Prior to the introduction of mineral fertilizers,
all the N and other nutrients used to grow rice
in flooded soils were provided by irrigation
water, sediments, biological N 2 fixation, and
animal manure. N fertilizer use became
widespread following the invention of the
Haber Bosch process, which is used to convert
atmospheric N 2 into mineral fertilizer.
Traditional rice varieties, however, did not
respond well to added fertilizer N. It was only
with the introduction of modern, N-responsive,
high-yielding varieties that the demand for N
and other nutrients became greater than that
which could be supplied from indigenous and
cycled sources. Over the past 30 years,
governments have subsidized N fertilizer
manufacture and distribution to safeguard food
supplies during periods of rapid economic and
population growth. This and the greater
convenience of mineral N fertilizer use have
led to a decline in the use of most organic
nutrient sources.
Where possible, nutrient sources such as
farmyard manure, straw, and green manure
should be used in combination with mineral
fertilizers to satisfy part of the rice crop’s
requirement for nutrients and to sustain soil
quality in the long run. In many areas, however,
the supply is not sufficient, and using organic
manure is more costly than applying equivalent
amounts of nutrients as mineral fertilizer.
Organic rice farming is practiced in small areas
but depends on the following:
the availability of organic inputs, and
a price premium to compensate for
smaller yields and greater production
costs.
So far, we have not seen convincing evidence
that the supply of nutrients from organic
sources to intensive rice-cropping systems can
be managed on a large scale. In most areas,
insufficient organic manure is available to
balance nutrient removal. The problem of