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By Ali Montazar, Irrigation and Water Management Advisor, University of California Cooperative Extension, Imperial, Riverside and San Diego Counties
A better understanding of how carrot crops use water and nitrogen may ultimately improve carrot production in the low desert areas of California.
Carrots are one of the 10 major commodities grown in Imperial County, California, with an average acreage of nearly 16,000 over the past decade, according to 2010-2019 Imperial County Agricultural Crop & Livestock Reports. The farm gate value of fresh market and processing carrots was about $66 million in 2019.
In the Imperial Valley, most carrots are typically sprinkler irrigated for stand establishment and subsequently furrow irrigated for the remainder of the growing season. However, there are fields that are irrigated by solid set sprinkler systems during the entire crop season (Fig. 1).
Nitrogen (N) and irrigation management in carrot production systems is critical for increasing the efficiency of crop production and decreasing costs and nitrate leaching losses. The N needs of carrots for optimum storage root yield depend on the climate, soil and residual soil N from the previous season. To accomplish greater nitrogen and water efficiency, more accurate crop water use information of carrots is required with respect to different soil types, carrot crops, weather and farming practices. Utilizing more accurate estimates of crop water consumption and N uptake may have a significant impact on water quality issues and on soil water and N availability, potentially increasing the economic sustainability of carrot production.
An ongoing study at the University of California particularly seeks to quantify and fully understand carrot production issues under current management practices, and to fill knowledge gaps for nitrogen and water management in carrots through conducting experimental trials in the low desert of California.
Field Experiments
Field experiments were conducted at the University of California Desert Research and Extension Center (DREC) located in Holtville, California, during the 2019-2020 crop season (Fig. 2). The trial consisted of two sprinkler irrigation regimes and three nitrogen strategies. In addition, measurements were conducted in five commercial fields in the Imperial Valley with various soil types and under sprinkler and furrow irrigation. Stand establishment was accomplished by sprinklers at the experimental sites.
The actual crop water consumption (actual crop ET; ET stands for evapotranspiration) was measured using the residual of the energy balance method with a combination of surface renewal and eddy covariance equipment (fully automated ET tower in Fig. 3). As an affordable tool to estimate actual crop ET, Tule Technology sensors were also set up at all experimental sites. The Tule ET data were verified using the ET estimates from the fully automated ET station. Soil moisture sensors were installed at multiple depths to monitor soil water potential on a continuous basis (Fig. 3). In both the DREC trial and the commercial sites, actual soil nitrate content and the total N in the plants (tops and roots) were measured several times per crop season.
Results
The common irrigation practice in carrot stand establishment is to irrigate the field every other day during the first two to three weeks after seeding. Carrots germinate slowly, and hence, the beds need to be kept moist to prevent crusting.A comparison of applied water and crop water consumption indicates that the carrot fields could be overirrigated by three times of crop water requirements during the stand establishment.
A wide range in the length of the crop season (seeding through harvest) was observed, ranging from a 128-day period in a processing carrot to a 177-day period in a fresh market carrot. Theseasonal crop water consumption varied between 12.5 inches and 16.6 inches at the experimental sites (Fig. 4a). The results clearly demonstrate that carrot fields may have variable irrigation water requirements depending upon early/late planting, processing vs. fresh market, irrigation practices, length of crop season and soil type. A peak daily crop water use of 0.21 inches on March 23, 151 days after planting, was observed in a fresh market carrot field (Fig. 4b).
Water stress should be avoided throughout the carrot growing cycle. The critical period for irrigation is between fruit set and harvest. Sprinkler irrigation may be considered as a more effective irrigation tool when compared with furrow irrigation. More frequent and light irrigation events are possible by sprinkler irrigation. Over-irrigation of carrot fields increases the incidence of hairy roots, and severe drying and wetting cycles result in significant splitting of roots. Sprinklers reduce salinity issues, which is important since carrots are very sensitive to salt accumulation.
The irrigation water that needs to be applied in an individual field depends on crop water requirements and the efficiency of the irrigation system. Assuming an average irrigation efficiency of 70 percent, the approximate gross irrigation water needs of carrot fields in the low desert would be 1.5 – 2.1 ac-feet/ac (pre-irrigation is not included). Pre-irrigation along with proper irrigation scheduling over the season may effectively maintain crop water needs and salinity in carrots.
The preliminary results of this study demonstrate a notable amount of N uptake both in the roots and tops at harvest time. For instance, a total N content of 286 lbs./acre in plants grown for a period of 164 days (149- and 137- lbs./acre in roots and tops, respectively) was observed in a fresh market carrot field (Fig. 5). The total N content was 297 lbs./acre in processing carrots (123 lbs./acre in roots and 174 lbs./acre in tops). Total N uptake in the roots and tops was similar, with rapid increase beginning 55 days and 45 days after seeding, respectively. The rate of increase in total N content in the roots did not decline near harvest in any of the experimental sites, while it declined in the tops for fresh market carrots beginning 125 days after seeding.
Nearly 50 percent of seasonal N accumulated in the tops and the roots occurred at 85-90 days after planting when the canopy is fully developed. An effective nitrogen fertilizer application could be splitting N application into 10-12 percent at planting, 30-35 percent as side dressing (in case of furrow irrigation) and the remainder through irrigation events. Assuming a 150–170-day period as a carrot crop season, it is recommended to apply the total nitrogen fertilizers by 15-20 days before harvest.
The results of this study illustrate that 45-55 percent of total N accumulated in the carrot plants is left in the fields as residual soil N right after harvest which could contribute as a source of nitrogen for the following season (Fig. 6). Further work is needed to quantify what fraction of N provided by the plant residues potentially contributes to the following season, particularly since there is a risk of leaching a portion of residual N due to heavy pre-irrigation in the late summer during land preparation.
