PHOSPHATE RETENTION BY SAND IN RELATION TO SEEDLING CULTURE

PHOSPHATES ARE known to be fixed by soil particles and by sand. When complete fertilizers are applied to soil, the phosphorus is one of the last elements to be leached out by rain. Similarly, the addition of nutrient solutions to sand cultures frequently results in retention of some phosphate by the sand. Although phosphates become fixed in a form that is insoluble in water, they still may be available to plants through the activity of the roots. Among the many soil studies on phosphate retention, the work of Ford (1933), Perkins and King (1936), and Beater (1937) may be mentioned. In regard to the sand culture of seedlings, the importance of phosphate retention became evident in connection with certain experiments for the control of damping-off. It was noted that seedlings made as good growth in sand previously used with a complete nutrient solution, washed in water and then supplied only with KNO3, as in new sand with both KNO3 and Ca (H2PO4)2 supplied. Later it was found (Dunlap and Livingston, 1937) that considerable phosphate remained in the moist sand mass in a porous flower pot under continuous subirrigation, even after the K and NO3 had been completely removed by evaporation and water movement. As preliminary experiments, the growth of seedlings in new sand with only KNO3 added was compared with growth in the same kind of sand but with both KNO3 and Ca(H2PO4)2 supplied as plant nutrients. In this way, young plants of celery, lettuce, spinach, and tomato were found to respond markedly to the additional application of Ca(H2PO4)2. On the other hand, barley, cabbage, flax, garden pea, kochia, and zinnia showed little difference in size of the seedlings when Ca(H2PO4)2 was included as a nutrient. In this connection the P-content of seeds is known to vary considerably between different species. This paper reports the results of experiments on phosphate retention by sand as indicated by the early growth of plants from seed. A white, insoluble sand (Berkeley No. 18 mesh, Pennsylvania Glass Sand Corporation), practically neutral in reaction, was used as the culture medium for the plants. Over one hundred cultures of about 125 tomato seedlings each were grown in the greenhlouse at the Connecticut Agricultural Experiment Station during the winter of 1937-1938. There were two series of cultures. The first consisted of new sand entirely, with various nutrients [mostly KNO3 with varying amounts of Ca(H2PO4)2] added. After the plants in the first series were harvested, the same sand was washed and used over again in the second series of cultures to which only KNO3 was added in most cases. Some of the cultures in each series contained sand that had been washed in 8 per cent HCl. In addition to the green' Received for p}ublication October 25, 1938. house cultures, laboratory studies were made to determine the relative rapidity and degree of phosphate retention when different types of sand were treated with solutions of Ca(H2PO4 )2. Except for certain modifications, the method for growing the experimental plants was the same as that described by Dunlap and Livingston (1937). The weights recorded below refer to the total, fresh, stem and leaf material produced by the entire culture 28 days after sowing. Obviously the fresh-weight data obtained from the plants in the first series of cultures was in no way related to phosphate retention by the sand, since pbosphates are available even after fixation. In the first series, cultures without added nutrients and receiving only tap water produced 5.5 gm. of fresh material. Those receiving a single-salt application of 1 gm. (about 0.01 gm.-mol.) of KNO3 per culture produced an average fresh weight of 21.8 gm. Different applications of Ca(H2PO4)2, in addition to the above amount of KNO3, per culture at time of planting gave the following increases in fresh weight: