Foliar Urea Led India Castor Trial as Nano Urea Showed No Clear Yield Edge

A three-season field trial in southern India found that applying half of the recommended nitrogen dose to soil and replacing the remaining split applications with two foliar sprays of conventional urea produced the highest rainfed castor yield, profit and energy efficiency among eight tested strategies.

The strongest treatment averaged 1,546 kilograms of castor seed per hectare across the 2022, 2023 and 2024 rainy seasons. That was 19.1% above the 1,298 kilograms per hectare recorded under the conventional all-soil fertilizer recommendation and 10.3% above the comparable nano-urea treatment at 1,402 kilograms per hectare.

Those percentage differences are Agrilinkage calculations. The statistical result is more cautious: the authors reported that the top conventional-urea treatment, the nano-urea treatment and the standard fertilizer treatment were in the same statistical group for pooled seed yield. The trial therefore supports an economic and efficiency advantage under its conditions, but not a clear claim that conventional foliar urea always produces a higher yield.

How the field trial was organized

Researchers at the University of Agricultural Sciences in Bangalore tested eight nitrogen-management modules in a randomized complete block design with three replications. Each plot measured 32.4 square metres, and the same hybrid castor variety, ICH-66, was grown over three monsoon seasons.

The standard treatment supplied 40 kilograms of nitrogen per hectare, with half applied at sowing and the rest split between 30 and 60 days after sowing. Alternative treatments placed 50%, 75% or 100% of the recommended basal nitrogen in soil, then used either a 2% conventional urea solution or nano urea at 2 millilitres per litre as foliar sprays at 30 and 60 days.

The leading treatment used the full recommended basal amount, 20 kilograms of nitrogen per hectare, followed by the two 2% urea sprays. Despite its label of 100% basal nitrogen, it did not apply the full 40-kilogram conventional recommendation to soil. It supplied the complete recommended basal half and replaced the later soil splits with foliar feeding.

Yield varied sharply with rainfall

The top treatment produced 1,827 kilograms per hectare in 2022, 1,180 in 2023 and 1,631 in 2024. Rainfall during the crop periods was 1,491 millimetres, 712 millimetres and 1,439 millimetres respectively. The low-yield 2023 season therefore coincided with less than half the rainfall of 2022, illustrating why a three-year average is more informative than a single season.

The nano-urea version of the same basal strategy averaged 1,402 kilograms per hectare. The conventional soil-only recommendation averaged 1,298 kilograms. A treatment with phosphorus and potassium but no nitrogen produced only 178 kilograms per hectare, confirming that nitrogen was a dominant production constraint at this site.

The profit result was clearer than the yield ranking

The 2% foliar-urea strategy cost an estimated $470 per hectare and generated $1,174 in gross returns and $704 in net returns. The conventional soil-only recommendation cost $457, generated $986 in gross returns and returned $529 net. On the study's prices, the foliar strategy increased net returns by about 33%, matching the authors' calculation, while its benefit-cost ratio rose from 2.16 to 2.50.

The comparable nano-urea treatment cost $480 per hectare and produced $586 in net returns with a benefit-cost ratio of 2.22. Conventional urea therefore had the better economic result in this experiment, even though pooled seed yields were statistically similar.

These prices cannot be transferred directly to another season or region. Fertilizer prices, labour for two spray operations, castor seed prices, equipment and exchange rates would change the result. Farmers also need enough leaf area, suitable weather and application equipment to deliver foliar nitrogen without excessive loss or crop injury.

Nitrogen efficiency involved a trade-off

Lower basal rates produced the highest partial factor productivity because yield was divided by a smaller nitrogen input. The 50% basal plus nano-urea treatment reached 106.7 kilograms of seed per kilogram of nitrogen, compared with 32.5 under the conventional soil-only program. But its seed yield and net return were below the leading full-basal plus conventional-urea treatment.

This is a common nutrient-management trade-off. The treatment that uses each kilogram of nitrogen most efficiently is not necessarily the one that maximizes total harvest or farm income. The right choice depends on fertilizer availability, cash constraints, rainfall risk and the value of additional output.

The leading foliar-urea treatment also recorded the highest total nitrogen uptake at 34.9 kilograms per hectare and an oil content of 42.7%. Its energy output reached 58,726 megajoules per hectare, about 20% above the conventional treatment's 48,881, while energy input was lower at 11,481 versus 12,370 megajoules per hectare.

What the carbon figures do and do not show

The authors calculated a cradle-to-farm-gate carbon balance using published emission factors for inputs, field operations and transport. Higher-yield foliar treatments produced a lower carbon footprint per kilogram of castor seed than the nitrogen-free control because emissions were spread across much more output.

That does not mean foliar urea eliminates nitrogen pollution or greenhouse-gas risk. The trial's carbon accounting used coefficients rather than direct field measurements of ammonia loss, nitrate leaching or nitrous oxide emissions. The authors also observed changes in the soil nitrogen balance and cautioned that foliar substitution can draw down the soil nitrogen pool.

How far the result can travel

The experiment used one castor hybrid on a sandy clay loam Alfisol near Bangalore, with farmyard manure applied to every plot. It does not establish the same optimum for heavier soils, different hybrids, mechanized farms or much drier environments. Spray concentration, water quality, nozzle coverage and weather at application can also change foliar performance.

The practical finding is narrower and more useful: under three variable rainfed seasons at this site, two conventional foliar urea sprays paired with the recommended basal soil nitrogen produced the best combined yield, net return and energy result. Nano urea did not show a clear pooled-yield advantage and was less profitable at the study's prices.

Primary source: Frontiers in Plant Science field study published 28 October 2026.

Methodology and disclosure: Agrilinkage recalculated percentage differences from the pooled three-year values, checked the authors' significance groupings before comparing treatments, and separated measured field results from coefficient-based carbon accounting. The cover and body image is an original Agrilinkage conceptual illustration, not a photograph of the trial.