Fertilizers deliver nitrogen in four main chemical forms: nitrate, ammonium, ammonia, and urea. Each behaves differently in soil, moves differently through water, and becomes available to plants on a different timeline. Most fertilizers contain one or more of these forms, and the form you choose affects how much nitrogen your plants actually get versus how much is lost to the environment.
Nitrate: The Form Plants Actually Use
Nitrate is the primary form of nitrogen that plant roots absorb. It dissolves completely in water and moves freely through the soil with moisture, which makes it immediately available to crops but also vulnerable to being washed away. Heavy rain can push nitrate down through the soil profile and into drainage channels or groundwater, a process called leaching. This is the single biggest cause of nitrogen loss in sandy, coarse-textured soils.
Because nitrate travels so easily with water, straight nitrate fertilizers work best when applied close to the time plants need them. Applying nitrate months before planting, especially on lighter soils, is essentially pouring money into the water table.
Ammonium: Stays Put in the Soil
Ammonium carries a positive electrical charge, which means it clings to clay particles and organic matter in the soil rather than washing away with water. Think of it like a magnet sticking to a refrigerator. This makes ammonium far more resistant to leaching than nitrate.
The tradeoff is that ammonium isn’t the finished product. Soil microorganisms gradually convert ammonium into nitrate through a process called nitrification, and that conversion speed depends heavily on temperature. Nitrification ramps up significantly as soil temperatures rise from about 40°F to 95°F. Below 50°F, the process slows dramatically, which is why fall fertilizer applications are timed for cool soils: keeping nitrogen in the ammonium form longer means less of it leaches away over winter.
Soils with more clay tend to convert ammonium to nitrate faster because clay holds moisture that supports the microorganisms doing the work. Sandy soils, with their lower water-holding capacity, slow the process down somewhat but also can’t hold onto the nitrate once it forms.
Anhydrous Ammonia: Highest Nitrogen Content
Anhydrous ammonia is a gas at room temperature, stored under pressure as a liquid, and contains about 82% nitrogen by weight, making it the most concentrated nitrogen fertilizer available. It has to be injected into the soil at a depth of 6 to 8 inches using specialized equipment. Once underground, the ammonia gas reacts with soil moisture and converts to ammonium within hours.
Application requires care. If the injection furrow isn’t sealed properly, or if the soil is dry and cracked, ammonia escapes back to the surface before it can convert. Wing sealers and closing disks on the injection equipment help prevent this. Fall applications should only go on medium to heavy-textured soils (silt loam or heavier) when soil temperatures at 4 inches deep have dropped below 50°F. Sandy soils are off-limits for fall ammonia because the resulting nitrate will leach through before spring planting.
Urea: The Most Common Solid Fertilizer
Urea is the most widely used solid nitrogen fertilizer in the world, containing about 46% nitrogen. It goes through a three-step conversion before plants can use it. First, naturally occurring enzymes in the soil break urea down into ammonia. Then the ammonia reacts with soil water to form ammonium. Finally, soil microbes convert the ammonium to nitrate.
The vulnerability with urea is that first step. When urea is spread on the soil surface without being worked in or watered, the ammonia produced can escape directly into the air. Research from Montana State University found that ammonia losses from surface-applied urea ranged from 1% to 44% across 21 field trials. The variation is enormous because losses depend on temperature, humidity, wind, and how quickly the urea gets incorporated. Even a light rain or shallow tillage within a day or two of application can dramatically cut volatilization losses.
Organic Nitrogen Sources
Organic fertilizers deliver nitrogen in more complex molecular forms that must be broken down by soil biology before plants can access them. Blood meal is one of the richest, with an analysis of roughly 12.5-1.5-0.6 (N-P-K), releasing its nitrogen slowly over two to six weeks. Feather meal contains about 12% nitrogen and breaks down somewhat faster than blood meal.
Manure and compost supply nitrogen along with phosphorus, potassium, and organic matter, but the nitrogen content varies widely depending on the source animal and how long the material has been composted. One important consideration: if you add organic matter that hasn’t fully decomposed, such as fresh straw or unfinished manure, soil microbes will pull nitrogen out of the surrounding soil to fuel the decomposition process. This temporarily leaves less nitrogen for your plants, not more.
Slow-Release and Coated Fertilizers
Coated fertilizers are typically urea granules wrapped in materials designed to meter out nitrogen over weeks or months rather than all at once. The earliest version, developed in the 1960s, used sulfur coatings. Sulfur-coated urea was a step forward, but imperfections in the coating often caused a “burst effect,” releasing too much nitrogen immediately when the coating cracked on contact with water.
Modern coatings use synthetic polymers, often polyurethane-based, that are more resistant to water penetration. The release rate depends on coating thickness and soil temperature, with warmer soils speeding things up. Some newer formulations use layered coatings with a water-attracting inner layer and a water-repelling outer layer, extending release to 60 days or more. These products cost more per unit of nitrogen, but they reduce the number of applications needed and cut nitrogen losses from leaching and volatilization.
How Nitrogen Form Affects Losses
Each form of nitrogen is vulnerable to a different type of loss. Nitrate leaches with water. Urea and ammonia can volatilize into the air. And all forms can ultimately be converted by soil bacteria into nitrous oxide, a potent greenhouse gas, or into nitrogen gas that simply drifts away.
Nitrification inhibitors are additives that slow the conversion of ammonium to nitrate, keeping nitrogen in the less mobile ammonium form for longer. They work by suppressing the soil bacteria responsible for the conversion. When paired with fall-applied anhydrous ammonia, these inhibitors can meaningfully reduce nitrogen losses over winter. Field studies have shown that certain inhibitors reduce nitrous oxide emissions by 16 to 18% and improve nitrogen use efficiency by 7 to 13%, depending on the specific product.
Soil texture plays a major role in which losses dominate. Clay-rich soils hold ammonium well but also support faster nitrification, which can increase nitrate leaching once conversion happens. Sandy soils let both nitrate and water move through quickly, making timing and form selection even more critical. Matching the nitrogen form to your soil type, climate, and planting schedule is the most effective way to ensure that the nitrogen you pay for actually reaches your plants.