Horticulture Principles and Practices

Necrosis
Death of tissue associated
with discoloration and
dehydration of all or parts
of a plant organ.
also mobile in the plant and is regularly recycled from older parts to younger growing
parts. It has a tendency to be rendered readily unavailable (fixed) in the soil. It is most
available at a pH of 5.5 to 7.0.
POTASSIUM (K) Sandy soils may be deficient in potassium since the element is readily
leached. Potassium is absorbed by plants in its ionic form (K ). It is a catalyst for enzyme
reactions and is also important in protein synthesis, translocation, storage of starch, and
growth of meristematic tissue. Whereas nitrogen and phosphorus are converted into
compounds for plant growth, potassium occurs in the plant tissue as a soluble inorganic
salt. It is very mobile. Luxury consumption of potassium is common in plants, even
though the element is required in large amounts. When potassium is deficient in the soil,
the root system and stems become weak and prone to lodging; yield is reduced. Readily
visible deficiency symptoms vary among species. Some plants show marginal burning
of leaves (marginal necrosis), speckled or mottled leaves, interveinal chlorosis, and leaf
curling. These symptoms occur in older leaves and spread upward. Potassium is removed
by plants, but it is also prone to fixation and leaching, especially in soils low in organic
matter and from soilless growing media.
The most abundant monovalent ion in plants is K , whose concentration may be
equal to or more than that of nitrogen. Plants such as carnation may have as high as 9
percent potassium on a dry-weight basis. Roses have about 2 to 3 percent potassium on
the average. Potassium appears to have a role in nitrogen metabolism; when deficient,
plants show a high level of water-soluble nitrogen. When ammonium forms of nitrogen
are used to fertilize plants in case of deficiency, the deficiency symptom is intensified
and the plants become severely injured. This injury may be due to the accumulation of
nitrogen (that has not been changed into protein) to excessive and toxic levels.
Secondary Nutrients Calcium (Ca) Calcium is not only an essential plant nutrient
but is also used in correcting soil acidity so that other soil nutrient elements can be made
available to plants in appropriate amounts. It is absorbed as Ca 2 ions by plants. Calcium
is important in cell growth and division, cell wall formation (calcium in the form of pectate),
and nitrogen accumulation. The element also forms organic salts with organic acids
in plants. For example, in dumbcane (Dieffenbachia) calcium forms calcium oxalate,
which is irritating to the tongue and throat of humans when ingested.
When deficient, plant tissue formation is incomplete. The terminal bud may cease
to grow, leaving a blunt end. Deficiency symptoms for calcium are manifested frequently
as defective terminal bud development. The margins of young leaves may not form,
resulting in strap leaves. Slight chlorosis followed by brown or black scorching of new
leaf tips and die-back of growing points are also characteristic of this deficiency. Roots
grow poorly and are short and thickened.
MAGNESIUM (MG) Magnesium is released when rock minerals such as dolomite, biotite,
and serpentine decompose. Absorbed as Mg 2+ ions, magnesium is the central atom in the
structure of a chlorophyll molecule. It is also essential in the formation of fats and sugars.
Magnesium is mobile in plants, and thus deficiency appears first in older leaves.
Large amounts of potassium ions may interfere with its uptake due to ion antagonism
created by this situation. This antagonism is prevented when the potassium to magnesium
ratio in the growing medium is about 3:1 to 4:1. There is interveinal chlorotic mottling or
marbling of the older leaves, proceeding to younger leaves as the deficiency intensifies.
SULFUR (S) Sulfur is obtained primarily from the decomposition of metal sulfides in
igneous rocks. It occurs in the soil as sulfates and sulfides, as well as in humus. It is
absorbed by plants as sulfate ions (SO 2
4 ). The unique flavors of certain vegetables, such,
as onion and cabbage, as well as other cruciferous plants, are due to certain sulfur
compounds. Sulfur is an ingredient in vitamins and amino acids. The dominant symptom
of sulfur deficiency is chlorotic foliage. In addition, the stems of affected plants are
weak, thin, hard, and woody. Sulfur is not usually added as a fertilizer element but is
added indirectly when sulfate forms of other elements are applied. Sulfur is also available
from air pollution.
112 Chapter 4 Plant Growth Environment