2017 ASHS Annual Conference

Application of fertilizer after planting young pecan trees is a common practice in establishing orchards. The recommended application rate is 229.5 kg N ha^-1 for young trees. A common thought when applying fertilizers is to apply more than recommended in order to ensure trees will not suffer from nutrient deficiency. However, higher than recommended application rates can potentially negatively affect root development and lead to fertilizer burn. In addition, over-application of fertilizer can lead to eutrophication of groundwater and nearby water bodies. In this experiment we applied five levels of N to 40 newly established pecan trees (n=8) at the Texas A&M University pecan research orchard and installed root observation tubes on half the trees per N treatment. Subsequently, we measured tree diameter growth, leaf gas exchange, and leaf N concentrations at regular intervals for two years. The five levels applied were relative to the recommended rate (1 N), at 0 N (no fertilizer), 0.25 N, 0.5 N, 1 N and 2 N. There was no significant impact of N fertilization rate on tree diameter growth or leaf gas exchange. Root demographics data show that maximum root growth occurred in March and April. During the observation period, new root production was greatest in the 1 N treatment, while roots exposed to 1 N or double the recommended rate (2 N) had a significantly shorter lifespan. This resulted in reduced net root length production in the 1 N treatment. These patterns were consistent at two soil depths, 14-28 cm, and 98-112 cm. There was no benefit to applying more than the recommended rate in terms of aboveground growth. The data suggest that 0.5 N was an adequate rate for maximizing above- and belowground growth, which could lead to significant saving for the growers as well as the environment.