Permanent Wilting Percentages of Soils Obtained from Field and Laboratory Trials.

In experimental work on the soil-water relations of plants in the field, it is advantageous to have some knowledge of the water-holding characteristics of the soil before the field trials are started. If the moisture equivalents or the field capacities and the permanent wilting percentages are known, an irrigation program for the various plots can be laid out in advance, with assurance that the unirrigated plots will not be allowed to remain dry too long and that the other plots will be watered according to, plan. The difference between the field capacity and the permanent wilting percentage may be taken as the percentage of water readily available to plants. The moisture equivalent may be used as a measure of the field. capacity of most mediumand fine-textured soils (10). Since our experimental evidence has shown that the permanent wilting percentage is the critical soil moisture content denoting the lower limit of readily available moisture (11), the advantage of having accurate information concerning this constant is apparent. Sometimes, however, the permanent wilting percentages obtained in the laboratory or greenhouse, either by direct or indirect methods of determination, do not agree closely with the results obtained in the field. We believe that the permanent wilting percentages obtained directly in the field or in the laboratory are more satisfactory than those obtained by indirect methods. Calculation of the permanent wilting percentage from the moisture equivalent has proved to be notably untrustworthy in this respect (8). In certain cases field results also have not agreed closely with the permanent wilting percentages obtained by growing plants in containers, using a stock supply of soil previously obtained from the area on which the experiments were conducted, particularly when the soil was not uniform in texture or structure in the lower depths. The fact that these discrepancies sometimes occur led to a re-examination of our methods of making permanent wilting percentage determinations in an effort to find out what factors were responsible, and, if possible, how the difficulties might be overcome. These studies led to a general realization that in most cases the direct determination of the permanent wilting percentage by growing plants in containers gave results in close agreement with those in the field; in others, satisfactory agreement could not be obtained. Methods of finding the permanent wilting percentage