Agriculture Reference
In-Depth Information
determined the OM content of 43 sites, covering 33 rural properties in the State of Mato Grosso,
Brazil. The soils of these sites were classified as Oxisols and the OM content varied from 8 to
31 g kg
−1
(0.8-3.1%) with an average value of 21.5 g kg
−1
(2.2%). These authors reported that the
average OM content of these soils was good but the mineralogy of highly weathered tropical soils
such as Oxisols is rich in oxides and hydroxides of Fe and Al. These oxides and hydroxides did not
permit adequate mineralization of OM and a major part of OM remains in an inactive form (Lopes,
1983).
From their OM content, soil can be classified as mineral soils (<l00 g OM kg
−1
soil), organic soils
(100-250 g OM kg
−1
soil), or peat soils (>250 g OM kg
−1
soil) (Bailey et al., 1991). In Brazil, there
are about 35 million hectares of lowland, locally known as “varzea.” These areas are distributed
throughout the country and generally have favorable climatic conditions for crop production. Owing
to poor drainage, lowland rice is a suitable crop to plant on these soils during the rainy season.
During dry periods, other crops can be planted in rotation, provided there is adequate drainage.
Cu r rently, <2 million hectares of the varzea land is under rice cultivation. Fageria et al. (1997)
also determined the OM content of varzea soils of the Mato Grosso and Mato Grosso do Sul States
of Brazil (Table 4.1). Data in Table 4.1 show that the OM content in these soils varied from 5 to
257 g kg
−1
(0.5-25.7%) in the top 0-20 cm soil layer, 1 to 37 g kg
−1
(0.1-3.7%) in the 20-40 cm soil
depth, 21 to 148 g kg
−1
(2.1-14.8%) in the 40-60 cm soil depth, and 31 to 50 g kg
−1
(3.1-5.0%) in the
60-80 cm soil depth. The averaged values of the SOM content of the two states were 46 g kg
−1
at
the 0-20 cm soil depth, 12 g kg
−1
at the 20-40 cm soil depth, 17 g kg
−1
at the 40-60 cm soil depth,
and 7 g kg
−1
at the 60-80 cm soil depth. Overall, the OM content in the top layer (0-20 cm) was
higher compared to the lower soil depths. The turnover of organic carbon depends on environmental
conditions, such as aeration, temperature, and water content, and for this reason, the concentration
of organic C is higher in the topsoil layer (Mengel et al., 2001). Additionally, most of the plant
residues are added on the soil surface and their decomposition can increase the top layer OM con-
tent. Cochrane et al. (1985) reported the OM levels in soils from the lowlands of tropical America.
These authors classified soils as high, medium, and low in OM if they contained >45, 45-15, and
<15 g kg
−1
, respectively. According to this classification, the average OM content in the soils of the
two states (Mato Grosso and Mato Grosso do Sul, Brazil) in the 0-20 cm depth, 46 g kg
−1
was high.
In the lower soil depth, the values of OM content were low. These results showed that there is great
variability in the OM content of varzea soils. Fageria et al. (1991) also reported ample variability in
the OM content of varzea soils of the Goias State of Brazil.
Some soils such as Histosols, humic Oxisols and Ultisols, and Andisols contain very high OM
and create problems for crop production (Sanchez and Miller, 1986). Vast areas of deep organic
soils classified as Histosols cover 32 million hectares of Southeast Asia (Driessen, 1978). Many of
these soils have more than 20% C to a depth >50 cm and attempts to cultivate them have met with
very limited success (Dent, 1980). The main constraints are poor foothold for roots, H toxicity, and
TABLE 4.1
OM Content (g kg
−
1
) of the Varzea Soils of the States of Mato Grosso
and Mato Grosso do Sul, Brazil
Soil Depth (cm)
Minimum
Maximum
Average
0-20
5
257
46
20-40
1
37
12
40-60
21
148
17
60-80
31
50
7
Source:
Adapted from Fageria, N. K. et al. 1997.
Commun. Soil Sci. Plant Anal.
28:37-47.
