Preliminary studies of the exuded plant sap and the relation between the composition of the sap and the soil solution

Samples of the sap of corn, sorghum, and Sudan grass were obtained by cutting the stalk of plants near the surface of the ground and collecting the sap which exuded from the stump ends. Determinations of pH, specific resistance, and of phosphorus, calcium, nitrate nitrogen, silica, and chlorides were made on a number of the samples of sap obtained. Studies were made of the relation between the concentration of various elements in the plant sap and in the soil solution obtained from the soils upon which the plants were growing. Other experiments included studies of the composition of the plant sap of corn, sorghum, and Sudan grass as affected by three different degrees of soil acidity and studies of the composition of sap samples collected on successive days. The main results may be summarized as follows: 1. The sap of corn was found to contain an average of about 3,700 p.p.m. total solids of which approximately one-third was in the inorganic form. 2. The total electrolyte content of the sap as measured by conductivity determinations was found to be less in nearly all cases than that of the displaced soil solution. 3. The total phosphorus content of the sap of corn, sorghum, and Sudan grass was found to range between about 150 and 450 p.p.m PO4. Of this concentration generally over two-thirds and in some cases all were found to be in the inorganic form. 4. The concentration of silica in the plant sap was found to average about 250 p.p.m. Of calcium, about 80 p.p.m. and of chlorides about 90 p.p.m., while nitrates were found to vary from a trace to 344 p.p.m. 5. Phosphorus and silica were found in much higher concentrations in the plant sap than in the displaced soil solution, "the concentration factor" for the former ranging from 552 to 4,967 and for the latter from 15.2 to 34.8. On the other hand, chlorides and calcium were found in considerably lower concentrations in the plant sap than in the soil solution. Nitrates were higher in some cases and lower in others in the plant sap than in the displaced soil solution. 6. The data indicate that there may be a correlation between the concentration of phosphorus in the plant sap and in the soil, solution and soil extract. 7. The pH values of the sap of corn, sorghum, and Sudan grass were found to be about the same, varying from about 4.40 to 4.80. Differences in soil acidity from pH 4.60 to 6.60 were not found to affect the acidity of the sap of any of the three species. 8. The relatively poorer growth of sorghum and Sudan grass than of corn on acid soils could not be explained on the basis of the inability of the former plants to accumulate calcium or phosphate in their saps when grown in acid soils. 9. The H-ion concentration of the soil was not found to affect the concentration of calcium, phosphorus, and chloride in the plant sap within a range of pH 4.60 to 6.60. With increased pH values of the soil a slightly greater concentration of silica was found in the plant sap. 10. Samples of sap from the same plants collected on successive days were found to remain quite constant in total phosphorus for a period of 4 days. The inorganic phosphate, however, was found to increase as bleeding proceeded, while the organic phosphate tended to decrease. Chlorides were found to increase in the sap as bleeding continued, but calcium and silica were not found to show any consistent variations. The significance of these data in relation to the absorption and accumulation of inorganic elements in the plant sap is briefly discussed. It is also pointed out that studies of the plant sap obtained by bleeding may be found useful in various studies of soil-plant relationships.