368 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [ III, IV.

lopment. As a consequence, the percentage of nitrogen and phosphorus, expressed
in terms of dry matter, gradually falls from a maximum value in the earliest vege-
tative stage to a minimum value at maturity. The amount of nitrogen and phos-
phorus absorbed by a gramineous pasture plant is limited by the amount that can
be rendered available by the soil during the vegetative phase. If nitrogen is
available and soil moisture conditions are favourable tillering proceeds to the limit
of the available nitrogen supply and to a lesser extent to the limit of the available
phosphate supply.

In temperate regions and winter rainfall climates a long period of slow
vegetative growth precedes the reproductive phase, and it is probable that under
such conditions the normal pasture could secure the necessary nitrogen and minerals
from the soil at a sufficiently rapid rate to keep pace with the somewhat slow photo-
synthetic activity of the plant except on the poorest types of soil, and except during
late spring or early summer when the flush of growth occurs.

An application of nitrogen just prior to this stage normally increases both the
yield of dry matter and the protein content of the herbage in Britsh pastures.
In tropical and sub-tropical regions of summer rainfall and winter drought, growth
commences with the onset of the wet season towards midsummer, and is exceedingly
rapid from the outset on account of high temperatures and favourable soil humidity
and the demands of the pasture for nitrogen and phosphate are correspondingly
great.

Very little is known of the rate at which nitrification or denitrification proceeds
in pasture land under tropical conditions, or of the precise effects of. comparatively
heavy tropical rains and occasional periods of drought on the available nitrogen
and phosphate supply of the soil. It is possible that the demands of the rapidly
growing pasture for nitrogen and phosphorus may, for various reasons, exceed the
rate at which supplies are made available in the soil, in which case, the amount
of tillering will be adjusted to the available nitrogen supply.

Photosynthetic activity, however, on account of high temperature and favour-
able light conditions continues at a relatively high level under tropical and sub-
tropical conditions, with the final result that the nitrogen and phosphorus content,
and to a lesser extent the silica-free ash content, expressed in terms of dry matter
produced, will be relatively low.

It is possible, therefore, that the explanation may be found in terms of differ-
ence in the effect of the two contrasting types of climate on the uptake of nitrogen,
phosphorus and other minerals on the one hand and assimilation of carbon on the
other, the carbon-nitrogen ratio tending to be high in tropical and sub-tropical
climates, and low in the temperate and winter rainfall climates.