Installment 6
What is the difference between available minerals and total minerals in the soil?
There are about 4 million ponds of various minerals in an acre of soil. Some are potentially plant nutrients, some are toxic, and others are pretty much neutral to plant growth. A very large percentage of those minerals are in the form of very stable crystals. These form the soil substrate or soil parent materials. In the crystalline form they are not available to plant roots. The action of natural environmental acids (rainfall, etc) and organic acids (from plant roots, micro-organisms, animal waste), combine to slowly degrade these crystals to a form that plants can take up as nutrients. These ‘available’ minerals are a very small fraction of the total mineral content of the soil. Determining the makeup of that small fraction of available nutrients – that is the purpose of the standard soil test.
The remainder of the non-degraded, crystallized mineral nutrients represent a potential reservoir of plant nutrients. The rate at which these reserve nutrients become available depends on natural processes and on the level of biological activity in the soil. Depending on the level of nutrient in the parent material of the soil, significant amounts of nutrient (esp potassium, calcium, magnesium) may become available in the soil, as the season progresses.
Since available minerals are such a small fraction of total minerals, lab testing for total nutrients in the soil is not all that economically viable. Still, it would be useful to know the main parent materials comprising the soil (and the ones that are absent). Fortunately, this sort of general information about the total mineral composition of your soil can be obtained at no charge from USDA soil classifications found on soil maps. See here and here for more info on soil maps and soil classifications from USDA.
When thinking about the relationship between total (parent material) and available soil minerals, it is important to remeber the major role soil micro-organisms play in making plant nutrients available. The X factor in determining what minerals will be available to your crop in any given season is the amount of organic matter and soil biology on your own farm. The addition and conservation of organic matter, along with the establishment of robust, aerobic microbial populations in the soil can make a much larger portion of ‘reserve’ minerals available for plant use over time. Knowledge of these relationships help the experienced farmer to do their final ‘calibration’ of what inputs are needed on their own farm from year to year.
Next Installment
How does plant testing fit in with soil testing?