Humankind is faced with the continuing challenge of sustainably growing sufficient food to feed an ever-growing population. The United Nations Food and Agricultural Organization [1] citing a United Nations World Population Prospects report predicts that the earth’s population will reach 9.1 billion by the middle of this century.

Much of this growth will be in nations whose populations now suffer from malnutrition or outright starvation. In addition to a growing population, the increase in people will demand more food, more meat, and higher quality food because it will be more urban and wealthier according to FAO. Their estimate is that increased demand will require current food production to rise by 60 percent.

The challenge becomes more acute when it is understood that the land area for growing food is not expanding. Indeed, urban growth onto farmland and loss of arable (farmable) soil by wind and water erosion are reducing the available land area most suitable for farming. More land can be brought into production, but with a potentially high environmental and monetary cost.

The United Nations projects that there is a significant land reserve in the world that could be brought into production if needed, particularly in sub-Saharan Africa and Latin America, but there are significant constraints to bringing these lands into production [2]. 

As production per acre has increased on the best agricultural lands, less productive lands have been removed from production. If these lands are brought back into production, the environmental costs may be significant. One of these costs is accelerated soil erosion.

What is soil erosion, and how can soil be managed sustainably to meet the challenge of food for all?  Soil erosion is a two part process that occurs when soil is detached (part 1) from one location and transported (part 2) to another. Soil erosion may occur at geologic rates and at accelerated rates in excess of the natural rates.

The amount of soil eroded is a function of the erosivity (energy) of wind or water, the erodibility or susceptibility of soil to the erosive forces of wind or water, the steepness of the slope on which the soil resides and the amount and kind of cover on the soil surface. Geologic soil erosion is a natural process that is part of the earth’s cycle of mountain building and destruction. Geologic erosion created the Grand Canyon and the dunes in White Sands National Monument in New Mexico.

Geologic erosion is not a process that is managed for agriculture. Accelerated erosion is soil erosion at rates greater than geologic erosion. It is a process that must be managed, controlled and minimized if soil and water resources are to be protected for our long-term benefit. Accelerated erosion occurs when the energy from rain or wind contacts bare soil, detaching bits from the surface and those bits are transported to another location.

 

soil erosion 1 

 

Detachment and transport are two natural processes that become a problem when poor soil management leads to accelerated rates of soil erosion. Detachment of soil particles from the soil surface is primarily due to raindrop impact on the soil surface. Raindrop impact can also transport bits of soil downhill. Raindrop detachment is a function of the amount and intensity of rain. The potential for accelerated soil erosion is greatest in locations where there is much rain or the rain is intense.

Examples of high intensity rains are mid-western thunderstorms and tropical rainstorms. Detachment also occurs when water flows over the soil surface. Land slope determines the water flow rate. Surface flow may become concentrated in shallow channels called rills. Water flow in rills has the capacity to detach and transport soil particles. If rills are allowed to persist and grow, they can deepen and widen until they are gullies that can’t be crossed by farm equipment. At that point, soil erosion is severe.

 

soil erosion 2

 

Wind erosion is also a combination of detachment and transport of particles. Detachment by wind is primarily a function of wind speed. Where wind speeds are greatest, the potential for accelerated soil erosion is greatest. Detachment and transport by wind are by three processes, rolling, saltation and suspension.

The largest soil particles are rolled along the soil surface. These particles don’t move far because of their size. Saltation, bouncing of particles along the soil surface and at heights of a few feet above the surface moves large amounts of medium-sized particles short distances. The smallest soil particles are detached and transported long distances by suspension. Particles suspended high in the atmosphere that originated in China have been detected in Hawaii and particles that originated in Africa have been detected in South America.

This detachment and transport of soil particles creates two problems. The first occurs at the detachment location where plant nutrients, organic matter and mineral particles are removed from the soil mass. Over time, the soil thickness and the volume of soil available for plant roots to explore for nutrients and water is reduced. In the extreme, the soil thickness is reduced so that insufficient depth, nutrients or water is available for plant growth. In parts of the world where soil erosion is severe, the eroded land is abandoned because farming is impossible on the thin soils.

Detached particles are transported away from the detachment site and are often deposited in waterways where they become pollutants. Soil, that essential material for plant growth is transformed into sediment, which clogs streams, reduces reservoir capacity, and fertilizes lakes and ultimately the ocean. In the case of wind erosion, soil becomes dust, reducing visibility and becoming a health hazard for those breathing it.

 

soil erosion 3

 

Accelerated erosion is not one problem, it is two, reducing the productivity of soil at the location that loses material and reducing the quality of the air and water where the soil is deposited. Reducing accelerated soil erosion requires that soil be protected from the energy of wind and rain. Keeping the soil covered, completely or in part, is an effective method for protecting the soil from the detaching and transporting power of rain and wind. Plants, especially growing plants are particularly effective.

Growing plants have roots that bind soil particles together, increasing the size of groups of soil particles, called aggregates. In addition, roots slough cells and exude organic materials that become food for soil microorganisms that in turn produce gummy substances that bind individual soil particles into aggregates that strengthen the soil and protect the soil from the erosive force of wind and water.

Well-aggregated soils are porous, promoting infiltration of water into soil and reducing surface runoff that can transport soil away. While roots are promoting soil aggregation below the soil surface, plant parts on the soil surface are protecting the soil from raindrop impact or wind that detaches and transports soil particles. Plants and plant parts on the soil surface help to reduce the velocity of water flowing over the soil surface. Slower flowing water has less capacity to detach and transport soil than fast flowing water.

Living plants support both above and below ground soil conservation. Cover crops, those that are planted to protect soil from erosive rain and wind are effective in promoting soil conservation.

 

soil erosion 4

 

 

Dead plants, often referred to as mulch, protect the soil surface from raindrop impact and the erosive force of wind. Soil cover, regardless of the kind is an essential part of soil conservation. In this International Year of Soils, it is important to remember that soils are a critical part of food production. Along with water, sunlight, and good farmers, soils are an essential part of growing food, feed and fiber. Good soil management maximizes crop yield while protecting the resource from degradation. Producing sufficient food for the world’s growing population while conserving and protecting soil resources necessary to produce the food is a challenge that requires our attention.

 

[1]http://www.fao.org/fileadmin/templates/wsfs/docs/expert_paper/How_to_Feed_the_World_in_2050.pdf.

[2] Alexandratos, N. and J. Bruinsma. 2012. World agriculture towards 2030/2050: the 2012 revision. ESA Working paper No. 12-03. Rome, FAO. 

Figure 1. Erosion in the Dunnigan Hills, Yolo County California. Soil removed from the hillsides is deposited on the young wheat seedlings. (photo by the author)

Figure 2. Severe gully erosion, Canberra Australia. (photo by the author) 

Figure 3. Dust accumulated on a picnic table after a single dust storm event. (photo by the author).

Figure 4. Cover crop in a young vineyard protects the soil from raindrop impact and overland flow, reducing the potential soil erosion. (photo by the author)