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Plant Care Articles

Why Do I Need to Fertilize My Plants?

Robby Flannery, Ph.D.

All plants need fertilizer in some form or another in order to complete their life cycles. In nature plants will most often acquire nutrients by absorbing them through their roots as organic matter decomposes in the soil. Some plants have gone to other extremes to get their nutrients: Venus Fly Traps have modified leaves that will snap shut to trap an insect. The modified leaf would then go about digesting the insect to extract and absorb the nutrients. Luckily, insect sacrifice is not discreetly necessary to successfully fertilize the plants in your garden. There are many fertilizers and techniques that can be used to bring about a productive and thriving garden.

What is fertilizer?

Plant fertilizers are sometimes called plant food. This is an unfortunate moniker since it is misleading. Plants make their own food through the process of photosynthesis. Photosynthesis results in the production of sugar (think maple syrup, sugar cane or sugar beets; all very sugary plants or plant parts). That sugar is burned off to produce energy in the same processes that animals use for the same purpose. If fertilizers are not food than what are they? Fertilizing a plant is analogous to giving a plant its daily multivitamin. The plant that lacks sufficient nutrients may survive for a long period of time, but it would never be very healthy. This would be like a person who consistently eats unhealthy food that has little nutritional value. Sure, the unhealthy food would keep the person alive, but that person would have poor health.

What happens to plants with poor nutrition?

Poor plant health is manifested in plants differently, but there are some symptoms that are fairly universal amongst plants. Plants with poor nutrition will likely have stunted growth. They won’t grow as tall as they potentially could. They will have short stems with few leaves on them. For example, Charlie Brown’s Christmas tree looks like it could use some fertilizer. Plants that have been subjected to severe poor nutrition for a long time may drop their leaves and potentially die. Also, plants with poor nutrition may never flower and thus fruits, or will produce very few flowers and fruits. This is especially undesirable if a gardener is growing tomatoes or another crop. It’s frustrating when all you want is tomatoes and your tomato plant never produces any. Another symptom that many plants have as a result of poor nutrition is discoloring of the leaves. The most prominent discoloration is a yellowing (known in horticulture as chlorosis, or chlorotic leaves) of the leaves. Sometimes the undersides of the leaves may turn a purplish color. Other times the newer leaves are yellowish, and the older leaves are still green, and vice versa. With some training, you can figure out which specific nutrient is lacking just by the coloring of the leaves and which leaves are discolored.

What nutrients do plants need?

There are 17 nutrients that plants need, but some are much more important than others. Two of the nutrients (carbon and oxygen) come from the carbon dioxide that plants “breathe” in through their leaves. Hydrogen comes from the water that they absorb through their roots. The other 14 nutrients are called mineral nutrients and are what most people would associate plant fertilizers with. The top three mineral nutrients are nitrogen (N), phosphorus (P) and potassium (K). Of those top three nutrients, plants “crave” nitrogen above all others. A complete fertilizer has all three of the top plant nutrients. The ratio of concentrations of nitrogen, phosphorus and potassium in a fertilizer is known as their NPK ratio. You would see this ratio on the side of a fertilizer container, and it would appear as three numbers (e.g. 5-10-10). Each number represents the percentage of the fertilizer that is either N, P or K. A 5-10-10 ratio stands for 5% nitrogen, 10% phosphorus and 10% potassium. The other 75% could be other nutrients (sulfur, magnesium, calcium, iron, etc.) or inert compounds. A complete, balanced fertilizer includes all three of the top nutrients and they will have the same (or close to the same) concentrations. A 10-10-10 fertilizer is an example of a complete, balanced fertilizer.

How do you fertilize your plants?

Fertilizers come in many different forms, and they all have their pros and cons. For every type of fertilizer it is important to follow the directions that come with it. You can have too much of a good thing. If a little of a good thing is good for the plant, then A LOT of a good thing must be GREAT for the plant is the wrong type of thinking when it comes to fertilizing. First off, too much fertilizer may be more nutrients than the plant needs or can absorb at one time. The extra fertilizer may find its way into the runoff where it will pollute the environment. This is a waste of money and is harmful to the local waterways. Too much fertilizer can harm more than the environment, though. Providing too much fertilizer to your plants can cause damage to the roots and leaves of the plants. It can potentially cause the flowers to prematurely fall off the stem. In extreme cases of too much fertilizer, a plant can die. This is why it is always important to follow instructions when it comes to how often and how much fertilizer should be used.

The type of fertilizer you use is dependent on what you want to achieve. If you want to provide nutrients to the plant immediately, the best course of action is to use a fast-release water soluble fertilizer. Water soluble fertilizers can come in small granules of salt or concentrated liquids. Both examples need to be diluted before use. These fertilizers can be dissolved/diluted in a watering can or can be administered by using a garden hose attachment that automatically doles out the correct concentration of nutrients by simply turning on the hose. For the gardener who wishes for a low maintenance but less hands-on approach to fertilizing, there is slow-release fertilizer. These fertilizers usually come in pellets and will slowly break down over time, which slowly releases nutrients to the soil. Most fast-release fertilizers will be available for absorption by the plant for two to six weeks (of course this is dependent on the type of plant and soil). Slow-release fertilizers can provide nutrition for eight weeks to nine months. All of these fast- and slow-release fertilizers are examples of chemical fertilizers, which if used properly can provide excellent results with minimal to no impact on the environment.

The use of organic fertilizers is another option for fertilizing your plants. Examples of organic fertilizers are compost, fish meal, bat guano, manure, bone meal, sewage sludge and so on. While many of these examples have ready to use nutrients available the moment you dig them into your soil, much of the nutrition is bound up in organic compounds that will not be available for plant absorption until the organic compounds are broken down. This is why organic fertilizers tend to promote more microbial activity in the soil (microbes are what catalyze the breakdown of the bound nutrients). Not only do many organic fertilizers add organic matter to the soil, but they also act like slow-release fertilizers. Organic fertilizers may be more bulky and weigh more than soluble fertilizers, but they also need to be used less frequently. Since their instantaneous impact on the environment is low, it is more difficult to pollute with organic fertilizers.

How do you make your own fertilizer?

Compost tea is a relatively newcomer to the organic fertilizer market. Luckily, it is easy to make on your own. First, you would need access to finished compost or manure. Composting is simple to do once you know how, and a complete set of instructions would be beyond the scope of this article. There are some tips that are good to keep in mind, however. Avoid using meat products in your compost bin. This means no bones or leftover meat, cheese or other dairy products and so on. Your compost bin should vegetarian (to an extent; egg shells are very welcome in compost). If your compost smells funky, chances are it’s either too wet or has too much nitrogen. If it’s very wet, either water less or aerate the pile more by mixing. If you fear that it’s too much nitrogen, add more grass clippings or straw to the pile.

Once you have your finished compost, you will need to steep it to make your tea. This literally is the same as steeping a tea bag in water. You can put a few shovels of compost or manure in a burlap sack and then steep it in a clean trash can filled with water for about 30 minutes. Your tea will be ready for your plants. You can also use a 5-gallon bucket. Fill the bucket with compost until ¾ of the bucket is filled. Pour water in the bucket until it is just below the rim of the bucket. This will need to steep for longer than the previous procedure. Allow the mixture to sit for one week. After one week you can dilute the tea by using 1 part tea for every 10 parts water. Be sure to mix the solution every day. It’s important to not allow the mixture to sit for too long though. There will be fewer nutrients available to the plants the longer it steeps. In other words, the tea will lose its potency over time. There are many different procedures that you can use to make compost tea, but ultimately it will produce a nutrient enriched solution that you can use to water your plants.