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How to choose garden fertilizers

Fertilizer is generally needed in the vegetable garden.
Fertilizer is generally needed in the vegetable garden.
Kim Willis

Do you need to fertilize your plants? When a homeowner walks into the garden department of the local store he or she is often confronted with a long row of bagged fertilizers. It seems like a daunting task to pick the type of fertilizer that will be best for your situation. And some people are confused about organic matter and compost and their relationship to fertilizer.

Fertilizer and compost or organic matter are both good for gardens and you don’t have to choose one over the other. In fact the best gardeners use both. Compost and organic matter worked into soil improve your soil, helping it hold water and nutrients and making it comfortable for plant roots. Organic matter and compost help supply soil organisms like bacteria that are beneficial to plant growth. But compost and various types of organic matter may not supply some nutrients missing in your soil.

Plants manufacture their own food by photosynthesis so it is technically wrong to call fertilizer plant food. But in order to manufacture their food plants need certain elements from the soil. The type of rocks that were broken down and the decayed organic matter in an area determine the type of soil that area has and the amount of essential minerals and other nutrients the soil holds. Something will grow in almost any soil but it may not be good soil for what you want to grow. And nutrients can be depleted from soil by plants or from water leaching them out of the soil.

The type of rocks that broke down in an area to form soil also determine the soil pH or it’s alkalinity or acidity. (Other things that happened to soil in an area such as the long use of certain fertilizers can also affect soil pH.) Sometimes all the elements necessary for plant growth can be in the soil but plants can’t use them because the pH level determines whether plants can take those nutrients from the soil. To understand soil pH better please read this article.

Do you need fertilizer?

Different types of plants use soil nutrients in different amounts. When the same plants grow in the same spot for a long time the elements they need may become depleted, which leads to poor plant growth. While nitrogen may be returned to soil from the air and decomposition of organic matter some other soil elements are not replenished naturally.

Chances are good that vegetable gardens and plantings of annual flowers need fertilization. These plants use soil elements up quickly and because they grow rapidly need fertilization. Roses generally need fertilization. Lawns may need some fertilization, especially in fall to promote good root growth. Fruit trees and things like strawberries and grapes generally need some fertilization to perform at their best.

Some plants do better without extra fertilization, they are generally plants that evolved in places with poor soil and are very efficient in using what is available. Some perennials and herbs fall into this category. When given fertilizer they may grow differently than expected and are more prone to disease. Other plants may have a high requirement for certain elements, called trace elements for good growth.

It’s important to know the species of plants you grow as perennials, trees and shrubs in the landscape and what their needs are for fertilization. Most references will give guidelines for fertilizing common plants. Some plants like acidic conditions such as blueberries, azaleas and holly. Using an acidic type of fertilizer helps them grow. When a plant is said to be a “heavy feeder” it generally will need extra fertilization.

Too much fertilizer is bad for the plants and bad for the environment. You can always give plants more fertilizer but if you over fertilize plants you may burn the roots, cause leggy, weak growth, and more pest problems. Having a soil test done every few years will let you know what elements may be needed in your soil. Plants that are growing well, especially things like trees and shrubs, may not need fertilization every year.

Types of Fertilizers

Most fertilizers are made of synthetic ingredients. The plants use these ingredients just as they do the elements found in nature. It’s no different than you using a vitamin pill instead of consuming foods with the vitamins and minerals you need. Most modern fertilizers release elements slowly, over a long period of time. Slow release fertilizers are less likely to burn plants but try to keep them off leaves and apply only as directed.

Some fertilizers are still quick release, which means the nutrients are all available at once. This means a quick green up but less margin for error in applying before the plants are chemically burnt. They must be applied more frequently than slow release fertilizers.

Liquid fertilizers also give plants nutrients immediately. Liquid fertilizers must be mixed and applied frequently. Liquid fertilizers are good for container plants but difficult to use on large gardens and lawns.

There are fertilizers formulated for certain plants, such as tomatoes or roses. These contain the major ingredients found in all fertilizers and also certain trace elements those plants are known to need. If you have problems with a certain type of plant these may help.

Organic fertilizers are things like bone meal, blood meal, and manure. Organic fertilizers sold in stores should have the same labels as other fertilizers. Most are high in one nutrient only and you may need to buy a combination of things. New organic fertilizers with more balanced ingredients are also on the market.

Some people also fertilize with fresh manure or compost. Both are great for adding organic matter to the soil but used as a fertilizer they have drawbacks. Manure and compost vary greatly in nutrient content from batch to batch. You need a great volume of manure or compost, six or more inches, to equal a sprinkling of slow release fertilizer. Fresh manure can burn plants and it can be the source of food borne illness if used on food plants. Manure may also bring weed seeds to the garden and is hard to spread on lawns.

Choosing and using fertilizer

All fertilizer bags are required by law to have several things on the bag. They must have three numbers on the bag or container. These numbers signify how many pounds of the major elements plants need, nitrogen, phosphorus and potassium, in that order, are in the bag. Nitrogen is first because it is the most important element for plant growth. Phosphorus in fertilizer is banned in some states such as Michigan because it pollutes water so that place will be represented by a 0.

Fertilizer bags must have a list of active ingredients, the weight of the bag and instructions for proper use. If there are pesticides in the fertilizer, such as a weed killer, the pesticide name and the pesticide registration number must also be on the bag.

The three numbers on the bag are based on 100 pounds of fertilizer. A bag marked 15-20-15 would have 15 pounds of nitrogen, 20 pounds of phosphorus and 15 pounds of potassium per 100 pounds of fertilizer. If the bag was a 50 pound bag then you would have about seven and a half pounds of nitrogen, (and half the other minerals), in that bag. The rest of the weight would be made up of fillers, micronutrients, pesticides or other things.

Most fertilizers are also labeled garden fertilizer, lawn fertilizer, flower fertilizer etc. These have different ratios of the major ingredients. These are the easiest fertilizers for gardeners to use. Just choose the fertilizer for the type of plants you are growing and follow directions. Flowers and vegetables can generally use the same fertilizer, but lawn fertilizer is too high in nitrogen for vegetables and flowers.

It is possible to buy fertilizer ingredients like phosphorus separately at farm supply stores but they are sold in large bags that would take home gardeners many years to use. Buy only the amount of fertilizer you can use in one year. Only buy and use phosphorus if a soil test indicates your soil is deficient in it.

Some fertilizers have weed killers or insect killers added to them. Read the label carefully and follow directions exactly when applying these. Never use weed killing lawn fertilizers on gardens or flower beds. These kill broad leaved plants, which are also the plants in your garden and flower beds. After applying these products keep children and pets away for the time recommended on the bag.

When to apply fertilizer

Apply slow release fertilizers to vegetables and annual beds at planting time. You may need additional applications in mid- summer. Most trees, shrubs, fruit plants and perennials should be fertilized in early spring. Do not give these plants fertilizer in late summer or fall as it may make them more susceptible to winter injury.

Mix in slow release fertilizers when planting containers or use a weak liquid fertilizer on them weekly. Lawns do well with an early spring and early fall application of fertilizer.

The fertilizers that home owners use are a major source of water pollution. Don’t get fertilizer on hard surfaces like sidewalks where it washes off into storm drains. Always follow label directions for the amount and way to apply the fertilizer.

Here are some additional articles you may want to read.

The difference between soil, compost, and planting medium

Home garden remedies that don’t work

You can see the author’s weekly garden blog at

You can contact the author at

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