If properly applied, urea and fertilizers containing urea are excellent sources of nitrogen for crop production.
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After application to the soil, urea undergoes chemical changes and ammonium (NH4 +) ions form. Soil moisture determines how rapidly this conversion takes place.
When an urea particle dissolves, the area around it becomes a zone of high pH and ammonia concentration. This zone can be quite toxic for a few hours. The free ammonia that has formed can kill the seed and seedling roots within this zone.
Fortunately, this toxic zone becomes neutralized in most soils as the ammonia converts to ammonium. Usually it's just a few days before plants can effectively use the nitrogen.
Although urea imparts an alkaline reaction when first applied to the soil, the net effect is to produce an acid reaction.
Urea or materials containing urea should, in general, be broadcast and immediately incorporated into the soil.
If applying urea-based fertilizer in a band, separate it from the seed by at least 2 inches of soil. Under no circumstances should urea or urea-based fertilizer be seed-placed with corn.
With small grains, you can generally apply 10 pounds of nitrogen as urea with the grain drill at seeding time, even under dry conditions. Under good moisture conditions, you can apply 20 pounds of nitrogen as urea with the grain drill.
Research from North Dakota State University indicates that, under dry conditions, urea can reduce wheat stands more than 50 percent (Table 5). This was for urea applied with a grain drill in a 6-inch spacing, at the rate of more than 20 pounds of nitrogen per acre.
University of Wisconsin research indicates that seed-placed urea with corn, even at low rates of nitrogen, is very toxic to the seed and greatly reduces yields (Table 6). However, when urea was side-placed as a 2-by-2-inch starter, researchers noted little, if any, damage (Table 7).
In Minnesota, good crop production usually requires an application of more than 20 pounds of nitrogen per acre. Farmers can avoid damage from urea by broadcasting most of the urea nitrogen fertilizer ahead of seeding. Data in Table 8 indicate that urea broadcast prior to seeding is equal to or more effective than similar ammonium nitrate treatments.
A 2-year study was conducted at the ICAR-Indian Institute of Wheat and Barley Research&#;s Research Farm in Karnal (Fig. 1), Haryana, India (29° 43&#; N; 76° 58&#; E, altitude 245 m above mean sea level) from to .
Figure 1Location and layout of experimental trial at resource management farm of ICAR-IIWBR, Karnal (The map was extracted by using shapefile tool in QGIS 3.8.2 software).
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The experimental site is situated in the sub-tropical and semi-arid climate of Haryana, India. The region experiences cold winters from November to January, followed by mild temperatures in February and March. Mean weekly meteorological observations were taken during the cropping season from November to April , and again from November to April , and were recorded at the meteorological observatory of ICAR-Central Soil Salinity Research Institute in Karnal. These observations are presented in Figs. 2 and 3.
Figure 2Mean weekly maximum and minimum temperature during the crop growing seasons (&#;21 & &#;22).
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Figure 3Mean weekly relative humidity and total rainfall during the crop growing seasons (&#;21 & &#;22).
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Weekly maximum and minimum temperatures ranged from 14.9 &#; to 37.8 &#; and from 3.5 &#; to 17.1 &#; in &#;21 and from 12.6 &#; to 40.6 &#; and from 4.9 &#; to 19.4 &#; in &#;22, respectively (Fig. 2). During the year &#;22, the maximum and minimum temperature during the post-anthesis stage was higher than the previous year. The weekly averages of morning and evening relative humidity varied between 50.4 to 100.0 and 20.7 to 92.4 percent in &#;21 and 49.3 to 100.0 and 13.1 to 91.9 percent in &#;22, respectively (Fig. 3).
The total rainfall received during the crop seasons was 124.2 mm and 111.2 mm during &#;21 and &#;22, respectively (Fig. 3). With the exception of an increase in the maximum (average of 11th to 16th meteorological weeks was approximately 3.1 &#;) and minimum (average of 11th to 16th meteorological weeks was approximately 2.7 &#;) temperatures, which caused terminal heat stress during the grain development stage in second year of study i.e. &#;22. While the other weather parameters were conducive to the growth and development of wheat.
Prior to commencing the present experiment, the field was cultivated with transplanted rice in the summer of using traditional farmers&#; practices to ensure soil fertility homogeneity across the experimental field. The initial physico-chemical properties of the soil at depths of 0&#;5, 5&#;10, and 10&#;15 cm were analyzed before the start of the experiment. The soil of the experimental site lies in inceptisol order which was sandy loam in texture, with sand content of 62.4%, silt content of 27.5%, and clay content of 10.1%. The values of other soil properties, such as pH1:2.5, electrical conductivity1:2.5, organic carbon, and available N, P, and K, are given in Table 1.
Table 1 Chemical characteristics of soil prior to the experimentation.Full size table
The field experiment was conducted during two winter seasons in &#;21 and &#;22 to evaluate the effect of different nano urea sprays on wheat and compare their outcome with recommended conventional urea in conservation tillage, with the goal of reducing dependence on conventional nitrogen fertilizers. Prior to the experimentation, the cropping sequence practiced in the experimental plot was puddled transplanted rice -conventional tillage wheat. The experimental design was a randomized block with eight treatments and three replications, using gross plot areas of 8.0 m&#;×&#;2.0 m and net plot areas of 6.0 m&#;×&#;1.6 m. Glyphosate herbicide was sprayed to remove weeds prior to sowing. Wheat variety &#;DBW-187&#; was line-sown at a rate of 125 kg/ha with 20 cm spacing, using a zero drill Kamboj happy seeder and leaving residue from the previous crop (puddled transplanted rice @ 6 t/ha). The sowing date was 1st November and 9th November for &#;21 and &#;22, respectively.
The experimental plots received different doses of nitrogen (with the recommended dose at 150 kg/ha) through prilled urea (46% N) as per the treatments outlined in Table 2. A full dose of phosphorus (60 kg/ha) and potassium (40 kg/ha) were applied at the time of sowing in all treatments by using single super phosphate (16% P2O5) and muriate of potash (60% K2O), respectively. Nano urea (an IFFCO product) with 40,000 ppm of nitrogen was sprayed as per the treatment. Nano nitrogen was applied at a rate of 4 ml of nano urea/liter of water with a product dose of ml/ha. Foliar spray of nano urea was carried out using knapsack power sprayer having two nozzle spray boom after proper calibration and in synchrony with seed drilling. The detailed timing and weather conditions are given in Table 3. Other agronomic practices were carried out as per the recommendation of ICAR-IIWBR, Karnal. In &#;21, six irrigations were applied at critical growth stages of wheat, while in &#;22, an additional irrigation was applied at the grain filling stage due to an increase in temperature during the later stages. Glyphosate 41% SL @ 2.5 l/ha was sprayed a week before sowing as chemical plowing for weed management. After sowing, post-emergence applications of clodinafop 15% WP @ 60 g a.i./ha and metsulfuron methyl 20% WP @ 4 g a.i./ha were used for proper weed management. Termite attack was controlled by spraying chlorophyrifos 20% EC @ 2.5 l/ha, and rust was managed by spraying Propiconazole 25% EC @500 ml/ha just before flowering. At maturity, the crop was manually harvested and sun-dried. Mechanical threshing was done using Hadamba wheat thresher. The threshed grains were weighed and converted into kilograms per hectare for the estimation of yield.
Table 2 Performance of nano-nitrogen fertilizer under conservation tillage (rice residue retention condition).Full size table
Table 3 Details of the weather conditions during foliar spray regime.Full size table
Data were collected on plant height, dry matter accumulation, leaf area index, grain yield, number of effective tillers per m2, thousand grains weight (TGW), leaf chlorophyll content or greenness, and spectral reflectance of leaves or NDVI values. Plant height and dry matter accumulation were measured at harvest. The leaf area index was measured by using the &#;Sun Scan Canopy Analyser&#; from three sites in each plot. The grain yield was calculated from the net plot area and then converted into kg/ha. The number of effective tillers per meter row length was counted at two places in each plot and converted to per m2. The TGW was calculated by taking random grain samples and counting them using a Contador electronic seed counter (Pfeuffer, Germany) and weighing them.
The readings of leaf chlorophyll content or greenness were taken with a handheld device Soil Plant Analysis Development (SPAD, Minolta Camera Co., Osaka, Japan) chlorophyll meter. Five readings were taken from newly and fully opened leaves of each plot with care so that the mid-rib of the leaf should not come under the eye of the instrument, and the average value was worked out.
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The readings of spectral reflectance of leaves or NDVI values were taken by moving the instrument over the plot at DC37 and DC65 stages20 using a handheld optical sensor &#;GreenseekerTM handheld sensor&#; which measures the reflectance of a given crop area over a 61 cm&#;×&#;61 cm area when the unit is positioned between 81 to 122 cm above the target area21. The NDVI is calculated from reflectance measurements in the red and near-infrared (NIR) portion of the spectrum.
$${\text{NDVI}} = \frac{{{\text{R}}_{{{\text{NIR}}}} {-}{\text{ R}}_{{{\text{Red}}}} }}{{{\text{R}}_{{{\text{NIR}}}} + {\text{ R}}_{{{\text{Red}}}} }},$$
where RNIR is the reflectance of NIR radiation and RRed is the reflectance of visible red radiation.
Composite soil samples were collected prior to setting up the experiment layout, and subsequent to crop harvest, samples were collected from each plot at three depths (0&#;5 cm, 5&#;10 cm, and 10&#;15 cm). These samples were then processed and subjected to analysis to determine the available nitrogen content. Other chemical properties such as pH, EC, organic carbon, available P, and K were also analyzed, but no significant differences were observed among treatments and are presented as supplementary information.
Soil pH and EC were determined in a 1 and 2.5 ratio of soil and water suspension. The oxidizable organic C was determined using22 wet-oxidation method. An alkaline permanganate method (Subbiah and Asija, ) was followed to determine the KMnO4-oxidizable N. Olsen&#;s extractable P (soil pH&#;>&#;6.0) was determined using the ascorbic acid reductant method23. Potassium (K) was extracted by neutral NH4OAc and detected using a flame photometer23.
Representative samples of crop produce (grain and straw) were collected randomly at harvest, dried, processed, and analyzed for nitrogen content. The amount of N removed by the crop was calculated by multiplying the percentage of nutrients with the yield of grain and straw separately and summing them up. The uptake of the crop was computed based on its dry weight as follows:
$${\text{Nutrient uptake }}\left( {{\text{kg}}/{\text{ha}}} \right){ = }\frac{{{\text{Nutrient content }}\left( \% \right) \, \times {\text{ Yield }}\left( {{\text{kg}}/{\text{ha}}} \right)}}{100}.$$
Nitrogen use efficiency is the yield increment per unit of N fertilizer given to the crop. It is measured in kg grain yield/kg nitrogen applied.
$${\text{Agronomic efficiency }}\left( \% \right) = \frac{{{\text{Yield}}_{{\text{fertilized plot}}} {-}{\text{ Yield}}_{{\text{control plot}}} }}{{{\text{F}}_{{{\text{applied}}}} }}.$$
Yield fertilized plot and Yield control plot are yields (kg/ha) when quantity of N fertilizer applied was F and zero; Fapplied is the total nitrogen (kg/ha) applied24.
Nitrogen use efficiency (NUE) can be separated into two components: nitrogen uptake efficiency (NUpE) and nitrogen utilization efficiency (NUtE). NUpE describes the amount of nitrogen that a plant can take up from external sources, while NUtE describes the plant's ability to assimilate and remobilize nitrogen within its own tissues. NUE25 is the product of NUpE and NUtE26,27.
$${\text{N uptake efficiency}} \left( {{\text{NUpE}}} \right) \, = {\text{ Nitrogen contents in plant}}/{\text{Nitrogen supplied}},$$
$${\text{N utilization efficiency }}\left( {{\text{NUtE}}} \right) = {\text{Yield}}/{\text{Nitrogen contents in plant}},$$
$${\text{NUE}} = {\text{NUpE }} \times {\text{ NUtE}}.$$
The grain protein content of wheat was estimated by using Infratec&#; (FOSS) instrument, a whole analyzer using near infrared (NIR) transmittance technology25.
Wheat grain yield was multiplied by the minimum support price for &#;22 (246.9 US $/ton) and &#;23 (251.9 US $/ton) (reference for MSP from PIB, GOI). The wheat straw yield was also multiplied by the market rate (100 US $/ton) and added to get the gross return. The cost of cultivation was calculated by considering field preparation, seed, fertilizer, irrigation, transportation, herbicide application, the cost involved in harvesting and threshing of produce, management charges, the rental value of land, interest on fixed capital, depreciation cost of implements, and farm buildings. The net return was calculated by subtracting the cost of cultivation from gross returns. The benefit&#;cost ratio was calculated by dividing the gross return by the total cost of cultivation. To convert into US$, the gross return, cost of cultivation, and net return were divided by the prevailing exchange rate ($1&#;=&#;Rs. 80).
A partial budget analysis was employed to assess the economic advantages of various nitrogen fertilizer application sources and timings in relation to wheat grain and straw yields. During this study, we identified the economically viable treatments by estimating their costs and benefits in accordance with the market prices for the specific research year. It's worth noting that experimental yields often surpass what farmers can realistically achieve when implementing the same treatments28. Gross and net benefits were computed using the following formulas:
(1) Gross returns&#;=&#;wheat grain and straw yield&#;×&#;Market price of respective yields,
(2) Net Benefit&#;=&#;Gross returns&#;Total variable costa.
aTotal variable cost was computed by deducting the fixed land cost of from total cost of cultivation.
The calculation of the marginal rate of return (MRR) involved dividing the change in net benefit by the change in variable cost, which represents the additional return gained by increasing the input. Before computing the MRR, a simplified dominance analysis, as outlined by Maize and Center in , was carried out to identify treatments that hold relevance for farmers in terms of earnings. A treatment is considered dominated when, despite incurring higher costs, it fails to yield a greater increase in net benefits. In such cases, it is dominated because there exists at least one other treatment with equal or lower costs that generates higher benefits. To conduct this analysis, treatments were arranged in ascending order of variable costs, and comparisons were made to determine whether an increase in costs was associated with a proportionally greater increase in net benefits.
Analysis of variance (ANOVA) and ranking of treatments was completed using Tukey&#;s Range test at 0.05 (5%) level of significance. The General Linear Model (GLM) Procedure in SAS®9.3 version 6.1. for Windows (Cary, NC, SAS Institute Inc., ) was used for statistical analysis.
All authors approve ethical responsibilities related with this manuscript.
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