Agriculture Reference
In-Depth Information
Nutrient stocks diminish with depth, often markedly in highly-weathered soils
paralleling the decline in organic matter concentrations. This is illustrated in the context
of organic matter studies in Table I.19 which lists the stocks of carbon and nitrogen
at selected depths in the oxisol profile presented in Figure I.33c. Other factors affecting
the magnitudes of useful soil reserves include the depth of the soil profile and
nutrient availability.
The 'plant-available' amount of a nutrient element in a soil is considered to be
the fraction of it that could potentially be taken up by plant roots and their associated
mycorrhizal fungi, at any given time. It must therefore be defined in relation to the
particular plant community or crop present and in relation to plant mycorrhizal status
and other factors affecting the ability of the root system to extract nutrients from the soil.
To be available, nutrient elements must be physically accessible, that is, not sequestered
within unweathered primary minerals or undecomposed organic matter. Further, nutrients
in parts of some profiles may not be accessible to roots because of genetic constraints.
For example, roots may not be able to grow to sufficient depths to exploit deeper
reserves. Inaccessibility may also result from the presence of impermeable pans or a layer
of permafrost, partial waterlogging leading to anoxia or undue soil dryness. In some
agricultural soils, past poor soil management practices may restrict the roots to the soil
volume above a compacted zone.
In non-calcareous soils, much of the store of readily plant-available basic cations
(Ca, Mg, and K) is retained at colloid surfaces and may be held in a state of equilibrium
with cations in the soil solution. The colloids involved are largely clay particles or
organic matter and comprise the major part of the whole-soil cation exchange capacity.
Nutrient and other elements may occur in soils in chemical forms that are insoluble
or can not be taken up by plants. This may occur because they are poorly soluble at
existing soil pH levels, as in the case of manganese considered above (Figure I.35), or are
occluded through reaction with other soil components. Low phosphorus concentrations
commonly limit plant productivity in many soils and phosphorus may be of particularly
