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Valerie T. Eviner, F. Stuart Chapin III, and Charles E. Vaughn
decrease losses through leaching and denitrification. Ammonium and urea additions should be
avoided in very basic soils, in order to avoid ammonia volatilization and nitrite accumulation, and
urea additions in acid and neutral soils should be
carefully applied and monitored in order to minimize pH increases and its negative effects. Ammonium additions should also be avoided in very
acid soils. High addition rates, or long-term additions will increase the effects of N salts on soil pH,
and a form with little impact on pH, such as
NH.N0 3 should be used.
Phosphorus
Control of Phosphorus Fertilizer Availability
The availability of fertilizer P to plants is limited
mostly by its solubility and its adsorption to soil
minerals. Adsorption is especially high in soils with
high clay contents and in weathered acid soils,
which tend to have high concentrations of aluminum oxides and iron oxides. Adsorption is higher
on 1: 1 clays such as kaolinite than on 2: 1 clays,
such as montmorillonite. The pH is a critical factor
determining adsorption. Although different minerals adsorb P maximally at different pHs, in most
soils P availability is maximal at pH 5.5 to 6.5. The
presence of divalent cations also can substantially
increase P adsorption. For example, clays saturated
with Ca 2 + have higher adsorbed P than those with
Na +. Thus, calcareous soils often have low P availability. Phosphate can also compete with other ions
for adsorption sites, so its adsorption is decreased
in the presence of OH-, H3Si04 , S04' and
MoO~ - . Phosphorus adsorption is more permanent
than N adsorption. Once P is adsorbed, it does not
become available to plants at any great quantities.
To make up for this, P fertilization must usually be
high in order to cause any appreciable increase in
P availability. For example, it is common to add P
at half the rate that N is added, although plant tissue
contains about 14 times more N than P.
In addition to adsorption, it is also necessary to
consider precipitation of the phosphate ions. Adsorption usually occurs once phosphate enters the
bulk soil, whereas precipitation occurs close to fertilizer granules. High concentrations of calcium,
aluminum and iron could cause substantial precipitation, while the use of other fertilizer salts, such as
(NH4hS04' NH.N03, NH4CI, KN03, K2S04, and
KCI, can decrease P precipitation.
It is important when comparing the response of
growth to P fertilization among different sites, that
one consider these different factors regUlating P
availability.
Common Phosphorus Fertilizers
There are two main types of P fertilizer: a watersoluble form that is immediately available to plants,
and P in rock phosphate, which is slowly available
in acid conditions.
Rock phosphate is a good way to provide a small,
continuous supply of P to a system. The P supply
to plants is optimized if rock phosphate is finely
ground and mixed into the soil, but it is only effective in acid soils (PH <6.0). It is most effective in
systems with warm, moist soils, and long growing
seasons. Application rates must be high to achieve
effective fertilization. It should be noted, however,
that long-term studies have shown that even after 4
to 10 years, a single application of superphosphate
can enhance P availability to a greater extent than
fertilization with rock phosphate (Bolland et al.
1989).
The water-soluble forms offer the advantage of
immediate availability of P, and are much more
comparable to forms of other nutrient additions
such as NH4N0 3 and KCl. In general, there are very
small differences in supply rate between the different water-soluble P fertilizers. Water-soluble P is
often added as single superphosphate or triple superphosphate, which are calcium orthophosphates.
These work well in nutrient limitation experiments
as long as the site is not calcium limited. These
have no effect on soil pH, and almost all of the P
is water soluble and plant available. Single superphosphate contains substantial S, which is beneficial for combined-nutrient additions, but is not a
satisfactory way to test for P limitation in soils that
may also be S limited. Triple superphosphate, on
the other hand, has very little S (0 to 1 %) and is
much better for such applications. Phosphorus is
often added as ammonium phosphates, but this is
not a satisfactory way to add P if trying to test for
N versus P limitation.
Phosphorus can also be added as a slow release
fertilizer. For example, Osmocote produces P 20 5
pellets.
Valerie T. Eviner, F. Stuart Chapin III, and Charles E. Vaughn
decrease losses through leaching and denitrification. Ammonium and urea additions should be
avoided in very basic soils, in order to avoid ammonia volatilization and nitrite accumulation, and
urea additions in acid and neutral soils should be
carefully applied and monitored in order to minimize pH increases and its negative effects. Ammonium additions should also be avoided in very
acid soils. High addition rates, or long-term additions will increase the effects of N salts on soil pH,
and a form with little impact on pH, such as
NH.N0 3 should be used.
Phosphorus
Control of Phosphorus Fertilizer Availability
The availability of fertilizer P to plants is limited
mostly by its solubility and its adsorption to soil
minerals. Adsorption is especially high in soils with
high clay contents and in weathered acid soils,
which tend to have high concentrations of aluminum oxides and iron oxides. Adsorption is higher
on 1: 1 clays such as kaolinite than on 2: 1 clays,
such as montmorillonite. The pH is a critical factor
determining adsorption. Although different minerals adsorb P maximally at different pHs, in most
soils P availability is maximal at pH 5.5 to 6.5. The
presence of divalent cations also can substantially
increase P adsorption. For example, clays saturated
with Ca 2 + have higher adsorbed P than those with
Na +. Thus, calcareous soils often have low P availability. Phosphate can also compete with other ions
for adsorption sites, so its adsorption is decreased
in the presence of OH-, H3Si04 , S04' and
MoO~ - . Phosphorus adsorption is more permanent
than N adsorption. Once P is adsorbed, it does not
become available to plants at any great quantities.
To make up for this, P fertilization must usually be
high in order to cause any appreciable increase in
P availability. For example, it is common to add P
at half the rate that N is added, although plant tissue
contains about 14 times more N than P.
In addition to adsorption, it is also necessary to
consider precipitation of the phosphate ions. Adsorption usually occurs once phosphate enters the
bulk soil, whereas precipitation occurs close to fertilizer granules. High concentrations of calcium,
aluminum and iron could cause substantial precipitation, while the use of other fertilizer salts, such as
(NH4hS04' NH.N03, NH4CI, KN03, K2S04, and
KCI, can decrease P precipitation.
It is important when comparing the response of
growth to P fertilization among different sites, that
one consider these different factors regUlating P
availability.
Common Phosphorus Fertilizers
There are two main types of P fertilizer: a watersoluble form that is immediately available to plants,
and P in rock phosphate, which is slowly available
in acid conditions.
Rock phosphate is a good way to provide a small,
continuous supply of P to a system. The P supply
to plants is optimized if rock phosphate is finely
ground and mixed into the soil, but it is only effective in acid soils (PH <6.0). It is most effective in
systems with warm, moist soils, and long growing
seasons. Application rates must be high to achieve
effective fertilization. It should be noted, however,
that long-term studies have shown that even after 4
to 10 years, a single application of superphosphate
can enhance P availability to a greater extent than
fertilization with rock phosphate (Bolland et al.
1989).
The water-soluble forms offer the advantage of
immediate availability of P, and are much more
comparable to forms of other nutrient additions
such as NH4N0 3 and KCl. In general, there are very
small differences in supply rate between the different water-soluble P fertilizers. Water-soluble P is
often added as single superphosphate or triple superphosphate, which are calcium orthophosphates.
These work well in nutrient limitation experiments
as long as the site is not calcium limited. These
have no effect on soil pH, and almost all of the P
is water soluble and plant available. Single superphosphate contains substantial S, which is beneficial for combined-nutrient additions, but is not a
satisfactory way to test for P limitation in soils that
may also be S limited. Triple superphosphate, on
the other hand, has very little S (0 to 1 %) and is
much better for such applications. Phosphorus is
often added as ammonium phosphates, but this is
not a satisfactory way to add P if trying to test for
N versus P limitation.
Phosphorus can also be added as a slow release
fertilizer. For example, Osmocote produces P 20 5
pellets.