Principles of Soil Management in Neotropical Savannas - Principles of Sustainable Soil Management in Agroecosystems - page 70

economic growth and welfare, which also created some environmental concerns.
Most of the ensuing degradation of natural resources, such as biodiversity loss and
CO 2 emissions, are direct consequences of removal of the native vegetation. In
addition, the attending agricultural management also causes drastic environmental
impact, especially on soil properties and water quality. The following sections dis-
cuss the most common land use systems in the Cerrado, different soil management
options, and impacts on soil properties.
3.2.1 B aSiC t enetS of S oil M anageMent in the C errado
Resck (1998) summarized that successful management of Cerrado soils requires a
proper combination of four practice sets. Each practice must be performed in an opti-
mal manner so that the other three are effective. Failure to do so almost always leads
to low or declining yields and soil degradation. Although the frequency and intensity
of each practice set vary among major land use systems, as discussed below, a brief
description of these practices follows.
3.2.1.1 Clearance of Native Vegetation and Correction of Soil Acidity
Removing native trees and shrubs is traditionally done by pulling a heavy metal
chain by two tractors, followed by raking of fell biomass in windrows and burn-
ing (Folle and Seixas 1985; Klink and Moreira 2002). Decomposition and burning
release nutrients from the biomass within a few months, causing nutrient and SOC
depletion, especially by the burning of residues that otherwise would have slowly
converted into humus. Liming is required to raise pH levels and replace exchange-
able Al 3+ , toxic to most crops, by Ca 2+ and Mg 2+ . Liming enhances CEC through the
dissociation of carboxyl groups of humic substances, an important process because
of low CEC of clay minerals in most Cerrado soils. However, excessive liming or its
poor incorporation by harrowing in a <20 cm layer can turn micronutrients insoluble,
induce severe deficiency, and produce low crop yields. Also, high Al saturation in the
subsurface horizons restricts root growth into the topsoil, exacerbating susceptibility
of crops to dry spells during the growing season. Thus, gypsum (calcium sulfate) is
often applied on the soil surface at the rate of 0.5-1.5 Mg ha -1 (Spehar 1998), because
its dissolution and subsequent leaching into subsurface layers can effectively replace
Al 3+ by Ca 2+ , enhancing root depth and agronomic productivity.
3.2.1.2 Fertilization
Cerrado soils being low or extremely low in all mineral nutrients, require corrective
fertilization to “build up” an initial nutrient bank/reserve, followed by replacement
of losses through crop harvest and/or erosion. The fertilization must be done only
after liming; otherwise, fertilizer nutrients are wasted either by leaching in soils of
low CEC or by high P fixation on positively charged sites of Fe/Al oxides at low pH.
Nonetheless, high soil pH by excessive liming also leads to micronutrient insolubility,
causing deficiencies that can only be corrected by deeper incorporation into the sub-
soil. Agronomic productivity in most Cerrado soils is mostly P-limited. Corrective fer-
tilization often involves applying rock phosphate on the whole area (typically <1 Mg