Agriculture Reference
In-Depth Information
chemical properties, which are influenced by SOM, are summarized in Table 4.2 and their detailed
discussion is given in the succeeding section.
4.6.1 a
vaIlaBIlItY
of
m
aCronutrIents
Soil organic C and N cycling are strongly linked to agroecosystems (Spargo et al., 2012). Practices
that build SOM tend to increase a soil's capacity to meet crop N needs through mineralization of soil
organic N. Furthermore, OM is a major indigenous source of available N that contains as much as
65% of the total soil P, and provides significant amounts of S and other nutrients essential for plant
growth (Bauer and Black, 1994). Also universally accepted is that the C fraction is used by microor-
ganisms as a major energy source for metabolic activity, in the process altering nutrient availability
(Bauer and Black, 1994). OM has many of the characteristics of an ideal N fertilizer. Organic N is
not readily leached or denitrified and its mineralization rate is dependent on many of the same fac-
tors that affect plant growth, such as temperature and water availability. The N-supplying power of
both OM and legumes is particularly important in today's economy, as the cost of N fertilizer has
increased dramatically in recent years. The National Research Council has estimated its availability
in the United States at about 3.4 million tons per year, approximately 15% of the total annual N
input to soil (Darst and Murphy, 1990). The level of OM is an excellent predictor of the amount of
total N in the soil. According to Kapland and Estes (1985), there was a linear relationship between
the total N and OM contents in 24 soils of New Hampshire. The linear regression model was total
g kg
−1
N = −0.25 + 0.056 (g kg
−1
OM). According to this equation, approximately 5.6% of OM was
made up of total N. This value is somewhat higher than the empirically derived value of 5% by Read
and Ridgell (1921). Essentially, the entire N in the plow layer of the soil, of the order of 93-97%,
occurs in organic combinations. Most of the remainder can be accounted for as nonexchangeable
(fixed) NH
+
at any one time (Stevenson, 1982).
The supply of N to a crop by OM depends on the rate of mineralization. Schepers and Mosier
(1991) reported that, for a given climatic region, a general estimate of N mineralization could be
made based on the SOM content. They estimated that, assuming 2% of the total organic N on the
surface, 30 cm is mineralized annually; a soil with 1% SOM content could be expected to mineral-
ize approximately 45 kg N ha
−1
year
−1
. It is important to remember that these are general estimates
because the amount of organic N made available through mineralization processes will vary greatly
over time due to factors such as temperature, precipitation, and tillage (Doran, 1980; Franzluebbers
et al., 1995; Wienhold and Halvorson, 1999).
Soil management practices such as the addition of OM to the soil may modify the amounts of
available P found in soils. Organic materials that are generally returned to the soil contain P, rang-
ing from 0.1% to 0.5% (Dalton et al., 1952; Singh and Jones, 1976). Broadbent (1953) presented a
comprehensive review on the role of OM in the release and tie-up of P from the soil. A number of
researchers report a decrease in P sorption by soils in the presence of OM (Frossard et al., 1986;
Kuo, 1983; Reddy et al., 1980). Sah and Mikkelsen (1989) reported that flooded rice soils without
added OM increased P sorption by 10-70% in half of the 10 soils of California. The common
belief of these workers is that OM decomposition produces organic acids, which form stable com-
plexes with Fe and Al and, consequently, block P retention by them. Other workers report that
OM increases P retention by the soil (Harter, 1969; Sen Gupta, 1969). Some have suggested that
this results from microbial assimilation. P becomes available after the decomposition of organic
P compounds following the death of microbial cells (Fageria and Gheyi, 1999). Diaz et al. (1993)
reported that P release from OM oxidation has been estimated at 72 kg ha
−1
year
−1
in Histosols of
the Everglades (USA).
Kapland and Estes (1985) reported that increases in OM levels resulted in corresponding
increases in exchangeable soil calcium, magnesium, potassium, extractable phosphorous (Bray 1),
and total nitrogen in agricultural soils from New Hampshire. Within terrestrial ecosystems, sulfur
(S) is a macronutrient that is required for the metabolism of all organisms, and it may be deficient in
