7.5: Flow Patterns at Large Scale

We have looked at carbon and nutrient balances on farms, but a larger-scale break in nutrient cycling occurs as agricultural products are shipped long distances across continents or even oceans. When you deliver a train or boat load of grain from the Midwest to the eastern United States, or even from Brazil to Asia, a lot of carbon and nutrients are flowing and don’t return. The international trade in agricultural commodities such as corn, soybeans and wheat means that significant quantities of the basic ingredient of soil health are shipped overseas. (It is worth noting because it takes so much water to produce grains: around 110 gallons (approximately 900 pounds) of water to produce one pound of corn grain or soybeans. In essence, water is also being shipped abroad embedded in the agricultural exports.)

Long-distance transportation of nutrients and carbon is central to the way the modern food system functions. On average, the food we eat has traveled about 1,300 miles from field to processor to distributor to consumer. Exporting wheat from the Pacific Northwest and soybeans from the Midwest of the United States to China involves even longer distances, as does importing apples from New Zealand to Los Angeles. The nutrients in concentrated commercial fertilizers also travel large distances from the mine or factory to distributors and then to the field, like potassium from Saskatchewan to Ohio, or phosphorus from Morocco to Germany. The specialization of the corn and soybean farms of the Midwest and the hog and chicken mega farms centralized in a few regions, such as Arkansas, the East Coast’s Delmarva Peninsula and North Carolina, has created a unique situation. This regional specialization of farms appears to make economic sense (or perhaps not?) but disrupts the nutrient and carbon cycles that maintain soil health. The nutrients from crop farms travel a long way to animal farms and are replaced with fertilizers from a completely different place. Meanwhile, the animal farms become overloaded with nutrients. The carbon exported from the crop farms never replaces the organic matter lost during the year.

Of course, the very purpose of agriculture in the modern world—the growing of food and fiber and the use of the products by people living away from the farm—results in a loss of nutrients from the soil, even under the best possible management. In addition, leaching losses of nutrients, such as calcium, magnesium and potassium, are accelerated by acidification, which occurs naturally or can be caused by the use of some fertilizers. Soil minerals, especially in the “young” soils of glaciated regions and in arid regions not subject to much leaching, may still supply lots of phosphorus, potassium, calcium, magnesium and many other nutrients even after many years of cropping. A soil with plentiful active organic matter also may supply nutrients for a long time. A mixed crop-livestock system that exports only animal products cycles the nutrients and carbon well, and it may take a long time to deplete a rich soil because few nutrients are exported with those products.

But for crop farms, especially in humid regions, the depletion occurs more rapidly because more nutrients are exported per acre each year. Eventually a continually cropped soil becomes nutrient depleted, and sooner or later you will need to apply some phosphorus or potassium. Nitrogen is the only nutrient you can “produce” on the farm: legumes and their bacteria working together can remove nitrogen gas from the atmosphere and change it into forms that plants can use.

The issue eventually becomes not whether nutrients will be imported onto the farm but rather what source of nutrients you should use. Will the nutrients brought onto the farm be commercial fertilizers; traditional amendments (limestone); biologically fixed nitrogen; imported feeds or minerals for livestock; organic materials such as manures, composts, and sludges; or some combination of sources? Some nutrient sources are nutrient dense and therefore efficiently transported and applied, like inorganic fertilizers. But they don’t provide the benefits of carbon, which is critical to the biological processes in the soil. Organic sources provide the benefits of both nutrients and carbon, but the nutrients are in low concentrations and expensive transport effectively restricts their application to nearby locations.

Finally, a few words about a positive aspect of large-scale nutrient flows. National and international food trade occasionally provides benefits by reducing the effects of regional micronutrient deficiencies in the importing country. For example, selenium is a trace element that most humans and animals acquire from the soil through their diets. European selenium intake is enhanced by importing wheat grown on U.S. soils, which are naturally higher in the nutrient.

Global Grain Trade And Nutrient-Carbon Flows

Several grain and oil crops are heavily traded around the world, including wheat, corn, soybeans, rice and oilseeds. They greatly impact the global flow of nutrients and carbon, the basic ingredients for healthy soil. The most prominent transfers involve corn and soybeans shipped from the Americas to East Asia, Europe and the Middle East. Why is that?

The import of these grain crops is heavily driven by higher demand for animal protein and edible oils as a result of rising living standards and diversifying diets. Growing these crops also requires a large land base that many countries do not have. Japan and Korea are populous but also quite mountainous. The Middle East and Mexico are dry, and Europe has more animals than it can feed from its own land. China is by far the largest grain importer, especially of soybeans, because of agronomic constraints and domestic policies that prioritize growing cereal crops that meet basic food security. A large portion of the crops that feed its animals are therefore imported. (China raises about half the world’s pigs.)

The Americas have a large agricultural land base and are the primary exporter of those grains, with the United States, Brazil and Argentina accounting for almost 90% of the soybean and 75% of the global corn exports that are mostly used to raise animals in other parts of the world (Figure 7.6). These countries also have policies that promote grain production and exports. U.S. grain areas have been fairly stable over the past decades, but the South American countries have met the higher global demand by putting extensive areas of grassland, savannah and even rainforest into crop production.

Wheat and rice are different, not only because they are mostly consumed directly by humans. Wheat export is more balanced among countries with Russia (20%), Canada (14%), the United States (13%), France (10%), Australia (8%) and Ukraine (7%) being the top exporters. Rice tends to be grown more for domestic consumption and therefore not exported as much, but India (30%) and Thailand (23%) are the main international suppliers.

With the large transfer of grains, and associated carbon and nutrients, from one region to another, deficiencies are created with the exporters and excesses with the importers. Evidence is emerging that the breadbasket soils in the Americas are becoming less healthy and have lost organic matter. The importing countries have many livestock farms that accumulate nutrients and have water pollution concerns. Hypoxia (dead zones) is an increasing problem in the seas around Japan, Korea, China and northern Europe.

Figure 7.6. Leading exporters and destinations of soybeans during 2016/2017. Carbon and nutrients are reallocated and impact soils on both sides. Source: USDA-ERS.