Soil salinity and sugarcane juice quality

Sugarcane (Saccharum spp. hybrids) juice quality is reduced by soil salinity. If the effect of salinity in commercial production is predictable, it will be possible to estimate juice quality in salt-affected fields prior to harvest. Variation in the effects of soil salinity on sugarcane juice quality in commercial production was assessed using 15-stalk samples harvested in 1992 and 1993 from a salt-affected commercial field of CP 70-321 sugarcane. Mean electrical conductivity (ECe) of saturated water extracts of 54 (1992) or 74 (1993) soil cores from 0–30, 30–60, and 60–90 cm depths were used to calculate mean ECe at each site, which ranged from 0.5–17.0 dS m−1. Most of the increase in ECe was due to increases in Na+ and Cl−. Magnesium, Ca2+ and K+ were also present. Stalk samples were harvested from 29 (1992) or 30 (1993) sites, with ECe ranging from 0.5 to 9.5 dS m−1. Each dS m−1 increase in ECe decreased Brix (% soluble solids in juice) and Pol (% sucrose in juice) by about 0.6%, decreased apparent purity (Pol as % of Brix) by 1.3% in 1993, increased juice conductivity 0.8 dS m−1, and increased cane residue (fiber) 0.5%. Effects of ECe on Brix, Pol and conductivity were very similar between years, indicating that the response of juice quality to salinity is predictable. This should allow development of sugar quality maps of commercial sugarcane fields for site-specific management decisions. Recoverable sugar yield per ton cane and per hectare were reduced by ECe in both years. In 1992, juice osmolality was less at the higher ECe sites, but in 1993 it was unaffected by ECe. About 90% of the osmolality of the juice was accounted for by the solutes analyzed (total sugar, Na+, K+, Ca2+, Mg2+, and Cl−). Potassium was the most abundant cation in the sugarcane juice (52.5 to 107 mmolc l−1). In 1992 there was a weak curvilinear (R2 = 0.35) increase in juice K+ as ECe increased, while in 1993 K+ tended to increase linearly with ECe (r2 = 0.20). Juice Na+ increased with ECe from 4.9 to 37.4 mmolc l−1. There were also increases in juice Mg+ (11.3 to 42.0 mmolc l−1) and Ca2+ (2.2 to 22.4 mmolc l−1), with increased ECe. Most of the increase in juice conductivity was due to increases in Cl− (30.8 to 106 mmolc l−1). For most attributes there were no significant differences between years. This study shows in greater detail than most previous studies how soil salinity affects juice ionic composition, osmolality, and the accepted industry measurements of juice quality, Brix, Pol, apparent purity, and conductivity.