Soil pH varies by up to half a point over the year. Soil pH tends to be higher (more alkaline) when the soil is cool, and lower (more acidic) in summer, when increased bacterial activity in warmer weather has an acidifying effect on soil. Factor this in when changing soil pH.
Garden soil pH is usually neutral to slightly acidic, pH 6.5-7.0. This also happens to be the ideal soil pH for vegetables. However, if you want to really tweak performance:
Green, leafy vegetables (like spinach and lettuce), cruciferous vegetables (like broccoli, cauliflower, cabbage, and kale), and other Fall Vegetables prefer a more alkaline soil, pH 7.0-7.2.
Fruiting plants, like nightshades (tomatoes, peppers, eggplants) and cucurbits (cucumbers, melons, squash) prefer a more acidic soil, pH 6.0-6.8. See Summer Vegetables for more information on the preferences of these summer favorites.
Adding Organic Matter Buffers
Soil Against pH Swings
Adding organic matter is an indirect method of soil pH adjustment. Organic matter “buffers” soil, especially sandy soil.
More About Soil
Facts About Soil
Gardening in Sandy Soil
Improving Clay Soil
The Soil Food Web
The higher the organic matter content of a soil, the more lime it takes to raise the soil pH 1 point, and the more sulfur it takes to lower the soil pH 1 point.
Plants grown in soil with a lot of organic matter have healthier roots. They’re able to extract enough nutrients from the soil even when the pH isn’t optimal.
In a healthy soil with adequate organic matter, changing soil pH may not be necessary, because plants continue to grow at pH levels that would stunt growth in leaner soils.
When you increase soil organic matter, you’re not really changing soil pH, you’re increasing your plants’ tolerance for acidic or alkaline conditions. For information on increasing soil organic matter, see Improving Garden Soil.
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