Yoshida - 1981 - Fundamentals of Rice Crop Science

180 FUNDAMENTALS OF RICE CROP SCIENCE
Root color is affected by age (see Chapter 1) and soil iron content. A root is
white when young and actively growing. When it is long and old, and when there is
adequate iron in the soil, it is normally brown because of iron oxide deposits
on the root surface. When the iron content in the soil is low, however, old roots
remain white, pale, or grayish. Under extremely reductive conditions some roots
or an entire root system becomes black, suggesting hydrogen sulfide injury. These
roots turn brown when they are exposed to air. Roots and the soil mass from
flooded soils may have a hydrogen sulfide smell. Hydrogen sulfide can be detected
by using a piece of filter paper that has been impregnated with saturated basic lead
acetate solution.
b. When to observe symptoms. The early stages of symptom development are
the best time for observing visible symptoms. It is of little value to observe plants
when the symptoms of the disorder have become so severe that the plants are
almost dead.
Zinc deficiency in lowland rice usually appears within 2–3 weeks after transplanting
or sowing. Later, the crop may recover spontaneously. When the deficiency
is severe, plants may die or the symptoms may persist until flowering or
longer. The problem field appears patchy because some plants die early while
others survive and grow.
Iron toxicity symptoms may appear on plants grown on sandy soils adjacent to
lateritic highlands 1–2 weeks after transplanting. Symptoms may also appear 1–2
months after transplanting and persist until flowering when iron toxicity occurs in
peaty or boggy soils.
c. Deficiency symptoms
Nitrogen. Plants are stunted with limited tillers. Except for young leaves which
are greener, leaves are narrow, short, erect, and yellowish green. Old leaves die
when light straw colored.
Phosphorus. Plants are stunted with limited tillers. Leaves are narrow, short,
erect, and dirty dark green. Young leaves are healthy and old leaves die when
brown colored. A reddish or purplish color may develop on the leaves if the variety
has a tendency to produce anthocyanin pigment.
Potassium. Plants are stunted, but tillering is only slightly reduced. Leaves are
short, droopy, and dark green. The lower leaves at the interveins, starting from the
tip, turn yellow and, eventually, dry to a light-brown color. Sometimes brown
spots may develop on dark-green leaves.
Sulfur. The symptoms are very similar to and almost impossible to visually
distinguish from those of nitrogen deficiency.
Calcium. The general appearance of the plant is little affected except when the
deficiency is acute. The growing tip of the upper leaves becomes white, rolled, and
-curled. In an extreme case, the plant is stunted and the growing point dies.
Magnesium. Height and tiller number are little affected when the deficiency is
moderate. Leaves are wavy and droopy due to expansion of the angle between the
leaf blade and the leaf sheath. Interveinal chlorosis, occurring on lower leaves, is
characterized by an orangish-yellow color.