Soil tests and analyses are of course performed after a soil sample is collected, but it is probably a good idea to consult with some labs and select one before you actually collect the sample. You should discuss your situation with the consultants or managers of the soil testing lab you chose to use regarding the sample collection process to insure that you provide adequate materials to the lab for analysis. They can also help you determine what you want the samples to be analyzed or tested for. This will depend to some extent on your location, the problems you have encountered (plant symptoms experienced in the field), and past fertilization history.

The 13 essential mineral nutrients for cotton include the macronutrients: nitrogen (N), phosphorus (P), and potassium (K); the secondary nutrients: magnesium (Mg), calcium (Ca), and sulfur (S); and the micronutrients: boron (B), copper (Cu), chlorine (Cl), iron (Fe), molybdenum (Mo), manganese (Mn), and zinc (Zn). Some sources now may also include sodium (NA), cobalt (Co), vanadium (V), and silicon (Si) among the list of essential plant nutrients. Because a nutrient is considered as being essential for cotton growth and development, it does not necessarily mean that you will need to test for or fertilize with each and every one of these. This too will depend upon location, plant symptoms, and fertilization history. The nutrients to test for in a standard soil analysis will normally include N, P, K, and pH (level of soil acidity or basicity). In some cases it will also be necessary to include Zn, Fe, and B.

In the western portions of the cotton belt it will always be important to evaluate soil salinity by measuring the electrical conductivity of the soil extract (ECe) and the Na levels, commonly expressed as an exchangeable sodium percentage (ESP) or the sodium adsorption ratio (SAR). Salinity levels can impact crop vigor and yield potential, particularly in the early stages of development. Sodium represents an aspect of the soil system that is evaluated chemically, but it's primary impact is physical in that high sodium concentrations cause a dispersal of soil particles which in turn results in a breakdown in soil structure, reducing water penetration and infiltration, aeration, and increasing crusting problems. Salinity and NA levels represent good examples of where it can be important to consider sample collection for a specific analysis. Instead of collecting a soil sample to a depth of six to 12 inches, it may be necessary to sample only the upper few inches of the soil surface when determining salt or Na levels. Relatively high concentrations of salt or Na in the upper few inches of the soil can have a severe impact on early seedling vigor (salt) or create soil crusting problems (Na).