Study Finds Land Fallowing Improves Soil Quality in PVID

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by Jeremy Cusimano, MS, UA Department of Soil, Water, and Environmental Science, Sharon B. Megdal and Jean E. McLain, WRRC, and Jeffrey C. Silvertooth, UA Department of Soil, Water, and Environmental Science

The Palo Verde Irrigation District in California sees extra benefits for transferring water through the use of land fallowing—soil quality and crop production go up. Growing population in the Southwest and increasing probability of Colorado River water shortages have many regions considering options for meeting current and future water demands.  One option to address urban water demands in the Lower Colorado River Basin is the transfer of water from agricultural areas to highly populated urban areas, otherwise known as ag-to-urban water transfers. By definition, water transfers include voluntaryagreements that result in a temporary or permanent change in the type, time, and/or place of use of water.  In a 2012 report, the Western Governors Association wrote, “Water transfers can be local or distant; they can be a sale, lease, or donation; and they can move water among agricultural, municipal, industrial and environmental uses.”

One option that has been implemented in Southern California involves the transfer of water that has been “saved” by agriculture through conservation measures.  To accomplish these Colorado River water savings, management strategies, such as fallowing or idling land, shifting to crops that consume less water, or pumping groundwater for irrigation, are commonly used.  Fallowing, defined as arable land not under rotation that is set at rest for a period of time ranging from one to five years, is often the method of choice when it comes to water transfers in the Desert Southwest, where crop production is particularly water intensive due to climate, irrigation method and crop selection.  

It should be recognized that large-scale, long-term water transfers that utilize fallowing have the potential to impart significant changes to local soil quality and crop production.  Studies have shown that land fallowing is beneficial to soil quality, crop production and overall long-term sustainability.  Therefore, land fallowing associated with ag-to-urban water transfers can be viewed as a sustainable land management practice that can be employed to reverse some of the negative effects of continued soil cultivation.

A crop rotation and water supply program implemented by the Palo Verde Irrigation District (PVID) and Metropolitan Water District (MWD) includes fallowing a large proportion of arable land within the Palo Verde Valley, Calif. each year as part of the agreement between the two agencies.  The 35-year program transfers up to 118,000 acre-feet of Colorado River water annually to MWD through fallowing of up to 28,000 acres annually in the Palo Verde Valley.  Under the program, water normally used to irrigate farmland within the valley portion of PVID is “saved” through land fallowing, and an equal amount of water is made available to MWD.  

The effects of short-term land fallowing on soil quality and crop growth following the completion of this water transfer had not been studied in the region until recently.  A study, funded by UA Technology and Research Initiative Fund, Water Sustainability Program, and conducted by Jeremy Cusimano for his UA master’s thesis, aims to quantify the effects of short-term fallowing on soil quality and agricultural productivity within the Palo Verde Valley.  Working with local growers, the PVID, and the University of California Cooperative Extension in Blythe, the study examined differences in soil fertility, microbial communities, and crop growth and development between fallowed sites and cultivated sites throughout the Palo Verde Valley.

For the study, composite soil samples were systematically taken from numerous fallow and arable fields throughout the Valley.  Soil samples were analyzed for plant-available nutrients, salt content and organic matter; while analyses of microbial functionality and diversity provided insight into biological shifts in the soils.  The same fields were then planted in the fall with broccoli, which was used to compare the effects of fallowing on plant growth.  Results revealed significant differences between fallow and arable fields in the Palo Verde Valley.

The study found that short-term fallowing had increased the quantity of carbon, or soil organic matter, and nitrogen, However, it was also found that fallowing increased salinity by approximately 35 percent in PVID soils.  Such an increase in soil salinity would be of concern, but producers can reduce salinity buildup by applying excess (5-20 percent) irrigation water to leach salts from the soil.  The project also revealed that microbial communities in fallowed soils had significantly greater microbial diversity and functionality, indicating that microorganisms had benefitted from the period of rest. Such increased microbial diversity can lead to improved nutrient cycling and concomitant benefits to crop production.  Lastly, improvements in soil quality associated with fallowing enhanced the growth and production of the fall broccoli crop.  Marketable yield and total plant biomass were both significantly higher after fallowing, with fallowed fields producing on average an additional 30 cartons of broccoli per acre when compared with fields that had been continuously farmed.

The study indicates that the PVID-MWD water transfer has been beneficial to soil quality within the PVID.  Local growers and land managers agree, stating that fields have shown a positive response after just a few years of fallowing.  According to longtime PVID farmer Bart Fisher, “Those of us who farm in the Palo Verde Irrigation District have been very pleasantly surprised by the noticeable increase in productivity of fields that have been in fallow for two to five years.  At this point, fallow has become an integral part of our crop rotations, and we use it as a productive soil management technique.”

The beneficial effects of fallowing on nutrient dynamics, microbial communities and crop growth, however, may be short-lived, and possibly affected by grower management decisions.  Therefore, it is important that growers and land managers understand the mechanisms driving improvements in soil quality following fallowing, as these fluctuations may require a change in soil management strategies.

Furthermore, additional irrigation water is needed to leach salts that accumulate during the fallow period.  For example, an alfalfa crop grown in the PVID would require approximately 12 additional acre-inches of water applied throughout the growing season, although the quantity of additional water needed is largely dependent on the crop, soil characteristics, and other environmental conditions. Consequently, the need for supplementary irrigation water supplies in the PVID may arise.

The study, while limited in scale, shows that water transfers that include the use of land fallowing have the potential additional benefit of inducing significant improvements to soil quality and crop production.  With the likelihood that fallowing programs will be adopted in Arizona in the near future, the information from this study highlights the importance of considering the implications of fallowing.  The results of the study provide a foundation for further research concerning water transfers and their implications on soil quality and crop growth.

For further information on this study, contact Jeremy Cusimano at jeremycusimano@email.arizona.edu.

*The authors would like to acknowledge Ed Smith, Dr. Vonny Barlow, Bart Fisher, Jack Seiler, and Chad Elliot for help with implementation of the study.