Sampling

It is important to understand how the sample collection can affect the quality of the results obtained. The ideal situation is one in which any fertiliser application is spaced out over time, until the balance between the nutrients in the soil is achieved. Thus, for example, the application of a single nutrient can affect the level of other nutrients, as in the case of potash (K) which, once dissolved, acts as an extractant and displaces other cations such as calcium and magnesium from the exchange complex.

The depth of the soil samples is also very important; it can be done in a simple way or in a stratified way where soil samples are examined at different depths. As a minimum, the first 6-7 cm should be studied, and if the stratified sample needs to be studied, another sample can be taken up to 12 cm and so on, 3 depths being more than sufficient. It is not possible to compare results from samples of different depths or strata. These layering techniques will give us information on how salt washes are occurring, for example, or if there are deficiencies of a nutrient such as phosphorus, which is not very mobile.

Following Stevens' examination of thousands of samples of different soil types, it has been found, irrespective of root depth, that the greatest exchange of moisture and salinity occurs in the first 6-7 cm of soil.

As the irrigation water equilibrates with the pore water in the first 6-7 cm, the differences between the two chemical species decrease and so do the exchanges that take place.

The sample must be obtained with tools that allow continuous replication of the same depth, for which specific drills are available.

In addition, the leaf part must be included in the sample and the laboratory must be the one to sieve the upper organic part.