Soil pH is a measure of acidity or alkalinity. This measurement can range from a very low pH of 1.0 to an extremely high pH of 14.0. 7.0 is considered neutral and several vegetables will thrive in soil with a neutral pH. Some vegetables do prefer a more acidic soil while others prefer a more alkaline soil. Fortunately most will grow and produce well in a pH range of 5.5 to 7.5. Areas with high rainfall, such as coastal Washington, tend to have acidic soils high in organic matter, while areas with lower rainfall, such as most of the mountain valleys in Montana, tend to have alkaline soils low in organic matter. Here in the valleys of the Flathead region, where most of us garden, soils tend to be somewhat to highly alkaline.
Before you amend your existing soil to plant vegetables it is a good idea to test the pH. An easy way is to purchase a test kit. Most of them are easy to use and give fairly accurate readings. Your county extension agent can do a soil test also. Once you have determined your soil pH and which crops you intend to grow, you can amend your garden soil. Adding lime will increase pH and make acidic soils more alkaline; adding sulfur will lower pH and make soils more acidic. Organic matter usually helps reduce alkalinity. Aged manure, pine needles, compost and coir dust are a few amendments that will help do this while they improve soil structure and encourage beneficial soil micro-organisms. If your garden is divided into separate beds or raised beds, it is easier to adjust the soil in each bed for what you plan to grow. I rotate my vegetable crops in a four-year rotation plan, so I try to keep a basic soil pH around 6.0 to 6.5. This way I can grow almost every vegetable, but I slightly adjust pH each year before growing a particular plant. For example, I add sulfur before growing potatoes. Conversely, I add a small amount of lime before planting any of the Brassica family (Broccoli, Brussels sprouts, cauliflower, etc.). Having soil pH correct for each crop will reduce pest and disease problems as well as increase yields.
Here is a listing of pH tolerance ranges for specific vegetables. (The optimum pH for each is usually the median between the two extremes.) I have listed vegetables preferring more acidic soils first and those preferring more alkaline soils last:
Potatoes, 4.5-6.0; sweet potatoes, 5.6-6.5; horseradish and rhubarb, 5.5-6.8; butternut squash, carrots, corn, eggplant, lettuce, peanuts, peppers, pumpkins and watermelon, 5.5-7.0; cucumbers, garlic, winter squash, and tomatoes, 5.5-7.5; celery, 5.9-6.9; soybeans and strawberries, 6.0-6.8; onions, radishes, shallots and spinach, 6.0-7.0; beets, any of the Brassica family, peas, summer squash, Swiss chard, and zucchini, 6.0-7.5; okra, 6.0-8.0.