Understanding Different Types of Fertilizers

Choosing fertilizers for garden and lawn care is no simple matter. Head off to the garden center and you're staring down rows of products; choose the wrong one and you'll do more harm than good. Buying the right fertilizer takes some knowledge and the patience to carefully read and understand product labels.

Do You Need Fertilizer?
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. Different types of plants use elements 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.

Some plants do better without extra fertilization. These are generally plants that evolved in places with poor soil and are very efficient in using what is available. If you fertilize these plants, they may grow differently then you expect or be more prone to disease. Other plants may have a voracious appetite for certain elements that exist naturally in smaller quantities in soil, called trace elements.   

Before you set out for fertilizer, it is important to perform a soil test done and to learn about the plants you have in your garden to see if they have special needs. Too much fertilizer is bad for the plants and bad for the environment. Rain runoff filled with fertilizer can turn marginal areas into a haven for weeds or promote bacterial growth in waterways. The rule of thumb: use only what you need and as little as it takes to get the job done.

Look at the Bag
All fertilizer bags are required by law to display certain information. They must have three numbers that indicate how many pounds of the major elements that 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. 

The three measurement 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 you're buying a 50-pound, then you would have about 7 1/2 pounds of nitrogen, 10 pounds of phosphorus and 7 ½ pounds of potassium. The rest of the weight is made up of fillers, micronutrients, pesticides or other materials.

If you had a soil test done and the recommendations were to apply 3 pounds of nitrogen per 1,000 square feet, and your lawn measures 2,000 square feet, you would need about a 25-pound bag of a fertilizer with a nitrogen number of 23 or 24.  

Fertilizer bags must also include a list all 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.

Types of Fertilizers
Most fertilizers are made of synthetic ingredients. The plants use these ingredients just as they do the elements found in nature. Most modern fertilizers release elements slowly, over a long period of time. Slow-release fertilizers are less likely to burn plants, but your should still try to keep them off leaves and stems and apply them only as directed.

Some quick-release fertilizers are still available that make the nutrients all available at once. This means a quick green up but less margin for error in applying before the plants suffer chemical burns. These fertilizers must also be applied more frequently than slow-release fertilizers.

Liquid fertilizers also give plants nutrients immediately. Liquid fertilizers must be mixed and applied frequently. They're 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 along with certain trace elements those plants are known to need. If you have problems with a certain type of plant, these may help, but only after a soil test shows a specific nutrient deficiency.

Organic fertilizers are things like bone meal, blood meal and manure. Bagged varieties sold in stores will carry the same labels found on other fertilizers. Most organic fertilizers are high in one nutrient only, and you may need to buy a combination of them to meet plant needs. 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. Manure may also bring weed seeds to the garden and is hard to spread on lawns.

Manure can't be used with vegetables shortly before harvest due to the danger of bacterial illnesses spreading into the plants. Organic fertilizers can also put your plants at risk, as their odors encourage wildlife to investigate your yard, which can lead to seeds and plants being dug up or devoured.

Choosing and Using Fertilizer
Most fertilizers are labeled garden fertilizer, lawn fertilizer, flower fertilizer, etc. The main difference between them is the ratio 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 potassium 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, because the nutrient value will break down quickly.

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 without discretion, which just happens to be the plants in your garden and flower beds.

After applying fertilizer, keep children and pets away for the time recommended on the bag. Clean up any fertilizer that lands on hard surfaces, such as driveways or walkways, to keep it from washing into storm drains and polluting waterways.