Urea dominates global nitrogen fertilizer use because it packs more nitrogen per kilogram than almost any other solid option and is cheap to manufacture. But its drawbacks, especially ammonia gas losses and inconsistent performance in wet or alkaline soils, have pushed farmers and researchers toward a broad menu of alternatives. These range from straightforward product swaps like calcium ammonium nitrate and ammonium sulfate to biological strategies like cover crops and microbial inoculants, and even emerging options such as fertilizers recovered from human urine. The best choice depends on the crop, climate, soil chemistry, and whether the goal is cutting costs, reducing emissions, or both.
Calcium Ammonium Nitrate and Other Nitrate Fertilizers
Calcium ammonium nitrate, usually called CAN, is probably the most direct swap for urea in temperate climates. It supplies nitrogen in both ammonium and nitrate forms, meaning part of its nitrogen is immediately available to plant roots without the soil conversion step urea requires. In grassland trials comparing the two, urea and CAN produced similar annual yields with no significant difference, though urea had a slight edge in spring and CAN performed marginally better in summer. Urea also came out cheaper per tonne of grass produced.1Soil Use and Management. Can the agronomic performance of urea equal calcium ammonium nitrate across nitrogen rates in temperate grassland? In a silage-crop study, ammonium nitrate boosted yields significantly more than urea, with a nitrogen use efficiency of about 36% compared to roughly 21% for untreated urea.2Biogeosciences. Nitrogen use efficiency and N2O and NH3 losses attributed to three fertiliser types applied to an intensively managed silage crop
The environmental picture flips, though. CAN tends to produce considerably more nitrous oxide, a potent greenhouse gas, than urea under the same conditions. Across multiple UK and Irish grassland sites, the direct nitrous oxide emission factor for CAN averaged about 1.5% of applied nitrogen, while urea averaged around 0.25%, a roughly sixfold difference.3PubMed. Reducing nitrous oxide emissions by changing N fertiliser use from calcium ammonium nitrate (CAN) to urea based formulations This tradeoff, higher efficiency but higher greenhouse-gas emissions, is one reason regulators in some countries have started nudging farmers away from CAN rather than toward it.
Ammonium Sulfate
Ammonium sulfate is a two-for-one product: it delivers both nitrogen and sulfur, making it attractive on sulfur-deficient soils. It accounts for roughly half of all sulfur fertilizer used worldwide.4PubMed Central. Use of ammonium sulphate as a sulphur fertilizer: Implications for ammonia volatilization One of its widely cited advantages is that it acidifies the soil more than urea or ammonium nitrate, which sounds like a downside but can actually free up phosphorus and micronutrients that are locked in alkaline soil.5Soil Science. Comparison of Ammonium Sulfate With Other Nitrogen and Sulfur Fertilizers in Increasing Crop Production and Minimizing Environmental Impact: A Review
That acidification is a double-edged sword. On soils that are already acidic or low in clay and organic matter, ammonium sulfate can push pH down fast enough to require extra liming. Research measuring how much lime different fertilizers demand found ammonium sulfate needed roughly 1.6 to 2.3 times the lime of ammonium nitrate or urea, depending on the soil.6Soil Science. THE EFFECT OF DIFFERENT AMMONICAL NITROGEN SOURCES ON SOIL ACIDIFICATION Ammonia volatilization is another concern: losses stay under about 5% of applied nitrogen on soils with a pH below 7, but on alkaline soils (pH above 7) losses can range widely and sometimes reach 20 to 40% of applied nitrogen.4PubMed Central. Use of ammonium sulphate as a sulphur fertilizer: Implications for ammonia volatilization The cost per unit of nitrogen is also higher than urea’s, so ammonium sulfate makes the most economic sense where sulfur is genuinely needed.
Anhydrous Ammonia
Anhydrous ammonia is a gas injected directly into soil, and it remains one of the cheapest nitrogen sources per unit of actual nitrogen delivered. It is especially popular in large-scale corn production across the U.S. Midwest. The catch is logistics: it requires pressurized tanks, specialized equipment, and trained applicators. Timing matters too. Research across multiple site-years in Illinois found that spring-applied anhydrous ammonia required about 26 fewer kilograms of nitrogen per hectare than fall-applied ammonia to achieve similar yields, because fall-applied nitrogen has months to leach or convert to gas before the crop can use it. Spring application also returned about $150 more per hectare in net profit.7University of Illinois Urbana-Champaign. Effects of Anhydrous ammonia application timing on corn yield and profitability For farmers already set up for it, anhydrous ammonia is a genuinely competitive alternative to urea, particularly where soil conditions allow injection.
Enhanced Efficiency Fertilizers
Rather than replacing urea entirely, a growing market of products modifies it. Controlled-release urea wraps standard urea granules in coatings, often sulfur, polymer, or a combination, that slow the rate at which nitrogen dissolves into the soil. The idea is to match nitrogen release to crop uptake, reducing waste. A large meta-analysis focused on maize found that sulfur-coated urea cut nitrous oxide emissions by about 22%, while sulfur-plus-resin coatings increased yield by roughly 11% and polymer coatings boosted nitrogen use efficiency by about 35%.8Industrial Crops and Products. Controlled-release urea derived from various coating materials on the impacts of maize production: A meta-analysis
Biochar-based controlled-release fertilizers are another variant gaining attention. These combine biochar, a charcoal-like material made from organic waste, with nitrogen. Economic analysis suggests break-even prices around $1 per kilogram for large-scale production, which gets competitive with conventional options, though feedstock costs dominate the economics and make up about 50 to 60% of total production cost.9Waste and Biomass Valorization. Economic Assessment of Biochar-Based Controlled-Release Nitrogen Fertilizer Production at Different Industrial Scales
Chemical Inhibitors Added to Urea
A closely related strategy is to add chemical inhibitors to standard urea rather than coating it. Two classes dominate: urease inhibitors, which slow the enzyme that converts urea to ammonia in the soil, and nitrification inhibitors, which slow the conversion of ammonium to nitrate. The most common urease inhibitor is NBPT; the best-known nitrification inhibitor is DMPP. A meta-analysis across many crops and regions found that using these inhibitors increased crop yields by about 7.5% and nitrogen use efficiency by about 13% on average.10Agriculture, Ecosystems & Environment. Meta-analysis of the effect of urease and nitrification inhibitors on crop productivity and nitrogen use efficiency
The two inhibitor types work differently and have different emission profiles. In a maize field study, adding a urease inhibitor to liquid fertilizer cut ammonia emissions by about 13% but actually increased nitrous oxide by the same amount. Adding a nitrification inhibitor, by contrast, slashed nitrous oxide by about 31% while boosting yield by about 21%. Combining both together reduced total nitrogen losses while improving yield and efficiency.11PubMed Central. The Combined Use of Liquid Fertilizer and Urease/Nitrification Inhibitors on Maize Yield, Nitrogen Loss and Utilization in the Mollisol Region On wet, temperate grassland, urea with both NBPT and a nitrification inhibitor brought nitrous oxide emissions down to essentially background levels.3PubMed. Reducing nitrous oxide emissions by changing N fertiliser use from calcium ammonium nitrate (CAN) to urea based formulations These products occupy an interesting middle ground: they keep urea’s cost advantage while addressing its biggest environmental weaknesses.
Organic Amendments and Animal Manures
Manures and composts are the oldest nitrogen sources in agriculture and remain relevant, especially in integrated systems that combine organic and synthetic inputs. Poultry manure, for instance, raises soil pH, organic matter, and multiple nutrient levels as it decomposes. In a maize trial, combining split urea applications with poultry manure produced higher yields than either input alone.12PubMed Central. Effects of Different Rates of Poultry Manure and Split Applications of Urea Fertilizer on Soil Chemical Properties, Growth, and Yield of Maize Vermicompost, produced by earthworm digestion of organic waste, similarly contributes nutrients and improves soil structure.13Journal of Soil Science and Plant Nutrition. Recycling Manure as Vermicompost: Assessing Phosphorus Fertilizer Efficiency and Effects on Soil Health Under Different Soil Management
Organic amendments are not a like-for-like urea replacement. Their nitrogen content varies batch to batch, release timing is hard to predict, and the sheer volume needed to match synthetic rates makes transport expensive. They can also produce higher nitrous oxide emissions than synthetic sources: poultry litter in one multi-year study lost about 4.5% of its nitrogen as nitrous oxide, far higher than any inorganic fertilizer tested.14PubMed. Atmospheric emissions of nitrous oxide, methane, and carbon dioxide from different nitrogen fertilizers For these reasons, organic amendments work best as a partial substitute blended with synthetic nitrogen, not as a standalone replacement on most commercial farms.
Biological Nitrogen Fixation and Green Manures
Certain bacteria can pull nitrogen directly from the air and convert it into forms plants use. The best-known example is the partnership between legumes (beans, clover, alfalfa) and rhizobia bacteria that form nodules on their roots. Growing a legume as a cover crop or green manure and then tilling it in before the next cash crop returns that fixed nitrogen to the soil. A rotation trial with maize and oilseed rape found that incorporating a green manure allowed farmers to cut synthetic nitrogen application by 25 to 30% during the following crop without any significant yield loss, while roughly doubling the agronomic efficiency of whatever synthetic nitrogen was still applied.15PubMed Central. Green Manure Amendment Can Reduce Nitrogen Fertilizer Application Rates for Oilseed Rape in Maize-Oilseed Rape Rotation
Free-living nitrogen-fixing bacteria like Azotobacter species do not need a legume host. They live in the soil and fix atmospheric nitrogen on their own, and they can also promote plant growth through other mechanisms like producing plant hormones.16PubMed Central. Nitrogen Fixing Azotobacter Species as Potential Soil Biological Enhancers for Crop Nutrition and Yield Stability Commercial Azotobacter-based inoculants exist and are used particularly in South and Southeast Asia, though the amount of nitrogen they fix in the field tends to be modest compared to synthetic inputs.17PubMed Central. Azotobacter: A potential bio-fertilizer for soil and plant health management The practical ceiling of biological fixation is a supplement rather than a full replacement for synthetic nitrogen in high-yield grain systems.
Insect Frass and Novel Organic Products
One alternative that sits between traditional organics and synthetics is black soldier fly frass, the excrement left over from using fly larvae to process food waste. It is a dry, granular material with a more predictable nutrient content than most animal manures. In maize field trials, frass applied at the same nitrogen rate as urea produced about 7% higher grain yields and boosted nitrogen uptake by roughly 29%. Its agronomic nitrogen use efficiency at low application rates was more than double that of a standard commercial organic fertilizer.18Frontiers in Plant Science. Exploring Black Soldier Fly Frass as Novel Fertilizer for Improved Growth, Yield, and Nitrogen Use Efficiency of Maize Under Field Conditions The waste-processing industry already produces the larvae for animal feed; the frass is essentially a byproduct looking for a market, which keeps costs down. Scale is still limited, but this is one of the more promising next-generation organic alternatives.
Nutrient Recycling From Human Urine
An idea that sounds strange but has solid science behind it: recovering nitrogen and phosphorus from human urine and using them as fertilizer. Urine contains most of the nitrogen and phosphorus that humans excrete, and separating it at the toilet before it reaches wastewater treatment plants makes recovery far more practical. One approach is to precipitate struvite, a mineral containing nitrogen, phosphorus, and magnesium, which can recover about 90% of urine’s phosphorus.19Periodica Polytechnica: Chemical Engineering. The utilization of struvite produced from human urine in agriculture as a natural fertilizer: a review Another is nitrified urine fertilizer, where biological processes convert urine’s ammonia into stable nitrate. In greenhouse trials with ryegrass, both struvite and nitrified urine fertilizer performed as well as conventional mineral fertilizers, with crop nitrogen recovery reaching 72 to 75%.20PubMed Central. Plant uptake of phosphorus and nitrogen recycled from synthetic source-separated urine Lettuce trials have confirmed that a potassium-struvite precipitate from urine produced growth comparable to standard NPK fertilizer.21Frontiers in Sustainable Food Systems. Exploring the potential of human urine derivatives in circular agriculture: a case study on lettuce The barrier is infrastructure: source-separated toilets and processing facilities are still rare outside pilot programs in parts of Europe and sub-Saharan Africa.
Foliar Feeding as a Delivery Strategy
Sometimes the alternative to urea is not a different product but a different delivery method. Foliar fertilization, spraying nitrogen solution directly onto leaves, bypasses soil losses entirely. A two-year wheat trial found that foliar spraying with urea-ammonium nitrate solution at just 96 kilograms of nitrogen per hectare achieved grain yields nearly identical to conventional soil fertilization at 160 kilograms per hectare, a 40% reduction in total nitrogen input with no meaningful yield penalty.22Journal of Agriculture and Food Research. Application of the full nitrogen dose at decreasing rates by foliar spraying versus conventional soil fertilization in common wheat Foliar feeding requires more frequent passes through the field and specialized sprayer settings, so it adds labor. But for high-value crops or fields with high nitrogen-loss risk, it can slash input costs while maintaining output.
Green Ammonia and the Future of Nitrogen Production
Most discussion of urea alternatives focuses on what goes on the field, but a parallel revolution is underway at the factory. Conventional ammonia production, the first step in making urea, relies on natural gas and produces significant carbon dioxide. Green ammonia replaces natural gas with hydrogen generated by renewable electricity, eliminating those emissions at the source.23Communications Sustainability. Green ammonia presents an opportunity to advance energy and food system sustainability in India The finished product is chemically identical to conventional ammonia and can be turned into the same range of fertilizers. The constraint is cost: electrolytic and biochemical routes currently run two to three times the price of conventional production and demand vastly more land and water.24PubMed Central. Low-carbon ammonia production is essential for resilient and sustainable agriculture Modeling suggests that even with optimistic technology improvements, electric Haber-Bosch and direct electrocatalysis are unlikely to undercut the median historical price of centralized ammonia production without policy support or supply-chain disruptions that raise fossil-based prices.25Nature Food. Cost-competitive decentralized ammonia fertilizer production can increase food security Green ammonia may reshape the emissions profile of nitrogen fertilizer within a few decades, but it will not change what farmers apply to their soil in a meaningful way: the end product is still ammonia-based nitrogen.
Regulation as a Driver of Adoption
One of the strongest forces pushing farmers toward alternatives is policy. Germany offers a striking case study. Between 2016 and 2022, ammonia emissions from urea fertilizer dropped sharply, accounting for about 83% of the total ammonia reduction from synthetic nitrogen sources over that period. The main policy tools were straightforward: reducing overall urea application and requiring that urea either be incorporated into soil immediately after spreading or be treated with a urease inhibitor.26PubMed. Successful NH(3) abatement policies and regulations in German agriculture The EU’s National Emission Ceilings Directive sets ammonia limits for member states, which effectively forces a shift from untreated urea toward stabilized products, CAN, or organic sources. Similar regulations are emerging in parts of Asia and Latin America where urea has traditionally been the overwhelmingly dominant product.
These regulations matter because farmers tend to stick with what they know unless economics or rules push them otherwise. Urea remains the cheapest nitrogen source per kilogram in most markets, and that price advantage is hard to overcome with agronomic arguments alone. When policy requires inhibitor treatment or immediate incorporation, the cost gap between plain urea and alternatives narrows, making the switch more palatable. The German experience suggests that regulation paired with practical options, rather than blanket bans, is the most effective path.