What are the differences among the many forms of fertilizer that you see on shopping aisles? What is a granular fertilizer? What about water-soluble? What are slow-release, timed-release, and controlled-release? What is an organic fertilizer? If you have questions about the numbers on the bag, read What's in a Package of Fertilizer? first. Then read below for more about the different forms of fertilizer.
What is the difference between granular and liquid fertilizer?
Granular fertilizers are solid granules, while liquid fertilizers are made from water soluble powders or liquid concentrates that mix with water to form a liquid fertilizer solution. Plants quickly take up most water-soluble fertilizers, while granular fertilizers need a while to dissolve or decompose before plants can access their nutrients.
These are meant to be worked into the soil or sprinkled around plants. They last 1 to 9 months, depending on the type. Because they must first break down before releasing nutrients, it usually takes a watering and a few days to begin to see their results. It's often good to work them into the ground before planting. You may be familiar with common granular farm-grade fertilizers such as 8-8-8 and 10-10-10.
High-tech Granular Fertilizers
Miracle-Gro® Shake ‘n Feed® Tomato, Fruit & Vegetable Plant Food, have "slow-release," "timed-release," or "controlled-release" properties, synonymous terms meaning that they release their nutrients slowly over a period of time. They are coated to control the rate of release typically from 2 to 9 months, depending on the formula, whereas uncoated (and cheaper) farm grades last only 2 to 4 weeks. They depend on moisture and temperature for release. The nice thing about coated granular fertilizers is that you apply them only once or twice during the growing season and they gradually release nutrients in tiny doses as roots can take them up. Little is wasted and they last longer, so even though their price is higher, they are actually more economical than uncoated, farm-grade types. When applied at the right rate and in places where they don't wash away, these high-tech products are more environmentally sound because they are less likely to contribute excess nutrients to any runoff that ends up in local streams.
Organic or Natural Granular
Organic fertilizer comes from an organic source such as manure, blood meal, cottonseed meal, feather meal, crab meal, or others, as opposed to synthetic sources. There are also some natural fertilizers that are not organic, such as Greensand, which contains potassium, iron, calcium, and other nutrients. These are considered okay for organic gardening because they are not synthesized, but come from natural mineral-rich deposits in the earth. Organic fertilizers depend on the microbes in the soil to break them down into digestible bits for plants. Organic fertilizers tend to encourage soil microbes, earthworms, and other flora more than synthetic fertilizers do, because most organic fertilizers don't add excess salts and acid to the soil. Microbes aren't very active when the soil is colder than 50 degrees, and according the USDA, a rule of thumb is that for every 18-degree increase in soil temperature, the microbial activity doubles. Therefore, you have to be careful about applying too much organic fertilizer, because if microbial activity releases more nutrients than the plants take up and the soil can hold, the excess can wash away. Typically, a gardener might overload the soil in spring, when it is cold, but won't know it until summer warms things up and the tomatoes are all leaf with no fruit!
Liquid fertilizers, such as Miracle-Gro® Water Soluble Tomato Plant Food and Miracle-Gro® LiquaFeed® Tomato, Fruits & Vegetables Plant Food, are water-soluble powders or liquid concentrates that mix with water to make a fertilizer solution. They usually require a hose-end sprayer or watering can. The liquid nutrients generally last 1 to 2 weeks, so you need to reapply often. The advantage of liquid fertilizers is that they are quickly absorbed, so plants get their benefits soon after you apply them. They are great as a starter solution and for a quick boost during the growing season. These fertilizers are also useful to supplement granular fertilizers for potted plants because frequent watering of containers leaches nutrients. Fish emulsion is a popular organic liquid concentrate fertilizer, but be forewarned that if raccoons and other critters are a problem in your garden, you my find some digging around fresh applications until the fishy aroma is gone.
Can I get by with one general-purpose fertilizer, or do I need different types?
By enriching the soil with plenty of compost and organic matter, you will use less fertilizer. Vegetables need plenty of phosphorous and potassium, so the fertilizer you choose should contain plenty of each. For the longest-lived vegetables, such as tomatoes, you may prefer to use an organic fertilizer or a coated high-tech product that releases nutrients slowly over a period of time. You can supplement with a liquid food. Organic fertilizers are suitable for everything.
When in doubt, buy the fertilizer that offers the greatest number of nutrients and a nitrogen level of at least 3 percent. See The Basics of Fertilizing for a little more help on which crops need the most (and least) fertilization.