ONLY RECENTLY HAVE we come to appreciate the services of soil acidity in mobilizing—making available—many of the nutrients in the rocks and minerals of the soil.
When we learned that soils are less productive in giving us legumes and other protein-rich forages, according as they become more acid—either naturally or under our cultivation—we came to the conclusion that soil acidity is the cause of this trouble. We now know that a plant puts acid into the soil in exchange for the nutrients it gets. It is that same acid held on the clay that weathers the rock fragments and serves to pass their nutrients on to the clay, and from there on to the plant. The coming into the soil of excessive acidity is merely the reciprocal of the going out of the fertility. Nature’s process of feeding the plants, and thereby the animals and us, is one of putting acidity into the soil from the plant roots in order to break out of the rocks what nutrients they contain for nourishment of all the different life forms.
When we put lime rock on the soil as a fertilizer supplying calcium to our legume crops, we know full well that this rock reacts with the acid-clay of the soil. The acid goes from the clay to the lime rock which, being calcium carbonate, breaks down to give carbonic acid while the calcium is absorbed or taken over by the clay. While the calcium goes on to the clay to be available there for the plants, the carbonic acid decomposes into water and carbon dioxide as gas. Since this gas escapes from the soil, this escape takes away the acid, or, as we say, “it makes the soil neutral.” The benefit to the legume crop from the application of this lime rock to the soil does not rest in the removal of this soil acidity. Rather, it rests in the exchanging of calcium as a nutrient to the clay which was holding the acidity or hydrogen, a chemical element that is not of direct nutritional service.
Soil acidity has been breaking potash rock down chemically too. During all these past years the potash feldspars have been undergoing weathering attacks by soil acidity. On this rock the acid clay carries out its weathering effects in the same way as it does for lime rock, except that it trades acid to the feldspar and takes potassium unto itself in exchange. Magnesium rock, as we have it in dolomitic limestone is also broken down by the acid clay. By this same process the clay becomes stocked with magnesium. This is then more readily exchangeable and available to the plant from the clay than it would be if the plant root were in direct contact with the rock fragment itself. By exactly the same mechanism we can expect phosphate rock to be made available for the plant’s use. It is in these processes by which the acidity of the soil is beneficial. If the soil contains the two colloids, clay and humus, which can hold acidity, and then if that soil has scattered through it fragments of lime rock, of magnesium rock, of potassium rock, of phosphate rock or in fact of any rock with nutrients, it is the soil acidity that mobilizes to the clay the calcium, the magnesium, the potassium, the phosphorus, or all the other nutrients respectively for rapid use to the plants. This is nature’s process of providing plant nutrients on the clay of the soil in available form. By it Nature has stocked our moderately acid soils with fertility. It was that condition of our virgin soils that prompted population to seek new lands in the first place.