While most gardeners understand that fertilizers supplement the nutrients available to plants from the soil, agreement isn't as clear about the best formulas or the best types of fertilizers to use. More is not always better in the garden, since an overabundance of one nutrient can lead to a shortage of others, so starting with a soil test is the best way to determine the most appropriate formula for your garden. However, there are some general rules that can help you decide until the test results come back.
Grading Your Fertilizer
When you buy a bag of commercial mixed fertilizer, it comes with three numbers on the package. Those numbers are a national standard of labeling that indicate the percentage of nitrogen, phosphorus and potassium, or potash, found, by volume, in the bag. For example, a 12-6-6 label lets you know that 12 percent of the volume is nitrogen, 6 percent is phosphorus and 6 percent is potassium. A "general purpose" fertilizer is one that will meet the needs of most plants from beginning to end of the growing season.
Garden experts generally recommend that, for growing vegetables, the nitrogen levels be less than or equal to those of phosphorus. Depending on the results of your soil test, this might mean that your garden will do well with a 5-10-5 or a 10-10-10 mixed fertilizer, advises Colorado State University.
Nixing Negative Nitrogen Effects
Although nitrogen is the most needed nutrient for plants, too much can lead to disappointing vegetable garden results. Excessive nitrogen produces lush, lavish foliage, which will make your landscape look inviting, but it can delay the production of fruits and vegetables. Again, depending on the results of your soil test, you can use about 3.2 ounces for a 100-square-foot garden plot while the plants are young and developing. You can continue to add nitrogen, as necessary, throughout the growing season, but be careful to avoid direct contact with the plants.
Producing Positive Phosphorus Effects
Phosphorus is crucial to crop yield, so you can use more of it to increase the productivity of established plants. Following the package directions, add a 3-20-20 blend when you plant your veggies, Sunset suggests.
Using Fertilizer to Your Best Advantage
Testing your soil is important, but it isn't necessary every single growing season, unless you make major changes and amendments in between. Every four to five years, use a test kit from your local garden center to determine exactly what nutrients your soil contains and which are missing. If you'd rather throw caution to the winds, as a rule, you can apply 1 pound of 10-10-10 mixed fertilizer to every 100-foot row in your garden, or double that for a 5-10-5 grade.