if we are not addressing this problem,” Van Straaten told the first International Workshop on Alternative Potash at MIT on Nov. 11.

African farmers are replacing just 10 percent of the potassium being removed with each harvest, creating “a recipe for disaster,” David Manning, professor of soil science at Newcastle University in the United Kingdom, said. Consumers in the Northern Hemisphere may not be paying a sufficiently price for products such as bananas to allow these farmers to replenish their soils with fertilizer, he suggested.

“The current price is too high for many farmers. Demand is high because there is not enough potash going onto the land, so there’s a bottom line here that world production of potash would have double to meet the present day’s population. And that is clearly not possible,” Manning said.

Need for alternatives

Potash is one of three main fertilizers needed for farming. The others are nitrogen and phosphorus. Representatives from the U.S., Canada, France, England, Brazil, the Ivory Coast, Malawi, and Tanzania gathered at MIT Nov. 10-12 for the public workshop and several closed sessions examining the scientific and economic status of efforts to develop alternatives to traditional potassium chloride salts.

Africa has faced famines and malnutrition in part because of low fertilizer use, according to Mshindo Msolla, who is based in Tanzania with the African Fertilizer and Agribusiness Partnership. “This particular organization is trying to ensure that fertilizers are timely and delivered in the right quantities, which are soil- and crop-specific. Currently we are supporting these particular activities in Tanzania, Mozambique, Ghana, Nigeria, and the Ivory Coast. … I’m here because potash deficiency is becoming very common in East Africa and specifically so for Tanzania as well,” he said. “We don’t have the resources, so we really need to look for these alternatives, alternative K [potassium] sources, using the locally available silicate minerals.”

Potash is plentiful in the Northern Hemisphere, where producers in Canada, Belarus, and Russia dominate the world market. Manning said three countries produce more than 70 percent of the world’s potash, with little or no production in Africa, South Asia, and Australasia. “We have got a serious problem of geographic distribution,” he said. Potash shortages affect parts of the Southern Hemisphere from Africa, to Brazil, to India and China.

Farm to market

Manning suggested consumers think of the connection between the food they buy in grocery stores and the fertilizer needs of the world’s farmers. Crops such as bananas sold in the U.S. and Europe are imported from countries with hot climates. “When we take a crop from the soil, we are taking nutrients from the soil. We are actually mining when we eat our food,” he explained. “We need to reflect upon [this] when we are buying those supermarket products. … Are we actually paying a price that is enough for the farmers to put the nutrients back in the soil?” Manning asked. A price war over bananas in the UK leads him to doubt whether farmers are being paid enough to get the potash that they need, he said.

“Mining of fertilizers is an absolutely essential aspect to support human life,” Manning said. The United Kingdom recently approved an underground potash mine deep in the North York Moors National Park to York Potash Ltd., which is a wholly owned subsidiary of Sirius Minerals PLC. York Potash will be extracting polyhalite, a mineral with agriculturally significant amounts of potassium, magnesium, sulfur, and calcium. “Polyhalite was mined in the U.S. in the 1920s, is now seen as a new product, and as well as potassium, it’s adding calcium, magnesium, and sulfate, all of them beneficial,” Manning said.

Internationally, the current price for potash is about $300 per ton, Van Straaten noted. As of early November, he said, colleagues in Nairobi, Kenya, told him potash was going for $578 per ton there, while colleagues in Kampala, Uganda, told him it was unavailable.

Dependence on imports

While the potassium salts mined in the Northern Hemisphere work well for countries there, alternative sources may be needed for the Southern Hemisphere. Both soils and the rock beneath them differ in tropical climates from those in the Northern Hemisphere.

This year, agricultural crop production in Brazil will reach more than $110 billion. Brazil’s fertilizer consumption in 2014 reached 15 million tons of phosphorus and nitrogen, and 6 million tons of potash. But Brazil is dependent on imports for 90 percent of its potash and fertilizer costs have risen over the past two decades from 5 percent to 30 percent of agricultural costs.

Eder Martins, a researcher with the Brazilian Agricultural Research Corp. (Embrapa) in Brazil, noted studies have identified five different types of sources of potassium. New mapping techniques can point to which potash sources are most appropriate for which agricultural regions. One promising avenue is potassium from syenite, a mineral rock that contains up to 95 percent potassium feldspar.

MIT alternative

Researchers in the lab of Antoine Allanore, the Thomas B. King Assistant Professor of Metallurgy at MIT, working with partners at Embrapa in Brazil, have developed and tested a new hydrothermal technique for producing hydrosyenite, a potassium feldspar-based fertilizer. Greenhouse tests conducted by Embrapa’s Cerrado Ecoregional Center in Brazil showed the alternative potassium fertilizer developed at MIT performed better on corn (maize) plants than potassium chloride salt fertilizer. The MIT process uses mechanical grinding and hydrothermal treatment of potassium feldspar. “From an industrial standpoint, benefitting from the fact that this microstructure reacts better to mechanical treatment means lower energy, which means lower cost, which means more scalability,” Allanore said. The MIT research was funded by Brazilian minerals company Terrativa.  “This new technology can change the production of potash in Brazil,” Martins said.

An advantage of hydrosyenite, or hydropotash, for the Cerrado area of Brazil, Allanore said, is that it also makes available silicon and other elements from the feldspar. The Cerrado soil has a desirable pH to capture aluminum so that it doesn’t interfere with plant growth. “At end of day, you don’t need to separate all the elements that constitute K-feldspar,” he said.

Cost estimates suggest a hydrosyenite plant requires one-third to one-half the capital needed for a new potassium chloride (KCl) mine. Operating costs are expected to be comparable to that of traditional KCl producers. “However, there is a big difference, which is the fact that now we don’t need to go after traditional resources. We can actually directly implement this plant in a region close to the farmers who don’t have access to KCl because of other reasons than the price of production in Canada or Russia, such as transportation and infrastructure.”

African engagement

“We see a huge engagement from the representatives from Africa, which identifies clearly the same problems as in Brazil from an economic standpoint, and we are curious about how the development we’ve made in Brazil could be translated in Africa … because the farmers are different, the crops are different, the soils are different,” Allanore said. “That’s one of the benefits of having this workshop; we can share experience from different regions all over the world, different economic situations, and then translate that into actions.”

Lisa Stillings, a research geologist with the U.S. Geological Survey and the University of Nevada at Reno, presented a detailed overview of various silicate mineral dissolution rate studies, both her own and those of other researchers. Not all silicate minerals contain potassium, so not all are appropriate as a potassium fertilizer source. Stillings said her research on feldspar, a potassium-bearing silicate, showed that on the surface of the dissolved feldspar mineral, the leach layer is thicker at more acidic solutions.

A recent paper by Allanore group postdoc Davide Ciceri demonstrated through microfluidic experiments that feldspar interacting with a strong acid can release sufficient quantities of potassium for agriculture.

Benedict S. Kanu, an agricultural specialist with the African Development Bank in the Ivory Coast, asked about the implications for Africa. Stillings noted that grinding the mineral to the finest particle size possible would increase the potassium dissolution rate. “Acidic conditions, if that’s possible, and if there are any organic acids, that might also help to remove potassium from the structure,” she added. Lab experiments show that continually altering soil conditions can recreate initial dissolution after mineral-based fertilizer is applied. Adding humic acid is one possible route to increase acidity.

Involving farmers

University of Guelph professor Peter Van Straaten specializes in agrogeology. “Agrogeology is a bridging science, but it’s also part of a strategy of low external input, sustainable agriculture,” he said. Bananas grow well on potassium rich volcanic rocks, such as sanidine, while corn does poorly on deeply weathered granite. Van Straaten has worked on projects in sub-Saharan Africa, Brazil, and Indonesia. “We try to involve farmers at the onset of a project, not the end,” he said.

Potassium salts (KCl) have over 60 percent K2O and are mined from ore that contains 20 to 30 percent K2O. Potassium silicates, such as potassium feldspar, contain 8 to 16 percent K2O but have low solubility, Van Straaten said. (The alternative hydrosyenite developed at MIT contains about 12 percent K2O.) “The challenge is to increase the rate of K release,” he said. British researcher Manning said feldspar, for example, has a low dissolution rate compared with other potassium bearing minerals. Leucite is 10,000 times more soluble than potassium feldspar, he said. “These are the minerals we want to go for and that’s because their dissolution rates are so high,” he said. Such resources are however not always as large, abundant, and pure as K-feldspar, a reality that often prevents a cost-effective exploitation at large scale.

“If we could process K (potassium) silicates locally at low cost, with appropriate technologies, there would be an abundance of K resources available to farmers, but it’s not that easy,” Van Straaten said. New geophysical tools are enabling rapid identification of soils that require potassium fertilizer.

Biological approaches

Van Straaten offered an alternative for dissolving unreactive phosphate mineral rock using a mush formed by Penicillium mold and Aspergillus niger fungus, which produces citric acid from the waste. The citric acid at least partially dissolves apatite mineral, which contains phosphorus but not potassium. The process is being used in Indonesia to make a biophosphate product. “It could be replicated in many places,” he said. Many processes used for producing phosphorus could also be used for producing potassium, he added.

Assistant Professor Edith LeCadre of Montpellier SupAgro
 and INRA in France presented research on the role of plant roots in dissolving potassium from feldspar in the soil. The immediate area of interaction between plant roots and soils is known as the rhizosphere. “Roots can weakly modify the physics, the chemistry, and the biology just near the roots, so it’s only a few millimeters around the roots. But in this critical zone, you have an interaction between soil minerals and plants,” she said. Roots, for example, release organic carbon that can be used by bacteria and fungi. Her research showed bioavailabilty of potassium after seven days from three different mineral rocks: potassium feldspar, syenite, and albite. “Plants can absorb much more potassium in the presence of soil,” she said. Contributors to this effect include the buffering effect of the soil and organisms present in the soil.

LeCadre worked with colleagues in Brazil and the Republic of Congo. Experiments near Sao Paulo, Brazil compared areas with reduced rainfall to unaltered areas and estimated the importance of potassium for plant production. The researchers also studied the effect of substituting sodium chloride (ordinary table salt) for potassium. “Sodium can, in a certain extent, replace potassium,” she said. The researchers also measured root depth.

Chinese model

Professor Jian-Ming Liu described an alternative fertilizer now being produced in China. His “soil conditioner” was licensed in China in 2012 after a decade of tests on 80 crops in 27 provinces, he said through a recorded presentation. Tests demonstrated improved quality and yield in tea plants and apple trees.

Liu, who is a research professor at the Institute of Geology and Geophysics at the Chinese Academy of Sciences, applies a pressurized steam curing process to potassium-rich silicate rocks to make the multi-element fertilizer.

“Plant roots can secrete root acids such as citric acid, acetic acid, which can easily flow along the micropores and dissolve the microcrystals of the product and absorb all the elements in the microcrystals,” Liu explained.

The Chinese product is unlikely to be effective for Brazil, however, Allanore said. “The chemistry principles behind the processing developed by Professor Liu and our group are the same; however, the processing parameters are very different and therefore will lead to a very different product. The soil conditioner product is typically very diluted in potash content, which is not good from a commercial perspective, and the product also goes for much longer time of processing and much more addition of additional activators, which at the end of the day makes it suitable for some very specific regions or specific markets but is probably not transposable. We cannot translate that very well in the context of Brazil, for example, where you have big farmers, huge scale, and big consumers.”

A role for silicon

Plants like rice need silicon as well as the three major fertilizers, Manning noted. Both potassium and silicon occur in silicate minerals, but from a geochemical viewpoint they differ in their behavior.

The sugar cane harvest in Thailand, for example, removes about 220 pounds, or $50 worth, of potassium for every 10 tons of sugar cane harvested, Manning said. “You’re taking the potassium away. You need to put potassium back and if you don’t do that, then potassium is being mined with every crop that’s taken away,” he said.

Manning noted the UK grows about 6 million tons of potatoes in a good year. “That’s $9 million worth of potash taken from our soils, and we put that back. We do replenish our soils with the nutrition that’s taken away,” he said.

Publication does not imply endorsement of views by the World Economic Forum.

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Author: Denis Paiste writes for MIT News.

Image: A farmer operates a tractor while tilling soil to grow potatoes. REUTERS/Louafi Larbi 

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