Nitrogen deficiency in soil is one of the most critical aspects of plant growth that a gardener can control; watering being the other one. Of all the nutrients, nitrogen is the most difficult to manage.
In this post I will discuss everything you need to know so that you can prevent nitrogen deficiency in your soil.
Why is Nitrogen Important?
Taken from my book: Soil Science for Gardeners.
Nitrogen is the most important nutrient for you to be concerned about as a gardener. Next to carbon, hydrogen, and oxygen, nitrogen is the most abundant nutrient in plants and it is the nutrient that is most likely to be lacking in soil, especially in cooler climates.
Nitrogen is used as a building block for all kinds of large molecules. Enzymes, which are a special type of protein, contain quite a bit of nitrogen. Since enzymes cause chemical reactions, they control just about everything that happens in a plant, from photosynthesis to growing. Nitrogen is also part of DNA, RNA and chlorophyll—all vital for plant growth.
Commercial fertilizer can contain a variety of different chemical forms of nitrogen including ammonium (NH4), nitrite (NO2), nitrate (NO3), and urea (CH4N2O), but plants are only able to use nitrate and ammonium directly.
Why would fertilizer contain nitrite and urea if plants can’t use them? The answer involves an important concept in gardening. Microorganisms in the soil are able to convert one form into another and this is happening all of the time.
Ammonium, nitrite, nitrate, and urea are all very soluble in water and once dissolved, they move along with the water. Rain and irrigation easily washes nitrogen out of the soil and into rivers and lakes, causing pollution.
What does all this mean for the gardener? Nitrogen is a critical nutrient for your plants but it is very unstable in soil. It is easily and quickly converted from one form to another and it is easily washed away. Your plants can have lots of nitrogen available in the morning and after a good rain have a shortage. As a gardener you never know how much nitrogen is available to your plants because the amounts change so quickly. Nitrogen is the nutrient that is most likely to be in short supply for your plants.
Although nitrogen is important, too much is not good either, because it creates soft, weak growth which is more prone to disease and pests, slows ripening of crops, delays hardening off for winter, and can affect flavor in vegetables. Too much also encourages growth of roots, stems and leaves instead of flowers and fruit.
Soil Test for Nitrogen
Nitrogen is very important, so why not test for it?
Most routine soil tests do not include a test for nitrogen. The reason is that the nitrogen in soil changes too fast. By the time you get your results, the nitrogen in soil has changed. You can get soil tested for nitrogen, but you need to freeze the soil sample, and the test is more expensive.
But ….. they have seen DIY soil test kits in nurseries and garden centers that will test for nitrogen. That is true, but the results are almost useless. They are not very accurate, and they don’t provide an actual numerical result. They only provide a range, something like high, medium and low. If you get a low reading, you don’t really know how low it is, so you don’t know how much nitrogen you need to add to bring it up to a medium level.
The other issue with these kits is that they normally measure nitrate – they don’t measure all of the plant available nitrogen in soil.
The main problem with measuring nitrogen is that it changes very fast. Is a gardener really going to test weekly and make adjustments?
Detecting Nitrogen Deficiency
Instead of testing soil, you can watch your plants. Low nitrogen levels will show up in plants as poor growth and yellow leaves. But many other things can also cause poor growth and yellow leaves. Plant growth can be used to verify a known deficiency, and it can be used to limit possible problems, but it can’t be used to identify a deficiency.
Preventing a Nitrogen Deficiency in Soil
A gardener has a few options for managing the nitrogen levels in soil using fertilizers.
Fertilize With Synthetic Fertilizer
The time honored tradition is to apply fertilizer a couple of times a year. Spreading it on lawns in spring certainly increases the nitrogen levels resulting in grass growing greener and faster. Adding it to vegetable crops in spring and mid summer will help them grow.
Your landscape plants don’t need to be fertilized unless you know of a specific deficiency.
Containers tend to be watered a lot and need regular fertilizer, and synthetic works best for them.
By the way, synthetic fertilizer used correctly does not kill soil microbes – it actually helps them grow.
Fertilize With Organic Fertilizer
Add a layer of compost or manure as a mulch. This will slowly release nitrogen over several years and provide enough for most plants.
It might not be enough for vegetables or containers unless you use a lot more than just an annual mulch.
Best Way to Prevent Nitrogen Deficiency
The best way to prevent nitrogen deficiency is to ignore the nitrogen. Instead, focus on building better soil.
As a gardener you are very limited in what you can do directly to manage nitrogen levels. The good news is that you don’t have to, if you let nature do it for you.
Think about this for a minute – nobody worries about nitrogen levels in a forest or on grasslands. Nobody fertilizes. It all just grows on its own and your garden can work the same way.
In healthy soil, the combination of microbes and organic matter, provides a slow but steady stream of nitrogen for plants. If you increase the organic level in soil, the microbes will come, and take care of things for you. How do you do this? You focus on creating healthy soil. As your soil improves, it will contain more organic matter and more microbes, which in turn provides more nitrogen.
Rules For Creating Healthy Soil
- Disturb the soil as little as possible – don’t dig or rototil.
- Don’t compact the soil – stop walking on it.
- Keep the soil covered – use organic mulch or growing plants (cover crops).
- Add organic matter – use organic mulch, compost, manure and leave plant residue on the ground.
- Keep growing things – plant roots add organic matter and help aerate the soil.
Creating healthy soil takes time, but if you follow the above rules it will get better every year. As the soil health improves, so will the nitrogen level.
Containers and vegetable gardens are a bit different. Containers need regular fertilizer because the frequent watering washes the nutrients out of the pot. When you grow vegetables you want a good yield now, not in 4 or 5 years. In a new garden, fertilizer is a good option. As soil health improves you will need less and less each year.
Creating healthy soil is explored in much more detail in my book called Soil Science for Gardeners. It even has a section for measuring the current health of your soil and another one to help you develop a personalized Soil Health Improvement Plan.
Stay Away From Gimmicks
The gardening world is ready to sell you all manor of useless or nearly useless products. Stay away from them.
Lots of new gimmicky additives are available for your soil; mycorrhizal fungi, humic acids, vitamins, compost tea, and molasses, The science does not support the use of these products. They may add some small benefits, or they may do absolutely nothing, but they are not needed to have healthy soil.