Rocks for Crops

In 2011, after giving a talk about low carbon emission steel making, MIT metallurgist Antoine Allanore was approached by a Brazilian iron ore miner with a proposition: turning rocks into fertilizer.

“Okay, I’m listening,” said Allanore, Thomas B. King Assistant Professor of Metallurgy, who specializes in sustainably extracting metals from ore.

The miner wanted to make potassium-based fertilizer, often called potash since farmers used “pot ash” as their source of potassium before industrialization. “Man has known for a long time that ashes from wood fires make crops grow better,” Allanore says.

Now Allanore is scaling his patent-pending potash extraction process in preparation for the launch of a pilot production plant in Brazil. He is also collaborating with an agronomy agency there to test his product on hectares of crops. “After so much hard work in the lab, it’s fantastic to see crops growing,” he says.

Allanore’s technology has the potential to remove a significant barrier to agricultural expansion in Brazil caused by difficulties in securing and making use of traditional potash products. More broadly, however, his work could also have an influence on agriculture in other areas of the world struggling to improve crop yields, such as some areas of China, Africa, India, and South Asia.

Modern potash production has focused on the mining of potassium-based salts from dried-up seas. The salts met the needs of farmers in the United States and northern Europe, where the soil is rich with silica-based clay that retains potassium as the salts rapidly dissolve during rainstorms. But in tropical soils, rain washes the potassium away.

Another problem is that the key mines supplying today’s potash market are located in Canada, Russia, and Belarus. Few suppliers control the market, and small farmers in developing nations don’t have much bargaining power. These farmers also pay to ship the salts overseas, and then from seaports to remote interior farmlands, a costly, difficult prospect given poor road and railway conditions in the developing world. “These regions need their own sources of potassium, and their own fertilizers that match their soils,” says Allanore.

The rock presented to Allanore in 2011 was potassium feldspar, a strong, insoluble, granite-like rock found around the globe. Allanore needed to find a way to get at the potassium inside, about a fifth of the rock’s composition. But he also needed to be sensitive to the realities of farmers in remote areas of Brazil or Africa, who may have limited access to energy or water. “I like electricity and could use it,” he says, “But electricity isn’t readily available in remote areas.”

Instead, Allanore applied heat, at a mid-range temperature of 200ºC, and calcium oxide from limestone. The resulting chemical reaction breaks down the feldspar into smaller rocks, exposing potassium that slowly dissolves in water. These rocks also contain other components, such as silica, that are good for enriching tropical soil. “For use in the north, we’d get rid of the silica and leave behind potassium that would dissolve faster,” says Allanore. “But in Brazil, the silica will help form a better soil over time.”

In trials in greenhouses in Brazil, Allanore’s potash rocks fertilized crops as well as salt-based potash. Trials are underway to compare performance in fields of potassium-dependent crops that are important to Brazil, such as soybean and sugar cane, with results coming in a year or so. “It will be a practical demonstration of whether our material is actually a solution to the potash problem in Brazil,” says Allanore.

Allanore’s potash rocks could also be produced in Africa and other tropical regions using local feldspar and limestone, though similar field tests would need to be done since each region grows different crops.