When Should I Add Nitrogen to My Garden?

The question of when to add nitrogen to a garden is a complex one. Nitrogen levels in soil change frequently, and they are affected by factors like temperature, soil drainage, and the plants the soil is supporting. This makes it hard for any expert to hand down universal advice about exactly when a garden needs nitrogen. Still, it’s possible to help gardeners figure it out for themselves. This bulleted list offers a quick summary of scenarios that indicate you may need to add nitrogen. 

Your garden might need more nitrogen if:

• You have just grown a heavy-feeding crop
• You are about to grow a heavy-feeding crop
• Your soil does not contain much humus
• You have added a carbon-rich material that is easily broken down, such as sawdust
• Plants show signs of nitrogen deficiency
• You have harvested annual vegetables multiple times without adding organic matter
• A lab soil test indicated a need for more nitrogen

A heavy-feeding crop is one that uses a lot of nitrogen, such as broccoli, corn, or large-fruited peppers. 

Let’s look with more detail at how a garden’s particular conditions affect the optimal timing of its fertilization.

When in a Crop’s Life Cycle Do You Need to Add Nitrogen? 

It’s a good idea to look up plant-specific advice, but it’s commonly done early, such as at or before the time of planting. Nitrogen is essential for plants to produce chlorophyll, which in turn allows them to feed themselves and produce robust leaves and stems. 

However, by the same token, it’s important to be sure you really do need nitrogen before adding it, since an overabundance of this element can cause your plants to focus only on producing leaves and stems. This means the plants will place limits on their root growth and the production of flowers and fruit, which is disappointing for a gardener looking to harvest carrots or peppers. Worse, excess nitrogen will run off in rain or irrigation water, causing problems for human health (such as infant methemoglobinemia, “blue baby syndrome”) and water ecosystems (such as the dead zones in the Great Lakes, the Gulf of Mexico, and elsewhere).

When Should You Choose Synthetic or Organic Sources of Nitrogen?

If you are using a soilless medium, or if you are growing in containers and instant plant growth is more important than the long-term health of your soil, synthetic nitrogen fertilizer will give you robust results. However, in any scenario where you want to keep growing plants in living soil over the long term, it’s best to use organic sources of nitrogen that support the health of the whole soil ecosystem. 

In the long term, you can treat soil-building the way nature does, adding layers of plant trimmings on top of your soil to decompose in place. Make sure you add more than just the left-over parts of plants you’ve harvested; you need to bring in extra plant matter to replace the nutrients that ended up on your dinner table. You should also be aware that if you need fast results from a garden without existing stores of humus, you will need to add well-composted materials for the first three years. It can take that long for the nutrients in your trimmings to become available to your living plants.

With synthetic fertilizer, the results occur more quickly but the balance of the carbon and nitrogen cycles is disrupted. Large quantities of ammonium and/or nitrate are added to the soil at once. The plants take up as much of the nitrogen as they can, but much is left over. Some reverts to gas and is wasted. Some washes away to pollute waterways. And the rest is consumed by soil microbes at a rapid pace. 

Unfortunately, since the microbes need carbon in order to use the nitrogen, they also consume the stores of carbon that are the soil’s fertility. At the same time, since the plants no longer need the microbes as a nitrogen source, they stop exuding the carbon sugars that were the foundation of humus production. This process is so devastating that even if the plants grown from the fertilizer are composted in place, they do not contain enough carbon to replace what was lost. The gardener will now need to add more fertilizer next year to match this year’s results, and the cycle of depletion will continue from there. 

How Do Growing Conditions Affect the Timing of Your Nitrogen Application?

The natural process of nitrification (that is, of nitrogen being converted into a form usable to plants) happens in soil that is moist, well-aerated, and warm (above 10 degrees Celsius, or 50 degrees Fahrenheit). Organic fertilizers, therefore, should only be applied once the soil has warmed up for the growing season. You need the soil microbes to be awake in order for them to do the work of converting your nitrogen.

Synthetic fertilizer depends less on temperature but more on water. There are two primary forms of synthetic fertilizer: 

• Granular fertilizer: apply the day before a light rain is forecast so it will be dissolved into the soil but not washed away. If the weather doesn’t cooperate, you can water it in yourself instead.
• Liquid fertilizer: apply exactly what you need as you need it; this form doesn’t need rain to bring it down to the plant roots, but any excess that is not used immediately by your plants will leach away

If your soil is water-logged, applying nitrogen fertilizer in any form is likely not a good idea. Your plants may indeed be experiencing nitrogen deficiency in these conditions, but since the waterlogging prevents their roots from easily taking nitrogen in, any you add has a high risk of being converted to a gas or washing away. You need to address the drainage and aeration of your soil before you can worry about the nitrogen.

Legumes and Nitrogen Fixing