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IX. Nitrogen-Supplying Capacity and Fertilizer Recommendations
K. L. SAHRAWAT
nutrients such as P and K. The main problems seem to stem from the fact that
nitrogen availability to plants is governed by several environmental and soil
factors which are not taken into account whenempirical procedures for determining available N are used. However, tests for predicting the nitrogen-supplying
capacities of soils are important for the efficient use of fertilizer N. It is always
good to have a test which can provide a rough estimate of the pool of available N
in soils so that fertilizer N can be applied to achieve a given yield of rice. This
can be illustrated by an example taken from soil test crop-response project work
in India (see Velayutham, 1979; Randhawa and Velayutham, 1982). The alkaline permanganate digestion method (Subbiah and Asija, 1956) was used for
estimating the available N in soils for several crops, including rice (Ramamoorthy and Velayutham, 1976; Venkateswarlu, 1976). Based on the data obtained
for rice grain, it was established that approximately 1.5-1.8 kg N/ha is needed
for every 100 kg of grain in the alluvial soils of Delhi. From past experience it is
known that about 26% of the available N (determined by the alkaline permanganate method) is taken up by the rice crop. For example, if the available N in a soil
as measured by this method is 250 kg/ha, only about 65 kg of this pool will
appear in the rice plants. Based on the yield target, the amount of fertilizer
nitrogen (corrected for a use efficiency of 3040%) required for wetland rice can
be calculated. This test gives a rough guide for making fertilizer N recommendations which should result in the more efficient use of fertilizer than in cases
where the nitrogen-supplying capacity of the soil is not taken into considerdtion.
Velayutham (1979) has summarized the Indian work on soil test crop response
for rice. Briefly, the yield target and the required fertilizer nitrogen for achieving
the yield target can be calculated from the following equations:
T = ns/(m-r)
F = rns/(m-r)
where T is the yield target in 100 kg/ha, n is the ratio of percentage conmbution
from soil and fertilizer N, r is the N requirement in kg/ha of grain production, m
is the ratio of N requirement and contribution from fertilizer N, s is the soil test N
value in kg/ha, and F is the fertilizer nutrient rate in kg/ha. This scheme seems to
provide a fair degree of approximation for efficient use of fertilizer N considering
the N-supplying capacities of soils.
Another example for fertilizer N recommendation based on the N-supplying
capacity of rice soils is from the work done at the International Rice Research
Institute in the Philippines by Ponnamperuma and his colleagues, who used the
anaerobic incubation method to measure levels of available N. They sampled rice
fields in 13 provinces in the Philippines and, based on the analysis of 483 soil
samples, the available N in these soils ranged from 10 to 637 ppm. It was
NITROGEN AVAILABILlTY INDEXES FOR RICE
possible to separate low- and high-N-supplying capacities using the anaerobic
incubation test (Ponnamperuma, 1978; Castro, 1979). Based on the results of
potentially mineralizableN, Ponnamperuma (1978) formulated a rough guide for
the fertilizer nitrogen requirements of a crop of 5 tons/ha for Philippine wetland
1. Soils that needed no fertilizer nitrogen (available N > 150 ppm)
2. Soils that needed 50 kg N/ha at panicle primordia initiation (available
N = 100-150 ppm)
3. Soils that needed about 50 kg N/ha at planting and about 50 kg N/ha again
at panicle primordia initiation (available N=50-100 ppm)
It was further observed that at eight locations in a province, rice yields of
4.5-7 tons/ha were obtained on soils containing more than 155 ppm available N;
zinc was applied but N was not (IRRI, 1974; Castro, 1979). These two examples
illustrate the principle of basing the recommendation of fertilizer nitrogen needs
of rice on the available N results, and it seems to be a step in the right direction. It
is, however, recognized that these recommendationsmust be modified from time
to time to reflect experience gained.
The high cost of fertilizer nitrogen combined with the need for increased yields
of rice has stimulated research on methods of using soil and fertilizer nitrogen
efficiently. For the judicious and efficient use of fertilizer, a measure of the
nitrogen-supplying capacity of soils is prerequisite because rice soils vary widely
in their capacity to release ammonium nitrogen when submerged. Our fertilizer
recommendations will be only as precise as our methods for measuring the
amounts of available soil nitrogen. Because of the fact that one-half to two-thirds
of the nitrogen used by the rice plant, even in well-fertilized paddies, comes from
soil nitrogen through mineralization, research on methods for measuring the
nitrogen-supplying capacities of wetland rice soils assumes still greater importance.
For devising effective methods for measuring available nitrogen in soils, it is
essential that the factors that affect mineralization and the availability of soil
nitrogen to the rice plant be well understood. Soil and environmental factors that
affect the mineralization of soil organic nitrogen are fairly well documented.
However, with the present state of knowledge it is not possible to quantify (1)
how the texture and the mineralogical makeup of a soil affect the release of
nitrogen under submerged conditions, or (2) how the mineralization of soil
organic nitrogen is affected by the presence of the rice plant. Attempts should be
made in the future by using stepwise regression analyses to separate the effects of
K. L. SAHRAWAT
different soil characteristics on nitrogen mineralizationin flooded soils for different regions with due consideration of taxonomic criteria. It is envisaged that a
knowledge of the environmental (such as temperature and moisture) and soil
factors that affect mineralization will be useful in developing improved anaerobic
incubation tests and, eventually, in developing models for measuring soil nitrogen mineralization rates and hence nitrogen-supplying capacity under field conditions, No data are presently available on the measurement of soil nitrogen
mineralization rates in the field or on the comparative evaluation of mineralization rates as measured in the laboratory and in the field. Anaerobic incubation
tests have shown potential as indexes for soil nitrogen availability to rice in a
large number of greenhouse pot experiments and a few field experiments. Improved incubation tests should be devised by consideringthe climatic conditions
of a region, especially soil temperature. The anaerobic incubation method is
quite versatile in that it is very responsive and amenable to ranges in temperature.
Research is also needed for the development of standardized methods for (1)
determining the optimal time of soil sampling, (2) sampling soil (especially for
submerged paddy fields after land-preparatory operations), (3) preparing soil
samples for laboratory and greenhouse work, and fmally (4) use in laboratory
and greenhouse expriments. Recent work has revived interest in using organic
matter (as measured by organic C and total N) as the index of soil nitrogen
availability to wetland rice (Ponnamperuma, 1978; Ponnamperuma and Sahrawat, 1978; Sahrawat, 198Oc, 1982d, 1983b). However, researchers usually
have not obtained consistent results. It is known that both the quantity and the
quality of organic matter affect the mineralization and availability of soil nitrogen. Little is known about the quality of organic matter (except for the C/N ratio)
in wetland rice soils, which is indicated by our inability to answer simple questions even with our present knowledge about organic matter. For instance, (1)
What are the criteria for characterizing the quality of organic matter in relation to
its contribution to mineralizable nitrogen in submerged soils? and (2) How does
the quality of organic matter affect mineralization and soil nitrogen availabilityto
rice? More knowledge about the quality of organic matter, and especially about
the fraction that contributes to soil mineralizable N pools, should improve our
capability to use this simple index for predicting soil nitrogen availabilityto rice.
Considerableresearch efforts have been devoted to the development of chemical indexes for assessing available soil nitrogen in soils, but these indexes have
not been tested extensively for rice soils. Ideally, chemical indexes that extract
the soil organic nitrogen fraction, which is the source of mineralizable nitrogen
through the biological process, should be satisfactory in assessing the nitorgensupplying capacity of a soil. However, these conditions are usually not met and,
in the case of many chemical indexes, their chemistry is not fully known.
Alkaline pennanganate digestion is the most extensively used chemical method,
especially in India, for assessing the availability of soil nitrogen to rice. Recent
NITROGEN AVAILABILITY INDEXES FOR RICE
knowledge about the chemistry of the method has shown that it exhibits a better
potential for predicting soil nitrogen availability to wetland rice than to upland
crops (Sahrawat and Burford, 1982). Simple models based on regression equations relating potentially mineralizable nitrogen (as measured by biological indexes) with chemical indexes and/or soil characteristics should be tested for their
suitability to predict nitrogen availability to rice, since they have a sound basis
(Stanford, 1977; Stanford and Smith, 1978; Sahrawat, 1983b).
In view of the diverse soil and climatic conditions (where rice is grown) that
affect soil nitrogen availability, it is quite probable that a single index of nitrogen
availability will not find universal acceptance. Research on nitrogen availability
indexes for wetland rice soils compared to arable soils is still in infancy, and it is
hoped that this article will stimulate research in this area which holds considerable promise for the efficient use of fertilizer nitrogen as well as for devising
conservative soil management and cultural practices to regulate soil nitrogen
release in connection with nitrogen uptake by the rice plant. International cooperation is desirable for extensive testing of the promising indexes of nitrogen
Part of this work was done at, and supported by, the International Rice Research Institute, Los
Banos, Philippines. I am grateful to Dr.F. N. Ponnamperuma, Principal Soil Chemist, IRRI, for his
valuable suggestions. I thank Dr.C. W. Hong for his helpful review of this article.
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Absorbed-ion activity, 224-226
Acetochlor, 268, 276, 282, 284, 287
AD-2, 268, 280
AD-67, 268, 280, 305
Aegilops bicornis, 167
Aegilops bicuncialis, 167
Aegilops caudata, 158, 167, 171, 172
Aegilops columnaris, 167
Aegilops comosa, 167
Aegilops crassa, 167
Aegilops cylindrica, 167
Aegilops heldreichii, 167
Aegilops juvanalis, 167
Aegilops kotschyi, 163-164, 167
Aegilops longissima, 167
Aegilops mutica, 167
Aegilops ovata, 159, 167, 172
Aegilops sharonensis, 167
Aegilops spelroides, 165, 167
Aegilops squarrosa, 163, 167
Aegilops triaristata, 167
Aegilops tricuncialis, 167
Aegilops umbellulata, 167
Aegilops uniaristata, 167
Aegilops variabilis, 163, 167
Aegilops ventricosa, 167
Alachlor, 268, 275, 276, 282-287, 297, 302
Alexandergrass, 270, 283
Allium cepa, 270
Amaranthus retrojlexus, 270
Ammonium production, 417-421
Ascorbic acid, 288
Asulam, 268, 290
Atrazine, 268, 369
Avena fatua, 270
Avena sativa, 270
Barban, 268, 271, 277, 282, 299
Barley, 105, 108, 110, 114-115, 122-123,
270, 278, 281, 282
Bamyardgrass, 270, 286
common, 116, 127-129
field, 129-130, 270, 275, 276, 281
Beet, 270, 291
Bensulide, 268, 285
Bentonite, 237, 248
Beta vulgaris. 270
Bluegrass, Kentucky, 270, 276-278
Brachiaria plantaginea, 270
Butachlor, 268, 276, 285, 287
Butam, 268, 278
Buthidazole, 268, 278
Butylate, 268, 277, 281, 297, 306
Calcite, 72, 86
Carbon distribution, 26
Carrot, 270, 291
Cation exchange, thermodynamics,
2 15 -264
CDAA, 268, 279
CDEC, 268, 282
Centrosema pubscens, 25
CGA-43089, 268, 272, 274, 280, 283-285,
290, 293-301, 308-309
CGA-92194, 268, 272, 283, 286, 293-301
Chenopodium album, 270
Chlornitrofen, 268, 289
2-Chlomthyl phosphoric acid, 181
Chlorsulfuron, 268, 278, 282-283, 285, 286
Cisanilide, 268, 275, 278
Claviceps purpurea, 194
potksium exchange, 215-264
white, 42, 43
Competition selection, 105-1 11
Concep I, 297
Concep 11, 297
herbicide antidote, 274-289, 297, 299-310
Cotton, 103, 117, 120, 131-133, 275, 286
Crop evolution, annual, 97-143
Cucurbita foetidissima, 317, 319-331
Cycloate, 268, 277, 280, 282
Cytoplasmic sterility, 157-169
Epronaz, 268, 278
EPTC, 269, 271-281, 284-287, 291,
Erosion, reclamation, 38
Ethephon, 181, 182
Ethofumesate, 269, 279, 280, 285
Feldspar, 67-68, 82
Flowering behavior, 183-185
Fluazifop-butyl, 269, 279, 285
Foxtail, green, 270, 275, 286
2,4-D, 269, 270-272, 288, 369
Daucus carota, 270
DCPA, 268, 278
Diallate, 268, 276, 282
Diclopop-methyl, 268, 278, 283, 285
Diethatyl, 268, 276, 282, 284
Dimefuron, 268, 278
Diphenamid, 268, 275, 278, 280, 283
Diuron, 268, 286
Dowco, 221, 268, 278
Echinochloa crus-gali, 270
cross-fertilization factors, 183- 190
cytoplasmic differentiation, 164- 166
cytoplasmic sterility, 157-169
fertility restoration, 169-180
hybrid seed production, 191-196
rice mutations, 383-413
Glycine mar, 130, 270
Glycine soja, I30
Gossypium barbadense, 131
Gossypium hirsutum, 131, 270
Gourd, buffalo, 317, 319-331
Gypsum, 72, 73
H-26910, 269, 276, 282
Haynaldia villosa. 167
Hedysarum coronarium, 33
Helianthus annuus, 133