Root distribution within a soil profile affects the extent and depth of N and soil water utilization by a plant. We determined the effects of N fertilization on spring wheat ( Triticum aestivum L.) root growth and soil water use, and determined if genotypic differences existed in root growth and soil water use in response to N fertilization. Eight genotypes (G) of hard red spring wheat were grown under three N fertilizer rates (0, 67, 134 kg N ha− 1 ) at two locations in western Minnesota in 1982 and 1983. Major soil types were a Wheatville silt loam (coarse‐silty over clayey, frigid Aeric Calciaquoll) and a Forman clay loam (fine‐loamy, mixed Udic Argiboroll). Root sampies were obtained from all genotypes at the soft‐dough growth state in 15‐cm increments to a depth of 120 cm. Soil water use was determined weekly by neutron attenuation during the growing season on three genotypes in 1982 and five genotypes in 1983. Although a significant N × G interaction occurred for root length, certain similarities existed among genotypes. Root length was significantly (a = 0.05) increased in the top 30 cm by applying 67 kg N ha− 1 but remained the same or decreased at 134 kg N ha− 1 . Below the 30‐cm depth, 134 kg N ha− 1 generally suppressed root growth more than the 0 and 67 kg N ha− 1 rates. Genotypes did not differ in total amount of available soil water (0 to 120 cm) used, but differences in depths from which soil water was extracted were observed in response to N fertilization. Depth of soil water use and root growth were influenced by N fertilization rates for some genotypes. We suggest that high rates of N fertilization may inhibit deeper root growth and hence decrease the potential use of deeper soil NO 3 and water reserves.
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Comfort et al. (1988) studied this question.