Are you dreaming of a beautiful, abundant vegetable garden where your harvest basket overflows with produce? To get your garden off on the right foot, we need to make sure that our soil has enough nitrogen.
Why do plants need nitrogen?
Nitrogen is 1 of the big 3 macronutrients that plants need to grow, the other 2 being phosphorus and potassium.
Plants need nitrogen to create amino acids that are needed to produce proteins and enzymes, and a few other necessary functions. Specifically, they need higher amounts of nitrogen when growing green leaves in the vegetative stage.
But it’s not just plants that need nitrogen. Healthy soil that teems with beneficial microbes need nitrogen too.
Beneficial microbes break down organic matter into nutrients for your plants, aerate the soil, and support and protect your plants against pests and diseases. A healthy garden comes from healthy soil.
Unlike the other macronutrients, nitrogen is not found naturally in soil. It’s actually found in the air.
Rain and lightning bring nitrogen down from the air into the soil. Nitrogen-fixing plants (like legumes and clover) absorb nitrogen and fix it into their roots.
When the plant dies and the roots decompose, the nitrogen is released into the soil where other plants can take advantage of it.
What happens when there’s too much nitrogen in soil?
Too much nitrogen means that the plant can’t properly absorb other nutrients and thus starve and die.
When there’s too much nitrogen in the soil, leaves develop “burnt” tips, where it looks like someone took a flame to the edge of the leaves. At higher levels, leaves turn yellow and pale as they suffer from nutrient deficiency.
The earlier you can catch it, the more likely you are to save the plant. If it’s in the ground, you’ll want to give the area 3-4 frequent shallow waterings to carry the excess nitrogen deeper into the soil.
If it’s a houseplant, give them a few thorough waterings, making sure that water comes out the drainage hole.
The only way you get too much nitrogen in the soil is by adding too much synthetic nitrogen in fertilizers. Switching to organic fertilizers like compost will prevent this from happening, as compost contains lower but sufficient amounts of nitrogen.
How do I know if my plants need more nitrogen? (Nitrogen-Deficiency)
The classic sign of nitrogen deficiency is yellow leaves at the peak of its growing season. When nitrogen is limited, plants will scavenge it from older leaves to feed to new leaves.
Yellow leaves can also be a symptom of too much nitrogen, but if you haven’t fertilized lately (or within a few weeks if you’ve used a slow-release fertilizer), then it’s more likely nitrogen deficiency.
How can I test my soil for nitrogen deficiency?
To test your nitrogen levels, contact your local university extension office. They’ll send you a kit.
The instructions may vary depending on how they test your soil, but in general, you’ll dig up soil samples from various parts of your garden or lawn, mix them together, put them in the provided container, and send them off to the lab. They’ll send you a report.
University extension offices are fantastic resources for gardeners as they keep up with the latest research and you can go to them for specific advice.
You can also buy a soil-testing kit from commercial companies.
You can also catch nitrogen deficiencies by looking at which weeds are popping up in your garden and lawn. Some weeds only grow in soil with low nitrogen content, like clover.
How can I add more nitrogen to my soil?
Compost is the best source for nitrogen and fertilizer, as it contains a wide variety of nutrients that your plants need to grow. Plus, 1 application of compost will last months as the beneficial microbes transform the organic matter into nutrients.
There’s 3 types of compost you can use:
- Green manure: made from plant matter like grass clippings, leaves, garden scraps, and kitchen scraps
- Livestock manure: from the aged manure of livestock like chickens, rabbits, cows, and horses (don’t use human, dog or cat manure)
- Worm castings: made from vermicomposting, where worms digest plant matter and add a bit more microbial action
Some plants have evolved to get their own nitrogen, hosting specialised bacteria that transform nitrogen gas from the air into a solid form in root nodules. The plant then feeds the bacteria sugar and other necessities in a symbiotic relationship.
Plant legumes like peas and at the end of their life cycle, cut back the foliage but leave the roots. As the roots decompose, the nitrogen will be released back into the soil. Plus, you’ll have fresh peas.
Rotate legumes through your vegetable garden before planting heavy feeders like tomatoes or spinach, or plant legume or clover cover crops in the autumn when the growing season is winding down or as soon as the ground dethaws to prepare this year’s garden soil.
Fresh Grass Clippings
Fresh grass clippings are a fantastic source of nitrogen. Just by leaving fresh grass clippings where they fall, you’ll recycle 25% of the cuttings’ nutrients back into your lawn [PDF]. Or apply them like a mulch around your garden plants or compost them.
Alfalfa meal is made from drying adult alfalfa plants and pressing them into cubes. This process makes them unlikely to contain weed seeds.
They have an NPK ratio of 3-1-2 and also contain a lot of micronutrients. Best yet, they’re high in triacontanol, a natural growth hormone that encourages healthy root growth in young plants.
Alfalfa meal can also be pretty cheap when you buy the bags intended for livestock. Alfalfa meal for gardening is higher priced, although it may contain less weed seeds.
Their major drawback is that it’s a food source for rodents like mice and rats, so if you struggle with rodents, you will want to try something else.
Like alfalfa meal, blood meal is also high in nitrogen and is a traditional method for increasing nitrogen in the soil.
It’s made from powdering dried blood that’s a byproduct of slaughterhouses. As a bonus, the smell of blood will scare away squirrels, deer and voles.
Blood meal and bone meal are different. Bone meal is made from the bones and is used to increase phosphorus in the soil.
The drawbacks of blood meal include:
- It smells like blood
- You need to be more careful with application, as too much can cause nitrogen burn. It will, however, provide nitrogen pretty quickly.
- Low quality or blood meal from abattoirs with sickly or poorly treated animals are a risk to your health.
- Wear a mask when handling dry blood meal. You don’t want this stuff in your lungs.
Alfalfa cubes (as mentioned above) are an excellent safe and vegetarian alternative for blood meal.
Fish and fish emulsion are both high in nitrogen. While there’s a lot of myths circulating about how fish emulsion is a superior fertiliser, it’s really just on par with other forms of composts.
That still means that fish emulsion is great for your garden, although when you have a large garden, it’s harder to justify how expensive it is.
You can buy a lot of manure compost for the same price as a bottle of fish emulsion.
If you have houseplants or a small container garden, then fish emulsion is a good organic option for you.
If you have a larger garden, then you can get the same benefits at a lower price by burying fish remains in your garden beds. Indigenious Peoples across North America have used this same technique for thousands of years.
The drawback is that you need to bury them deep enough, or cover with a cage, to prevent animals from digging up your garden to get to the fish.
Synthetic nitrogen comes in synthetic fertilizers in the form of a water-soluble nitrate, nitrite, and ammonium. Synthetic nitrogen bypasses the need to break down organic matter into nutrients.
Horticultural research has shown that plants don’t seem to discriminate between natural and synthetic forms of nitrogen, but beneficial microbes do.
Synthetic nitrogen has many disadvantages:
- Kills off beneficial microbes that would otherwise feed and protect your plants, aerate the soil and prevent soil erosion. Meaning you need to apply more and more synthetic fertilizers to get the same results.
- Leeches away quickly, so needs more applications.
- Spoils groundwater and rivers, causing algae blooms and affecting aquatic life.
- Synthetic fertilizers typically only contain the 3 macronutrients when plants need 17 different nutrients to grow healthy.
- Easy to overapply and burn your plants.
For the long-term health of your yard and garden, go with compost. (You can even apply it to your lawn with a broadcast spreader.)
Spent Coffee Grounds
I too grew up hearing that you should add your spent coffee grounds to your garden. But spent coffee grounds aren’t universally good for your garden.
In fact, spent coffee grounds can prevent germination or cause a phytotoxic response in many species, like broccoli, leeks, radishes, asparagus, and some flowers and ornamental plants.
Some other species, like cabbage and soybeans, love spent coffee grounds, showing improved yield and germination rates.
There’s a lot we don’t know about how spent coffee grounds affect the soil and plants. If you do still want to take advantage of your spent coffee grounds, here’s a few general guidelines to follow to avoid the worst effects:
- Don’t apply spent coffee grounds to the soil around your plants. Compost them first.
- Whether hot composting or vermicomposting, add less than 20% of the total volume. So if your compost is 10 gallons, spent coffee grounds should only make up 2 gallons or less. Make sure to use lots of carbon!
- When using in a vermicompost, consider allowing spent coffee grounds to sit and degrade for 21 days before feeding them to worms, as one study [PDF] found that this helped vermicompost reach peak temperatures earlier and the compost went through more stages of decomposition than without pre-composting.
- And do not feed hot coffee or hot coffee grounds to plants or compost. Let them cool first, or you’ll damage the plants and the microbiology.
Here are some ways you can organically be sure that your plants get enough of this vital nutrient:
With these 6 easy methods of adding nitrogen to your garden, you’ll be able to grow that abundant harvest without burning your plants to cinders.
Want to learn more? I go more into the [#disadvantages of coffee grounds here.]