Soil pH affects the activity of soil microorganisms and many of the chemical reactions that occur in the soil. The availability of N, P, K, S, Ca, Mg, and Mo increases as soil pH increases from pH 5.0 to 7.0. The availability of Fe, Mn, B, Cu, and Zn, on the other hand, decreases. Chlorine is relatively unaffected by soil pH. The effect of pH on the availability of N arises mainly from the influence of soil pH on microbial activity. Most of the N and S in soil resides in the organic fraction and is released in available form as crop residues are decomposed microbially. The effect of pH on the availability of the other nutrients is governed by the chemical reactions that take place between these nutrients and soil colloids. One of the principal reasons for liming Wisconsin soils is to reduce the potential for manganese (Mn) toxicity. On the other hand, a deficiency of Mn can occur in high pH soils. For this reason, soil pH has a pronounced influence on the growth and yield of most crops.