Agriculture Reference
In-Depth Information
severe Cu deficiency. Definitely too much of a good thing! Another case of too much SOM occurs
in certain well-drained soils of the tropics classified as Humox or Humults. These are Oxisols or
Ultisols with more than 16 kg C m
−2
to a depth of 1 m, exclusive of surface litter (Sanchez and Miller,
1986). Their topsoils have organic C contents ranging from 4% to 8% and show a strong black color.
Although initially attractive to farmers, the areas of these soils in Brazil are under extensive cattle
grazing, subsistence farming, or are not farmed at all, while adjacent nonhumic Oxisols and Ultisols
are commercially farmed (Sanchez and Miller, 1986). Two factors are thought to account for the low
fertility of these high OM content soils: manganese toxicity and high lime requirement. Their high
SOM contents promote the reduction of manganese compounds to Mn
2+
that can become toxic to
many plants (Pavan and Miyazawa, 1984). To overcome Mn toxicity, these soils generally have to be
limed to a pH of 6.2. The amount of lime required to raise the pH from 4.6 to 6.2 is astronomical and
certainly not economical. A third example relates to the better known group of soils derived from
volcanic ash that have high topsoil SOM contents of 10-20% C. Plants grown on these soils exhibit
N and P deficiencies that are completely out of proportion with the soils' total N and P contents. The
dominant allophane minerals of Andisols are intimately mixed with SOM and fix large quantities
of P, which inhibit N mineralization (Bornemisza and Pineda, 1969).
4.5 OM VERSUS SOIL PHYSICAL PROPERTIES
Physical properties of soils are those characteristics, processes, or reactions of a soil that are caused
by physical forces and that can be described by, or expressed in, physical terms or equations (Soil
Science Society of America, 2008). Examples of physical properties are soil texture, structure or
porosity, bulk density, and water-holding capacity. The soil physical properties mainly influence
air-water relations in the soil, which in turn affect the growth of plants. The addition of OM to soil
improves these physical properties. With the improvement of the soil physical properties, there is an
improvement in soil quality that consequently improves crop productivity (Stevenson, 1982; Bauer
and Black, 1994). The main soil physical, chemical, and biological properties that are influenced or
are having a positive correlation with SOM are summarized in Table 4.2 and their detailed discus-
sion is given in the succeeding section.
TABLE 4.2
Soil OM Function in the Soil for the
Sustainability of Cropping Systems
Changes in Soil Property in Favor of
Sustainability of Cropping Systems
Soil Property
Physical
i. Texture
ii. Structure
iii. Bulk density
iv. Water-holding capacity
Chemical
i. Availability of macronutrients
ii. Availability of micronutrients
iii. Cation exchange capacity
iv. Aluminum toxicity
v. Allelopathy
vi. Heavy metal toxicity
Biological
i. Nitrogen mineralization bacteria
ii. Dinitrogen-fixing bacteria
iii. Mycorrhizae fungi
iv. Microbial biomass
