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15 VII. SOIL-PLANT RELATIONS A. Basic Aspects The soil and plant components of the soil-plant system are individually com- plex, as is evident from the preceding sections. The combination of these two com- ponents increases the difficulties of understanding and generalization. Both systems, independently and together, are dynamic. Plant growth requires the continuous net removal of ions from the soil into the plant. On the other hand, changes in mois- ture and the removal of ions by the plants continually change the quantity of the ions available to the plant. B. Competing and Carrier Cations The kinds and amounts of the complementary ions affect the availability of a given ion (48, 87). Two types of processes can be distinguished: the exchange reac- tions governing the distribution of ions between clay and solution (37, 48) and the competitive effects during the course of ion absorption by plants (17, 20, 21, 27, 28). Since several cations compete for the same carrier site, increasing the concentration of one should decrease the uptake of others in the same group. Examination of this hypothesis in greenhouse and field experiments has shown this to be true within certain ranges. Increasing the calcium concentration in nutrient solution from zero to two milliequivalents per liter reduces the uptake of strontium (43). Further increases in calcium reduce strontium uptake only slightly. A fourfold reduction of strontium uptake in field experiments appears to be the maximum that can be achieved by the addition of calcium to acid, low-calcium soils. Even smaller reduction occurs in soils richer in calcium. The addition of stable strontium has little effect on radioactive strontium up- take because of the similarity of strontium to calcium and the thousandfold greater abundance of calcium in soils (68, 127). In one experiment, no effect of stable stron- tium was observed (43) and a slight increase in strontium-90 uptake was found in another experiment (104). It was postulated that small increments of strontium dis- placed some of the strontium-90 from the exchange complex into solution. It is esti- mated that five tons of strontium amendments per acre would be needed to reduce the strontium-90 uptake appreciably (104). A depressing effect of potassium on plant uptake of calcium, magnesium, and strontium has been observed (47, 54, 65). Potassium treatments decreased stron- tium uptake 20 per cent in wheat plants (47) and 40 per cent in radish plants (54). In a field comparison of plant concentrations of different elements with the cor- responding soil concentrations (57), it was found that varying levels of calcium and
16 magnesium brought about only slight changes in the strontium content of four pasture species. However, potassium and sodium reduced the strontium content of blue- grass as much as 34 per cent and that of redtop 51 per cent, whereas sodium addi- tions increased strontium in Korean lespedeza. Similar observations have been made on the uptake of cesium-137. When soil potassium is low, additions of potassium reduce the cesium uptake, but additions of stable cesium often increase cesium-137 uptake, presumably by displacement of ex- changeable cesium-137 into solution (84). Rubidium, ammonium, and calcium in- creased cesium uptake 8, 3, and 1-1/2 times, respectively, but when carrier cesium-137 was used, practically no effects of these ions were observed (128). C. Distribution Factors Because of the similarity in chemical behavior between certain fission products and certain essential elements, fission-product uptake is often reported relative to the uptake of the chemically similar essential element. The "Observed Ratio" (OR) (18), or the "Distribution Factor" (DF) (66), for strontium is the ratio of stron- tium to calcium in plant or plant part divided by the ratio of strontium to calcium in the nutrient medium. The term "discrimination factor" is expressed in the same manner but usually applies to a single step in the various successive processes that determine the over-all relative distribution of the two elements between substrate and tissue. In nutrient solution experiments, when only the plant discrimination processes are measured, the strontium/calcium DF is close to 1. 0 (67, 105). This indicates little discrimination between strontium and calcium and is true for most of the plant parts except the roots, where DF values as high as 6. 0 were observed for low- solution concentrations of strontium. The average DF values of rubidium/potassium and cesium/potassium for millet, oat, buckwheat, sweetclover, and sunflower plants were 0.85 and 0.20, respectively (67). This indicates some discrimination by the plant against rubidium and more against cesium. Alfalfa and wheat grown on eight soils (98), wild plants and corn grown on soil in a radioactive waste disposal area (6), and beans grown on a Sassafras sandy loam with added calcium (105) had strontium/calcium DF values close to 1.0, indicating little discrimination in soil reactions. However, the DF values can vary within a given plant, ranging from 2. 6 for corn flowers to 0.5 for corn grain (6). The calculated DF will vary to some extent, depending upon the method of extracting the cations from the soil. Based on the amounts of strontium-89 and cal- cium-45 added to Cinebar soil, the DF for beans ranged from 0. 64 to 1. 2 (43). Another experiment with strontium-89 and calcium-45, using a dilute calcium chlo- ride extract of the soil, gave an average DF of about 0. 7 (111). Discrimination factors from 0. 8 to 1. 6 were found for strontium/calcium in eight grasses and eight legumes grown in three soils, using ammonium acetate for extraction (138). In a study of soils and vegetation in a disposal area (33), the best soil index of strontium-90 uptake by plants appeared to be concentrations of strontium-90 in the saturation ex- tract. Others (112) have also suggested that a water extract may provide a better measure of the availability of strontium and calcium in the soil than the exchangeable fraction.
17 Barley, buckwheat, and cowpeas grown on an Evesboro sand gave a cesium/ potassium DF of 0. 02, based on the amount of radioactive cesium added to the soil and the acid-soluble potassium (66). The range was from 0. 06 to 0. 77 for wild plants and corn grown in a radioactive disposal area (6). Upland rice, wheat, and beans grown on a Japanese soil gave much lower DF values â0. 002 to 0. 003 (41). Discrimination factors of 0. 02 for barium/calcium and 0. 4 for rubidium/potassium were also found for the barley, buckwheat, and cowpeas grown on the Evesboro sand (66). Discrimination in plant uptake of strontium and calcium is usually slight in pot experiments, except for roots, but apparent discrimination against either stron- tium or calcium can occur in the field. The strontium-90 is normally concentrated near the soil surface, or in the plow layer, the distribution of exchangeable calcium in the profile usually is nonuniform, and the root zone varies with the plant species and with soil conditions. D. Effects of Clays and Anions Twice as much strontium is taken up from illite clay suspension as from bentonite clay (63), which indicates that bentonite holds strontium more strongly than illite. Clays have more of an effect than just as an anion, for the aluminum concen- tration affected calcium uptake from calcium sulfate but not from calcium clay (64). Anions have differential effects on various species. In tobacco, calcium uptake is nearly the same from the carbonate, sulfate, or phosphate salt, but alfalfa seems to prefer carbonate to sulfate or phosphate as a calcium source (102). Strontium up- take by bean plants in one soil was reduced 40 per cent by calcium carbonate but only 15 per cent by calcium sulfate (105). Bicarbonate in nutrient solutions (32) reduced strontium uptake by 70 per cent, rubidium by 43 per cent, ruthenium by 24 per cent, and cerium by 19 per cent. Cesium was the only ion studied in which the uptake was not adversely affected. Thus, the reduced strontium uptake from alkaline calcareous soils may be due to a bicarbonate ion effect as well as to a calcium effect. Strontium added to soils as the sulfate, oxalate, hydroxide, fluoride, carbonate, or phosphate was one tenth as available to plants as strontium added as the chloride or nitrate (132). Also, calcium sulfate and calcium carbonate were more effective than calcium chloride in reducing strontium uptake. This indicates an effect of solu- bility, since all unavailable strontium salts are of low solubility. Massive doses of phosphate have reduced strontium uptake 50 per cent on alkaline soils but have given no reduction on acid soils (132). Other studies (14, 53) report conflicting results from the addition of phosphate. Hydroxyapatite and fluoroapatite are insoluble calcium phosphates that exist under alkaline conditions. The strontium analogs of these compounds are expected to be similarly insoluble (26).
