Huating Dou

Potassium Management for Optimizing Citrus Production and Quality

Abstract Potassium (K) is highly mobile in plants at all levels, that is, from individual cell to xylem and phloem transport. This cation plays a major role in (1) enzyme activation; (2) protein synthesis; (3) stomatal function; (4) stabilization of internal pH; (5) photosynthesis; (6) turgorrelated processes; and (7) transport of metabolites. Citrus trees generally do not show visible deficiency symptoms across a wide range of K status in the leaves, except when the leaf concentrations drop below 3-4 mg kg21. However, fruit quality is quite sensitive to varying levels of K availability. High levels of K cause large fruit size with thick and coarse peel. In contrast, K deficiency produces smaller fruits with thin peel. With regard to juice properties, K nutrition has a significant role in juice acidity; that is, high juice acidity with high K availability, while low K availability causes decrease in juice acidity. High K availability in the soil can reduce the uptake of other cations, primarily magnesium, calcium, and ammonium N. In this paper, the available information on the effects of varying availability of K on the fruit yield, postharvest quality of fruit, as well as juice quality is summarized. The current recommendations on the application of soil and leaf analysis for evaluation of the K nutritional status and guidelines for K fertilization are also discussed.

An Evaluation of Sources of Nitrogen in Shallow Groundwater Using 15N Abundance Technique

A 15N abundance technique was employed to identify the source of NO3-N in groundwater under three commercial citrus production sites in central Florida. Water samples were collected from 0 to 300 and 300 to 600 cm depths in the surficial aquifer and analyzed for NO3-N and delta N-15 (δ 15N). Groundwater samples were also collected in a residential area adjacent to one of the citrus groves and analyzed for NO3-N and δ 15N. The δ 15N values were in the range of +1 to +10% in both depths underneath the citrus groves. The range of δ 15N measured in this study represents the range expected for groundwater that was impacted by NO3-N originated from mineralization of organic N from the soil as well as from the crop residue. There are occasional high δ 15N values which are indicative of the effects of NH3 volatilization losses of applied fertilizer N. The range of δ 15N values for groundwater samples collected from the residential area adjacent to the citrus groves was very similar to that from the groundwater underneath the citrus groves. Thus, the source of NO3-N that impacted the groundwater under the citrus groves also impacted the groundwater in the adjacent residential area.