Principles of Soil Management in Neotropical Savannas - Principles of Sustainable Soil Management in Agroecosystems - page 71

ha -1 ), but because of low solubility, its use is recommended only for pastures, forest
plantations, and perennial crops, which can absorb P over longer terms than annual
crops. Soluble phosphate sources must always be applied in restricted soil volumes
(e.g., planting lines or seedling pits) in order to reduce the strong specific sorption to
Fe/Al oxides, and there is a growing trend to observe the same practice for all soluble
nutrient sources. After a “nutrient bank” is created, soil nutrient levels must be deter-
mined by soil testing every other year, or more frequently for intensive cropping, and
supplemental fertilizer used as needed.
3.2.1.3 Soil Tillage and Seedbed Preparation
These practices are usually initiated after clearance of native vegetation and residue
burning. In many cases, a tractor-pulled rake or tiller gathers residues in windrows
for burning, thus causing the initial disruption of soil (excluding soil upheaval when
trees and shrubs are pulled off with a chain). Also, every time lime is applied on
soil surface, it must be incorporated with disk plows or harrows to a 0.2-m depth
or deeper, followed by another harrowing. Alternatively, farmers use a heavy disk
plow ( grade pesada ) not only to incorporate lime but also for a routine seedbed
preparation. These operations can be repeated in different frequencies and intensi-
ties depending on the crop and soil management, varying from several times per
year to only once every 14 years or more. Soil tillage for weed control is also com-
mon in coffee stands and small farms by tractor or animal/manual driven opera-
tions. As a rule of thumb, tillage on an annual or higher frequency strongly disrupts
the larger macroaggregates and promotes compaction of most Cerrado soils (Resck
1998; Resck et al. 2001), and also causes significant SOC loss (Zinn et al. 2005b) by
oxidation of crop residues and coarse-sized SOC hitherto occluded inside aggregates
or within the interaggregate pores (Zinn et al. 2011; Pes et al. 2011). These degrada-
tion processes are especially severe when the heavy disk harrow is used rather than
a plow or a harrow (Silva et al. 1994; Resck 1998).
3.2.1.4 Crop Rotations
Numerous benefits of crop rotations for diverse regions and soils are widely known
by farmers and researchers. In the Cerrado, crop rotations are especially impor-
tant to improve plant nutrition and to maintain SOC at adequate levels. Upland rice
( Oryza sativa L.) and corn ( Zea mays L.), the most common crops grown during the
early years of the Cerrado agriculture, generate enough harvest residues to main-
tain adequate mulching, and whose quality allows relatively slow decomposition and
proper humification. However, after P buildup by corrective fertilization, N becomes
the limiting nutrient, since N fertilizers are among the most expensive in Brazil. On
the other hand, soybean varieties specifically adapted to the Cerrado region supply
all their N requirements by symbiotic N fixation. However, the amount of crop resi-
dues are low and C/N ratio is narrow. Thus, adequate mulching is not feasible, and
SOC can decrease drastically under continuous monoculture. Therefore, alternating
cereals and legume crops, such as corn/soybean [ Glycine max (L.) Merrill] or corn/
beans ( Phaseolus spp.), or even soybeans and pastures (see Section 3.2.7), is a use-
ful strategy that is becoming more and more common. Planting cover crops such as