A study of the Scottsburg experiment field indicates that a toxic substance is present in the soil and that the Fe present is mostly in the ferric condition. The conditions which cause oxidation of Mn to the dioxide form also change most of the ferrous Fe to ferric, in which form it is not available to plants at about p H 6.0. This condition is counteracted by the reducing action of manure but not by commercial fertilizers. The manure added to the soils studied did not greatly change the percentages of Fe and Mn present in the soil, but their less oxidized forms seem to be more favorable to plant growth. This change is believed to have been the main cause for an annual increase of about 1/3 in corn and wheat yields and an approximate doubling of clover yield. The presence of MnO2 can usually be recognized by the formation of a green color when the soil is tested by the potassium thiocyanate method. The MnO2 often present in soil will cause a fading out of the red color of Fe(CNS)3, if there is less available Fe than Mn present. The KCNS color standard method thus may indicate a basic soil, whereas in reality it is acid.