An indication of the nutrient holding capacity is the laboratory measured cation exchange capacity of the fine earth. The active mineral soil material, designated clay, is less than 0.002 millimeter in diameter, and trends in exchange capacity are generally shown by the trends in clay percentages. Organic matter content also contributes to cation exchange capacity, and the highest exchange capacity is in horizons that contain the most organic matter. The surface layer of the soils that were characterized have about 15 milliequivalents of exchange capacity per 100 grams of soil. Exchange capacity decreases in subsoil horizons, except where clay increases substantially.

Soils are acid or basic depending on the basic cation content of the exchange complex. When the cation exchange capacity of the clay and organic matter is dominated by hydrogen and aluminum ions, the soil is acid and pH is low. Conversely, the pH is high and the soil is neutral to alkaline when the complex is dominated by basic ions, mainly calcium. The humid climate in Centre County leads to precipitation and vegetation that deplete the soil of soluble calcium ions. Figure 19 illustrates the changes in exchangeable calcium as depth increases for Laidig, Leck Kill, Hagerstown, and Morrison soils. The amount of exchangeable calcium indicates the soil chemistry of these soils. Hagerstown soils contain a moderate amount of exchangeable calcium, perhaps because of long term application of lime and because they formed in parent material high in calcium. Morrison and Laidig soils formed in material low in calcium and have essentially been depleted of all calcium. The Leck Kill soils have apparently had an application of lime to the surface in the recent past. The pH ranges from 4 to 5 for Morrison, Laidig, and Leck Kill soils and from 5 to 7 for Hagerstown soils. The soils that have low pH are also low in cations; Hagerstown soils, however, are moderate in magnesium, 2 to 5 milliequivalents per 100 grams in the subsoil. The extractable sodium cation is very mobile, and because of the humid climate in Centre County and the resulting leaching there is less than 0.1 milliequivalent of sodium per 100 grams of soil in all horizons analyzed. Potassium extracted is generally less than 0.2 milliequivalent but is as much as 0.7 milliequivalent per 100 grams in the subsoil of the Hagerstown soil.

10/15/98