A Method for Controlling the pH of Nutrient Solutions in Sterile Sand Cultures 1

T apparatus described in this paper was devised for the purpose of studying the influence of pH on the nitrogen-fixing ability of certain Rhizobia. Difficulties were encountered in obtaining normal root growth, presumably because of lack of balance of the nutrients in the solutions used. The experiment has been discontinued temporarily, but in view of the fact that good pH control was attained under conditions that should make possible continuous sterile irrigation it is thought that a brief description of the apparatus may be of interest to others doing similar work. A diagram of the apparatus is given in Fig. ~.2 The nutrient solution was placed in the 4o-liter carboy A and the Mariotte flask arrangement was used to maintain a constant head. From this carboy the solution moved through glass and rubber tubing to the 5-inch doublewalled irrigator pot B.3 The soil cavity of the irrigator pot was filled with well-washed white silica sand.4 The porous cup C embedded in the sand was connected to the reduced pressure line D by means of glass and heavy-walled rubber tubing. A test tube sample trap was inserted in the outflow line at E. The nutrient solution in the interwall cavity of the irrigator pot was usually at or slightly above atmospheric pressure. A partial vacuum pressure was maintained in the outflow line D and hence also in the porous cup. Under the action of this pressure differential the nutrient solution flowed from the porous inner wall of the pot, through the sand, out through the porous cup C and the brass drainage pipe D to the 4o-liter containers F and G. The inclined brass drainage pipe D was ~ inch in diameter and 7 feet long and had spouts for attaching ~4 of the pH control units. The nutrient solution for any pot, after passage through the sand, could be sampled for pH determination by clamping off the flow line on either side and substituting an empty test tube for E. The porous cup was specially made to provide a ~ ~ inch space at the sides and bottom for the quartz sand, this arrangement hus permitting approximately radial flow from the pot wall, through the sand, to the inner cup wall. A circular glass plate was placed in the bottom of each pot so as to cut off the moisture supply from the lower surfaces. This was done to keep the flow more nearly radial and to prevent excessive transfer of liquid through the sand at the lower levels