Total Applied Energy Versus Cavitation Intensity: How Soil Aggregate Stability Can be Accessed Using Ultrasound?.

The soil aggregate stability has been intensively studied in Soil Science, because soil aggregation is closely related to important soil attributes and processes, such as: soil porosity, water infiltration, soil aeration, hydraulic conductivity, surface sealing, compressibility, soil erodibility and carbon stabilization. Using ultrasonic energy to assessing the soil aggregate stability, the energy level needed for breaking and dispersing the aggregates is easily determined. Moreover, it is possible to have an overview of the soil aggregate hierarchy. Expressing the ultrasonic energy in J mL^-1 of soil-water suspension or J g^-1 is the main way the applied energy has been shown. We hypothesized that this is not a suitable way, since the same total applied energy can be obtained from different combinations of sonication time and power level [E(Joules) = P(Watts).t(seconds)]. Thus, this work aimed to evaluate the effect of interaction between time and ultrasound output power on the stability of soil aggregates. It was used a sonicator probe-type insert (2.5 cm) in a soil suspension (10 g oven dry-basis of aggregate sized between 4 and 8 mm in 250 mL of distilled water). The soil aggregates were obtained from an Oxisol (0-5 cm layer) under coffee production in Brazilian Savanna and submitted to ultrasonic irradiation under different output power levels and times: (i) 20 W during 500 s; (ii) 30 W during 333.33 s; (iii) 40 W during 250 s and (iv) 50 W during 200 s. In all these conditions the total applied energy was 10,000 J (40 J mL^-1 or 1,000 J g^-1). After each sonication, the soil aggregates were wet-sieved (2 mm, 1 mm, 0.5 mm and 0.25 mm sieves), the mass of aggregates and or particles that were retained in each sieve was oven-dried (105° C) for 48 hours, weighted and finally calculated the amount of aggregates in each size fraction (4-2 mm; 2-1 mm; 1-0.5 mm; 0.5-0.25 mm; and < 0.25 mm). Our results show that despite the total applied energy was the same, different responses on soil aggregate stability were obtained. The cavitation intensity seems to be more efficient or has more effect on soil aggregate breakdown and dispersion. This statement is true once that the binomial “high output power and short-time” had more effect that “low output power and length of time”, although the combination of these conditions had produced the same amount of energy. Financial support: FAPEMIG, CNPq, CAPES and PROPP/Federal University of Uberlandia.