It is essential to photosynthesis, one of the most critical processes in the entire world by which energy is captured from the sun and used to run the organic "motors" of every living cell-that is, to produce food.
Oddly enough, for all its essential nature, phosphorus is not readily available in soils except within a rather narrow pH range. When a pH of S.O or lower is reached, phosphorus is chemically trapped by aluminum compounds and converted into highly insoluble, fixed forms which are unavailable to plants. Iron is captured in a similar way.
When pH goes up and calcium is present in generous amounts, phosphorus reacts with it to form other highly insoluble compounds. Superphosphate is commonly applied to the soil to supply phosphorus. If it contains any amount of fluorine (as is sometimes the case), and the soil pH is 7.8 or above, fluorapatite, the most insoluble of all phosphate compounds, is formed.
We need not go into the chemistry and interactions of all elements essential to plant growth; the important fact is that availability of nutrients in soil depends so directly on pH that adjusting this factor is something every gardener should know how to do. All of the mineral elements plants need for growth are available between readings of 6.0 and 6.9. Even above and below this range, the minerals are available to a certain extent, so that if small errors are made in reading the tests, it is not too serious.
Here, then, is a basic principle in managing soils-keep pH between 6.0 and 6.9 for all plants classed as circumneutral and you won't go far wrong. This assumes, of course, that the vital food elements were either in the soil when you tested it, or that you will supply them.
PLANTS WITH DIFFERENT NEEDS
When you examine the list of plants and their soil preferences given in the Appendix, you will see that some of them do best at pH readings below 6.0 while a few are able to tolerate alkalinity above 7.5 to 8.0. Why these exceptions?
Here is one of those mysteries that makes soil such a fascinating study. These plant exceptions need nutrient elements which, according to the pH theory, should not be available at the readings listed for them. The answer is that soil is not a uniform, homogeneous mass, like a great blob of plastic with every molecule made up of identical atoms. Instead, soil is a composite: a great macrocosmos and microcosmos rolled into one. Within the same grain of soil can be found acid and alkaline elements existing side by side, "buffered" from attacking each other by a series of checks and balances that allow them to act according to their individual reactions.