Ammonium Sulfate Fertilizer (21 0 0): Uses on Rice and How It Differs from Urea

Ammosul comes as small white or beige crystals that dissolve fast in water. Its price is tracked every week, right beside urea and complete.

It is a nitrogen fertilizer first. But the sulfur it carries is the reason to pick it over urea on some farms, so it pays to know when that sulfur matters.

What is in 21 0 0 fertilizer

The grade 21 0 0 means 21 percent nitrogen and no phosphate or potash. Many brands on the FPA list of registered fertilizers also declare 24 percent sulfur. So a 50 kg bag of fertilizer 21 0 0 holds 10.5 kg of nitrogen and 12 kg of sulfur.

All of its nitrogen comes in the ammonium form. That matters in a flooded paddy, as you will see below.

Why the sulfur matters

Sulfur helps rice grow and mature evenly. It also speeds up chlorophyll and protein making. A crop short of sulfur shows yellow young leaves, fewer tillers, fewer and shorter panicles, and late maturity. Since the young leaves yellow first, sulfur deficiency looks different from nitrogen deficiency, which starts on the older leaves.

How ammonium sulfate fertilizer differs from urea

Urea carries more than twice the nitrogen per bag, so it usually costs less per kilo of nitrogen. Mosaic, a fertilizer maker, notes that more concentrated sources often make the better choice when a field needs nitrogen alone. Ammonium sulfate earns its place when the soil also lacks sulfur.

The two also lose nitrogen differently. Urea fertilizer first has to turn into ammonium in the soil, and that step can release ammonia gas. Ammonium sulfate skips that step. Even so, its ammonium can still escape as gas if it sits on the surface of an alkaline soil. So work it into the soil, or apply it just before irrigation or rain, as you would with urea.

The acidifying effect of ammonium sulfate

In warm soil, microbes turn ammonium into nitrate. That process releases acid, and after repeated use it lowers the soil pH. Mosaic explains that the acid comes from this change, not from the sulfate. It also says ammonium sulfate makes more acid than the same nitrogen from ammonium nitrate, because all of its nitrogen goes through that change.

On soils that are already acidic, keep an eye on pH. For corn, pH 5.4 is the critical level, so lime below it. A soil analysis stays valid for up to three years. A soil test every few seasons tells you when to lime before the pH drops too far.

How to use ammonium sulfate fertilizer on rice

Because its nitrogen is already ammonium, rice farmers often apply it to flooded fields, Mosaic notes. Nitrate fertilizers lose nitrogen to the air in waterlogged soil, so they suit paddies poorly. Ammonium sulfate can fill any of the usual nitrogen splits: early, at mid tillering, or near panicle initiation.

It makes the most sense early. Sulfur goes on at 0 to 14 days after transplanting (DAT), or 7 to 10 days after sowing (DAS). So on soils short of sulfur, ammosul in the first dose covers both nutrients at once. The full timing is on our fertilizer for rice page.

Swapping urea for ammosul

Say your plan calls for 1 bag of urea at active tillering. That is 23 kg of nitrogen. To get the same from 21 0 0 fertilizer, divide 23 by 10.5 to get about 2.19 bags, or 2.2 in round numbers. At the September 2026 averages, 2.19 bags cost about PHP 2,658 against PHP 2,320.69 for the urea. For the extra PHP 338 or so, you also get about 26 kg of sulfur. Our fertilizer calculation guide shows the same math for other grades.

What about ammonia nitrate fertilizer?

Many people search for 'ammonia nitrate fertilizer', but the product is ammonium nitrate. It carries about 34 percent nitrogen, half as nitrate and half as ammonium. It is a common fertilizer in some countries, but not here.

In 2002 the FPA issued Memorandum Circular No. 3, series of 2002. It cancelled the registration of solid ammonium nitrate as a fertilizer and banned its importation for farm use. Agriculture officials gave national security as the reason and pointed to bombings in Bali and in Zamboanga del Norte, according to a Philstar report at the time. Before the ban, some farmers used it on mango.

Republic Act 9516 of 2008, the law on explosives, also covers nitrates and other chemicals that can be used to make explosives, and importing them takes a PNP license. The 2016 rules on controlled chemicals list ammonium nitrate as high risk, so making, trading or buying it needs a PNP license too. For mango growers, officials named potassium nitrate and calcium nitrate as replacements. Our page on the Fertilizer and Pesticide Authority explains how to check a registration.

Keep your nitrogen plan straight in anee.io

Mixing urea and ammosul in one season makes the math easy to lose. In anee.io, the farm inventory keeps each fertilizer as its own item in 50 kg bags, with its own price. The cropping calendar dates each dose from the lot's own day zero, so you see which bag goes on which day.

Ask Anee, the smart farm technician, to convert a urea plan into ammosul for your lot. She answers in Tagalog or English and shows the bags and the cost.

Questions about ammonium sulfate fertilizer