Can Diesel Exhaust Fluid Be Used As Fertilizer?

Diesel Exhaust Fluid or DEF is mostly urea, the same ingredient in many nitrogen fertilizers, but DEF should not be applied as fertilizer as it is not tested for use on crops. The product is very pure and generally only contains two ingredients; urea and water. 

DEF is designed to be used in diesel engines to reduce the amount of harmful emissions produced by the truck. 

DEF is not regulated for use in the garden although it is non toxic and the composition is tightly defined as 32.5% urea and 67.5% deionized water. 

The other reason to choose agricultural urea in your garden is because it is much cheaper than DEF. 

Urea is one of the cheapest fertilizers available to gardeners and it is abundant in home and garden stores. You can purchase 10 pounds on Amazon for less than $30. This is more than most gardeners will need in a season. 

When you purchase DEF you are paying for the Urea and the water. 

Granular urea is cheap and keeps indefinitely when stored in a dry, sealed container. 

Is DEF Cheaper Than Fertilizer?

Some gardeners argue that DEF is cheaper than other fertilizer because it can be purchased for less than $10 a gallon. This is misleading as it is comparing DEF to prepared liquid fertilizers like Miracle Grow which provide much more than nitrogen. 

Liquid fertilizers are generally used when you are looking for a temporary boost of nitrogen or you are treating a specific area that has a nitrogen deficiency. A liquid fertilizer is a fast acting solution that must be applied regularly. 

Nitrogen fertilizers are very inexpensive at less than $3 per pound at the time of writing. They can be applied as a granular or liquid fertilizer depending on your desired result. 

Many gardeners use nitrogen fertilizer on lawns to “green-up” the grass at the beginning or end of a season or when there has been little rain. 

A liquid fertilizer will “green-up” a lawn more quickly than a granular fertilizer because it will release all the nitrogen to the grass at once. The fertilizer will need to be applied roughly every two weeks during the growing season to maintain the nitrogen levels. 

Granular fertilizer breaks down over the course of several waterings so it will not need to be applied as often as a liquid fertilizer. 

Over application of nitrogen can result in fast lawn growth and require more frequent mowing. 

Creating Liquid Fertilizer with Urea

If you are looking for a liquid fertilizer, you can dissolve urea in water at home to apply the fertilizer more quickly. 

You can do this by adding the granules to warm water and stirring it well over the course of a few hours. 

For a pure nitrogen fertilizer, be sure you are using pure urea without any added minerals or other compounds. This should dissolve completely in water. 

Be careful as the mixture may become cold as the urea reacts with the water. Do not mix this with your hands, use gloves and a stick to mix. 

The general rule is to add about 1 to 3 tablespoons per gallon of water, but you can use the amount that has been recommended by your soil test. 

Too much nitrogen can cause nitrogen burns on your grass. 

Why Use DEF as Fertilizer?

DEF is sometimes used as a fertilizer because it contains only two ingredients: water and urea. 

Urea is a common fertilizer that is the preferred compound for adding nitrogen to soil for grass and crops. 

If you have ever used a commercial fertilizer it likely contained urea. Products like weed and feed, miracle grow and turf builder all contain urea. 

Urea is also loved by gardeners as it keeps grass green through the growing season and as far into the winter as possible. Urea is used heavily in golf course maintenance and manicured lawns. 

Nitrogen needs to be applied regularly to maintain green lush lawns. 

It is a cheap form of nitrogen that is used heavily in agriculture. 80% of the world’s urea is used in agriculture. 

It is commonly believed to be produced from livestock urine, but it is actually chemically derived from ammonia. It was an important compound in the history of chemistry as it was first discovered in the urine in mammals and then was created in a lab. 

Other Sources of Nitrogen

If you are looking to add nitrogen to your garden you can use commercial fertilizers like urea or adding compost, blood meal, animal manure, or planting nitrogen fixing crops like alfalfa that will add nitrogen to your soil over the growing season. 

Nitrogen is a difficult nutrient to keep in the soil because it is highly mobile in the soil meaning it runs out with water and is used quickly by the plants that grow in the soil. 

Before you assume that the problem is a lack of nitrogen you should consider a soil test to get an idea of what your soil needs. 

Using organic soil amendments in addition to commercial fertilizers that can build the soil health year after year and require less amendment each growing season. 

Using Diesel Exhaust Fluid As Fertilizer

While it is unlikely to be harmful to your plants or the environment, DEF is not a cost effective fertilizer. DEF is a combination of urea and water in a fixed percentage. 

If you are looking to “green-up” your lawn quickly with a liquid fertilizer consider buying granular urea and mixing it with water at home. 

If you choose to use DEF as a fertilizer be sure to do the math on how much nitrogen you are applying to your plants. Too much nitrogen can burn your plants and cause yellow spots in your lawn. 

It is important to note that DEF is currently in high demand and the supply is limited, so the price of DEF is likely to keep going up, and urea is very shelf stable and can be used year after year.