After treating your grass, you might have some extra lawn fertilizer lying around. It makes sense to avoid waste by finding another use for the leftovers.
So, can you use lawn fertilizer in your garden? No, you should not use lawn fertilizer in your garden. Lawn fertilizer has a much different nutrient profile (content and ratio of nitrogen, phosphorus, and potassium) than garden fertilizer. Lawn fertilizer may also contain herbicides or pesticides, which can harm your plants or prevent pollination by killing bees.
You can always check to make sure that your lawn fertilizer contains no herbicides or pesticides. However, it will still contain the wrong ratio of nutrients for your garden (most likely, too much nitrogen and little or no phosphorus or potassium).
Let’s take a closer look at why you might not want to use lawn fertilizer in your garden, some considerations for fertilization, and some alternative methods you can use.
Nutrients in Lawn Fertilizer Versus Garden Fertilizer
There are many nutrients that plants need for growth. However, the “big three” that you find in most fertilizers are nitrogen, phosphorus, and potassium (often denoted NPK or N-P-K on packaging).
Lawn fertilizers and garden fertilizers have very different compositions. Even more confusing is that different brands of fertilizer can contain very different nutrient profiles.
This can be useful if you know what your garden needs. However, it can also cause nutrient imbalances if you add fertilizer blindly.
Let’s talk about the big three nutrients and their appearance in different types of fertilizers.
The “Big Three” Nutrients (NPK)
On a fertilizer package, you will sometimes see numbers like “5-10-15”. These numbers correspond to “N-P-K”, or nitrogen-phosphorus-potassium.
The “5” means that the fertilizer contains 5% nitrogen by weight.
The “10” means that the fertilizer contains 10% phosphorus by weight.
The “15” means that the fertilizer contains 15% potassium by weight.
So 100 pounds of “5-10-15” fertilizer contains 5 pounds of nitrogen, 10 pounds of phosphorus, and 15 pounds of potassium (the rest is material that does not supply any of these three nutrients).
For a more in-depth discussion of nutrient content, check out my article on NPK ratios.
Nitrogen is a nutrient that is important for all plants. Specifically, it supports green growth (leaves, shoots, etc.) However, too much nitrogen will encourage green growth in a plant at the expense of flowers, fruit, and vegetables.
As you can imagine, lawn fertilizers contain plenty of nitrogen to make grass grow greener. If you were to use this fertilizer in your garden, your plants may grow tall and green, but fail to produce flowers or fruit.
Phosphorus is a nutrient that is important for photosynthesis and root development. It also helps plants to resist disease. Keep in mind that some lawn fertilizers contain no phosphorus at all!
Potassium helps to transport water and other nutrients throughout a plant’s tissues. It also contributes to strong roots and stems.
As we will see, some lawn fertilizers contain no trace of at least one of the big three plant nutrients. This is not a recipe for success in your garden! For more information, check out my article on low-nitrogen fertilizers.
Nutrients in Lawn Fertilizer
Lawn fertilizers are meant to stimulate growth of the grass on your lawn, keeping it looking nice and green. Generally, this means that they will contain plenty of nitrogen, but not much or any phosphorus or potassium.
Let’s take a look at a couple of lawn fertilizers, along with their nutrient content.
Milorganite Slow-Release Nitrogen Fertilizer
Nutrient Content: 6-4-0
This means that for every 100 pounds of this fertilizer, we get 6 pounds of nitrogen, 4 pounds of phosphorus, and no potassium at all. If you substituted this for your garden fertilizer, you would eventually end up with a potassium deficiency in your soil.
To prevent this, you would need to add extra potassium to your soil, either by using compost or another fertilizer. For more information, check out my article on potassium deficiency in plants.
Sta-Green Lawn Fertilizer
Nutrient Content: 29-0-5
This means that for every 100 pounds of this fertilizer, we get 29 pounds of nitrogen, no phosphorus at all, and 5 pounds of potassium. If you substituted this for your garden fertilizer, you would eventually end up with a phosphorus deficiency in your soil.
You would also likely end up with excessive nitrogen in your soil. As mentioned earlier, this would cause excessive green growth in your plants, at the expense of flowers and fruit. Your tomato plants would get tall and leafy, but bear few flowers and fruit!
As you can see from these two examples, lawn fertilizers are generally unbalanced in their nutrient content, skewed heavily in favor of nitrogen.
Nutrient content also varies quite a bit from one brand to another. For example, we would need to use 500 pounds of Milorganite to get the same amount of nitrogen as 100 pounds of Sta-Green (five times as much)!
Lawn fertilizer may be fine for your lawn, but it may not be wise to substitute it for garden fertilizer to grow your tomatoes or other fruits and vegetables. Think of it like this: a screwdriver works fine for putting a screw into wood, but you will have a tough time using a screwdriver to put a nail into wood.
Nutrients in Garden Fertilizer
Garden fertilizers are meant to supplement nutrients in your garden soil, which are used up each year by plants as they grow.
Generally, this means that they will contain a mix of all three of the big three nutrients. However, the ratios can vary quite a bit, from an even split to some nutrients being two or three times as abundant.
Let’s take a look at some garden fertilizers, along with their nutrient content.
Jobe’s Organics– Vegetable & Tomato
Nutrient Content: 2-7-4
This means that for every 100 pounds of this fertilizer, we get 2 pounds of nitrogen, 7 pounds of phosphorus, and 4 pounds of potassium.
This fertilizer has low nitrogen content, so it might be good to use if the nitrogen level in your soil is high. This could happen if you were to use excessive nitrogen-rich organic material in your garden, such as compost made from grass clippings or manure.
Espoma Garden-Tone All-Purpose Food
Nutrient Content: 3-4-4
This means that for every 100 pounds of this fertilizer, we get 3 pounds of nitrogen, 4 pounds of phosphorus, and 4 pounds of potassium.
This fertilizer has a fairly balanced nutrient mix, with slightly less nitrogen than phosphorus and potassium. This might be a good choice if the nutrients in your soil are in balance with one another, but you will be planting “heavy feeders”, such as corn, tomatoes, and cabbage.
Scott’s All-Purpose Flower & Vegetable
Nutrient Content: 10-10-10
This means that for every 100 pounds of this fertilizer, we get 10 pounds of nitrogen, 10 pounds of phosphorus, and 10 pounds of potassium.
This fertilizer is completely balanced, but provides more of each nutrient per cup of fertilizer than Espoma or Jobe’s. However, be aware that fertilizers with higher nutrient content contain more salts.
If you apply too much of these salts at once, they can prevent plants from absorbing water, leading to plant burn.
Miracle Gro All-Purpose Plant Food: 24-8-16
This means that for every 100 pounds of this fertilizer, we get 24 pounds of nitrogen, 8 pounds of phosphorus, and 16 pounds of potassium.
This fertilizer contains much more nitrogen than phosphorus or potassium. It would be suitable for plants that are in the greening stage (growing shoots and leaves). As mentioned above, the high nutrient content also means a high salt content, which can cause plant burn if applied recklessly.
Herbicides and Pesticides in Lawn Fertilizer
As mentioned earlier, lawn fertilizers may also contain herbicides (to kill weeds) and pesticides (to kill grubs) to protect your lawn and the grass healthy.
However, these herbicides can be harmful to some of the plants in your garden. So, be careful about applying lawn fertilizer to your plants or your garden soil.
Also, remember that pesticides do not discriminate – they can also kill beneficial insects in your garden. This includes bees, which are important for pollination of your plants.
If you find that you have a problem with a lack of bees, it might be a good idea to go pesticide-free in your lawn and garden. It is even better if you can convince your neighbors to do the same.
Tips for Using Garden Fertilizer
If you decide to use garden fertilizer instead of lawn fertilizer, there are a few things to keep in mind as you get started.
Get a Soil Test First
The first thing you should do is to get a soil test. This will help you to determine whether you even need to fertilize your lawn.
If there are any nutrient deficiencies, you will know what to supplement. If there is a pH imbalance, you will be able to address that as well.
For more information, check out my article on soil testing.
Check the Label Before Buying Garden Fertilizer
Always read the label before you buy fertilizer. The package will tell you the ratio of nutrients (NPK), which you must know in order to put the right nutrients into your soil. This will also depend on the results of your soil test (mentioned above).
If your soil indicates low levels of phosphorus in your soil, then you will want the 2nd number (P) to be higher than the other two (N and K).
The package will also indicate how much fertilizer to use in a given area. Make sure that you calculate the area correctly. If you are only fertilizing half of your garden, adjust your dimensions accordingly. Also, make sure to use the same measurement units as those listed on the package!
Fertilize When the Soil is Moist
If you fertilize when the soil is dry, the salts from the fertilizer can build up to high concentrations in certain areas. When it rains, these salts can be washed into a plant’s roots, which can kill the plants due to high nutrient concentrations.
Make sure to water a bit or wait for some rain before you fertilize, and then water it in afterwards. Otherwise, you risk burning your plants – for more information, check out my article on over fertilizing.
Replace Organic Material
Remember that fertilizers replace nutrients in the soil, but not organic material. Think of it this way: if you take a vitamin pill every day with water, but don’t eat any food, you will eventually starve. You might not get a nutrient deficiency, but you aren’t getting enough calories to survive.
In a similar vein, if you constantly use fertilizer without adding any compost or manure, your soil will become thin and will have difficulty supporting your plants. It is also more likely to dry out, due to an inability to retain water.
If you find that you have a problem with this, check out my article on treating dry soil.
Hopefully, you have a better idea of just how different lawn and garden fertilizers are, and why they are not interchangeable. Don’t forget to check your lawn fertilizer to make sure that it contains no pesticides, so that you don’t kill bees or beneficial insects in your garden!
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