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Maximising nitrogen use efficiency in field crops

At a world scale, human activity has increased the flux of nitrogen (N) two times, particularly executed by large scale fertilizer manufacturing in the world. The total N applied to agricultural crop land all over the world only 5–15 per cent is eventually converted into human food. In field crops nitrogen use efficiency (NUE) range between 35–65 per cent. We should have to select the fertilizer depends on unit cost of nutrient present in it and its agronomic efficiency under a given situation. Fertilizer is a valuable input and steps should be taken to minimize its losses and to improve its uptake and consumption by the crop plant. The role of N, though the process of biological N fixation and in manures and inorganic forms such as salt petre, in promoting plant growth and increasing crop yields followed in the 1840s, through the work of Boussingault in France, Liebig in Germany and Lawes in England.
Nitrogenous fertilizers
Nitrogenous fertilizers are those fertilizers that provide nitrogen to the crops on their application. When nitrogenous fertilizer applied in soil most of the plants species recover only 25-35 per cent nitrogen. Losses of nitrogen occur in the form of ammonia volatilization, immobilization to organic forms, denitrification, run off and leaching.  Care should be taken in choosing the type of fertilizer as well as the timing and method of nitrogenous fertilizer application.
Choice of the nitrogenous fertilizer
In selecting the fertilizer care should be taken to select such a type of fertilizers that have minimum interaction with soil and the time and mode of application should be such as to ensure minimum immobilization of nutrients present in the fertilizers.
1. When applied in submerged soil conditions as in the rice crop, the ammonical and ammonia producing fertilizers like urea are most appropriate and suitable then other farms of nitrogen. As most stable form of nitrogen is ammonia when applied under such conditions.
2. For nitrogen application in the acidic soils, the ammonia producing nitrogenous fertilizers are more appropriate especially during rainy season because soil particles adsorbed ammonium form of nitrogen, that’s why losses due to leaching are reduced. Ammonium form of nitrogen adsorbed on soil particles released gradually for nitrification and then available to the crop plants.
3. Application in highly acidic soils, the nitrogenous fertilizers which are usually low in ammonium, but high in nitrate should be used on these type of soils as these types of fertilizer helps to increase the soil pH.  
4. Application in alkaline soils of low rainfall areas, preference is given to nitrate fertilizers over urea or ammonical fertilizers as these soils the loss due to may be volatilization when soil is alkaline.
Management of nitrogenous fertilizers
1. Nearly all the nitrogenous fertilizers are highly amenable to losses and most of the crops required nitrogen during their entire growth duration, application in splits is necessary to ensure maximum utilization of fertilizer by crop plants.
2. More number of splits may be given for perennial as well as long duration crops.
3. Losses of nitrogen fertilizers are more in sandy soils with low cation exchange capacity (CEC) and high temperature as compare to clayey soils. In this scenario more number of splits are required to minimize losses of nitrogen fertilizers from coarse texture and light soils.
4. For rice cultivars of medium duration, nitrogen based fertilizers should be applied in three splits, 1st- as basal dose, 2nd- at maximum tillering stage and 3rd - at panicle initiation stage.
5. In coarse textured sandy or loamy soils, the basic dose of nitrogenous fertilizers should be applied in 3-4 splits at different growth stages of rice crop.
6. Due to water stagnation after sowing/planting in such areas/lands application of nitrogen is not feasible. Application of neem or tar coated urea as basal dose is more appropriate form of nitrogen in such conditions.
7. In double-cropping systems, the fertilizer is calculated for the system and half of nitrogen may be applied in the organic form, according to the requirement of the first crop.
8. Losses due to broadcast application of nitrogen fertilizers to the flooded soils are up to 70 per cent. These losses majorly occur in the form of leaching.
Measures to reduce the losses of N from applied urea
1. The losses of nitrogenous fertilizers can be minimized by adjusting the time of application to synchronize with maximum plant uptake period and can also applying the fertilizer in two or more split doses of fertilizer.
2. Nitrogenous fertilizer should be applied early in morning or in the evening when the temperature of the atmosphere is not too high and just after the application the field should be irrigated immediately.
3. Areas of clayey soils having CEC of 10cmol/kg of soil are recommended for the application of urea briquettes and urea super granules.
4. Before application to field it is better to mix urea with moist soil. This will prevent the loss of nitrogen.
5. Urea is mixed five times of its weight with neem cake as it prolongs the availability of nitrogen to the crop plants.
6. For submerged soils, urea coating with kerosenated coal tar before mixing with neem cake is given preference as compared to simple mixing with neem cake. The dose of kerosene and coal tar is 2 kg coal tar mixed with 100 kg urea dissolved in 1 liter kerosene.
7. Urea coating with neem water (containing 5 per cent neem tri-terpenes) at concentration of 1% and dried under shade for 1-1.5 h before application in direct-seeded rice improve nitrogen use efficiency.
8. As far as possible, application of urea may be through plough sole placement or deep placement. In light soils deep placement of super granules and drilled urea during the last ploughing followed by flooding and planting is beneficial. Between four holes of transplanted rice super granules or urea briquettes may be placed, while lac coated or sulfur coated urea may be broadcast on the soil surface.
9. Losses in the form of ammonia under high pH conditions can be controlled by proper placement of urea. As far as possible, the ammonium fertilizer should be avoided. If there is no alternative to ammonium fertilizers, the fertilizer should be placed at least 4-6 cm below the surface.
10. Exogenous application of urea solution may be used in the situations where immediate results of applied nitrogen are to be required for crops. Concentration of urea solution may be increased upto 15 per cent when we have to apply it through power sprayers.
The writers are from the Department of Agronomy, University of Agriculture Faisalabad, Pakistan.


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