Disponibilidade e acúmulo de potássio, cálcio e magnésio em plantações de eucalipto em diversos sítios florestais

Abstract

The knowledge of the alterations of the nutrient contents in the soil compartments, caused by the variations in the changeable and non-changeable forms over time and the relation of these nutrients with the nutrient export by the eucalyptus, may provide safe indications as to the use of soils and, therefore, of the forest sites studied and, consequently, allow a better adequacy of the forest management systems. So, this study had the following objectives: evaluate the release of K, Ca and Mg in non- changeable forms in several classes of soil cultivated with eucalyptus; evaluate the relation between the contents of K, Ca and Mg in eucalyptus and the changes occurred in the changeable and nonchangeable forms of these nutrients in the soil; evaluate the soil capacity to supply such nutrients during a growth cycle of the eucalyptus (2, 5 and 7 years) and verify which of the eight sites selected for the study would present greater potential to provide nutrition for the future cycles. The work was carried out in an Euclyptus grandis settlement located in the region of Santa Bárbara, Minas Gerais. The sites selected for the study comprised five classes of soils, in which the eucalyptus was planted in 1996. In 1998, it was demarcated an experimental parcel of 60 x 60 cm, in each site, with 600 trees in the spacing of 3 x 2 m. In these same parcels, soil and plant collections were carried out in 2001 and 2003, resulting in three evaluations (1998, 2001 and 2003), corresponding to the ages of 2, 5 and 7 years of settlement. Chemical analyses were performed in the samples of each age and also soil analyses referring to: total acid attack, extraction with boiling HNO3 2 mol/L, extraction with cation exchange resin. Four trees of each parcel and of each age were cut for the determination of the dry matter production. Each tree was divided in wood, bark, leaves and branches so that the chemical analyses of the plants could be performed. The organic surface layer and the sub-forest were quantified by the use of a square of 0,5 x 0,5 m randomly cast in the four quarters of the parcel. The correlations between the different forms of K, Ca and Mg in the soil and the correlations between the production of dry matter and the plant content of K, Ca and Mg and the contents of K, Ca and Mg in the soil were calculated. With the results achieved, it was possible to conclude that: successive extractions with cation exchange resin were enough to extract only available forms of K and changeable forms of Ca and Mg; the soils that presented potassium minerals in the silt fine sand and clay fractions were those that presented the highest contents of K; modifications in the forms of K, Ca and Mg occurred in the period from 2 to 7 years of the eucalyptus growth; all the sites studied presented soil with low capacity to supply K, Ca and Mg; the sites that presented higher production of dry matter were not the same that presented the highest contents of K, Ca and Mg in the soil; Mg was the nutrient with the lowest demand by the eucalyptus and the one that presented the lowest contents in the soils; the sites presented different behaviors as to the contents of K, Ca and Mg and, therefore, they should be managed according to each one’s specific need; the physical characteristics of the sites Cururu and Valéria II P2 may be a barrier for future productions; Ca will be the most troublesome nutrient for the sustainability of the sites Valéria II P1, Valéria II P2, Vargem Alegre and Egas and the balance of nutrients showed that fertilizations should be carried out in all the sites, aiming a higher production in the next cycle.