Abstract
The present paper attempts to estimate the contribution made by diffusion dispersion, and mass flow to the NO_3-N supply to plant roots in a field condition, and the minimum NO_3-N concentration of the soil solution for the maximum growth of corn plants. We employed a single root model, and the results obtained are summarized as follows: 1. The transport of NO_3-N to the roots was mainly achieved by mass flow in the high level of soil NO_3-N content while in the low level of soil NO_3-N it was achieved by diffusion and dispersion. 2. The contribution of diffusion and distribution to the NO_3-N transport was closely related to the root density. Dispersion contributed most to the NO_3-N transport in the low root density while diffusion made the biggest contribution to the high root density in clay soil. 3. In sand soil which was low in initial moisture content (pF2) compared with clay soil, dispersion was the main mechanism of NO_3-N transport at the same level as the NO_3-N content of clay soil. 4. In a field condition, the minimum NO_3-N concentration of the soil solution for plant growth may be determined by such factors as the water and nitrogen requirement of plants, the NO_3-N transport rate to the roots, and by the soil texture, etc. In addition to these factors, it was found that the minimum concentration is governed by the root density. It also seemed probable that the minimum NO_3-N concentration for plant growth is the lowest NO_3-N concentration of the soil solution which does not make the NO_3-N concentration on the root surface zero.
