Effects of plant growth on the dynamics of the soil solution composition in the root zone of maize in four Japanese soils

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Abstract
The effects of plant growth on the dynamics of the chemical composition of the soil solution were studied by growing maize (Zea mays L.) in four Japanese soils. A pot experiment was carried out with eight treatments: 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 solutions were sampled nondestructively from the root zone in situ, five times during the 50-day growth period. In soils supporting active plant growth, electrical conductivity and concentrations of Ca, Mg, K, and NO3 in the soil solution decreased and the pH increased significantly with plant growth. Spatial differences were also observed: soil solutions 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 concentrations, the composition of the soil solution was also affected by plant growth: the activity ratio for K (ARK), an index of the soil cation ratio and availability of K, increased temporally and spatially, suggesting that an ARK gradient was formed and developed in the root 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 were different among ions. Decrease of N in the soil solution was almost equal to the amount of plant uptake, whereas, decrease of K amounted to less than one-fifth of the K taken up, suggesting intensive replenishment of K from the exchange sites of soil. On the contrary, the decrease of Ca in the soil solution was more than ten times the amount of plant 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 consequently, their availability for plants were affected by the concentration of NO3, which was the dominant anion as well as the dominant N source taken up in large amount by plants.