QUALIDADE DE SOLOS CULTIVADOS COM HORTALIÇAS NO DISTRITO FEDERAL

Abstract

Vegetable production is among the most important agricultural activities in the Distrito Federal, Brazil, and in many regions all over the world. Due to its perishability and to reduce costs and losses during transportation, growing areas develop mainly around large cities and close to consumer market. Soil quality degradation is often associated with the intensification of agriculture. Growing vegetables under conventional system is related to intensive soil preparation often performed using machinery and heavy implements and application of high fertilizer doses. Such intensive land use can give rise to imbalance in chemical and physical soil attributes, with direct effects on its biology. Thus, monitoring soil quality and adopting sustainable management practices are essential to provide safe food production, with high yield, without compromising the other soil functions and its quality. The objective of this work was to diagnose the soil quality cultivated with vegetables. Soil samples were collected in 29 sites traditionally cultivated with vegetables, at 0 to 20 cm depth. 120 soil samples were also collected in areas of native vegetation or with minor anthropic intervention to establish Quality Reference Values (VRQ) for trace elements of environmental relevance, as well as to use as references for interpreting the results of analysis of areas under agricultural production. In addition to soil chemical analysis, six more indicators were selected to compose the diagnostic study, namely: soil electrical conductivity, VRQ, degree of flocculation, active carbon (Labile), soil proteins, soil microbial respiration. The results obtained were ranked by means of quality indexes to categorize agricultural areas according to main soil function, that is to produce food. For this purpose, the average quality index (IQSm) was estimated considering the average of the variables of all sampled areas. The results indicated an improvement in soil fertility with acidity correction, however the fertilizer contents were often unbalanced, with excess of potassium, calcium, magnesium and micronutrients, except for boron that showed different pattern, as does phosphate, were the soil content were below critical level. Excessive fertilization had direct effects on the electrical conductivity and that could be a major constraint for some group of vegetables. Increases on trace element (heavy metals) levels were observed only for those with micronutrient function. The intensive tillage and the excess of cations promoted a considerable increase on dispersed clay of cultivated soils in relation to native areas. The content of active carbon and proteins were also lower, as a result of inadequate management of soil organic matter, with little diversity of cultivated plants, excessive soil preparation and unbalanced fertilization. The estimated average IQS for agricultural areas scored 63. The excessive content of potassium and some available micronutrients followed by poor biological quality of the soils account for such low score. Improving the soil quality of the studied areas involve some changes on the management of the variables that received lower scores, especially the ones associated with biological activity of the soil.