The most common question I receive is some variation of this: "What about nutrients in the soil that are not available?"

That question throws me for a bit of a loop, because the very purpose of soil testing is to find the availability of nutrients. Let me try to answer this question three times, in progressive order of complexity.

1. "What about nutrients in the soil that are not available?"

Answer: Soil tests already take availability into account. So one doesn't need to consider the term "availability." Just look at the number on the soil test and compare it to the guideline value that you choose. I think the most accurate values to compare to are the MLSN guidelines.

2. "What about nutrients in the soil that are not available?"

Answer: Marschner's Mineral Nutrition of Higher Plants has a chapter conveniently entitled Nutrient Availability in Soils. It begins:

The most direct way of determining nutrient availability in soils is to measure the growth response of plants by means of field plot fertilizer trials. This is a time-consuming procedure, however, and the results are not easily extrapolated from one location to another. In contrast, chemical soil analysis -- soil testing -- is a comparatively rapid and inexpensive procedure for obtaining information on nutrient availability in soils as a basis for recommending fertilizer applications.

Soil testing already provides information on availability.

3. "What about nutrients in the soil that are not available?"

Answer: Bah! Humbug!

See these links for more detail:

• "Available" calcium, soil pH, and fearmongering
• The unambiguity of nutrient availability indices
• Bah! Humbug! Why the concept of "exchangeable" vs. "available" nutrients is a fallacy
• It's Redundant: more about nutrient availability indices