Root growth as a function of ammonium and nitrate in the root zone

We examined the effect of soil NH 4 + and NO 3 − content upon the root systems of field‐grown tomatoes, and the influence of constant, low concentrations of NH 4 + or NO 3 − upon root growth in solution culture. In two field experiments, few roots were present in soil zones with low extractable NH 4 + or NO 3 − ; they increased to a maximum in zones having 2μg‐N NO 3 − g −1 soil and 6 μg‐N NO 3 = g −1 soil, but decreased in zones having higher NH 4 + or NO 3 − levels. Root branching was relatively insensitive to available mineral nitrogen. Plants maintained in solution culture at constant levels of NH 4 + or NO 3 − , had similar shoot biomass, but all root parameters – biomass, length, branching and area – were greater under NH4 nutrition than under NO 3 − . These results suggest that the size of root system depends on a functional equilibrium between roots and shoots (Brouwer 1967) and on the balance between soil NH 4 + and NO 3 − .