Pest management is cheaper and easier on land that is well suited for a particular crop. Lettuce planted in fields infested with lettuce pests or fields with poor nutrient balance will suffer yield loss. Choose fields for lettuce planting carefully, taking into account the field's cropping history. Also take into account pest problems that may originate in adjacent crops or fallow ground such as root knot nematode, green peach aphid, loopers, or problematic weeds. Avoid fields that are heavily infested with weeds such as common groundsel, prickly lettuce, and sowthistle that are hard to control with herbicides labeled for lettuce.
When selecting fields for lettuce, check the soil and available water for the following:
- Mineral levels. Check for excessive salt, sodium, boron, and other mineral imbalances.
- Soil pH. Lettuce grows best at pH 6.0–6.5.
- Herbicide residues. Residues may inhibit seedling growth. Assess plantback intervals of herbicides used in the prior crop and if there may be a risk of carryover of harmful residues.
- Soil type. Lettuce can grow in a variety of well-drained soils, however, it does best on fertile, high organic matter soils with good water-holding capacities. Heavy clay soils require very careful water management, and root development may be problematic in fields with shallow hardpans, compacted layers, or high water tables. Sandier soils usually have higher root knot nematode numbers than loamier soils. Avoid fields with major variations in soil type that make herbicide application difficult because rates must be adjusted for soil type.
- Irrigation water. If the quality of the irrigation water is unknown, test for pH, salinity, and specific ions.
- Root knot nematodes. Test for nematodes in Central Valley and desert areas before planting lettuce if they have been a problem in a previous crop.
- Verticillium dahliae. Assay soil for microsclerotia if Verticillium wilt has been a problem in a previous crop.
Also check field records for:
- Agronomic information. Determine past lettuce varieties that have been planted, including their planting and harvest dates and yields. See if the field has supported successful production.
- Cropping history. Identify previous crops that are known hosts of lettuce pests.
- Surrounding crops and areas. Check for cultivated crops that harbor weeds, invertebrates, and diseases problematic to lettuce.
- Rotational crops. Consider planting a rotation crop to keep difficult-to-control pests and weeds from increasing their numbers.