Ask a Master Gardener: How do I test, adjust pH in my garden?


Garden compost

Llyn Doremus shovels compost out of her backyard bin to use on her raspberry plants in this April 3, 2009 photo. Compost contains lots of beneficial microbial activity and nutrients for your plants.



    Master Gardener Kathleen Bander is a resident of Bellingham and life-long gardener. For more information on Whatcom County Master Gardeners, go to Ask a Master Gardener appears in The Bellingham Herald monthly. If you have a gardening question you'd like answered in the column, please email it to

Question: I am planning my first vegetable garden and know that soil is the paramount thing, so can you give me pointers about the pH levels and what I will need to do?

Answer: You're steps ahead knowing that soil is your most important consideration at this point. And the level of acidity or alkalinity in your soil (the pH level) has everything to do with how your garden will flourish. That's because some plants like a more acidic soil: blueberries, blackberries, strawberries, radishes and watermelon. Most other vegetables, including beans, tomatoes and lettuce, like alkaline conditions.

A soil's pH is the result of interaction among native rocks, plants and weather conditions over many years, and varies with climate and physical surroundings. Our moist climates in the Northwest usually produce pH between 4.0 and 5.5. Generally, a soil is said to be alkaline if it measures 6.0 to 7.5. It is acid if it falls between 4.5 and 4.8.

Here's the first step for the home gardener. Test your soil. At a nursery or big-box store, purchase a home color kit. They're fun to do, and done correctly can give you accurate readings for pH in your soil. Once you've obtained the test results, you will do one of two things: nothing, or lower the acidity.

By nothing, I mean that if your soil tests on the acidic side and you want to plant blueberries or any other acid-loving plants, do nothing. But above all, don't add lime. These plants need acid in the soil.

On the other hand, if your vegetable beds test low, under 6.0, you need to bring the pH up. Begin by adding a good amount of compost, then use dolomite lime; follow the packaging directions carefully.

Because we have so much precipitation in the Northwest, you'll need to test your vegetable garden's pH every other year, and make any adjustments to keep it as close to 7.0 as possible.

Q: If I were a potter, the clay soil in my backyard would be a gold mine, but all I want to do is grow some of my own flowers and vegetables. Any suggestions about how to amend the clay into something that plants would like?

A: Let's start with the good news: Clay soil has the highest nutrient content of all the three major soils - sand, silt and clay. Many of the minerals contained in clay soil aren't washed away like they are in the other two soils.

The bad news, of course, is that most plant roots don't like clay soil, as they find it too difficult to punch through the harder material. However, there are ways to amend the soil, so don't despair.

Increasing the soil's organic matter is the most important step. Add lots of compost, grass clippings, sand or peat moss. Concentrate on the addition of compost, as it contains lots of beneficial microbial activity and nutrients for your plants.

If all this seems like a little too much work, you can always build raised beds and fill them with a good quality soil. Most plants do quite well with only 8 inches of soil, so they'll grow above your clay soil. You can either just mound the good soil up, or enclose it in wood or rock to keep the soil from escaping. Just make sure there's 8 inches of soil to keep your plants happy and growing.

Q: I'm planning my garden and might want to grow beans to dry. Can that be done in our climate?

A: The quick answer is yes! And it's a good way to go. Dry beans provide as much protein as eggs and cottage cheese, and they have the added benefit of fiber and a wide array of minerals.

You need to choose bean varieties that suit our climate. A few are China yellow, Indian woman and black turtle beans. These all look like regular snap beans, but they quickly grow too big and tough to eat as pods.

All beans will produce dry beans, but the specific types to grow if you want a good crop of dry beans need to be planted a little later than most other beans, so they'll have time to dry in the early fall weather. Sow them no sooner than two weeks after the last frost date.

Never soak bean seeds in water prior to planting them; the water deprives the embryo of oxygen. Fertilize lightly, as overfertilized plants grow monstrously large with few beans.

Leave drying pods on the vines as long as you can, but harvest them before the serious rains start. To remove the beans from the pods, put the pods in a pillow case and walk all over it. Or tie the pillow case tightly shut and run it through a clothes dryer cycle on medium heat.

Store the beans in a cool, dry, dark place.

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