No abstract

Several types of volcanic tuff are found in the Canary Islands, where they are popular with growers as a cheap and abundant alternative for soilless culture. Textures and composition vary according to type, but all have in common a high, buffered pH and a P sorption capacity of agricultural importance. Four tuffs, two coarse black unweathered tuffs and two fine red weathered (alterated) tuffs, from four different localities, were selected to evaluate changes in P sorption after a preplant treatment with phosphoric acid to achieve pH 6. Granulometry, pH, and exchange capacity (CEC) were determined; neutralization curves showed that both red tuffs needed more acid to correct pH. Treated tuffs were fertigated during five weeks with a standard nutrient solution to determine short-term effects on the solution P content retention; rockwool slabs were used as control. Drainage was collected daily, and phosphorus content was determined. In red tuffs, there was a strong initial P release that gradually declined and was practically null at the end of the trial. The black tuffs did not change the P level of the solution during the first week, but afterwards they exhibited an increasing sorption inversely proportional to the decrease in the P level of the solution. Results showed a strong relationship between weathering (significantly more than particle size alone) and the amount of P sorbed from the nutrient solution. However, the present trial shows that preplant phosphoric acid treatment alone is not enough to sustain the necessary P level of the nutrient solution. For the black tuffs studied, the solution needs extra P (particularly for high demand crops), and for the red tuffs, part of the phosphoric acid should be replaced by nitric acid in order to avoid the strong initial release of P that unbalanced the nutrient solution composition.

In the eastern Venezuelan Llanos, the drilling fluids composed of wastewater (WW), accompanying hydrocarbons and wastes from oil activity are deposited in a system of waterproofed pits. Later, by means of siphons, the hydrocarbon phase of the WW is separated. WW are typically very salty and contain suspended and dissolved solids, residual hydrocarbons, and chemicals used in hydrocarbon extraction. So they are transported to a pit and treated with a flocculating agent and lime. Once carried out, the flocculation-coagulation operation and pH correction, WW are released to the environment. The savannas where the treated water are irrigated, in addition to the oil operation, present an intense agricultural and livestock activity, and rest on aquifers that are partially replenished with the percolation waters near the treatment pits. Therefore, this research will aim to analyze: i) the levels of soluble salts and heavy metals in the percolation waters collected in lysimeters located in savanna soils adjacent to oil wells after irrigation with WW treated with flocculating agents, and ii) the estimation of time which these salts take to be dislodged from the soil. The waters from the lysimeters did not report high levels of soluble aluminum or heavy metals since their soluble forms were precipitated by liming. Likewise, the coagulation-flocculation process removed the high levels of barium from the untreated WW. The contributions of sodium and calcium from the WW have a liming effect on the acidity of these soils. The sodium levels contributed by the WW could represent an environmental risk, fortunately, the good internal drainage of these sandy Ultisols allows their removal. Studies with lysimeters showed that the percolation waters of areas irrigated with flocculated water have a similar sodium content to that of control lysimeters six months after treatment.