Manure Cut Fertilizer Needs in a Shallot Trial. One Season, One Field, One Soil.

A field trial in Bengkulu, Indonesia, found that composted cattle manure at 10 tonnes per hectare allowed growers to cut inorganic NPK by 25 to 50 percent without losing bulb yield, and that the reduced-fertilizer plots stored better. The highest fresh bulb yield in the trial, 15.56 t/ha, came from manure plus 75 percent of the recommended NPK rate, 18.3 percent above the full-dose reference [2].

That is the finding. The caveat is the size of the evidence. This was one randomized complete block design, six treatments, four replications, one Ultisol, one season, one location. The abstract reports no second site, no second year, and no measurement of why the storage result happened. Treat it as a well-designed single experiment that generated a hypothesis, not as a rate recommendation.

What they actually did

The design matters, because it determines what can be attributed to what. Five treatments held manure constant at 10 t/ha and varied the inorganic NPK rate: 100, 75, 50, 25 and 0 percent of the local recommendation. A sixth treatment, full-dose NPK with no manure, served as the reference [2].

That means the headline comparison, manure plus 75 percent NPK against the reference, changes two things at once. The winning plot had manure and less inorganic fertilizer. You cannot tell from that comparison alone how much of the 18.3 percent gain came from the manure and how much from the reduced fertilizer rate. What the design does let you see is the shape of the response within the manure series, where the manure base is fixed and only the NPK rate moves.

The storage result is cleaner. Weight loss after eight weeks of ambient storage fell from 22.2 percent in the reference to 15.1 percent with manure plus 50 percent NPK, and the share of marketable bulbs rose from 63.9 to 82.2 percent [2]. But the same abstract reports that adding manure to the full NPK dose raised dry bulb yield without improving storability. Manure on its own, at full fertilizer, did not help storage. The improvement tracked the reduction in inorganic nitrogen, not the presence of manure. That is an inference from the pattern, not something the authors state, but it is the pattern the numbers show.

The mechanism, as far as anyone knows

The abstract does not propose a mechanism for either result, so anything here is reasoning from soil chemistry rather than from the paper's data.

The site is an Ultisol, an acid, weathered soil. Shallot production on these soils leans on heavy inorganic fertilization [2]. Manure raises soil organic carbon and pH, and both were higher in every manure treatment in this trial [2]. Higher pH reduces aluminium toxicity and improves phosphorus availability on acid soils. Organic matter improves aggregate structure and water holding, which matters for a shallow-rooted bulb crop. These are general effects, not findings from this paper.

The nitrogen productivity figure is worth reading carefully. Partial factor productivity of nitrogen rose from 88 to 192 kg/kg when NPK was halved [2]. PFP-N is yield divided by nitrogen applied. Halving the nitrogen rate while holding yield roughly constant doubles that number by arithmetic. It is a real measure of how much nitrogen the system needed to produce a tonne of bulbs, and it is the number that matters for fertilizer cost and for nitrogen losses. It is not independent evidence that the manure made the plants use nitrogen more efficiently.

For storage, the plausible story is that heavy inorganic nitrogen pushes late-season vegetative growth and delays bulb maturation, giving thinner skins and higher water content that drive weight loss in humid storage. Cutting nitrogen by half would reduce that. The trial did not measure skin thickness, bulb dry matter, or disease incidence during storage, so this remains a guess.

One ambiguity: the abstract reports a dry bulb yield of 12.40 t/ha against a fresh yield of 15.56 t/ha. That ratio is far too high for bulb dry matter, which in shallot is typically around a fifth of fresh weight. Dry bulb yield here almost certainly means cured or dried bulb weight after the tops and outer skin are removed, not oven-dry matter. The abstract does not define it.

What the evidence does and does not support

Supported: on this soil, in this season, a manure base of 10 t/ha with 50 percent of the recommended NPK matched the yield of full-dose NPK without manure, and stored better. Manure with 75 percent NPK exceeded it. Soil organic carbon and pH rose in all manure plots [2].

Not supported: that these rates transfer to other soils, other seasons, or other shallot varieties. Not supported: that manure alone can replace inorganic fertilizer. The 0 percent NPK treatment was run, but the abstract does not report its yield, which is the number that would answer the question most growers actually ask. Not supported: that manure improves storability. The full-NPK-plus-manure treatment did not store better [2].

The wider literature offers only loose support. A separate study on garlic, a different Allium species, found that cattle manure increased protein, total sugars, phenolics, flavonoids and antioxidant activity relative to an unfertilized control [1]. That is a different crop, a different soil, and a comparison against no fertilizer rather than against mineral fertilizer. It is consistent with the idea that animal manure supports bulb crop quality. It does not corroborate the shallot yield or storage numbers.

Where the evidence runs out

The trial was not repeated. One site, one season, four replications. Yield responses to manure depend heavily on soil type, manure source and how well the manure was composted, and the abstract gives no analysis of manure nutrient content beyond the application rate. A grower on a different soil has no basis for assuming the same substitution rate.

The storage result rests on a single eight-week window under ambient conditions in a wet tropical lowland. Storage losses depend on temperature, humidity, bulb curing and pathogen pressure, none of which are reported. Whether the 7-point reduction in weight loss would hold in a drier or cooler store is unknown.

No mechanism was measured. No soil nitrogen dynamics, no bulb quality traits, no disease assessment. The paper reports outcomes, not causes.

The economics are absent. Manure at 10 t/ha is a substantial material and labour cost. The abstract reports no cost comparison, so the claim that this is a saving is not established by the data presented.

And the 0 percent NPK treatment, the one that would tell you how far the substitution can go, is not reported in the abstract at all. Until that number is visible, the honest summary is this: on one acid tropical soil, in one season, cutting inorganic NPK by half while adding manure did not cost yield and appeared to improve storage. That is worth testing. It is not yet worth planning a season around.