A Review of Shade Tree Nitrogen Fertilization Research in the United States

Shade tree nitrogen fertilization research in the United States, beginning in the 1920s, is reviewed. The early empirical studies demonstrated that shade tree growth increased in response to N applications, that N source did not significantly affect growth; that surface applications were as effective as soil injection or drilling, and that N application rates of 29 g N/m2 (6 lb/100 ft2) were near optimal. Also, resulting from these studies were recommendations for dormant-season (early spring, late fall) N application times. The trees in these studies were planted on relatively close spacings and the experiments were conducted over long periods of time. Thus, the results were most likely confounded by "shared root zones." Due to root growth out of the "treated" areas and into adjacent plots, roots from a single tree could be exposed to two or more treatments. Two additional confounding factors were that many studies were established on soils with high native fertility and that grass competed for water and nutrients. These factors may explain the general absence of treatment differences in these studies. Later studies conducted with 15N showed low N uptake during the dormant (leafless) season, bringing into question the practice of dormant-season N applications. Additional research is needed with tree-lawn systems to define N uptake patterns, seasonal N uptake potential, and N cycling among the system's components.