EFFECTS OF PLANT-GROWTH ON THE DYNAMICS OF THE SOIL SOLUTION COMPOSITION IN THE ROOT-ZONE OF MAIZE IN 4 JAPANESE SOILS

The effects of plant growth on the dynamics of the chemical compositio n of the soil solution were studied by growing maize (Zea nays L.) in four Japanese soils. A pot experiment was carried out with eight treat ments: four soil materials with and without plants. In each pot, four samplers with a looped hollow fiber (LHF-samplers, Yanai et al. 1993: Soil Sci. Plant Nutr., 39, 737-743) were installed and the soil soluti ons were sampled nondestructively from the root zone in situ, five tim es during the 50-day growth period. In soils supporting active plant g rowth, electrical conductivity and concentrations of Ca, Mg, K, and NO 3 in the soil solution decreased and the pH increased significantly wi th plant growth. Spatial differences were also observed: soil solution s in the upper parts of the pots were significantly less concentrated in some elements than those in the lower parts. As in the case of conc entrations, the composition of the soil solution was also affected by plant growth: the activity ratio for K (AR(K)), an index of the soil c ation ratio and availability of K, increased temporally and spatially, suggesting that an AR(K) gradient was formed and developed in the roo t zone by the larger uptake of K compared with Ca and Mg. The effects of plant uptake on the decrease of the soil solution concentration wer e different among ions. Decrease of N in the soil solution was almost equal to the amount of plant uptake, whereas, decrease of K amounted t o less than one-fifth of the K taken up, suggesting intensive replenis hment of K from the exchange sites of soil. On the contrary, the decre ase of Ca in the soil solution was more than ten times the amount of p lant uptake. This phenomenon was considered to be due to the decrease in the anion concentration rather than to the plant uptake of the ion, suggesting that cation and anion concentrations are interrelated. In conclusion, cation concentrations in the soil solution, and consequent ly, their availability for plants were affected by the concentration o f NOB, which was the dominant anion as well as the dominant N source t aken up in large amount by plants.