Agriculture Reference
In-Depth Information
Soil pH is one of the most important chemical properties in determining the availability of
macro- and micronutrients. Soil pH buffering caused by the protonation and deprotonation of min-
erals and organic materials reduces the change in soil pH when acids or bases are added to the soils
(Weaver et al., 2004). Aitken et al. (1990) reported that the use of multiple linear regression analysis
showed that organic carbon was most significantly variable, accounting for 78% of the variance
in the pH buffering capacity, although clay accounted for 32% of the variance. The high organic
C content will also determine CEC and have a large impact on the pH buffering capacity (Giesler
et al., 2005).
Liming acid soils is an effective and dominant practice to improve the mineralization rate of
SOM. When soil pH is lower than 5.5, the microbial activities of nitrifying bacteria are lower and
the SOM mineralization rate is reduced. In dry bean grown in an NT system in Brazilian Oxisol,
the author detected N deficiency 2 weeks after germination in plots that did not receive lime (pH
5.2), but in plots that received lime (pH 6), N deficiency was not observed. In this case, it is hypoth-
esized that OM mineralization liberated sufficient N in the plots that received lime. According to
Andrew (1978), the H
+
ion is particularly important in legumes grown without fertilizer N. It affects
rhizobial survival and multiplication in soils, root infection and nodule initiation, legume rhizo-
bial efficiency, and nutrition of the host plant (Foy, 1984). Franco and Munns (1982) reported that
decreasing the pH of a nutrient solution from 5.5 to 5.0 decreased the number of nodules formed by
dry bean. Ve et al. (2004) also reported that the degree of humification was positively associated
with exchangeable Ca
2+
and Mg
2+
(main component of dolomitic lime), suggesting a positive influ-
ence of these basic cations on the turnover of young OM.
Ammonification and nitrification are two important reactions in soils affecting N availabil-
ity. Ammonification is the biological process leading to ammoniacal nitrogen formation from
nitrogen-containing organic compounds, whereas nitrification is a biological oxidation of ammo-
nium to nitrite and nitrate. Ammonification can occur over a wide range of soil pH, but nitrification
is markedly reduced at pH values <6.0 and >8.0 (Alexander, 1980). Alexander (1980) reported that
nitrification of organic materials stops at pH below 4.5. Morrill and Dawson (1967) reported that
soil pH is the best indicator of nitrification in various types of soils. Haynes and Sherlock (1986)
and Clough et al. (2004) reported that liming enhances nitrification and that cumulative N
2
O emis-
sions under field capacity conditions are reduced with liming. Clough et al. (2003, 2004), however,
reported that under saturated soil conditions, the cumulative fluxes of N
2
O and N
2
are much greater
than under field capacity conditions and are enhanced following liming. Since pH has a potential
effect on N
2
O production pathways and the reduction of N
2
O to N
2
, it has been suggested that liming
may provide an option for the mitigation of N
2
O from soils (Stevens et al., 1998).
4.8.5 u
se
of
o
rGanIC
m
anures
The use of organic manures and/or crop residues improves the OM content of soils (Rochette and
Gregorich, 1998; Singh et al., 2004a). These management practices bring several favorable physical,
chemical, and biological changes in the soil-plant systems and consequently improve the sustain-
ability of cropping systems. The beneficial effects of these management practices are discussed in
the succeeding sections.
4.8.6 u
se
of
C
over
C
rops
/G
reen
m
anurInG
Cover crops are defined as the close-growing crops that provide soil protection, and soil improve-
ment between periods of normal crop production, or between trees in orchards and vines in vine-
yards. When plowed under and incorporated into the soil, cover crops may be referred to as green
manure crops (Soil Science Society of America, 2008). The positive role of cover crop/green manur-
ing in crop production has been known since ancient times. The importance of this soil-ameliorating
