To grow big, healthy vegetables bursting with vitality, the soil has to provide all the nutrients they require. Even with the full sun most vegetables need, and no matter how careful you are to weed and water them, if the soil is not "right" they will not do well. They might survive, hanging on just long enough to produce a small crop of leaves or fruit, but they will not be the picture of energy you saw in the catalog photo. Feed your soil and it will feed your plants.
What to Test For
The soil requirements of vegetables cover a wide range. More than 16 nutrients are needed, including nitrogen, phosphorus, and potassium (the big three, the NPK numbers you see on fertilizer bags), plus calcium, magnesium, sulfur, iron, manganese, zinc, copper, boron, molybdenum and chlorine. Sodium, silicon and nickel are essential for some vegetable species and beneficial for many others. Cobalt is needed if legumes are to fix nitrogen in the soil.
Many annual vegetables are known as "heavy feeders" because they need nutrients in such large quantities. Corn is famous for needing large amounts of nitrogen in the soil, but others include, beets, broccoli, Brussels sprouts, cabbage, cantaloupe, cauliflower, celery, collards, cucumber, eggplant, endive, kale, lettuce, parsley, pepper, pumpkin, radish, spinach, squash, sunflower, tomato and watermelon.
The Importance of pH
The acid-alkaline balance is soil is a vital part of plant nutrition. It is measured on what is known as the pH scale, which runs from 0 to 14. A neutral pH is 7.0. Blueberries grow best around pH 4.5 to 4.8, and potatoes are often grown at a pH of 5.4 or less, but these are the extremes---most annual vegetables like a pH between 6.5 and 6.8, just on the acid side of neutral.
Soil pH is important because it controls the availability of plant nutrients. Your soil can be rich in all the nutrients, but if the pH is "wrong" your plants cannot take them up. Many gardeners consider soil pH to be the most essential part of a soil test.
NPK Is Not Enough
The "big three" plant nutrients are nitrogen, phosphorus, and potassium, the NPK numbers. They are certainly essential, but if the other macro- and micro-nutrients are missing you will soon see that your plants need more than NPK. Compost is a great way to add organic matter to your soil, and it sustains and feeds soil life---the billions of creatures, from bacteria to earthworms and beetles, that create all the lesser-known nutrients that plants need.
Getting a Soil Test
Soil testing laboratories can be found by contacting your county Cooperative Extension office, listed in your local phone book. What usually happens is that they will sell you a soil sample kit, which comes with instructions, which vary from state to state and even county to county, depending on local soil conditions.
Follow the instructions, and in due course you will receive the soil test report showing the amounts of macro- and micro-nutrients in your soil, the soil pH reading, and (usually) the soil organic-matter content. There will also be a section with recommendations for how to amend your soil to bring it up to a satisfactory level.
After the Test
When you have added to your soil all the amendments recommended by the test results, wait for at least one week before planting. Soil amendments are rarely "instant," and your seeds and transplants will do much better if you wait for the amendments to take effect.