Yoshida - 1981 - Fundamentals of Rice Crop Science

146 FUNDAMENTALS OF RICE CROP SCIENCE
the degree of which depends on soil properties. Soils with montmorillonite clays
tend to have higher recovery rates than those with kaolinite clays or allophane.
For soils with low nitrogen-holding capacity, split applications of fertilizer
should result in a higher nitrogen recovery and, hence, a higher yield than a basal
application. On the other hand, split applications may not be any better than a basal
application in soils where the applied ammonia is well held by clays. In Japan,
zeolite, which has a cation exchange capacity of about 80–160 meq/100 g, is
applied to increase the soil’s nitrogen-holding capacity.
3.7.7. Efficiency of fertilizer nitrogen for grain production
Agronomists usually express the efficiency of fertilizer nitrogen in kilograms of
rough rice produced per kilogram of nitrogen applied. Physiologists, on the other
hand, define the efficiency of nitrogen utilization in kilograms of rough rice
produced per kilogram of nitrogen absorbed.
These two efficiencies can be related by introducing a third parameter, percentage
of nitrogen recovery:
(3.22)
The percentage of nitrogen recovery varies with soil properties, methods,
amounts, and timing of fertilizer applications, and other management practices. It
usually ranges from 30 to 50% in the tropics (Prasad and De Datta 1979). The
percentage of nitrogen recovery tends to be high at low levels of nitrogen and when
nitrogen is placed deep in the soil or topdressed at later growth stages.
The efficiency of utilization for grain production in the tropics is about 50 kg
rough rice/kg nitrogen absorbed, and this efficiency appears to be almost constant
regardless of the rice yields achieved. In temperate regions, the efficiency appears
to be about 20% higher than in the tropics (see Table 3.12 and Section 3.7.6.).
Using values for the recovery percentage and utilization efficiency obtained for
the tropics, the efficiency of fertilizer nitrogen can be calculated as:
Efficiency of fertilizer nitrogen = (0.3 ~ 0.5) × 50
= 15-25 kg rice/kg applied N. (3.23)
These values are found in agronomic experiments (Prasad and De Datta 1979).
3.7.8. Grain yield and fertilizer requirements
Grain yield ( Y ) can be analyzed as yield obtained without fertilizer nitrogen ( Y 0 )
and yield increase obtained by fertilizer application ( D Y F ):
(3.24)