Agriculture Reference
In-Depth Information
Tillage operations are major factors involved in soil structure degradation on arable lands (Kay,
1990). Tillage management can have a profound impact on soil structural properties. Conservation
tillage can improve the structure of arable lands. The results of several studies showed that surface
soil in zero-tilled plots had significantly greater aggregate mean weight diameter and available
water capacity than soil that had been tilled (Mikha and Rice, 2004; Prakash et al., 2004; Gulde
et al., 2008).
8.2.2 C
hemICal
p
ropertIes
Soil chemical properties play a significant role in determining nitrogen use efficiency in crop plants.
Important chemical properties related to N availability, uptake, and use efficiency are soil acidity,
soil fertility, soil salinity and alkalinity, use of balanced nutrient supply, and reducing allelopathy.
These soil chemical properties should be favorable for plant growth to have higher nitrogen use
ef iciency.
8.2.2.1 Liming Acid Soils
Soil acidity is a major constraint to crop production for large areas worldwide. Soils become acidic
during geological evolution, especially in areas of high rainfall, because bases are leached to
lower profiles leaving surface layers acidic. Furthermore, acidity is associated with the release of
protons (H
+
) during the transformation and cycling of C, N, and S in soil-plant systems (Bolan
and Hedley, 2003). Sumner and Noble (2003) reported that top soils affected by acidity account for
30% of the total ice-free land areas of the world, with the Americas, Africa, and Asia accounting
for the largest portions. In tropical South America, 85% of the soils are acidic, and approximately
850 million ha of this area is underutilized (Fageria and Baligar, 2001a). Theoretically, soil acidity
is measured in terms of H
+
and Al
3+
concentrations in soil solutions. In Crop production, for crop
production, soil acidity involves many factors that adversely affect plant growth and development.
Plant growth on acidic soils can be limited by deficiencies of N, P, K, Ca, Mg, or Mo; toxicity of
H, Al, or Mn; reduced organic matter breakdown and nutrient cycling by microflora; and reduced
uptake of nutrients by plant roots and inhibition of root growth (Marschner, 1991). In Brazilian
Oxisols, deficiencies of most essential macro- and micronutrients have been reported for the pro-
duction of upland rice, corn, wheat, dry bean, and soybean (Fageria and Baligar, 1997a). The
positive effects of liming on crop growth may be associated with the amelioration of one or more
of the abovementioned factors (Haynes, 1984), and possibly from reduced weed growth (Haynes,
1984; Legere et al., 1994).
Soil acidity adversely affects the morphological, physiological, and biochemical processes in
plants and consequently the N uptake and use efficiency (Raven, 1975; Feldman, 1980; Foy, 1984;
Baligar et al., 1997; Grewal and Williams, 2003; Fageria and Baligar, 2005). External pH affects
root growth by influencing apoplastic auxin translocation from the zone of synthesis (i.e., root tips)
to the zone of root elongation (Raven 1975; Feldman, 1980). Excessive H
+
concentrations have been
reported to cause severe reductions in the shoot and root growth of wheat (Johnson and Wilkinson,
1993) and sorghum (Wilkinson and Duncan, 1989). Reduction in root growth and nodulation of
alfalfa in acidic soil has been reported by Grewal and Williams (2003). Apart from H
+
tox icit y,
excessive Al
3+
and Mn
2+
as well as deficiencies of nutrients have been found to be major factors
contributing to the reduction in plant growth and nutrient uptake in acidic soils (Foy, 1984). Figure
8.1 shows that the grain yield of dry bean decreased significantly with increasing Al levels in the
Brazilian Oxisol. Acidity also decreases N uptake and use efficiency by reducing N mineralization,
nitrification, and nodulation as well as by reducing the root development of crop plants (Grewal and
Williams, 2003; Menzies, 2003).
Liming is the most common and effective practice for reducing soil acidity-related problems.
Lime significantly increased grain yields of annual crops such as common bean, corn, and soybean
grown on Brazilian Oxisols (Fageria, 2001; Fageria and Baligar, 2001a, 2003b). Figures 8.2 through
