Fertilizer is something that many of us regularly use for our houseplants, gardens, and lawns. In theory, fertilizer should help your plants or lawn grow better. It seems easy; you go to the store, pick up a fertilizer that says it’s best for grass, houseplant, or vegetable garden on the front label, apply it per the instructions, and voilà, plants grow perfectly.
Unfortunately, the reality is not quite that simple.
Even if you’re using organic fertilizer products, it’s still not that simple. And applying fertilizers regularly over time can have significant negative impacts on your soil health. Soil science is complex, but the two basic things you need to know are:
- Synthetic fertilizers feed the plant, organic feed the soil.
- Nutrient excess can be just as bad as nutrient deficiencies.
Nutrient deficiencies can cause plants to be weak, turn yellow, not bloom, or create fruit. But nutrient excesses can make it look like you have a nutrient deficiency, when in fact, the nutrients may be available but inaccessible. An excess of one nutrient, like phosphorus, can block plants from absorbing other nutrients in the soil already.
The bottom line: before you apply any fertilizer, you should test your soil.
A home test kit purchased from the garden center will give you enough information to use fertilizer sustainably and efficiently for the average home gardener. The test kit will tell you the nutrient levels of nitrogen, phosphorus, and potassium, as well as the pH of your soil.
If you have significant problems, though, and can’t seem to grow anything very well, you may want to consider sending soil samples to one of the soil labs for more in-depth analysis. Many county extension offices can test your soil for you or give you information about sending it to the nearest soil lab.
Nitrogen is the nutrient that helps support lush green foliage growth. It’s essential for all plants. Legumes like peas, beans, and lupins are nitrogen-fixing plants, so planting them can naturally help improve your soil’s nitrogen levels. Tomatoes, peppers, corn, cucumbers, and squash use lots of nitrogen, so growing them can help lower nitrogen levels if they are too high.
If nitrogen levels are too low, what little nitrogen exists may be used up temporarily by bacteria in the process of breaking down organic material. High sources of nitrogen that you can add to the soil are blood meal, bone meal, coffee grounds, fish scraps, hoof and horn meal. You can also add nitrogen with inorganic fertilizer supplements; slow-release organic fertilizers are usually the best choice.
Too much nitrogen will inhibit the development of fruits and flowers, and it will often get washed away with rain. If it runs off into the local water systems, it can cause algae blooms in ponds and lakes that can choke out aquatic life.
Because garden plants use a ton of nitrogen, and it is highly water-soluble, so some may wash away, it is normal to need to apply more nitrogen every year, in addition to rotating nitrogen-fixing crops through your garden.
Phosphorus is the nutrient that supports the growth of flowers and fruits. The challenge with phosphorus is that it doesn’t get used up by plants in the same way that nitrogen does. It tends to stay in the soil for a very long time.
If phosphate levels get too high in your soil, it can inhibit plants from absorbing and using iron, zinc, and manganese from the soil, which are minerals they need to produce chlorophyll. If your phosphate levels are too high, you may need to apply iron and zinc as a foliar spray. Fortunately, plants absorb iron and zinc very well through a foliar spray method, so you can still grow healthy plants in high phosphate soil.
If phosphorus levels are too low, your plants may fail to bloom or produce fruit. You can naturally increase phosphorus levels by applying amendments like manure, bone meal, fish scraps, and cottonseed meals.
Overuse of manure in your garden can throw your phosphate levels out of balance over time, and it can be really difficult to get the balance back, so apply fertilizers, organic or inorganic, with high P levels (like bloom fertilizers) very carefully.
Potassium is important for photosynthesis, water regulation, and building strong cell walls in plants. Potassium is one of the nutrients that doesn’t tend to fluctuate as much as the other two.
However, potassium levels can get much lower in the soil your lawn grows on if you consistently remove grass clippings. If you’re using the grass clippings as compost in your garden, it’s good news for your vegetables, but you should make sure to check the soil nutrient levels around your lawn as well. Wood ash, corn fodder, kelp, or seaweed, and dried manure from beef cattle are high in potassium.
Too much potassium can disrupt plants’ ability to absorb and use calcium, nitrogen, and magnesium. Sometimes very rocky soil can be high in potassium.
What You Need to Know to Fertilize Vegetables
Vegetables, in general, require more fertilizer than trees and shrubs because they need a ton of nutrients to create all that nutritious produce for us to eat. But that still doesn’t mean you should just automatically apply a vegetable fertilizer to your garden every two weeks all summer long.
Your soil is a dynamic thing, and its chemistry changes from year to year as you grow things in it and add organic matter to it. So it’s important to monitor the changes in your soil over time, and adjust your use of fertilizer accordingly.
The best fertilizers for your vegetable garden will vary depending on the nutrient levels that already exist in your soil, what plants you’re trying to grow, and what the pH level is, which is why it’s so important to test your soil.
If your phosphorus and potassium levels are good, then you’ll likely just need to add a nitrogen fertilizer to your garden. Vegetables and annual flowers use a lot of nitrogen since they grow so much and produce so heavily in a relatively short period of time.
What You Need to Know to Fertilize Trees, Shrubs, and Lawn
Trees, shrubs, and your lawn need less fertilizer than your garden vegetables and flowers. As with fertilizing your garden, though, the best plan is still to get your soil tested to determine what the nutrient levels are and decide what fertilizer you should use to achieve the growth you’re looking for.
Trees & Shrubs
Trees and shrubs don’t generally need fertilizer. They have much deeper root systems and access to soil and nutrients that aren’t available to surface plants like most flowers and vegetables. They also have much broader root systems than most plants, which means they benefit from your fertilizer application on the lawn or even in your garden.
Check for problems like drainage, pest infestations, disease, less than ideal location, or breakage that may be affecting your trees or shrubs before you start applying fertilizer. If your trees and shrubs are having problems and you cannot identify the cause, it’s best to get a professional arborist to give you the best course of action.
If you have to apply fertilizer for trees and shrubs, it’s best to do it in really early spring or late fall, once all the leaves have turned color and are dropping. Don’t fertilize trees in mid-summer, as it encourages them to put out new growth, which may not toughen up enough before winter.
When you apply fertilizer to trees and shrubs, you should spread it evenly over the entire root area, which means going at least as far out from the tree’s base as the drip line, how far out the branches reach.
Similar to the garden, you should test the soil for your lawn. Generally, for the average homeowner, a test every 3-5 years is sufficient. You can do a test yourself or send off samples to one of the soil labs. The bonus of sending your soil samples into a lab is that you’ll get customized fertilizer recommendations to help you get the best results from your specific type of grass.