Foliar urea lifted rainfed castor yield in one Indian trial. The nitrogen arithmetic is harder to read.

A three-season field experiment on rainfed castor found that the best of eight nitrogen management modules was the full recommended basal nitrogen rate followed by 2% urea sprays at 30 and 60 days after sowing. That module produced 1,546 kg of seed per hectare, 42.7% oil content and 34.9 kg per hectare of nitrogen uptake [1].

That is not evidence that you can cut soil nitrogen and hold the yield. The winning module applied the full recommended basal rate and then added foliar sprays on top [1]. The paper frames the approach as partial N substitution through foliar application, but in the top-yielding treatment nothing was substituted. Nitrogen was added.

The distinction matters more than the yield number. The practical question for a rainfed grower is whether foliar feeding lets you use less fertiliser. This trial does not answer it. It answers a narrower one: given the full basal dose, does adding foliar nitrogen help?

What was actually tested

Eight modules, randomised complete block design, three replications, three consecutive Kharif seasons from 2022 to 2024 [1]. The modules crossed three basal nitrogen levels — 50%, 75% and 100% of the recommended basal N — with two foliar options, a 2% urea solution or nano urea at 2 ml per litre, applied at 30 and 60 days after sowing. Two references were included: the full recommended rate of fertiliser (40-40-25 kg N-P2O5-K2O per hectare) and a phosphorus-and-potassium-only control with no nitrogen [1].

The abstract reports results for the best module only. It gives no yields for the other seven, so the size of the gap between the winner and the rest is not visible.

Two comparisons do come through. Combined soil and foliar nitrogen beat soil-only nitrogen [1]. And 2% urea beat nano urea at 2 ml per litre for foliar feeding of this crop [1]. That second result is worth pausing on. Nano urea is promoted in India as a way to reduce bulk urea use. Here, on this crop, at this rate, it did not outperform a simple urea solution.

The mechanism

The paper starts from the fact that nitrogen has the lowest use efficiency of the major nutrients and multiple loss pathways, which makes it difficult to manage in rainfed castor [1]. Castor grown on rainfed kharif land typically gets its nitrogen as a soil application around sowing, followed by a season of intermittent heavy rain. Soil nitrogen is exposed to leaching, volatilisation and denitrification, and the plant's demand peaks well after the basal dose has been sitting in that soil.

Foliar feeding is meant to sidestep that. Urea dissolved in water and sprayed onto leaves is taken up through the leaf surface, so the nitrogen arrives when the plant is branching and setting spikes rather than waiting in the soil for weeks. The 30 and 60 day timings bracket that period [1].

The trial's own result fits foliar nitrogen working as a top-up rather than a replacement: the full basal dose plus sprays won, not a reduced basal dose plus sprays [1].

The efficiency numbers need care

The paper reports partial factor productivity of 67.8 to 106 kg of seed per kg of nitrogen under foliar feeding, against 32.5 kg per kg for soil-only nitrogen [1]. Partial factor productivity is yield divided by the nitrogen applied. Leave the foliar nitrogen out of the denominator and the ratio rises without any change in how efficiently the plant uses nitrogen. The abstract does not say how the denominator was calculated, so the comparison cannot be checked from the abstract alone.

The nitrogen balance figure is harder still. The authors report a positive nitrogen balance index of 10.7 to 12.1 under foliar fertilisation and describe it as depletion of the soil nitrogen pool [1]. A positive balance index normally indicates accumulation, not depletion. The abstract does not explain the sign convention, so this sentence cannot be interpreted as written.

There is a further inconsistency. The abstract gives the soil-applied nitrogen in the winning treatment as 20 kg per hectare, while the recommended fertiliser rate it is compared against contains 40 kg N per hectare [1]. The abstract does not reconcile these figures.

The paper also reports energy and carbon figures for the best module — 58,726 MJ per hectare and 1,947 kg carbon equivalent per hectare — and states that net energy and carbon gain improved [1]. The abstract does not separate inputs from outputs, so the net figures cannot be verified from it.

What other crops show

Better nitrogen timing holding yield at lower total nitrogen has support elsewhere, but not in castor. In maize and wheat, nitrogen-enriched biochar applied at 70% of the recommended nitrogen rate increased nitrogen uptake efficiency and grain yield relative to unamended controls, and the authors call for long-term, multi-site work before recommending it [7]. Different crop, different amendment, different mechanism.

In the Brazilian semiarid, legume-heavy cover crop mixtures fixed between 56 and 106 kg of nitrogen per hectare per year, amounts of the same order as those exported in fruit harvests [5]. Those are irrigated fruit systems, not rainfed castor, and the authors say agronomic and economic validation is still needed before recommending them to producers.

Closer to home, a single-season trial on rainfed mothbean in Gujarat found castor shell compost at 1 tonne per hectare broadly comparable to 20 kg N per hectare for several growth and yield traits [12]. One season, one site, and the interaction between manure and bioenhancers was not significant.

A three-year study of groundnut intercropped with castor under rainfed Alfisols found that treatment rankings were not stable across seasons, with highly significant year-by-treatment interactions [14]. That is a warning about pooled averages in exactly this kind of trial.

Where the evidence runs out

The castor result rests on one field experiment. Three seasons and three replications is a reasonable agronomic design, but the abstract reports no second location and no year-by-year breakdown. Given the seasonal instability documented in comparable rainfed systems [14], a pooled three-season average can hide a result that holds only in some years.

No test statistics appear in the abstract. The word significantly is used, but no p-values, standard errors or coefficients of variation are given, and yields for the seven losing modules are not reported.

The central practical claim — that foliar nitrogen allows partial substitution of soil nitrogen — is not supported by the treatment that won. The winner used the full basal dose plus foliar sprays. Whether 50% or 75% basal nitrogen plus foliar urea could match it is exactly what the trial was designed to test, and the abstract does not report those yields.

The nano urea comparison covers one product, one rate and one crop. It is not a general verdict on nano urea.

Nothing here transfers to food crops, to irrigated systems, or to soils and rainfall patterns other than those of the trial site. Castor is a non-edible oilseed grown for oil and industrial uses, and its nitrogen response is its own.

What a grower can take from this is narrow and useful: on rainfed castor, a foliar urea top-up at 30 and 60 days after sowing is worth testing alongside the full basal dose. What cannot be taken from it is a licence to cut soil nitrogen.