IRRIGATION OF SEED CARROTS ON A SANDY LOAM SOIL

Little research has been reported which quantifies the response of a carrot (Daucus carrota L. var sativa DC.) seed crop to water management. While the area of seed production of this crop in the United States is less than 3 000 ha, the return ranges from US $2 000 to $10 000 ha-1. Because of the need to mature and dry the seed on the plant, carrot seed is generally grown in areas with negligible summer rain and thus depends on irrigation to supply the crop water requirement. A study was conducted to determine the effect of irrigation water management on seed production and crop water use of carrots grown by the root-to-seed method. Two carrot types (Nantes and Imperator) were evaluated in 9 irrigation treatments over a three year study period. Irrigation treatments which replaced a percentage of the calculated crop evapotranspiration on either a daily basis or when a soil water depletion reached 30 mm were used. A trickle irrigation system with the laterals placed on the carrot bed was used to apply a uniform and accurate amount of water. There was a marked difference in the crop response to the water management of the two carrot types used. The Nantes type exhibited a positive response to moderate water deficits in terms of improved pure live seed (PLS) yield while the Imperator achieved its maximum yield when it was not stressed. Higher irrigation applications in the Nantes type resulted in reduced yields while the Imperator was not affected after its non-stress water requirement was met. Soil water data indicated that the most active zone of extraction of water was to a depth of 1.5 m in the soil profile. As the depth of applied water approached the crop water requirement, the depth of extraction was reduced. Increasing the frequency of irrigation also tended to reduce the depth of extraction of soil water. A total crop water use of approximately 550 to 620 mm was needed to achieve the best PLS yield which is roughly equal to potential evapotranspiration in the San Joaquin Valley, during the time that the crop water use was calculated. In such a climate, the irrigation interval should not exceed 3 to 5 days depending on the time of year.