For those not familiar with chemistry, you can rest assured that nitrogen is a completely natural and very common element in the environment. In fact, about 80% of the air you are breathing right now is made up of nitrogen.
And oddly enough, even with so much gaseous nitrogen around, plants get this important nutrient from the soil.
When the soil gets nitrogen depleted, your plants will suffer. Get to know the signs of nitrogen deficiency so you can take steps to keep your plants healthy if it crops up in your garden.
Why Plants Need Nitrogen
We may consider plants to be low-maintenance with simple needs like sunlight and water, but like all living things, plants need a pretty wide range of nutrients and minerals to grow properly and to thrive.
Nitrogen is necessary for the growth of leaves, stems and branches as well as maintaining a healthy level of chlorophyll.
This is the vital compound that gives all plants their signature green color, and is what the plant uses to convert sunlight (along with CO2 and water) into food. In other words, chlorophyll is behind the process of photosynthesis.
Symptoms of a Nitrogen Deficiency
The biggest symptom of nitrogen deficiency is a lack of vibrant green in the leaves. They’ll start to get pale (though not necessarily wilted), and there will be a reduction in overall growth of the plant.
If you look more closely, it will be the older leaves that start to turn yellow first. Yellowing of the younger leaves will mean your soil is probably lacking in iron or sulphur. Older leaves that yellow but hold on to their green centers would mean you need more magnesium.
Unfortunately, yellowing leaves is hardly a unique symptom and can be caused by other issues besides nitrogen. A lack of sunlight is a common one, and can be the first thing you try to fix before trying to deal with soil chemistry.
If improving the light source doesn’t help, it’s pretty likely a nitrogen issue.
Now that you’ve identified that your plants need more nitrogen, you have to choose a course of action to fix the problem. Basically, the cure for nitrogen deficiency is to improve the soil with a nitrogen-rich fertilizer.
Commercial fertilizer products usually have a set of 3 numbers, like 5-10-5 in their labeling, and you can use this to decide what is best for your specific needs. The numbers represent the ratios of nitrogen, phosphorus, and potassium (or N-P-K).
If you are wondering why the label isn’t N-P-P, it’s because they use the chemical symbol for each element to avoid confusing the two Ps. Look for a formula that has a high first number to bring in the most nitrogen, such as a blend intended for grass or other non-flowering plants (we’ll get to the flowering issue in a moment).
A purchased fertilizer is an easy choice, and probably the most convenient if you are dealing with indoor houseplants. For your outdoor garden, you can try some more natural options including compost or manure.
Not all natural fertilizer options are the same, and not all will give you that nitrogen boost you specifically need. For added nitrogen, go with these materials:
- Blood meal
- Chicken or horse manure
- Cottonseed meal
If you choose to add nitrogen with horse manure, make sure it has been allowed to age at least 3 to 4 months or it will “burn” the plants. Chicken manure is fine even when fresh, though it’s not as easy to find as other types of manure unless you are keeping your own chickens.
Backyard compost is a common choice but isn’t going to necessarily be high in nitrogen, and its nutrient composition can vary greatly depending on what material went into the compost pile to begin with.
Another way to naturally increase the nitrogen in your soil, particularly if you are dealing with large outdoor areas, is to spend a season growing a legume like peas or beans. These plants are known to “fix” nitrogen from the air into their roots, bringing the element back into the soil.
The key is to plant your peas, let them grow for the season, and then dig the plants back into the soil when they die. Don’t just pull them up. You want the nodules of nitrogen to stay in the soil along with the roots.
Having Too Much Nitrogen
As vital as nitrogen is, a plant can suffer from an excess of it as well. If the soil is too high in nitrogen, the plant isn’t necessarily going to be unhealthy, but it will put all its energy into producing new growth and leaves.
That can be just fine, as long as you are not trying to grow plants for flowers or fruit. Excess nitrogen will keep a plant from flowering, even though it seems healthy and thriving otherwise.
To encourage more flower or fruit production, you’ll want fertilizer that has a higher level of phosphorus rather than nitrogen.
Testing for Nitrogen
Whether you are trying to diagnose a nitrogen deficiency, or trying to see if your remedies have worked, you can do a soil test yourself rather than risk having your plants suffer even more. You can buy easy-to-use testing kits from any store that sells gardening supplies.
There are usually 2 types of kits, one that uses a dry paper strip and one that uses a liquid. Either one will have complete instructions on how to use, but the general principle is the same no matter what kit you use.
A dry strip kit will have you take a soil sample and add a certain amount of distilled water to it. Give it a shake and then dip the paper strip into the water.
The reagents in the paper will react with the nitrogen that has leeched into the water and give you a color change indicating the concentration.
With a liquid kit, you add water to your soil sample as well as a number of drops of the reagent. Give it a shake, and you then compare the resulting color of the water with the chart in the kit.
Here is a nice video that demonstrates how to use a liquid test kit for soil.
Comparing colors on a chart isn’t always the most precise, so don’t expect to get an exact numerical response to the amount of nitrogen in your soil. It’s more of a rough idea to determine if you are low or high, or just right.
Some kits can be used for other nutrients too, so you can check on your potassium and phosphorus levels while you are at it.
There really isn’t much you can do to prevent nutrient loss in the soil, as plants will constantly be using these elements as they grow. Adding fertilizer on a regular basis is the best course of action to keep the soil as rich as possible.
For more on helping your plants thrive, see my article on What Plants Need to Survive (and Thrive).
You can also rotate your plants around and give some garden beds a “rest” for some seasons, and perhaps add a few pea plants or clover to naturally improve the soil on the off years.