As garden vegetables grow, they undergo an intensive process that strips nutrients from the soil. Adding fertilizer keeps soil in optimum shape to feed your garden. Choosing the right type of fertilizer and adding the right amount depends on the soil and the plants you're growing. Soil tests are a great way to know what to add.

5-10-5? Deciphering the numbers

Commercial fertilizers are labeled with three numbers that indicate the fertilizer's nutrient ratio — for example, 12-12-12, 5-10-5, and 4-12-0:

  • The first number indicates the percentage of nitrogen (N)

  • The second number shows the percentage of phosphate (the type of phosphorus, P2O5)

  • Third number represents the percentage of potash (the form of potassium used, K2O.)

A 5-10-5 fertilizer contains 5 percent nitrogen, 10 percent phosphate, and 5 percent potash, and it's called a complete fertilizer because it contains some of each type of nutrient. In contrast, bone meal has an analysis of 4-12-0. It's a good source of phosphate but doesn't provide any potash.

Now that you understand the numbers, you have to choose between organic and chemical fertilizers.

Organic fertilizers

Most home gardeners can grow a perfectly beautiful and productive garden using organic principles. Organic fertilizers — animal and green manure, blood meal, fish emulsion, cottonseed meal, granite dust, and rock phosphate — have several advantages:

  • Many organic fertilizers contribute organic matter to your soil, improving its structure, feeding soil microbes, fighting fungal and bacterial diseases, and contributing micronutrients.

  • Most organic fertilizers supply a slow but steady diet for plants.

  • Some organic fertilizers, such as manure and compost, may be inexpensive — or free if you create them yourself.

  • Complete organic fertilizers, such as 5-5-5, are now more widely available and have higher concentrations of nutrients than in the past, making them easy-to-use alternatives to chemical fertilizers.

However, it's not all sun and roses when using organic fertilizers. Here are some disadvantages:

  • Some organic fertilizers, such as manures and compost, are bulky and difficult to store and transport.

  • Their slow release of nutrients, in some cases dependent on the action of soil microorganisms, may take too long to remedy a dire situation when an adequate nutrient supply is needed.

  • Many organic fertilizers are lower in nutrient content than their chemical equivalents, and the contents may vary depending on the weather and conditions where the fertilizers were produced. So you may not be exactly sure how much to put on your garden.

Chemical fertilizers

Chemical fertilizers are synthetically manufactured. They include elements such as sodium nitrate, potassium chloride, and superphosphate. Chemical fertilizers come in liquid, granular, powder, or pellet form. You can fertilize when you water with a watering can, using a liquid fertilizer. Or you can sprinkle some granular fertilizer around each plant.

Chemical fertilizers are widely available, less expensive than organic fertilizers, and quick acting, but the disadvantages of using chemical fertilizers far outweigh the advantages:

  • Chemical fertilizers add no organic matter to your soil and contribute nothing to improving soil structure: In fact, some research suggests that chemical fertilizers actually harm the microorganisms in the soil, making the soil less able to support long-term plant growth.

  • Chemical fertilizers are concentrated and fast acting, but they have no long-term benefits for your soil: It's like taking a vitamin versus eating a good meal.

  • Manufacturing chemical fertilizers requires large amounts of energy, usually supplied by nonrenewable resources: This massive use of energy increases pollution, global warming, and our carbon footprint. Even though some commercial organic fertilizers, such as rock phosphate and green sand, also are manufactured and require energy inputs, home gardeners can instead choose to use locally made compost and manures to get the nutrients they need for their gardens.