Water-Soluble Soil Tests for Crops

By Dr. Harold Willis

In order to get your soil into a proper balance of nutrients, you should have frequent soil tests made (at least once or twice a year, in spring and fall). The trouble with soil tests is that some are more reliable than others, and there are various ways of testing soil, some of which give accurate results but tell you little about what your crops really need. The type of soil test that gives the most useful information is a water-soluble test. This test tells how much nutrient is available to the plant at that time, rather than the total nutrients in the soil (but mostly unavailable). Most testing labs do not run water-soluble tests unless you request them.


Tests may vary slightly, but using one method (the LaMotte system), desirable water-soluble levels for major nutrients are:

  • 2000 pounds/acre calcium
  • 400 pounds/acre phosphate (P2O5)
  • 200 pounds/acre potassium
  • 40 pounds/acre nitrogen
Soil samples are prepared to run through analysis.

These figures do not translate to non-water-soluble tests and may be higher or lower than most experts recommend, but they do produce high quality crops. Generally, one should not worry about trace elements until the major elements are at proper levels.

Plant tissue testing, as done by most labs, is not as informative as water-soluble soil tests. Tissue tests only test the soluble contents of the cells. Some nutrients are part of the cell structure and are not soluble. Sometimes the soil may have plenty of nutrients, but they are not getting into the plant because of poor root functions or toxic soil conditions.

Source: How to Grow Super Soybeans