Effects of saline water irrigation on soil salinity, Pecan tree growth and nut production

Summary

Irrigated cultivation of pecans (Carya illinoensis K.) has increased dramatically in the Southwestern USA, yet their tolerance to salinity remains largely unknown. The first part of this study was conducted to assess if stunted tree growth reported in clayey soils is related to salinity, and the second part was to evaluate changes in soil salinity and the performance of 11 year old ‘Western’ trees irrigated with water of 1.1 dSm⁻¹ and 4.3 dSm⁻¹ for 4 years. The first study, conducted at a commercial orchard (49 ha) in the El Paso valley (TX), showed a highly significant correlation between tree trunk size and salinity of the saturation extract (EC_e) with r=−0.89. Soil salinity above which trunk size decreased in excess of the standard error was 2.0 dSm⁻¹ in EC_e from 0–30 cm depth, and 3.0 dSm⁻¹ in 0 to 60 cm depth with corresponding Na concentrations of 14 and 21 mmol l⁻¹. Excessive accumulation of salts and Na was found only in silty clay and silty clay loam soils. The second study, conducted at a small experimental field (1 ha), indicated that irrigation with waters of 1.1 and 4.3 dSm⁻¹ increased EC_e of the top 60 cm profile from 1.5 to 2.2 and 4.2 dSm⁻¹ and Na concentration in the saturation extract to 17 and 33 mmol l⁻¹, respectively. The leaching fractions were estimated at 13 and 37% when irrigated with waters of 1.1 and 4.3 dSm⁻¹, respectively. Tree growth progressively slowed in the saline plots irrigated with water of 4.3 dSm⁻¹, and became minimal during the 4th year. The cumulative shoot length over the 4 year period was reduced by 24% and trunk diameter by 18% in the saline plots relative to nonsaline plots. Irrigation with the saline water also reduced nut yields by 32%, nut size by 15% and leaflet area by 26% on the 4 year average, indicating that pecans are only moderately tolerant to salinity. The concentration of Na, Cl and Zn in the middle leaflet pair did not differ significantly between the two treatments. Soil salinity provided a more reliable measure for assessing salinity hazard than leaf analysis. However, soil salinity was found to be highly spatially variable following a normal distribution within a soil type. This high variability needs to be recognized in soil sampling as well as managing irrigation.