Supplement to Diagnosis and Management of Salinity Problems in Irrigated Pecan Production: Salt Leaching
In the publication prepared in 2006 with the above title (TWRI. TR-287), we described ways to diagnose salt problems which affect irrigated production of pecans. We also discussed the concepts of minimizing soil salinization, and ways to lower soil salinity. However, the materials presented were general and introductory in nature. This article is to supplement the previous publication with technical details, and practices of salt leaching during the routine irrigation as well as salt leaching irrigation for restoration. The aim of salt leaching is to keep soil salinity of the root zone below the level that trees can tolerate. The threshold salinity of irrigated pecans is in the range of 2 to 3 dS m-1 when measured in the soil saturation extract (Fig. 1), which is an official method of measuring soil salinity (Miyamoto et al., 1986). In the areas rich in gypsum, trees may tolerate higher levels of soil salinity, probably by 1 or 2 dS m-1. Calcium and sulfate ions are less harmful to pecan trees than sodium and chloride ions (Miyamoto et al., 1985). There are basically two ways to approach the task of salt leaching. The first approach is to maintain leaching following each irrigation so as to keep soil salinity in check. The first half of this article is devoted for describing ways to minimize soil salinization through this approach. The second approach is to let salts accumulate in portions of the orchards, then to flush salts out during the dormant period. This approach takes the reality into account; soil salinity levels vary widely even in a small orchard, and that it is more convenient to carry out leaching during the dormant period. Once a part of the orchard begins to be salinized, (meaning that soil salinity exceeds the threshold salinity level), growers need to carry out salt leaching irrigation for restoration. When dealing with restoration, the causes of salinization have to be identified prior to deciding the methods of salt leaching. This Fig. 1 Trunk cross-section as affected by salinity or Na concentrations in the soil saturation extract (Miyamoto, 1986). subject is discussed in the second part of this article.