Urea Fertilizer vs. Wood Ash: 7 Differences

Both urea fertilizer and wood ash are known to provide essential nutrients and growth benefits for gardens and lawns alike. When comparing the two, you’ll find some differences that create obvious advantages for each one, depending on the plant type. 

Wood ash is an excellent fertilizer for alkalizing soil. It is an excellent source of potassium but can only be applied by sprinkling the ash on the soil. This organic fertilizer can work even when dry or with little moisture. In addition, you need significantly less ash than urea in your garden.

In contrast, urea fertilizer does a great job of increasing acidity. Urea fertilizer is a synthetic or organic material that offers a high source of nitrogen and can be applied in solid, liquid, or spray forms. However, the solid form requires adequate moisture to work.

While both fertilizers are great resources for a healthy garden, using them may be detrimental to your plant’s growth if used on the wrong plant type. Read on for more differences between urea fertilizer and wood ash, so you can determine which fertilizer would benefit your garden.

1. They Contain Different Primary Nutrients

Most fertilizers are made up of three primary nutrients: nitrogen (N), phosphorus (P), and potassium (K). While many all-purpose fertilizers offer a healthy balance of all three nutrients, both urea fertilizers and wood ash provide a different approach—focusing on one key nutrient.

Urea fertilizer has an extremely high nitrogen (N) component, which is great for green, leafy growth, cell regeneration, and creating protein and amino acids.

Wood ash, on the other hand, has a high potassium (K) content, which is essential for moving water and other nutrients throughout the plant system to provide balanced, healthy growth. Potassium is key to successfully moving through the process of photosynthesis and can cause stunted growth if deficient. 

Additionally, wood ash is a good source of lime which helps balance the pH level of your soil, a necessary task for healthy plant growth. 

2. They Have Opposite Effects on Soil pH

Different types of plants have varying requirements for soil pH levels and nutrient content. Some require a high level of acidity, while others require soil that is more alkaline. Specific fertilizers like urea and wood ash are more appropriate for each of these needs.

Urea fertilizer increases the acidity of the soil. Because of its high nitrogen content, it quickly supplies ammonia to the soil, causing an acidic reaction. 

Some examples of plants that often prefer an acidic environment are:

  • Strawberries
  • Azaleas
  • Rhododendrons
  • Holly plants
  • Magnolia trees
  • Radishes
  • Sweet potatoes
  • Tomatoes
  • Cucumbers
  • Thyme
  • Oregano
  • Sage

In contrast, wood ash decreases the acidity of the soil, alkalizing it and increasing the pH level. Its ability to increase the soil’s pH level stems from its valuable calcium content.

Some examples of plants that prefer an alkaline environment are:

  • Broccoli
  • Garlic
  • Spinach
  • Lettuce
  • Artichoke
  • Arugula
  • Olive
  • Pomegranate
  • Aloe vera
  • Lavender

Determining your plant type and following up with a soil test will provide you with the context you need to know which kind of fertilizer and how much will work well in your garden. 

3. They’re Distributed Differently

One of the advantages of urea fertilizer is that it can be distributed in a variety of ways.

Urea fertilizer can be applied in the following ways:

  • Solid: Sprinkled directly on or into the soil, the solid form of urea fertilizer mixes with the soil as it’s watered. 
  • Liquid: By mixing the solid form with water, this solution can be applied by pouring directly onto the soil with a watering can.
  • Spray: Some plants do well with foliar spray. The plant’s foliage can be sprayed directly to soak up this nitrogen-rich nutrient by pouring the liquid solution into a spray bottle.

In comparison, wood ash is distributed by sprinkling the ash around the base of the plants and applying a thin layer over the top of the soil. No liquid or spray option is available for effective wood ash distribution, making urea fertilizer a bit more versatile in this regard.

4. Wood Ash Is Organic, Urea Fertilizer Is Usually Synthetic

It’s no secret that fertilizing with organic options is an excellent tool for maintaining an environmentally-friendly garden. Still, not all organic options are as affordable or effective as synthetic fertilizers.

Fortunately, wood ash is a great, low-cost way to fertilize without harmful added chemicals. In fact, it’s likely you already have some wood ash that you can use at no additional cost. Next time you clean out your fireplace, instead of throwing the ash away, hang on to it for a cost-effective, beneficial garden fertilizer.

The type of wood you burn in your fireplace is also important, as different types of wood offer varying benefits as fertilizer. I’ll discuss this more later in the article.

Urea fertilizer is most commonly a synthetic compound made up of inorganic materials and formulated into granules or prills. Less commonly, urea fertilizer can be made organically by using human or animal urine as fertilizer directly or combined with compost. 

When making organic urea fertilizer, it’s important to be sure that it doesn’t spill or leak into water systems, as it can be toxic and contaminating in some cases. The most effective and safe way to create organic urea fertilizer is to combine it with your compost heap and allow it to become part of the composting process, using the compost as one fertilizer when complete.

5. They Need Different Moisture Levels to Work

When wood ash is applied, it should be either completely dry or composted along with your compost heap. It immediately begins its work alkalizing and providing potassium to the soil with little moisture. Dry wood ash can also work to repel pests like slugs and snails.

Once it rains, the nutrients can be washed away, and new ash must be applied. Additionally, when wood ash gets wet, lye and salt are released, which can sometimes be harmful to plants. Because of this, it’s important to use the appropriate amount of wood ash so as not to create a toxic environment for your garden once it inevitably gets wet.

In contrast, urea fertilizer must be wet in order to fertilize the soil. Once urea fertilizer is dispersed, it begins to break down, and the ammonia will become unavailable to the soil. It’s essential to time urea fertilizer application with sufficient rainfall or be prepared to adequately water the garden yourself. 

The combination of fertilizer and water allows an essential chemical reaction to occur, releasing the nitrogen into the soil. 

6. They Have Varying Concentrations of Lime

Wood ash naturally contains a healthy source of lime. Lime is usually made of ground limestone and can be found naturally occurring in wood, particularly oak. Lime is beneficial to soil due to its ability to improve water penetration, allowing the plants to more easily receive the nutrients needed to thrive. 

Some synthetic urea fertilizers can be purchased with lime additives, providing both nitrogen and lime benefits to the soil. Still, both synthetic and organic urea fertilizers on their own don’t typically contain beneficial levels of lime. 

7. You Need Less Ash Than Urea to Fertilize a Garden

When it comes to applying fertilizer, wood ash and urea fertilizer require vastly different amounts to provide the essential nutrients they contain. 

The National Gardening Association suggests 2 pounds (907 g) of nitrogen per 1,000 square feet (92.9 sqm), while potassium should be applied at one-tenth of that amount

Because healthy soil requires so much less potassium than it does nitrogen, it’s necessary to apply more urea fertilizer (nitrogen-boosting) than wood ash (potassium-boosting). 

You should also consider the amount of nitrogen in the particular form of urea fertilizer that you choose. While all are notably high in nitrogen, some are lower than others, and this should be taken into consideration when determining the amount of fertilizer to apply.

Because wood ash is organic, it’s impossible to know the exact content of potassium in the substance. However, you can make an educated guess based on the type of wood that was burned. 

For example, wood ash that’s derived from oak usually has the highest potassium and lime content. The wood you burn should always be dry, untreated wood without any additional foreign materials. 

In order to maintain the integrity of your wood ash, don’t allow any impure materials to be burned in when making wood ash for your garden, including: 

  • Paper plates
  • Cardboard
  • Newspaper kindling
  • Other non-organic materials

These types of materials can interfere with the fertilizer and create more harm than good with potentially toxic additives.  


Urea fertilizer and wood ash are both excellent fertilizers for your garden, depending on the type of plants you’re growing.

Urea fertilizer is great for increasing the acidity and lowering the pH of your soil, allowing acid-loving plants like strawberries, azaleas, and rhododendrons to thrive. Its high nitrogen content supports green, leafy growth- especially in the grass of your lawn. 

Wood ash is terrific for providing a costless, organic fertilizer for your garden. Its high levels of potassium and lime increase the pH of the soil by alkalizing it and creating an ideal environment for garlic, spinach, lettuce, and more!

Alexander Picot

Alexander Picot is the founder of TheGrowingLeaf.com and its lead content writer. He created the website in 2022 as a resource for horticulture lovers and beginners alike, compiling all the gardening tips he discovered over the years. Alex has a passion for caring for plants, turning backyards into feel-good places, and sharing his knowledge with the rest of the world.

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