“Agriculture needs to get back to letting land lay out, or fallow,” a keyboard farmer posted on Arizona Farm Bureau’s Facebook page. The commenter went on to say it’s a type of rotation.

We try to respond to comments. But in this case, we reached out to an expert for his assessment.

Jeffrey C. Silvertooth, Ph.D. — a professor and Extension Specialist in Agronomy and Soil Science in the Department of Environmental Science at the University of Arizona — offered an informative response.

Soil scientists teach that crop rotations are a basic and fundamentally sound agronomic practice to maintain healthy soil. The practice of crop rotation is rather literal, meaning the rotational changing of crops in a given field or set of fields.

“A classic crop rotation example is the sequencing of cotton, wheat, and alfalfa that has been common in central Arizona for many decades,” Silvertooth said. “In this case cotton is the warm-season, broadleaf crop (a perennial plant grown as an annual); wheat is the cool-season grass crop, and alfalfa is the perennial deep-rooted leguminous crop. A good crop rotation system provides a complementary series of crops, ideally including a perennial legume crop.”

In other words, a crop counts as complementary when it differs from the crop before it in ways that matter: its season, its plant family, how deep its roots go, and what it takes from or gives back to the soil. The cotton–wheat–alfalfa rotation does all four.

One crop lays the groundwork for the next one's success,

Land Fallowing in the Desert Southwest

Fallowing in the desert Southwest is counterproductive.

Silvertooth explained that fallowing leaves a field completely open, void of any crop. Fallowing is not a common component of a crop rotation system, and in desert agriculture, it can be counter-productive, particularly to save water for irrigation.

In dry climates, leaving land fallow can cause salts to build up near the soil surface, because far more water evaporates than falls as rain, Silvertooth explained. “That is a particular problem in areas with drainage problems and higher water tables.”

Fallowing also has strong impacts on the soil microbiome — the whole community of tiny living things in soil, mostly bacteria and fungi but also protozoa, viruses and other microbes that help feed roots and keep crops healthy — as “the soil microbiome needs water to live and fully function.”

Silvertooth went further: “In irrigated crop production systems, farmers will fallow land to save water. That is common when irrigation water sources are reduced. In that case, desert farmers will typically fallow marginal land first and focus the water that is available on the fields they plant.

“Field experimentation on common agronomic crops have consistently shown steady, consistent crop growth throughout the season and yield response to irrigation water. Farmers generally understand this relationship very well. They know that if they reduce irrigation to a crop due to reduced water availability, they will experience a loss in crop vigor and yield. So, they focus their available water on the fields they plant, and they must be careful not to over-plant their water supply.”

Examples of fallowed land can be found today in Pinal County, where two irrigation districts have lost access to Colorado River water via the Central Arizona Project (CAP). As a result, approximately 50% of prime agricultural acreage in Pinal County is now fallow. That is not a choice, and it is not a favorable practice, particularly when land is leased or rented for farming, which is the case for many crop fields in Arizona.

Another good example of fallowing land can be found in the Imperial and Palo Verde Valleys of California. Irrigation districts in both valleys have sold Colorado River water to the Metropolitan Water District (MWD) in southern California. As a result, they have less irrigation water and in response they simply do not plant as many acres as they would if they had full water allocations. Thus, they are forced to fallow, but not for reasons of soil health. 

Land Brought Back After Fallowing

When desert land is brought back into production after fallowing, additional water must be applied to leach the salts accumulated at the soil surface and to rejuvenate the soil microbiome.

Fallowing does not rest or regenerate the soil. And it does nothing positive for the soil microbiome, according to Silvertooth.

“Thus, crop rotations are a good practice,” said Silvertooth. “Fallowing land is not part of desert crop rotations; it’s a negative consequence of losing access to irrigation water.”