Phosphorus Nutrition of Pecan

Abstract

A fundamental component of pecan management is maintaining phosphorus (P) nutrition throughout the tree’s growth, development and production. Pecans have difficulty maintaining adequate P concentrations when managed for high input production, so alternate methods of P fertilization in pecan are of interest in correcting deficiencies of the element. The role of P in root development of pecan is also of interest as increased fibrous root development observed in Arabidopsis in response to low P concentrations would be desirable for container production of pecan.

Phosphorus is relatively immobile within the soil profile making broadcast applications ineffective for rapid correction of P deficiency in pecans. Banded applications have been used to successfully correct P deficiency quickly. An experiment was conducted to determine effectiveness of banded P applications at differing rates within irrigated and non-irrigated plots on P movement within the soil, P uptake and movement within pecan trees, and yield and quality of nuts. On March 20, 2015, P at 0 kg ha-1 (0x), 19.6 kg ha-1 (1x), 39.2 kg ha-1 (2x), and 78.5 kg ha-1 (4x) was applied in bands of triple superphosphate to a ‘Desirable’ pecan orchard.

Soil test P decreased linearly over time in non-irrigated and irrigated environments when P was applied at 2x and 4x rates by 34.7% and 54.0% and 41% and 58.6%, respectively. There was no change in soil test P over time at the 0x application rate for both irrigation regimens. Soil test P decreased 44.4% for the 1x treatment in the irrigated plot but did not change in the non-irrigated plot. The largest linear decrease in soil test P from experiment start to finish was measured in the top 0–7.6 cm of the soil profile dropping 53.4% and 61.2% in the non-irrigated and irrigated plots, respectively. In contrast, soil test P did not decrease in the irrigated plot at the 15.0–22.5 cm soil depth for the entirety of the experiment but decreased linearly by 23.2% in the non-irrigated plot.

Increasing P application rate increased foliar P concentration quadratically in the non-irrigated plot, but only the 4x application rate increased foliar P modestly compared to the 0x control. In the irrigated plot, foliar P concentrations decreased linearly from 0.133% in 2015 to 0.121% in 2017 and foliar P concentrations were not influenced by P application rate. Foliar Fe and Cu decreased with increasing P application in the irrigated plot rate while foliar B increased. Foliar B concentrations were influenced by the application side of the P band in the irrigated plot as well, increasing on the proximal side and decreasing on the distal side compared to the application site. No differences in pecan yield or quality were observed in either irrigated or non-irrigated plots. Overall, P banding may not be the most sustainable way to increase concentrations of P quickly or to maintain foliar concentrations of the nutrient long term.

Pecan seedlings are known to have strong tap-rooting tendencies, a characteristic that can hinder containerized production of trees. In Arabidopsis, a reduction of P reduced vertical root development and encouraged root branching. An experiment was conducted to determine if decreased P rates would increase root branching and decreased tap rooting in container grown pecan seedlings. Beginning at 47 days after germination, varying P rates (0, 3.1, 7.75, 15.5, 23.25, and 31 mg L-1 P) in modified Hoagland’s solution were applied in 250-mL aliquots to randomly-assigned seedlings twice per week for the duration of the experiment. Seed retention or removal was added as a factor 47 days after germination as well. At the conclusion of the experiment, foliar P concentrations were within the recommended sufficiency range for all P rates and P rate did not influence plant dry weight or network surface area. Seed retention increased plant dry weights and network surface area but decreased foliar P concentrations. These finding indicate that seedlings with greater biomass would be produced if growers maintained seed attachment when transplanting, which would be beneficial for getting plants large enough to graft sooner in containerized production than those without seed attachment. Further research would be beneficial in determining the long-term effect of seed attachment in pecan seedling development and how long it is influential in growth characteristics. The increased fibrous root development observed in Arabidopsis in response to low P rate was not observed in pecan seedlings during the experiment.