Agriculture Reference
In-Depth Information
soil Al level for barley increased. OM such as sewage sludge or animal manures added to acid soils
improves fertility as well as reduces Al toxicity problems (Kapland and Estes, 1985).
Surface application or surface incorporation of OM also decreased phytotoxic subsoil Al
3+
activi-
ties because dissolved organic matter (DOM) that leached into the subsoil formed nontoxic Al-DOM
complexes (Hue, 1992; Liu and Hue, 1996; Hue and Licudine, 1999; Willert and Stehouwer, 2003).
The combined application of CaCO
3
and OM in lime-stabilized biosolids decreased subsoil acidity
and increased subsoil Ca saturation, compared with CaCO
3
alone (Tan et al., 1985; Brown et al.,
1997; Tester, 1990; Willert and Stehouwer, 2003). This effect was attributed to increases in Ca
mobility caused by Ca-DOM complexes (Willert and Stehouwer, 2003).
Aluminum-OM has been shown to be an important factor in determining pH buffering and the
relationship between pH and Al
3+
activity in soil solution (Bloom et al., 1979a). Bloom et al. (1979a)
also reported that the fraction of Al bound by OM is important in determining the quantity of Al
extracted from surface soils by neutral salts and that the exchangeable aluminum and effective
exchange capacity are defined by the cation and concentration of the extracting salt. Several inves-
tigations have shown that OM can influence the relationship between pH and the quantity of Al in
soil solution (Clark and Nichol, 1966; Evans and Kamprath, 1970). The addition of OM to an acid
soil decreases the concentration of Al in soil solution and also the effects of Al toxicity compared to
the original soil adjusted to the same pH. Field observations also indicate beneficial effects for the
addition of OM to highly acidic soils (Thomas, 1975).
Adams and Moore (1983) found that Al was toxic at a solution concentration of >0.4 µM in the
Bt horizon low in organic C (0.14-0.30%) but not in horizons higher in organic C (0.28-1.22%)
with the solution of Al between 9 and 134 µM. Ahmad and Tan (1986) found that adding OM (up to
10% wheat straw) can be as effective as adding lime in reducing Al toxicity to wheat and soybean
in acid soils. Bloom et al. (1979b) reported that the management of OM may be effective in lower-
ing Al
3+
activity in acid soil solutions. Additions of OM may, however, lower the apparent solubility
of Al(OH)
3
without lowering the Al
3+
activity. If the degree of neutralization of the added organic
acids is low, the release of H
+
ions from OM may result in a decrease in pH without a decrease of
Al
3+
. This effect is more important at higher pH values. Thus, the effectiveness of OM in lowering
Al
3+
depends on the soil pH and the base saturation of OM. Since equilibration with soil minerals is
very low, yearly additions of OM may be effective in reducing Al toxicity.
Thomas (1975) reported that OM, which usually accumulates near the surface of no-tillage (NT)
soils, tends to ameliorate the effects of soil acidity. Phytotoxic Al species have a strong affinity with
HA and form insoluble Al-HA complexes (Vance et al., 1996; Hiradate and Yamaguchi, 2003;
Yamaguchi et al., 2004). Since the formation of insoluble Al-HA complexes reduces the concentra-
tion of Al in soil solution, HA has a role in preventing Al toxicity in corn (Tan and Binger, 1986).
4.6.5 a
llelopathY
Allelopathy is defined as any direct or indirect harmful or beneficial effect by one plant on another
through the production of chemical compounds that escape into the environment (Rice, 1974).
IAS—The International Allelopathy Society (1996)—defined allelopathy as any process involv-
ing secondary metabolites produced by plants, algae, bacteria, and fungi that influence the growth
and development of agricultural and biological systems. This definition considers all biochemical
interactions between living systems, including plants, algae, bacteria, and fungi and their environ-
ment (Macias et al., 1988). Willis (1985) reported that the basic conditions necessary to demonstrate
allelopathy in natural systems are (i) a pattern of inhibition of one species or plant by another must
be shown, (ii) the putatively aggressive plant must produce a toxin, (iii) there must be a mode of
toxin release from the plant into the environment, (iv) there must be toxin transport and/or accu-
mulation in the environment, (v) the afflicted plant must have some means of toxin uptake, and (vi)
the observed pattern of inhibition cannot be explained solely by physical or other biotic factors,
especially competition and herbivory. However, Blum et al. (1999) reported that no study has ever
