Vegetables are living, growing things, and they need to be fed. The amount of nitrogen, phosophorous, potassium and trace minerals that plants need varies. (See below for list of heavy feeders.) But generally speaking, if you feed the soil, you feed your vegetables. If you do not feed your sol by adding compost or other fertilizer, eventually it will be depleted and your garden will become barren.
Get a Soil Test: If you are starting a new garden, consider sending a soil sample to the University of Minnesota for a soil test. The test will give you a general idea of your vegetable garden’s pH, its fertility level and whether there are trace minerals that you need to add. If your soil is acidic, the test will tell you if you need to add lime, which increases pH.
Add Compost: If you do not want to do a soil test and are not interested in adding special elements to your garden, here’s the basic recommendation for building soil fertility: Add compost. Compost improves fertility and tilth, the texture of the soil, Add 1 to 2 inches to the top of the soil and work it in 6 to 8 inches, if possible. I add compost to my vegetable beds in fall and then again in spring. Compost also makes a great mulch — you need 2 to 4 inches to suppress weeds.
Add fertilizer: Because vegetables take so much out of the soil, you may want to supplement your compost with some balanced fertilizer. There are organic and inorganic options at garden centers. Last year, I used Chickity Doo-Doo, which is (as you’ve probably guessed) made from processed chicken manure. It had lower levels of nitrogen, phosphorous and potassium at 5 percent, 3 percent and 2.5 percent respectively than many petroleum-based options (such as the classic 10-10-10 fertilizer), but everything grew well with a little chicken poo in the planting hole. You may want to consider a specialized fertilizer for the crops you are growing. For instance, there are several tomato-specific fertilizers that contain calcium, a nutrient tomatoes need more than other plants.
Consider a Cover Crop: If you are growing a larger garden, you may want to consider planting a cover crop. These “green manures” are nitrogen-fixing plants that squeeze out weeds and feed the soil. You can plant a spring-growing cover crop and then turn it over when you plant warm season vegetables or plant one later and leave it in place over the winter. The University of Minnesota Extension Service has a good fact sheet on cover crops.
Container Plantings: Because they require more water than in-ground or raised-bed gardens, containers also need more added fertilizer. You can make compost 30 percent of the volume of soil in your containers to give plants a good start. You will need to add fertilizer regularly to the container to keep your vegetable crops growing. Generally, adding fertilizer once every two weeks (maybe a bit more often during heavy growing times) will work, using organic or synthetic fertilizer.
Follow directions! With the exception of compost, which you can add in almost unlimited amounts, most fertilizers have the potential to burn plants, that is harm them by giving them too much fertilizer. So, follow directions on the packages for any fertilizers you add. Some gardeners recommend adding fertilizers at half the recommended rate, if you will be adding them regularly.
Nitrogen Needs of Vegetables
Nitrogen is the main element that plants need to grow, but how much they need varies. Among those with larger needs for nitrogen are: cucumbers, melons, greens, onions, corn and squash. Average feeders include tomatoes, peas and beans. Light feeders (or those that do well in lean soil) include herbs, potatoes, peppers, carrots and beets.
—Mary Lahr Schier
Follow Notes from Northern Gardener throughout January as we offer our series on 31 Days to a Great Northern Vegetable Garden. Tomorrow: Hoe, Hoe, Hoe.