While researching Southern California's water resources and its impact on the agriculture industry, I became intrigued about the potential of bentonite clay for transforming desert soil into rich farmland. Recently published research papers herald the benefits of bentonite as an amendment for desert soil in scientific field trials undertaken in Australia, Uganda, Thailand, Czech Republic, and more countries across the globe.

What really got my attention was an article published in the October 2020 issue of the prestigious scientific journal Nature. It provides empirical scientific data showing the beneficial effects of bentonite as a soil amendment for increasing crop growth and water conservation in arid regions of China. The field test commenced in 2011 by a team of scientists from Canada and China concluded that the "optimum rate of bentonite was 24 Mg-1 ha for all plant growth". Translating science-speak, this amounts to about 4.5 tons per acre. Bentonite clay is found all over the world in large quantities, it is easy to mine and process hence is relatively inexpensive with a price of about $100 per ton.

The results of the other research papers echoed similar positive results and documented that when bentonite is applied to desert sand the fertility is significantly increased resulting in higher crop yield while also retaining water more effectively.

Amazingly, I did not find any applied research, or pilot projects, about bentonite as a soil amendment for agriculture being conducted in the United States! Even the 2008 USGS chart (Figure 1) does not include agriculture as an "end use" for bentonite production.

The United States is the largest producer and consumer of bentonite and my discussions with senior management at the major bentonite producing companies revealed that increased agriculture yield and water conservation is a new and novel market for this $1.4 billion industry.

According to a February 14, 2022, report in Nature Climate Change, the current megadrought in the American Southwest has broken previous records for the extant driest 22-year period for the region in 1,200 years. One would think that California would take a leadership role in this emerging market since deserts make up about 25% of the state's total surface area and this re-emerging drought is placing unprecedented strain on its water resources threatening agricultural production. California is America's fruit and vegetable cornucopia producing over a third of the vegetables and two-third of the country's fruits and nuts for generating over $49 billion in cash receipts in 2020.

What is bentonite?

Bentonite usually forms from the weathering of volcanic ash, most often in the presence of water. Geologically it is a type of clay mineral mainly composed of montmorillonite that can absorb large amounts of water. Extracted bentonite is distinctly solid with a moisture content of approximately 30%. After being crushed it is subsequently dried to reach a moisture content of approximately 15%. Depending on the final application, bentonite is either sieved into granular form or milled into powder.

There are several types of this "clay with 1,000 uses" but sodium and calcium bentonite are the most in demand. Sodium bentonite can hold 15-20 times its weight in water and is used primarily as "mud" for drilling oil, sealing ponds, and used in cat litter. Calcium bentonite only holds 1-5 times its weight in water and is used in gardening as it is more stable and adds beneficial things to the soil as compared to sodium (salt) bentonite. Bentonite stores and releases water much easier than other types of clays and has hundreds to thousands of times more surface area than sand particles hence it improves nutrient holding capacity of soils and helps provide a better environment for soil microorganisms.

Bentonite structure and chemistry lesson

Sand particles are quite large with a smooth surface and a small surface area. Clay is the opposite and composed of tiny particles with a huge surface area compared to sand. As an analogy, imagine a basketball as a grain of sand and consider its outer surface area. Now consider how many golf balls would take up about the same space. According to the USDA, "clay particles may have thousands of times more surface area per gram than silt particles and nearly a million times more surface area than very coarse sand particles". Soil particles have tiny micropores on their surface which fill with water and sand particles have larger pores, but due to the smaller surface area, cannot hold as much water. Because of this 'surface area' phenomena, clay soils hold between three and six times the amount of water that the same volume of sandy soil holds.

To create a healthy, living soil it needs organic matter. This can be in the form of animal manures, compost, straw, etc. It needs to be broking down to feed soil microbes, which make the nutrients contained in the organic material available for plants.

Organic matter is made up of plant and animal residues in various stages of decomposition. The final stage - and most long lasting is humus, which is the residues of micro-organism activity, and is the most stable form of organic matter lasting thousands of years. All forms of organic material (decomposing to humus) are important additions to soil to feed micro-organisms. It is these creatures in their activity and life cycle which make nutrients in the organic matter available to plants. Under a microscope, humus is like a porous sponge holding onto water and nutrients, making them available to plants as required, and helps prevent leaching of nutrients.

Organic matter also significantly improves soil structure, allowing the penetration of air and water for soil roots to grow into voids created around pieces of organic material. A range of particle sizes is ideal for plant roots to grow. Lots of nooks and crannies created by big particles, with gaps in between them filled with small particles, creates pockets of air and water that plants need to thrive. Too many large sand particles allow water to flow straight through and too many small clay particles cause compaction and crusting. Ideal soil has a range of particle sizes and is generally referred to as loam.

Clay and humus, because of their electrical 'charge', hold onto nutrients in a way that sand cannot. Plant roots can remove these nutrients from clay and humus particles with a process called cation (pronounced cat-iron) exchange, and a high Cation Exchange Capacity (CEC) is the "secret sauce" for retaining nutrients in soil to mitigate leaching through with the water.

The clay mineral and organic matter components of soil have negatively charged sites on their surfaces which adsorb and hold positively charged ions (cations) by electrostatic force. This electrical charge is critical to the supply of nutrients to plants because many nutrients exist as cations (e.g., magnesium, potassium, and calcium). In general terms, soils with large quantities of negative charge are more fertile because they retain more cations

Mining and Processing

Bentonite is usually extracted by surface mining with bulldozers removing the overburden material above the clay which is then stockpiled. After the clay has been extracted from the pit, the overburden, subsoil, and topsoil are replaced, and the surface is contoured to blend with the surrounding terrain and drainage is reestablish throughout the area. With this technique, it is not uncommon for a pit to be opened, closed, and seeded within a year.

Reclaimed mine areas are monitored until self-sustaining plant communities have been established and the reclamation goals have been met. In many cases, well-planned and well-executed reclamation has improved habitat for wildlife, compared to pre-mining conditions. Creative ways in which this can be achieved include using topsoil-rich areas to assist in reclaiming topsoil-poor areas and creating local ponds and wetlands where none previously existed.

Following removal from the site, bentonite is hauled to a processing plant and stockpiled by quality. At the plant, clays of different qualities are frequently blended during processing to achieve more consistent product quality. The specific processing methods employed depend both on the nature of the crude bentonite and its intended end use. In general, processing is designed to maximize the dispersibility of the clay, increase its surface area, alter its surface chemical properties and, in some cases, increase its montmorillonite content. These goals, and thus the steps to achieve them, are often interrelated.

As an inveterate entrepreneur, I sense a huge opportunity for transforming fallow deserts into fertile farmland and tree farms with bentonite as a soil amendment and have dubbed this scheme as "Sand to Soil" ™.