Soil Testing for Nutrient Availability Procedures and Interpretation for California Vegetable Crop Production

Soil sample collection Nutrient content of soil can vary greatly by depth. The top 4-6 inches, the zone into which most fertilizer is placed and most crop residue is incorporated, often has much higher levels of organic matter, nitrogen (N), phosphorus (P), potassium (K) and micronutrients than the soil below. To accurately reflect overall nutrient availability a soil sample should be collected with a coring device that collects an equal amount of soil over the entire depth of the sample. In samples collected with a shovel the top few inches are usually over-represented, and the test results will exaggerate the actual fertility of the field. Variability across a field can also be an issue, particularly in fields with a range of soil textures or parent materials. To encompass field variability, a composite sample of at least a dozen soil cores should be gathered, representing all areas of the field. In fields in which significant soil variability exists, sampling different areas of the field separately may be warranted. This is particularly true of fields with areas that repeatedly have low yield; sampling low-yielding areas separately can identify problems that would otherwise be masked if only a single composite soil sample from the whole field was analyzed. The appropriate depth to sample depends on the crop to be grown. Sampling the top 6 inches may be appropriate for shallowly rooted crops such as onion. However, for most vegetable crops sampling the top 12 inches is preferable; this is particularly the case where furrow irrigation or buried drip irrigation is used; with both methods the top few inches of soil may be too dry for root activity for a significant portion of the season. Soil sampling usually takes place between crops, which in the California vegetable industry can be any time of the year. Most soil fertility properties are quite stable over time, meaning that sampling soon after terminating a crop and incorporating the residue will yield results similar to those obtained by sampling just prior to the establishment of the next crop. The exception is nitrate-nitrogen (NO3-N), which fluctuates significantly over relatively shorts periods of time due to the combined effects of soil microbial activity and leaching by rain or irrigation.