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Improving Nitrogen Use Efficiency in Rice Teekam Singh

Regional Rain-fed Lowland Rice Research Station, Gerua, Kamrup, Assam-781102, India Email: [email protected]

Introduction Global demand for foodgrains is rising because of population growth, increasing affluence and changing dietary habits. To meet this demand, the global food production needs to increase by 42% by 2030 and 70% by 2050 (FAO, 2009). By 2050 India will need 137.3 million tonnes (m t) of rice annually (CRRI, 2013) as against a 2013-14 production of 106.3 m t (GOI 2014). Thus an additional 31 m t of rice is required and to be produced from the same or even less cultivated area (presently 43.9 m ha) with lesser inputs, as the land demand for other human needs (industries, roads, railways, residences etc.) is also increasing. With the present growth rate of rice production (1.82%), India will be able to produce sufficient rice but a plateauing or even a negative yield trend has been reported in rice-wheat cropping system (Prasad, 2005), which is the backbone of the India’s food security. More than 50% of the rice cultivated in India is under rainfed conditions, of which 65, 27 and 8% is under lowland, upland and flood prone (deep water) conditions, respectively. The productivity of rice under rainfed lowland and upland situations is very low (less than 2 t ha-1) which is comparatively lower than that of the national average productivity of 2.42 t ha-1. There are several factors behind the low productivity of rice but the most important are the declining soil fertility and poor crop management practices. About 89, 80 and 50% of Indian soils are low to medium in nitrogen (N), phosphorous (P) and potassium (K), respectively (Motsara, 2002) while some secondary and micro nutrient deficiencies are also emerging. Management of nutrients should be given adequate attention to increase yields and sustain productivity. A traditional blanket fertilizer application cannot cope with the demand of intensive cultivation and diverse cropping systems with large variability in biophysical and socio-economic inputs in small and marginal land holdings.

Deficiency symptoms of Nitrogen in rice The most of the Indian soils are deficit in nitrogen. N deficiency is likely to occur in light textured sandy soils leached by heavy rainfall or excessive irrigation or waterlogged conditions. The soils having low organic matter are also deficit in N. The continuation of intensive cropping

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On the basis of fertility status, soils are classified into low, medium and high and a blanket recommendation is given for each category which results lower fertilizer use efficiency, imbalanced use of fertilizer and lower productivity. Thus, efficiently use of fertilizers and crop intensification together with varietal improvement and investment in irrigation will all contribute to increased rice supply in future.

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system exhausts more nutrients from the soil and resulting their deficiencies. N is an essential plant nutrient which has specific deficiency symptoms in rice plants like:- • Deficient plants appear stunted, thin and spindly. • The number of tillers per hill is severely reduced which ultimately reduce grain yield. • The deficiency symptoms appear first on older leaves and become more severe as N is

highly mobile in plants and under conditions of low soil supply it is easily mobilised from older to younger leaves.

• Affected leaves first become small and appear pale green. In later these yellow chlorotic leaves turn pale brown and eventually become dry and necrotic.

• In mature plants, pale green youngest leaves, yellow middle leaves and lemon yellow to pale brown older leaves may appears simultaneously.

Optimizing Nitrogen Use Efficiency It is most essential to apply nutrients at right time, right quantity and in right place as a best management practice for achieving optimum nutrient efficiency. N use efficiency for rice in experimental plots was 46% (Ladha et al., 2005), however, this may not accurately reflected the efficiency obtained at on-farm. N recovery in crops grown by farmers rarely exceeds 50% and is often much lower. The average N recovery efficiency for fields managed by farmers’ ranges from 20-30% under rainfed and 30-40% in irrigated conditions. The average N recovery for irrigated rice is 31% at farmers’ field. Nitrogen is an element that is easily lost from the soil in various forms. In warm climate gaseous loss of nitrogenous fertilizers is more common. Therefore, in Indian condition plants get only 30- 40% of total applied nitrogen fertilizer even if it is applied properly. As we know that nitrogen is required by plants up to flowering stage and nitrogen is leached down after it is dissolved in water, we can try to reduce its solubility in water in lowland situations. For this, urea the most commonly used nitrogenous fertilizer is coated with insoluble materials like neem cake, coal tar and even soil can also be used for this purpose. Thus, to achieve high NUE is a complex issue and governed by many soil, plant, climatic and management factors, it has to be handled in an integrated approach. The following approaches are suitable to increase NUE in rice.

1. Slow release N fertilizers Slow release N fertilizers offer a good option to reduce N losses from the system because their delayed N release pattern may synchronize with crop demand. The supply of N by single application of controlled release fertilizer is expected to satisfy plant requirements and maintain low concentrations of N in the soil throughout the growing season. As N is applied once, unlike multiple splits in conventional practices, labour and application costs will be reduced, N losses minimized and NUE increased, thus resulting higher grain yield. The following slow release fertilizers can be used or prepared to improve NUE in rice field.

(i) Coated urea Urea is commonly used N fertilizer by the farmers as its easy availability and having highest N content in solid form. Urea is easily soluble in water and chances to lose through leaching, with surface run off and ammonia volatilization. These losses can be minimized through coating the urea prills with neem cake and oil, karanj cake, coal tar, sulphur, and polymers and availability of N can be increased for the crop plants. The coated urea should be applied to the field as early as possible before transplanting of rice. The whole amount is given only once in the field as a basal dose. No other split dose is required for the crop. Coated urea dissolves slowly and make N available to plant in all the growth stages continuously. Farmers can save 30-40 kg N ha-1 or

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more while using coated urea in their rice fields without any yield disadvantage. Some of the coated urea can be prepared by the farmers on their own like neem, karanj and coal tar based coated urea. Urea (100 kg), well-powdered neem cake (30 kg) and about 1 litre of kerosene oil are required for preparation of neem coated urea. First, wet this urea with kerosene oil and spread neem cake powder over the heap of urea. Mix thoroughly all these ingredients so that neem cake powder adheres to urea granules. Neem oil (2%) is also used for coating urea. Presently many commercial neem based products are available for coating urea. Likewise 100 kg urea, about 5 kg of coal tar and 1.5 litre of kerosene are required for preparation of coal tar coated urea. First of all coal tar is melted and allowed to cool down. Then it is mixed with kerosene oil thoroughly. Urea is spread in a thin layer. The mixture of coal tar and kerosene oil is sprinkled. Mix them till urea granules are covered by fine layer of coal tar.

(ii) Chemical controlled release N fertilizers Urea is reacted with other chemicals to extend its insolubility in the soil and make it available for longer period. From the beginning, their high cost relative to other N fertilizers has limited their use in large scale in food crops. They are used in high value crops, vegetables, orchards and seed production. Examples- Isobutylidenediurea (IBDU), Urea-Formaldehyde, Methylene Urea, Urea-Triazones.

(iii) Soil urea mixture Urea and slightly moist soil are mixed in the proportion of 1:6 and kept for 48 hours. Spread this mixture in the field before transplanting rice. Soil of this mixture serves in slow release of nitrogen. Sometimes the moist soil blended with urea are converted into ball and spread over rice field which will place these balls in reduced zone and N remained available for longer period.

2. Nitrification inhibitors Another approach to maintain N in the soil as ammonium form is nitrification inhibitors added with the fertilizers. Nitrification inhibitors slow the conversion of NH4+-N into NO3-N and N remain available to the plant for longer duration. The application of these inhibitors could also have considerable influence on emissions of nitrogen dioxide (N2O) and methane (CH4) from soil. Many potential nitrification inhibitors such as ammonium thiosulfate, thiourea, dicyandiamide (DCD), nitrapyrin (NP) etc. are commercially available and suitable for use with solid chemical fertilizers in rice cultivation.

3. Improved management practices NUE can be improved by adopting proper soil, water, fertilizers and crop management practices that will maximize N uptake minimize N losses and optimize indigenous as well as applied N supply to the plants. A good crop husbandry provides favourable environment to the crop plants and improve the productivity. The common practice of surface broadcasting of N fertilizers increases N losses, particularly ammonia volatilization, from the soil and reduce NUE. Thus, site specific or deep or band placement of urea or urea super granules (USG) has been proven to increase NUE.

4. Integrated Nutrient management The judicious use of different source of nutrients to complement and supplement mineral nutrients to the plants is termed as integrated nutrient management (INM) or integrated plant nutrient supply system (IPNSS). The basic principle of INM is high nutrient use efficiency, maintenance of soil fertility, sustaining productivity and improving farmers’ profitability through judicious and efficient use of fertilizers, manures and biofertilizers to the extent possible. The

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application of farm yard manure, crop residue incorporation, green manuring etc. buildup soil organic carbon in all rice based cropping system which results improved soil physical properties.

5. Tools and technologies (i) Leaf colour chart (LCC) Leaf colour, a plant health indicator, has been found to be an ideal tool to optimize the N supply in rice. The LCC is an easy-to-use, inexpensive and accurate tool for determining N status in rice plants. The LCC depicts gradients of green hues that are based on wavelength characteristics of rice leaves from yellowish green to dark green. LCC readings should be taken 21 days after transplanting to panicle emergence and N applied when LCC show leaf colour below 3 (light colour varieties) and 4 (dark colour varieties) of six or more out of randomly selected 10 different fully expanded leaves then 17 kg/acre of urea is top dressed in rainfed lowland situations and 29 kg/acre urea for irrigated conditions. Repeat LCC readings at 7 days interval during kharif and 10 days in rabi season until flowering.

(ii) Remote sensing Remote sensing has great potential to delivered cost effective estimates of actual plant status. A high correlation exists between plant N status and normalized difference vegetation index (NDVI) obtained with ground based radiation sensors. The NDVI is based on the principle that growing plants absorb photosynthetically active radiation (PAR, 400-700nm), and reflect near infrared (NIR, 800-1000nm). The NDVI increases with increasing leaf greenness and leaf area. The greenness and leaf area is correlated with plant N status and can be used as a guide for recommendation of N application to a larger area.

(iii) Simulation models and decision support system Soil-crop simulation models are used in combination with field information and actual weather data to make nutrient prescriptions at the beginning of the growing season as well as in real time during crop growth. This technique offers possibilities for real time N management in prescriptive-corrective concepts in rice. Several dynamic crop growth simulation models such as CERES, SUCROS, WOFOST, APSIM and InfoCrop have been developed during the last decades that integrate the effects of different factors on productivity (Aggarwal et al., 2006). These models could be effective tools in achieving real-time N management in rice.

6. Basic principles for nitrogen application The following basic principles should be kept in mind during application of N fertilizers so that maximum efficiency of fertilizer can be obtained. • N should be applied in 2 to 3 splits doses and amount of nitrogen for top dressing should not

exceed 20-30 kg/ha at a time. • If soil is deficient in nitrogen, apply relatively more nitrogen at the time of transplanting and

it should be deeply placed. • If soil is rich in nitrogen, small amount of nitrogen should be applied at transplanting. • If soil is permeable, avoid high doses of nitrogen at any time of application. • Early application of nitrogenous fertilizers promotes tillering. • For less tillering rice varieties, more nitrogen at early stage of growth should be applied. • For long duration rice varieties, more nitrogen as topdressing at various stages of growth

should be applied. • The nitrogenous fertilizer should be incorporated 3-4 cm deep into the soil, so that

nitrogen loss can be reduced.

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• Late application does not increase tillering, but increases grain yield by producing heavier panicles.

• Late and heavy dose of nitrogen at panicle initiation more especially at booting stage, increases the protein content in grains.

• In case of an incidence of bacterial leaf blight disease, it should be given in more split doses with lesser amounts.

• Inclusion of legumes as crop or green manure in crop rotation. • Application of organic manures, crop residue and biofertilizers.

Conclusion An increase in crop productivity is necessity and improvement in NUE is also a worthy goal and fundamental challenge facing the fertilizer industry, and agriculture in general. The opportunities are there and tools are available to accomplish the task of improving the efficiency of applied N fertilizers. However, we must be cautious that improvements in efficiency do not come at the expense of farmers’ economic viability or environment. Judicious use of N fertilizers with best management practices at right time, right quantity and in right place with targeting both high yields and NUE, will benefit farmers, society, and the environment.

References Aggarwal PK, Kalra N, Chander S and Pathak H. 2006. InfoCrop: Adynamic simulation model

for the assessment of crop yields, losses due to pests, and environmental impact of agro-ecosystems in tropical environments. I. Model description. Agriculture system 89(1): 1-25.

FAO [Food and Agriculture Organization]. 2009. In: OECD-FAO Agricultural Outlook 2009-2018; Can agriculture meet the growing demand for food. Pp 52-53.

GOI 2014. Ministry of Finance, Government of India. Economic Survey 2013-14. Pp A-17. Ladha JK, Pathak H, Krupnik TJ, Six J and van Kessel C. 2005. Efficiency of fertilizer nitrogen

in cereal production: retrospects and prospects. Advances in Agronomy 87: 85-156. Motsara, MR. 2002. Fertility status of Indian soils. Fertilizer News 47(8): 15-21. Prasad R. 2005. Rice-wheat cropping system. Advances in Agronomy 86: 255-339.

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