Geoscience Reference
In-Depth Information
around pH 7 that phosphorus is most available. Some
adsorption of PO
4
3-
will also occur on clay surfaces at
neutral pH. In addition microbial activity will usually be
at a maximum at neutrality, resulting in increased
microbial mineralization, and the conversion of phos-
phorus from organic to inorganic form. The 'phosphorus
problem' is that soluble and plant-available PO
4
3-
is
present only in low concentrations in most soils, and is
quickly converted to unavailable forms. Micro-organisms
are known to play an important role in solubilizing PO
4
3-
from unavailable organic and inorganic stores. Large
numbers of soil and marine micro-organisms are able to
solubilize apatites and possess the enzyme phosphat-
ase which will release PO
4
3-
from organic phosphorus.
Mycorrhizal fungi can also form which help phosphorus
uptake by plants (p. 531). As PO
4
3-
is of limited solubility,
very little phosphorus is deposited in oceanic sediments,
when compared with the total biomass phosphorus. The
amount deposited in oceanic sediments roughly balances
the run-off from the terrestrial environment, which in
turn equals the global input from rock phosphorus via
weathering and mining. The oceans have a huge capacity
for the immobilization of phosphorus in sediment, and
act as the largest pool of global phosphorus. The action
of fish-eating seabirds in transferring marine phosphorus
from the sea to the land is brought about by the large
'guano' deposits off the coast of Peru. The birds eat
the fish, whose bodies are phosphorus-rich, and much
phosphate is contained in the birds' droppings. A large
tonnage of phosphorus is returned from the sea to the
land in this way, and presumably similar if less spectacular
returns are made in all coastal areas. However, terrestrial
phosphorus is still lost to the marine environment; one
calculation is that, whereas the world's rivers discharge 14
Mt of phosphorus into the oceans annually, seabirds can
return only about 70,000 t (0ยท5 per cent). This 'leak' of the
nutrient is a further aspect of the 'phosphorus problem'.
needed by plants. Thus nitrogen, phosphorus, potassium,
calcium, magnesium and sulphur are required in large
amounts. The minor nutrients are iron, manganese,
copper, zinc, molybdenum, boron, chlorine, cobalt and
selenium. The minor nutrients are also known as
trace
elements
. There are two important features of nutrients
which govern their cycling in ecosystems and their
behaviour in soils. These are, first, whether or not the
element participates in a cycle involving gaseous atmos-
pheric components, and, second, the chemical form by
which the nutrient is absorbed by the plant. In addition
to carbon (C), hydrogen (H) and oxygen (O), cycles which
involve gaseous components are nitrogen (N), sulphur (S),
chlorine (Cl) and selenium (Se). The major store of these
nutrients is the atmosphere, though only nitrogen and
sulphur are of major importance. The remaining eleven
nutrient elements have no gaseous form, though of course
they can and do exist in the atmosphere as dust. In this
group it is possible to distinguish between the
base cations
which are absorbed by plants as the positively charged ion
(cation), and those absorbed as the negatively charged ion
(anion). As we shall see, the distinction is vital for the
nature of the respective nutrient cycles. In the cation
group are potassium (K
+
), calcium (Ca
2+
), magnesium
(Mg
2+
), iron (Fe
2+
or Fe
3+
), manganese (Mn
2+
), copper
(Cu
2+
), zinc (Zn
2+
) and cobalt (Co
2+
). Nutrient elements
which cycle and are absorbed primarily in the anion form
are nitrogen (NO
3
-
), phosphorus (PO
4
3-
), molybdenum
(MoO
-
4
), boron (B(OH)
-
4
), chlorine (Cl
-
) and selenium
nutrient cycles on the basis of the main store (atmosphere
or lithosphere) and the main chemical ion in the cycle
(cation or anion). From the table it can be seen that the
metallic cations form a group, nitrogen and sulphur have
some general similarities (with some contrasts in detail),
and phosphorus has a unique cycle.
Base cations are those essential plant nutrients which
are absorbed as the positively charged ion (cation) and
which have no gaseous phase. From
Table 21.6
we see that
the main store is in rocks and minerals, and that the
group contains the elements potassium (K
+
), calcium
(Ca
2+
) and magnesium (Mg
2+
). Other minor nutrients or
trace elements which cycle in a similar way are iron (Fe
2+
or Fe
3+
), manganese (Mn
2+
), copper (Cu
2+
), zinc (Zn
2+
)
CYCLES
The essential nutrients needed for plant growth are
eighteen in number. Three of them - carbon, hydrogen
and oxygen - comprise over 90 per cent of plant tissue and
come from water and atmospheric carbon dioxide and
oxygen. The remaining fifteen nutrient elements come
largely from the soil, though there is the possibility of
some absorption through the stomata on leaves. The
fifteen soil-derived nutrients can be classified into
major
nutrients
and
minor nutrients
on the basis of the amounts
Store
Cationic
Anionic
Atmosphere
-
N, S
Lithosphere
K, Ca, Mg
P
