Lettuce in Arizona is produced on a range of soil types. The lighter textured soils may include loamy sands and sandy loams while the heavier soils would include clay loams. Much of the land used to produce lettuce or other cool season vegetables is used for other warm season crops during the spring, summer, or fall period. Crops typically grown between produce crops may include wheat, melons, safflower, cotton, or sudangrass.
Lettuce grows best in fields that are level and well drained. For this reason, land cropped to lettuce is typically deep chiseled and laser leveled every season. Deep chiseling helps facilitate good internal drainage on even the heaviest of soils and helps preclude detrimental salt accumulation in the soil profile. Lettuce is highly sensitive to salinity. Soils with an electrical conductivity of their saturated paste extract of two or less are highly desirable. Land used for lettuce is often pre-irrigated before land preparation is completed to facilitate salt leaching (movement with water below the root zone).
The seedbed used for lettuce should be tilled until it is soft and friable to a depth of 10 to 12 inches. Depending on soil texture, this is accomplished by moderate to extensive disking. Once the soil is of a desirable consistency, rows are listed, beds are shaped, and lettuce is seeded two rows per bed. In Arizona, lettuce beds are typically 8 to 10 inches high on 40 to 42 inch centers and are almost always laid out in a north-south direction. This orientation minimizes temperature and light differences between the two rows on each bed.
Lettuce produced on low desert soils requires from 150 to 300 pounds of nitrogen (N) to the acre for optimal yields. The actual rate will vary depending upon residual soil N, soil texture, irrigation and rainfall. Split applications of N are usually more efficient than a single preplant application because N in the soil is subject to leaching, denitrification (gaseous loss to the atmosphere), and other mechanisms of loss during the growing season. Generally, 50 pounds N per acre is considered adequate preplant N. Stand loss and stunting may result from excessive amounts of ammonium-N (especially on light or sandy soils).
Subsequent applications of N can be applied by sidedress or water-run and usually start after thinning and cultivation. Typically following thinning, lettuce will be cultivated and sidedressed with N. The amount and frequency of N application can be adjusted based upon a pre-sidedress soil nitrate-N test or a plant midrib tissue test. Lettuce grown in soils with a nitrate-N concentration of 20 ppm or greater in the top 12 inches will generally not respond to additional N fertilizer. Midrib samples are also useful during the second half of the growing season (after the 8 to 10-leaf stage). Generally, lettuce is not considered deficient in N if midribs nitrate-N concentrations exceed 8000 ppm. A second series of cultivation and sidedressing is usually conducted approximately 14 days following the first such operation, usually near early head formation. Additional sidedress applications of N may be applied if adverse growing conditions are encountered.
Recent research indicates that the use of controlled release fertilizers may represent a viable alternative to split application of conventional soluble N sources. There is considerable variation in N release rates and costs of controlled release N fertilizers, and individual products should be closely evaluated before they are utilized.
Phosphorus (P) fertilizer should be broadcast or banded 2 to 3 inches below and beside the seed row immediately before planting. Studies have shown that P applied later in the season is less effective than that applied preplant. Rates of P applied can be adjusted using a preplant P soil test. Lettuce in Arizona will require from 200 to 400 lbs P2O5 per acre. Because P applied by band is utilized more efficiently, generally only 50 to 60% of that typically applied by broadcasting is needed for optimal yields. Tissue tests are considered inconsequential because it is extremely difficult to correct P deficiencies mid-season. Nevertheless, these tests are useful tools to diagnose P deficiencies. Generally, whole plants with leaf tissue P concentrations of less than 0.38% or midrib P extracts less than 0.30% are considered deficient in P.
Potassium fertilization is generally not needed for lettuce production in Arizona. Most soils used for lettuce production in Arizona have exchangeable K levels, a clay mineralogy (mica), and K release rates that make a K response improbable. Additionally, most lettuce is irrigated with Colorado River water that contains approximately 5 ppm K or 15 lbs K per acre foot of water.
Although some growers commonly use micronutrient fertilizers, most recent research suggests that lettuce produced in Arizona does not show a positive yield and quality response. Therefore, the routine use of soil or foliar applied micronutrients cannot be economically justified. Micronutrient fertilization should be based on the actual diagnosis of a deficiency.
Cooperative Extension bulletin number 8922 "Fertilizing Head Lettuce in Arizona" provides tables for recommended fertilizer rates (based upon soil and midrib analysis) and detailed sampling procedures. Contact your local county Cooperative Extension office for a copy of this publication.