Environmental Engineering Reference
In-Depth Information
pounds slowly change into soluble orthophosphates (HPO
4
and H
2
PO
4
),
which can then be taken up by plants. Unlike the compounds of nitrogen,
amounts of soluble forms of soil phosphorus are small, its mobility in soil
is low and very little is lost to leaching or other processes. Unlike many
other crops, turf-grass does not need high levels of phosphorus in the
soil. Phosphorus deficiency mainly affects the generative parts of the
plant, such as the seed-head and seeds, which are not important in turf.
In fact, a slight deficiency is beneficial, because low levels of phosphorus
hold back production of seed-heads, which are unsightly and undesirable
in turf. Because symptoms of phosphorus deficiency seldom appear in
leaves, turf managers pay little attention to the level of soil phosphorus.
Potassium
Potassium is involved in growth processes and strongly influences stress
tolerance, especially cold-temperature tolerance. Researchers disagree,
but most evidence suggests that potassium is required by turf in quan-
tities about a quarter to a half those of nitrogen. Tropical grasses may
require less potassium than cool-season grasses, but not enough research
has been done to confirm this theory. Potassium ions are released to the
soil very slowly during the process of physical and chemical weathering
of minerals. Some soils contain large quantities of potassium, and others
very little. A portion of soil potassium becomes trapped or fixed between
layers of clay particles and is called the soil reserve. Potassium is usually
readily available in the soil solution or in an exchangeable form on the
soil colloids. Potassium occurs as a positively charged cation (K
þ
) and
usually constitutes a large portion of the soil's cation exchange capacity.
Potassium is relatively mobile but not as mobile as nitrogen. Manage-
ment of potassium is more critical than management of phosphorus but
still far less essential than management of nitrogen.
Calcium
Calcium exists in the soil as positively charged cations (Ca
2
þ
). Most soils
contain enough calcium to support growth of turf-grasses. Calcium is
applied (a process known as liming) to increase low soil pH rather than to
support plant growth. Calcium deficiencies in plants are extremely rare.
Magnesium
Magnesium, like calcium, exists in the soil as positively charged cations
(Mg
2
þ
). Deficiencies occur sometimes, especially on coarse-textured
