Mechanical analysis in some soils of the semi-arid region in Northeast Brazil: normal, calcareous, calcareous with soluble salts, and saline soils

Abstract

In this paper the author studies the dispersion of the 2 clay fractions of some soil samples of the semi-arid region in Northeast Brazil, aiming to test the effectiveness of sodium hydroxide and hexametaphosphate as dispersing agents in mechanical analysis of different kinds of soils, namely: normal calcareous, calcareous with soluble salts, and saline soils. Thirty three soil samples were used: 10 normal soils, 16 calcareous, 4 calcareous with soluble salts, and 3 saline soils. The following pre-treatments were employed: Treat. 1. no pre-treatment. Treat. 2. 10% HCl pre-treatment. Treat. 3. previous washing of sample with 60% alcohol ethylic. Treat. 4. previous washing of sample with distilled water. These pre-treatments were selected according to the nature of each group of soils, as follows: Normal soils: Pre-treatment 1. Calcareous soils: Pre-treatments 1 and 2. Calcareous with soluble salts: Pre-treatments 1, 2, 3, and 4. Saline soils: Pre-treatments 1, 3 and 4. Considering the results obtained with the 2 dispersing agents, and taking into account the following items: a) the pre-treatment used; b) the textural classification, as determined in the field; c) the highest value obtained for the 2 micra clay fraction. The author presents the following conclusions: I. Normal soils: The effectiveness of sodium hydroxide and sodium hexametaphosphate as dispersing agents is the same. II. Calcareous soils: Sodium hydroxide (Treat. 2) produced the best results. Except for one sample, the hexametaphosphate (Treat. 1) presented results in accordance to the field texture. III. Calcareous with soluble salts: Sodium hydroxide (Treat. 2) was again the best. Sodium hexametaphosphate (Treat. 3 and 4) was also effective. IV. Saline soils. a) Low content soluble salts: Sodium hydroxide and hexametaphosphate may be used without pre-treatment. b) High content of soluble salts: Hexametaphosphate (Treat. 3 and 4) is preferible. In this case sodium hydroxide cannot be used.