EFFECTS OF CHLORIDE AND SULFATE IN VARIOUS N AND K FERTILIZERS ON SOIL CHEMICAL PROPERTIES AND NUTRIENT CONCENTRATIONS IN DURIAN LEAF AND FRUIT

Durian growers in Eastern Thailand prefer potassium sulfate (K₂SO₄) to potassium chloride (KCl) even though potassium chloride is twice as cheap. It is widely believed that potassium sulfate will result in better fruit quality, including flesh color. In addition, some growers believe that chloride accumulation due to KCl application will be harmful to durian trees. The purpose of this investigation is to study whether sulfate fertilizers can be effectively replaced with chloride fertilizers. The effects of N and K fertilizers containing chloride and sulfate on electrical conductivity (EC), exchangeable K, extractable S, chloride in soil, and nutrient concentrations in durian leaf and fruit from a mature orchard at Chantaburi, Thailand were examined. The topsoil, a Typic Paleudults, was a sandy clay loam (pH 5.0, EC 195 µS cm^-1) with clay loam subsoil. The experiment consisted of six treatments, i.e. 1) Urea + KCl, 2) NH₄Cl + KCl, 3) (NH₄)₂SO₄ + KCl, 4) Urea + K₂SO₄, 5) NH₄Cl + K₂SO₄, and 6) (NH₄)₂SO₄ + K₂SO₄. Each treatment was repeated six times on six different trees. Nitrogen and K were applied at 1,500 g N tree^-1 year^-1 and 2,000 g K₂O tree^-1 year^-1, respectively. For all treatments, the amount of exchangeable K increased for three weeks before returning to approximately its original level. All six treatments also caused the EC to increase for two to three weeks before declining. The highest EC (450 µS cm^-1) was far below the unacceptable level, suggesting that sulfate fertilizers can be replaced with chloride compounds without harmfully affecting the EC of the soil. In contrast, all forms of N and K fertilizers did not significantly affect N and K concentration in durian leaves, whereas S was higher in treatments containing sulfate either with N or K. The highest leaf S concentration of 0.19% was measured in the (NH₄)₂SO₄ + K₂SO₄ treatment. For chloride, it was found that NH4Cl, either applied with KCl or K₂SO₄, resulted in the highest concentration of 0.12% chloride in leaves. The lowest value of 0.02% chloride was observed in treatments without chloride application. Nonetheless, it was found that higher levels of S and Cl in the leaves did not translate into a higher S and Cl concentration in durian fruit. In conclusion, this study suggested that KCl could be used as an effective replacement for K₂SO₄ in this soil.