19. Nutrient Manipulations in Terrestrial Ecosystems
How to Add Phosphorus Fertilizer
Even with the best P fertilization management,
plants often take up less than 25% of added P (Tisdale et al. 1993). In high-fixing soils, rather than
adding P in one application, several smaller, more
frequent applications can maximize plant P availability. If adding a water-soluble form, it is desirable to minimize contact with the soil, so large
granules should be used. In contrast, very fine granules maximize release of phosphate from waterinsoluble fertilizers (Young et al. 1985).
The best time to add water-soluble P is just before germination, because P availability rapidly decreases with time after fertilization (Bramley and
Barrow 1992).
Potassium
Control of Fertilizer Potassium Availability
The availability of K fertilizer is determined by
many different factors, and different soils can be
drastically different in the amount of fertilizer
needed to increase exchangeable K concentrations.
Potassium, like NUt, can be temporarily fixed onto
soils. However, at very high pH, soil fixation can
be permanent as K becomes trapped between the
collapsed silicate layers of clays. Wet soils substantially increase K fixation, and K is gradually released as the soil dries. Adsorption characteristics
of soils for K are opposite to those of P; the highest
K fixation is seen in 2: I clays with high illite, while
1: 1 clays, such as kaolinite, do not fix substantial
amounts of K. Fine soil textures increase fixation
onto soil, while aluminum ion (AI3+) concentrations decrease fixation. In very acid soils, high concentrations of AI3+ and magnesium ions (Mn2+)
inhibit plant K + uptake, and could interfere with
growth response to fertilization. Leaching losses of
K are relatively small in most soils, but could be
substantial in sandy soils, or humid tropical soils
with high rainfall.
Assessment of K limitation is more complicated
than for other nutrients because it is not a simple
function of K available in soil solution, but is more
dependent on the ratio of K + to Ca 2 + and Mg2 + ,
which compete with K + for entry into the plants.
So in soils with high calcium and magnesium,
higher amounts of K must be added to become
available to plants. In addition, when adding a com299
bination of nutrients that includes K, be sure to
avoid adding other nutrients complexed with Ca or
Mg (such as CaS04)'
Common Potassium Fertilizers
All commonly used forms of K fertilizer are equally
effective in supplying plant-available K. The major
factor determining the appropriate choice depends
on the other nutrients being tested or added. Potassium is most commonly added as KCl (muriate of
potash), which is readily dissolved in water. There
are some plants that are intolerant to Cl- , in which
case another form must be used. Potassium chloride
also has limited applications in long-term fertilization, because Cl- accumulation can be detrimental to plant growth or microbial activity. Potassium
sulfate (K 2 S0 4 ) is often used in Cl- -sensitive situations and is an appropriate choice for systems
where S is not limiting to growth.
Potassium carbonate (K2C0 3 ), potassium bicarbonate (KHC0 3 ), and potassium hydroxide (KOH)
are very effective fertilizers, and when used on acid
soils, they will decrease the leaching of cations. Potassium phosphates are good for gradual applications because they have more controlled solubility
than the other forms of K fertilizer. Other forms that
are commonly used are potassium magnesium sulfate and potassium nitrate. Long-term or high rates
of K additions usually use potassium nitrate
(KN0 3 ) or potassium pyrophosphate (~P207) in
order to avoid salt accumulation problems. Unfortunately, because these are complexed with the two
most commonly limiting nutrients, these forms
can't be used to separately test K, N, and P
limitation.
How to Add Potassium Fertilizer
Potassium fertilizers are most effective if added just
before or at the time of germination or onset of
growth. Higher leaching losses can occur if K is
applied long before plant uptake will occur.
Sulfur
Control of Sulfur Fertilizer Availability
Sulfate (SO~ -), like nitrate, can be readily lost
through leaching, especially in soils where monovalent cations predominate (K + , Na +). Adsorption
How to Add Phosphorus Fertilizer
Even with the best P fertilization management,
plants often take up less than 25% of added P (Tisdale et al. 1993). In high-fixing soils, rather than
adding P in one application, several smaller, more
frequent applications can maximize plant P availability. If adding a water-soluble form, it is desirable to minimize contact with the soil, so large
granules should be used. In contrast, very fine granules maximize release of phosphate from waterinsoluble fertilizers (Young et al. 1985).
The best time to add water-soluble P is just before germination, because P availability rapidly decreases with time after fertilization (Bramley and
Barrow 1992).
Potassium
Control of Fertilizer Potassium Availability
The availability of K fertilizer is determined by
many different factors, and different soils can be
drastically different in the amount of fertilizer
needed to increase exchangeable K concentrations.
Potassium, like NUt, can be temporarily fixed onto
soils. However, at very high pH, soil fixation can
be permanent as K becomes trapped between the
collapsed silicate layers of clays. Wet soils substantially increase K fixation, and K is gradually released as the soil dries. Adsorption characteristics
of soils for K are opposite to those of P; the highest
K fixation is seen in 2: I clays with high illite, while
1: 1 clays, such as kaolinite, do not fix substantial
amounts of K. Fine soil textures increase fixation
onto soil, while aluminum ion (AI3+) concentrations decrease fixation. In very acid soils, high concentrations of AI3+ and magnesium ions (Mn2+)
inhibit plant K + uptake, and could interfere with
growth response to fertilization. Leaching losses of
K are relatively small in most soils, but could be
substantial in sandy soils, or humid tropical soils
with high rainfall.
Assessment of K limitation is more complicated
than for other nutrients because it is not a simple
function of K available in soil solution, but is more
dependent on the ratio of K + to Ca 2 + and Mg2 + ,
which compete with K + for entry into the plants.
So in soils with high calcium and magnesium,
higher amounts of K must be added to become
available to plants. In addition, when adding a com299
bination of nutrients that includes K, be sure to
avoid adding other nutrients complexed with Ca or
Mg (such as CaS04)'
Common Potassium Fertilizers
All commonly used forms of K fertilizer are equally
effective in supplying plant-available K. The major
factor determining the appropriate choice depends
on the other nutrients being tested or added. Potassium is most commonly added as KCl (muriate of
potash), which is readily dissolved in water. There
are some plants that are intolerant to Cl- , in which
case another form must be used. Potassium chloride
also has limited applications in long-term fertilization, because Cl- accumulation can be detrimental to plant growth or microbial activity. Potassium
sulfate (K 2 S0 4 ) is often used in Cl- -sensitive situations and is an appropriate choice for systems
where S is not limiting to growth.
Potassium carbonate (K2C0 3 ), potassium bicarbonate (KHC0 3 ), and potassium hydroxide (KOH)
are very effective fertilizers, and when used on acid
soils, they will decrease the leaching of cations. Potassium phosphates are good for gradual applications because they have more controlled solubility
than the other forms of K fertilizer. Other forms that
are commonly used are potassium magnesium sulfate and potassium nitrate. Long-term or high rates
of K additions usually use potassium nitrate
(KN0 3 ) or potassium pyrophosphate (~P207) in
order to avoid salt accumulation problems. Unfortunately, because these are complexed with the two
most commonly limiting nutrients, these forms
can't be used to separately test K, N, and P
limitation.
How to Add Potassium Fertilizer
Potassium fertilizers are most effective if added just
before or at the time of germination or onset of
growth. Higher leaching losses can occur if K is
applied long before plant uptake will occur.
Sulfur
Control of Sulfur Fertilizer Availability
Sulfate (SO~ -), like nitrate, can be readily lost
through leaching, especially in soils where monovalent cations predominate (K + , Na +). Adsorption