Agriculture Reference
In-Depth Information
Relative content
0
N
S
P
Figure 4.2
Changes in relative content of soil N, P,
and S with depth.
100
is similar to N, the decline with depth is more abrupt because of the immobility
of phosphate ions in the soil (exceptions can occur in very sandy soils, such as in
the coastal regions of southwest Western Australia). Sulfur, like N, accumulates
mainly in the A horizon, but SO 4 2- ions produced by mineralization of organic S
may leach and accumulate in the subsoil (fig. 4.2). The distribution of most
cations—Fe 3 , Ca 2 , Mg 2 , Mn 2 , Cu 2 , and Zn 2 , for example—usually cor-
relates with clay accumulation in the soil profile. But cations such as Cu 2 , Zn 2 ,
Fe 3 , Al 3 , and Mn 2 , which have a high affinity for organic matter (section
2.3.4.2), also accumulate in the organic-rich parts of the soil profile.
4.2.1
The Inorganic Store
Plant roots normally access only a small fraction of the soil's inorganic store of
the macronutrients P, Ca, Mg, and K and the micronutrients Fe, Mn, Zn, Cu,
B, and Mo in a single growing season. This fraction is drawn from an available
pool , made up of ions in the soil solution and exchangeable ions adsorbed by clay
minerals and organic matter. The remainder of the inorganic store (the unavail-
able pool) exists as (1) sparingly soluble compounds like gypsum, calcite, and mag-
nesite, (2) insoluble inorganic compounds of P, Fe, Zn, and Cu, (3) nonex-
changeable ions in clay lattices (e.g., K ), or (4) insoluble oxides (Fe, Mn, and
Mo).
Inputs to the available pool occur by
1. weathering of soil and rock minerals (chapter 1)
2. rainfall and dry deposition (chapter 5)
3. mineralization of organic matter and excreta (chapter 2)
4. transfers from the unavailable pool (section 4.4.3)
5. application of manures and fertilizers (chapter 5).
4.2.2
The Biomass Store
The amount of nutrients stored in plants depends on the mass of vegetation pro-
duced. This varies greatly, for example, from a mature tropical forest to a culti-
vated crop, such as grapevines. The biomass C in a vineyard depends on many
 
 
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