Many soil grains are actually "sandwiches" with an acid core and a layer of alkaline material on either side. In acid soil, not all alkaline elements are neutralized, and vice versa. This is particularly true if the soil contains a good percentage of organic matter, the most powerful "buffer" of all.
Acid-soil plants owe their ability to grow at low pH readings to the fact that their roots can tolerate some free aluminum. At readings
below 5.5, aluminum is set free unless it combines with phosphorus. This does not mean that the ability of plants to resist aluminum is unlimited: many growers of camellias, rhododendrons, azaleas and other ericaceous plants keep applying aluminum sulfate to acidify soils only to find that their plants deteriorate. The roots may be blackened and injured to a point where they can no longer take up nutrients.
ACID-SOIL PLANTS USE AMMONIA
The various acid-soil plants do not need their organic foods broken down as completely as do plants that require a more alkaline soil. The forms of bacteria and fungi that break down protein to release nitrogen are sensitive to pH. At readings below 6.0 only forms that break the pH down to ammonia are active. At higher pH readings, other organisms carry the breakdown further to produce nitrate nitrogen which all plants can use.
Acid-soil plants-rhododendrons, camellias, kalmia, blueberries- are able to use ammonia nitrogen because mycorrhizae on their roots do the converting of ammonia into nitrate form. (For a full explanation of the role of mycorrhizae, see Chapter Eight on Microorganisms.) Plants in the circumneutral group, on the other hand, must depend upon soil bacteria to carry out this final nitrogen conversion stage. Since these bacterial forms work best in the soil at a pH of 6.0 or higher, circumneutral plants do not survive in acid soil.
ALKALINE AND ALKALI SOILS
At readings above pH 7.3 to 7.5, there is sharp reduction in the
number of species which will grow well. Most of those that do survive are plants with relatively light green foliage, such as alpines and
plants from relatively dry areas with bright sunshine. Probably they
are able to manufacture their food with less chlorophyll than is needed by plants from areas where sunshine is less intense. For this
reason, alpines and others need less of the elements such as iron, copper and manganese that become unavailable in highly alkaline soils.
Another factor involved is organic matter. Since at a high pH it
does not break down rapidly, it tends to accumulate. Particularly is
this true in less humid climates, where soils with high pH are usually found.