AVAILABILITY OF INTERLAYER AMMONIUM AS RELATED TO ROOT VICINITY AND MINERAL TYPE

We studied the release of nonexchangeable NH4+ from soils when cropped with ryegrass. The plants were grown in special soil containers in which part of the soil was not in direct contact with the roots. Nonex-changeable NH4+ was released only in the immediate vicinity of the roots where, it is suggested, the NH4+ concentration of the soil solution was depleted to a very low level and that this condition allows a net release of interlayer NH4+. From the four soils studied, only two released interlayer NH4+ although all contained appreciable amounts of nonexchangeable NH4+. In these two soils, a Fluvisol (river) and an Alfisol, 34 and 62%, respectively, of the total N uptake of the grass originated from the nonexchangeable NH4+ pool. These two soils contained vermiculite and were low in exchangeable K+, in contrast to the Fluvisol (sea) and the Vertisol, which contained no vermiculite and showed a high percentage of K saturation of the clay.