Again, the only way to conduct this type of soil evaluation is to get out into the field with your favorite soil probe, auger, or a shovel and collect a good set of samples. Commonly, a recommended frequency for sampling a given field is once every three to four years, assuming no nutritional or production problems develop. This type of soil sampling frequency is usually considered as a minimum for developing a soil fertility maintenance program.

Usually, it is recommended that a single soil sample should be collected for any given field or management unit. However, a single soil sample should consist of at least 25 individual cores collected from representative areas of the field or management unit, which are then mixed together into a common "composite" soil sample. How much of this sample to send into the soil testing lab will depend on the analyses to be performed and the specific lab, but usually about 20-30 ounces (volume) are required. Therefore, of the 25 or more soil cores which are collected from a field, management unit or problem area, only the amount needed to fill an appropriate soil sample container is actually collected from the composite sample and sent into the lab. Depth of sampling is usually at least six inches, but may extend to 12 inches or more, depending on the situation. Sampling technique (depth and placement) may also depend on whether the field is irrigated, bedded or flat, and also on what is needed from the analysis such as nitrate-nitrogen (NO3--N), sulfate-sulfur (SO42--S), phosphate-phosphorus (PO43--P), sodium (Na) or total salt concentration, etc.. In reference to sample frequency, depth, amounts of soil needed, and tests to run; this will all depend to some extent on the nature and intent of the sampling process as to whether it involves routine management or diagnosis of problem areas in a field.

The identification of a field or management unit for soil sampling relates to the evaluation of field conditions at the end of the recent cotton production season. Individual fields are usually the largest unit that is recommended for an individual soil sample. However, if there are parts of a given field which are obviously unique and may require specific management, they should be sampled independently.

Figure 2. (A) Soil sampling pattern within a relatively uniform field, and (B) soil sampling pattern within three distinct zones of a field, each having unique characteristics warranting a separate sample (approximately 25-30 cores/sample).

For example, a relatively uniform field (Figure 2A), can be easily managed in a consistent manner, and therefore could be sampled by collecting at least 25 soil cores from representative areas (each core marked with an o). Figure 2B, on the other hand, could represent a field that has three distinct areas (A, B, and C), each of which should be sampled independently (25 soil cores from each of these areas). How one delineates between these areas in the field should depend first of all on plant growth and performance, which could relate to our end of season evaluations discussed earlier. The results you get back from the lab will only be as representative as the sample you collect.