Water content as a deterministic factor in the assessment of cohesive character in soils of Coastal Tablelands (Northeast, Brazil)

Abstract

Cohesive character refers to soil horizons whose consistency ranges from very hard to extremely hard when dried, changing to friable until firm when moist. However, it is not precisely known what the soil water content value allows for identifying this attribute. Therefore, having information about the relationship between gravimetric water content and cohesion helps in diagnosing this attribute. Given the above, the following hypotheses were considered: i) the friable or firm consistency does not occur in soil water content near the slightly moist condition – the threshold moisture between moist soil and air-dried soil; ii) the consistency becomes very hard or extremely hard before the water content matches the air-dried soil – humidity of the aggregate balanced with the atmospheric conditions. Samples were collected in seven soil profiles in six Brazilian states. At the laboratory, the blocks were separated into their natural aggregates/clods. An evaluator performed the consistency tests in moist and dried soil conditions and a fraction of the clod was used to measure the gravimetric water content at the time of the test. The data were analyzed in completely randomized design, with seven treatments (seven horizon soils) and ten replications. Multivariate statistical methods and regression analysis were also performed. It was concluded that: i) in moist soil, friable or firm consistency does not occur in the water content near the slightly moist condition. The consistency friable was observed in water content closer to field capacity, ii) on the other hand, between intermediate and slightly moist, very firm and extremely firm consistencies were observed. In air-dried soil, the consistency became hard, very hard, or extremely hard before the soil reaches air-dried condition; iii) given the large variability consistency of the soils, it is prudent that soil taxonomy systems define the water content range in which the consistency assessment can be done to identify soils with cohesive character for moist and air-dried soil conditions.