Nutrient management practices for crops, in general, can be grouped into blanket and site-specific nutrient management practices.
1. Blanket Nutrient Management (BNM):
The BNM recommendations are, generally, based on approaches that do not account for the dynamic nature of crop response to the environment. These are developed for large tracts having similar climate and landforms by conducting a large number of field experiments to achieve high fertiliser use efficiency by applying nutrients in right quantity, using right method and source and at right time.
Blanket recommendations for NPK to rice crop vary greatly (60-30-30 to 150-75-75 kg N, P2 O5 and K2O) depending upon the system of rice cultivation and the agroecological region.
An over view of blanket nutrient management recommendations for nitrogen is given below:
Large field-to-field variability in soil fertility restricts fertiliser use efficiency. Improved cultivars cannot express their yield potential with lower quantity of fertiliser application than actual requirement. Higher quantity of fertiliser application than the actual requirement leads to low fertiliser use efficiency with blanket nutrient recommendation.
Several soil and agronomic approaches (split application of N, controlled irrigation, use of nitrification and urease inhibitors, slow release N fertilisers etc.) have been evaluated with varying success. Soil test-fertiliser recommendations have not been successful. As such, plant need based fertiliser application or site-specific nutrient management (SSNM) is crucial for realising high yield and optimum fertiliser use efficiency.
2. Site-Specific Nutrient Management (SSNM):
The SSNM is basically a systematic approach to apply sound agronomic management to small areas that can be identified as needing special treatment. The components of site-specific management may not be new, but now we have the capability to use them more effectively.
Systematic implementation of these practices into site-specific systems is probably our best opportunity to develop a truly sustainable crop production system. This has been shown by field trials in several Asian countries including India. Results of SSNM-based field experiments with rice-wheat system are given in Table 1.18.
Results of multilocation on-station trials have shown that grain yields of 15 to 17 t ha-1 yr-1 are attainable in the high intensity rice-wheat system. Highest grain productivity of > 16 t ha-1 at Modipuram and Ludhiana, 14 to 16 t ha-1 at Kharagpur, 12 to 14 t ha-1 at Faizabad, Varanasi, Pantnagar, Sabour, RS Pura, 10 to 12 t ha-1 at Ranchi and 8 to 10 t ha-1 at Palampur was recorded.
Average annual grain productivity was 13.3 t ha-1 out of which contribution of rice was 60 per cent and that of wheat 40 Per cent. Averaged over locations, SSNM brought about 34 per cent yield advantage over fanners’ nutrient use practices. Increase in expenditure due to fertiliser use with SSNM was Rs 4,167 ha-1 over farmers practice but generated additional produce valued at Rs 20, 530, excluding extra fertiliser cost resulting in benefit: cost ratio of 4.9.
Leaf colour chart:
For SSNM, a leaf colour chart (LCC), first developed in Japan in 1989, appears to be an ideal tool to optimise N use in rice cropping, irrespective of the source of N applied (inorganic, organic or biofertiliser). The concept of LCC is based on a close link between chlorophyll and N content of leaf. Hence, LCC can be used as quick and reliable tool for assessing leaf N status of crops at different stages.
The LCC contains gradients of green colour from yellowish green to dark green, based on wavelength characteristics of rice leaves and can guide N topdressing. The LCC has been improved and calibrated for Japonica, Indica and hybrid rice.
The LCC evaluates N status of rice crop cost effectively, avoiding leaf sampling, laboratory analysis and delay in receiving the results. Farmers can use leaf colour as a visual and subjective indicator of the rice crop’s N status and decide on the need for N fertiliser application.
As per site specific N management guided by LCC, N is applied to rice crop whenever intensity of the green colour of the upper most fully opened leaf is less than shade 4 on the LCC. It has already been recommended to the fanners of Punjab and TN.
Need based real time N management takes care of N supply to rice crop from all kinds and sources and thus leads to fertiliser use efficiency. Site-specific management strategies for P and K application are becoming available but not yet reached the rice farmer.