Integrated nutrient management (INM) aims at reducing the chemical fertiliser applied and improving its efficiency through combined use of different sources of plant nutrients such as fertilisers, organic manures, green manures and biofertilisers.
Source # 1. Organic Manures:
In view of the widening gap between nutrient supply and depletion and ecological sustainability, it is essential that chemical sources of nutrients be complemented/supplemented with other natural sources. FYM, compost, crop residues, non-edible oil cakes and byproducts from agro-industries are some of the major sources of organic manures.
Organic manures have profound influence on physical, chemical and microbial properties of soil leading to an ideal soil environment for nutrient availability to the crop. Organic manures are also good sources of micronutrients. Long term manorial experiments at Cuttack, Bhubaneswar, Hyderabad and Pantnagar showed that combined application of NPK fertilisers at optimum levels and FYM (5-10 t ha-1) increased grain yield by 0.4-0.7 t ha-1 over application of NPK fertilisers (Table 1.19).
It also resulted in increasing the availability of nutrients in soil at all locations. Contribution of FYM/comport to grain yield was in the range of 18-29 per cent, depending on soil properties. These results suggest great potential for improving the productivity of rice through combined use of chemical fertilisers and organic manures.
Source # 2. Green Manures:
Green manuring, including clippings from trees, an ageold practice has assumed importance in the recent years due to importance for INM in rice production. Based on the research information on green manuring in south Asia, Abrol and Palaniappan (1988) have listed sunnhemp, daincha, pillepesara, mungbean and guar as common green manure crops of wet season, contributing 8-21 t green matter and 42-91 kg N ha-1 and senji, khesari and berseem as green manure crops of the dry season, giving 12-29 t green matter and 67-75 kg Nha-1. Investigations at CRRI, Cuttack revealed that Sesbania aculeata and Sesbania rostrata grown for 45 days accumulated 17.5 and 18.8 t ha-1 green matter and contributed 62 and 61 kg N ha-1, respectively. When they were grown only for 30 days, N contribution was marginal (17-25 kg N ha-1).
Fanners, generally, grow green manure crops and incorporate in the puddle at the time of land preparation. This practice, however, adds to expenditure on growing green manure crop and loss of a short season crop, with limited irrigation water, during summer. Very often, there may not be any scope for a green manure crop during summer due to shortage of irrigation water.
As such, the alternative is that both rice and crops like sesbania are seeded together as intercrops or mixed crops and allowed to grow for about a month. Subsequently, co-cultured sesbania is knocked down by 2,4-D ester at 0.4 to 0.5 kg ha-1.
It reduces the weed population by around 50 per cent without any adverse effect to growing rice crop. Sesbania surface mulch decomposes very fast and supplies nitrogen to rice crop. A new term ‘brown manuring’ has been coined for such manuring system.
Experiments at DRR, Hyderabad show that an irrigated green manure crop maturing in 60 days (May-June) can provide both the advantages of grain and green manure for succeeding rice crop. Five dual purpose legumes: pigeonpea, crotolaria, clitoria, desmanthus and siratro grown during dry season after rice in rainfed lowlands yielded 2-4 t ha-1 of forage or forage with seed and the legume residue incorporated into soil returned 80-160 kg N ha-1, which produced rice yield comparable to 25-50 kg fertiliser N ha-1.
Studies at IARI, New Delhi indicated that incorporation of mungbean or urdbean residue after picking the pods could contribute up to 40 kg N ha-1. In acid lateritic soils of Sekhanpur (WB), Sesbania aculeata as green manure crop to transplanted rice resulted in highest grain yield (3.7 t ha-1) with only 30-15- 15 kg NPK ha-1 as against 3.2 t ha-1 with 60-30-30 kg NPK ha-1.
Source # 3. Biofertilisers:
Biofertilisers for rice may be broadly classified into the following four groups:
1. Nitrogen fixers:
Symbiotic: Rhizobium in legumes and Anabaena azollae in Azolla
Associative: Azospirillum
Free living: Azotobacter, BGA
2. P-solubilisers: Bacteria, fungi and actenomycetes
3. P-mobilisers: VAM
4. Organic matter decomposers: Cellulolytic bacteria.
i. Rhizobium:
Legumes derive about 80 per cent of nitrogen from their symbiosis with Rhizobium. There is a carryover of 35-65 kg N ha-1 from cowpea, blackgram, groundnut, gram and lathyrus and 60-120 kg N ha-1 from berseem to the succeeding crop in a cropping sequence.
Multilocational trials in the All India Coordinated Agronomic Research Project during 1977-83 revealed that the mean grain yield response to Rhizobium inoculation in leguminous pulse crops varied from 0.08 to 0.20 t ha-1. The advantage of nitrogen fixation through Rhizobium is often utilised by rice crop through leguminous green manuring.
ii. Azolla:
Azolla fixes nitrogen with the help of alga, Anabaena azollae, present in its leaf cavities. Favourable environment for its growth include continuous submergenace (5-10 cm), moderately acid to neutral soil (pH 5-7), adequate soil available P, moderate air temperature of 25-30°C, bright sunshine and longer duration of light, especially in rice-azolla duel cropping.
The biomass in Azolla increases by 2-6 fold every week, with average daily nitrogen of 2.0 kg ha-1. Azolla is grown as green manure crop before planting rice or as an intercrop (dual crop) with rice. Of the two methods, intercropping is more practicable and economical, whereas green manuring provides high nitrogen utilisation efficiency and crop yield.
However, green manuring has limited scope, as it requires adequate water supply for about a month before rice planting. Inoculation at the rate of 1-2 t ha-1 for green manuring and 0.5-1.0 t ha-1 for intercropping is recommended with P application at 10-15 kg P2O5 ha-1 in three splits.
Growing Azolla once before or after rice transplanting is equivalent to application of about 30 kg N ha-1 through fertiliser, giving 10-40 per cent higher grain yield than control. Azolla green manuring or one dual crop in dry season resulted in grain yield of 4.0-4.1 t ha-1 which was comparable with that due to 30 kg urea N ha-1.
Similarly, integrated use of Azolla, either as green manure or dual crop with 30 kg N ha”1 as urea was equivalent to 60 kg N ha-1. Azolla green manuring along with one dual crop of Azolla contributed 75 kg N ha-1 and resulted in rice yield of 5.1 t ha-1, which was higher than that due to 60 kg N ha-1 as urea (Table 1.20).
In spite of several advantages, Azolla technology has been adopted to a limited extent due to the need for good water control, non-availability of inoculum for large scale field application, difficulty in maintenance of culture, susceptibility of Azolla to pests and diseases, sensitivity to extreme temperatures and prevalence of wide spread P deficiency in soils.
iii. Blue-Green Algae:
On an average, BGA can fix about 25 kg N ha-1. Extensive field studies at CRRI, Cuttack showed fresh biomass production of 4.5-27.5 t ha-1 contributing to 5-30 kg N ha-1.
Rice field should be inoculated with 10-20 kg of dry BGA ha-1, a week after rice transplantation. Application of 20-40 kg P2O5 ha-1 in 3 splits ensures good growth and N fixation. Since non- indigenous strains often fail to establish, well adapted native strains are preferred for inoculation. Mixed strains are better than single strain inoculation. Fresh BGA inoculation is better than dry BGA.
Since N requirements of rice cannot be met through BGA alone, it should be supplemented with N fertiliser. Application of N above 30 kg ha-1 inhibits growth of BGA. Nitrogen from BGA becomes available to rice after its decomposition. The standing rice crop utilises less than 50 per cent BGA-N and the rest remains in soil to the succeeding crop.
Increase in rice yield due to BGA has been reported from several countries. Field studies in India showed an average 14 per cent increase in yield over control, which corresponds to 450 kg grain ha-1. Algal inoculation if successful, is equivalent to 20-30 kg fertiliser N ha-1 and can meet one-third N requirement of rice. Algalisation increases organic carbon, total N and available P of soil and leads to build up of soil fertility, besides improving soil physical condition.
Major constraints to the success of algalisation include uncertainty in establishment of inoculated BGA, soil acidity, P deficiency, high N content in water due to topdressing of N fertilisers and cloudy weather.
iv. Azospirillum and Azotobacter:
These have widely been used for inoculation in rice. Inoculation is done mostly by seed treatment or seedling root dip but application to main field is also effective. Success of inoculation mostly depends on the quality of inoculation. In well-drained soil of near neutral pH, free-living Azotobacter fixes about 15-25 kg N ha-1.
Azospirillum residing in close association with roots of monocot crops also fixes N from atmosphere in the range of 1.5-7.0 kg ha-1. As the contribution of N to rice crop is low, addition of N through fertilisers at moderate level of 30-45 kg ha-1 is inevitable. Higher N rates inhibit the activity of the bacteria.
Work during the past two decades indicates success of Azospirillum inoculation in 60 per cent of the cases with an yield increase of 5-25 per cent. Response of rice to Azotobacter inoculation, however, is inconsistent. Soil acidity, low organic carbon and available P in soil and inadequate quality control of the inoculam are the major constraints encountered in adoption of these bacterial biofertilisers.
v. Phosphate solubilisers and mobilisers:
A number of bacteria, fungi and actinomycetes, both aerobic and anaerobic, solubililse native and applied insoluble P by secreting organic acids and producing chelating substances. Their populations are more in the rhizosphere.
Normally, fungi release more P from rock phosprate than bacteria, whereas both the groups are equally effective in releasing P from tricalcium phosphate. Aeration, temperature (25-30°C), high organic carbon and total P favour the activity of these microorganisms.
Inoculation with P solubilising microorganisms is, generally, done through seed or seedling treatment, although soil application is also effective. Treating rice seedlings with Pseudomonas striate and Bacillus polymyxa resulted in 10-20 per cent increase in yield in the absence of applied P and 5-15 per cent increase with rock phosphate addition.
In all India Coordinated Agronomic Research Project, inoculation of rice with Bacillus megatherium and B.circulans produced similar yield as application of 50 kg P2O5 ha-1 through superphosphate.
Vasicular-arbuscular mycorrhiza (VAM), a fungal biofertiliser mobilises relatively immobile elements such as P, Zn, Cu, Mn, Fe and other micronutrients and speed up their uptake by crop plants. The VAM association is found in both upland and lowland rice. However, the association is more in upland rice.
Mobilisation of P by VAM is attributed to tapping of P from greater soil volume through hyphae and production of phosphatases. Marginal increase in yield of rice has been reported in India. The major limitation with VAM technology adoption is cumbersome process of inoculum production on living plants.