A teaspoon of healthy soil contains more microbes than there are people on the planet. Chances are, though, the spoonful you take from an agricultural field today will have fewer than that; the microbes representing the populations of some of the largest countries—say, China and India combined—might very well be missing.
This article from The Nature Conservancy magazine estimates that organic matter in cropland soil has decreased by 30 to 50% worldwide. Although healthy soils contain 5% organic matter or more, depleted soils might have just 2% or less, rendering them much less productive.
There’s a fairly easy fix. Rather than relying solely on chemical fertilizers, farmers can take steps to increase organic matter and soil microbes by diversifying their crops. The article makes a compelling case for the benefits of cover crops, which not only prevent erosion but also improve soil health. It also argues for diversity: the greater variety of plants that grow (and decay) in a field, the more varied the soil microorganisms and the more fertile the soil. Diversity can be accomplished either by planting different cover crops in the off season—clover in a corn field, for example—or by intercropping, planting more than one crop at a time in the same field.
Diversity isn’t something modern farming is particularly good at, though. There are more than 300,000 species of plants in the world; for humans, somewhere between 20,000 and 80,000 of those species are edible. But we really use only about 200 of them, and three—rice, maize, and wheat—account for 60% of the calories we obtain from plants. The UN’s Food and Agriculture Organization estimates that 75% of the world’s food comes from only 12 plants and five animal species, and that since the 1900s, we’ve lost about 75% of the agricultural diversity in our crops.
The trend toward monocultures presents other concerns besides soil health. Although this homogeneity enables the use of efficient farming methods and production on a large scale, it also brings risks. Disease or parasites can do more damage, more quickly, than when crops are diversified. It also leads to what researchers refer to as “genetic erosion,” leaving fewer types of plants for research or future development.
Diversification might not be profitable or even feasible everywhere, but several organizations are now teaming up to try to quantify the benefits. Many agricultural operations in the US plant just corn and soybeans, sometimes alternating them on a two-year rotation. The National Corn Growers’ Association and nonprofit groups like The Nature Conservancy have formed the Soil Health Partnership with a goal of gathering long-term data on the benefits of cover crops, reduced tillage, and diversified rotations to see how they affect not only soil performance but also a field’s profitability.