Ion Exchange in Soil‐Plant Root Environments: II. The Effect of Type of Clay Mineral Upon Nutrient Uptake by Plants

Abstract This investigation was designed to measure the effect of the percentage of kaolinite, montmorillonite, and illite clay minerals upon nutrient uptake by soybean plants. A kaolinite‐montmorillonite and a kaolinite‐illite series of clay mixtures was prepared through the range of 0 to 100% kaolinite‐montmorillonite, and 0 to 100% kaolinite‐illite. Each clay mixture was mixed with nutrient‐free quartz sand to give 15% clay by weight. The sand‐clay mixtures, saturated to 70% Ca, 10% Mg, and 5% K, were placed in root development pans, and soybean plants were planted and grown between the collodion membranes for 4 weeks as described in previous studies. This enabled a measurement of the effect of the type of clay mineral upon nutrient uptake. Within the kaolinite‐montmorillonite series, each increment of montmorillonite added altered the shape of the titration curve, and the exchange capacity in direct proportion to the percentage of montmorillonite clay present. This effect was also found for the kaolinite‐illite clay series. The experimental exchange capacity values of each clay mixture agreed with its calculated value, indicating that there was no gain or loss in the exchange capacities of the individual clay minerals upon mixing. Only 5% montmorillonite and 10% illite were necessary to dominate the chemical properties of the kaolinite clay minerals. The uptake of Ca and Mg by plants was reduced in the 2.5% M‐97.5% K mixture, relative to that which occurred in 100% kaolinite; however, increasing the percentage of montmorillonite above 5% increased the amounts of Ca, and above 10% montmorillonite clay Mg increased in direct proportion to the increase of montmorillonite clay. The uptake of K increased progressively with each increment of montmorillonite clay included in the clay mixture. Within the kaolinite‐illite series, the total amount of Ca, Mg, and K taken up decreased due to the inclusion of 2.5, and 5% illite; however, with increasing percentages of illite, these cations were taken up in proportionally larger amounts. The ratios of K/Ca + Mg in the tops of the plant were altered, increasing from 1.4 to 1.9 due to the inclusion of 2.5% montmorillonite. Similar changes in this ratio occurred in the kaolinite‐illite series. Kaolinite released Ca and Mg in greater quantities relative to K than did the other two clay minerals. The data indicate that small percentages of montmorillonite or illite dominated the exchange properties of kaolinite and the resulting uptake of nutrients by plants.