Saturday, December 5, 2009

Soil pH and Garden Success

When a garden hasn’t been doing well, one of the first conditions to check is the soil’s pH---its acid-alkaline balance. The pH scale goes from 0 to 14, with 7 being a neutral pH, measurements below 7 being acid, and above 7 being alkaline. Most garden veggies need a soil pH of between 6 and 7. Around here, the soil is naturally acidic, but many people routinely lime their lawns and gardens, without checking the soil pH first, and end up with a soil pH that is too high.

This might seem like an odd topic for a winter blog post, but correcting a pH that is either too high or too low takes time, so it should be considered now, or soon.

My friend who decided to plant blueberries in the space where she has had several years of a failed vegetable garden sent a soil sample in to the Extension Office for evaluation at the testing lab at UGA. The lab found that the pH of her soil is 7, which isn’t a huge surprise because she had been spreading ashes from her fireplace on the garden, which raises the pH. Even if she hoped to plant veggies there again, the lab would have recommended that she bring the soil pH down a little, but the blueberries she intends to plant prefer a pH closer to 5. That soil needs some work before it will be a good place for blueberries.

The lab sent information about how much sulfur to add in order to bring the pH down to a better level for blueberries, but it also recommended that the bushes shouldn’t be planted until 6 months after applying the sulfur. If she added the sulfur in October, that means blueberry-planting should wait until April. That is pushing the boundary for good planting time in other ways; as spring progresses and the weather warms, newly planted bushes are less likely to do well. They need time in the soil for their root systems to become established before being stressed by the heat of a Georgia summer.

What this means is that any soil that is very far off the optimal range for the plants that will be planted in spring needs to be tested now, so adjustments to pH can be made soon enough to benefit the plants.

For a pH that is only slightly too high, my county’s old (newly retired!) extension agent used to recommend mixing some peat moss into the soil as a quick fix. Coffee grounds can also help bring down a too-high soil pH, and I dump mine routinely around my azaleas and blueberries to help keep the soil around those acid-preferring plants in a good pH range for them.

An article by Lewis Hill, published in Robert Rodale’s The Best Gardening Ideas I Know (1983) includes a list of some garden plants and the pH ranges they prefer, and I’ve pulled some of the food plants from his list to post here:

pH 4 to 5: blueberry
pH 5 to 5.75: blackberry, grape, parsnip, plum, potato, pumpkin
pH 5.75 to 6.5: bean, citrus fruits, cowpeas, currants, gooseberry, oats, pepper, rutabaga, rye, squash, strawberry, tomato, turnip
pH 6.5 to 7: apple, beet, broccoli, buckwheat, butternut, chicory, chives, cucumber, eggplant, endive, kale, leek, muskmelon, onion, pea, peach, radish, raspberry, rhubarb, spinach, watermelon, wheat
pH 7 to 7.5: alfalfa, asparagus, barley, cabbage, carrot, cauliflower, celery, lettuce, nasturtium, parsley

This list may help other gardeners in planning what to plant where, but it is useful to remember that plants will still grow and produce in soil that is slightly outside their preferred pH range. This is lucky, since most of us grow many of these plants mixed all together in our gardens. However, for peak production, planting in soil that is actually the preferred pH works best.

When I plant potatoes in the spring, I will have added quite a lot of coffee grounds to their soil to bring the pH down a bit, but not all the way down to 5. Knowing the pH preferences of other crops helps me know what to plant after the potatoes in that space. For example, cabbages in that space would likely be a total bust, because their pH preference is so much higher (7 to 7.5!). When the potatoes are harvested, I will either let the nearby melons sprawl across that space, or I will plant a crop that prefers the 5.75-6.5 range, like beans or cowpeas.

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