SummaryTherelative productivity of certain soil series has been determined from an evaluation of certain physical characteristics of the soil profile which are measurable in the field. The evaluation has been based on the assumption that the most important physical properties of a soil affecting its productivity are its texture, its depth, and the goodness of its drainage.The basis of the evaluation formula is that a rating is given to each textural class, ranging from 20 for a medium loam to 3 for a gravel and 5 for a heavy structureless clay, and this rating is given for each inch down to the bottom of the profile, or to an arbitrary depth of 30 in., whichever is the shallower. These ratings are added together to give a textural value,V, for the soil. If there is any gleying in the top 30 in. this value is then reduced by multiplying by a factorG, which is less than unity and which is smaller the higher the top of the gley horizon. The yield, of wheat,P, as. measured by the weight of ears produced per unit area at any spot in the field was calculated by the formulaP = cVG, wherecis an arbitrary constant. This prediction formula accounted for 90–7 per cent, of the variance of the wheat yield in 1947, a year in which accurate estimates of the yield were made, but only 68 per cent, of the variance in 1948, a year in which the yield figures were subject to large errors. The formula was less satisfactory for the yield of straw.The formulaP = cVGwas more accurate thanP = cVGwhereVGis the textural value to the top of the gley horizon, a result interpreted as showing that wheat roots make some use of gleyed soi