All plants have different pH preferences. The pH level of the soil directly affects soil life and the availability of essential soil nutrients for plant growth. Whether you’re planting for commercial or recreational reasons, knowing the pH of your soil can help you choose the right mix of plants and allow the right treatment of your soil.
Soil life refers to the living organisms that that live in soil and break organic materials down into simpler forms. Soil bacteria, a microscopic soil occupant responsible for the decomposition of organic material into simpler nutrient forms that become food for plants, thrives at about 6.3 – 6.8 pH. Fungi, mould and anaerobic bacteria, on the other hand, prefer a more acidic environment, making acidic soil more prone to souring and putrefication.
Availability of Nutrients
While soil life plays an important part in fertilizing soil, the pH of your soil determines the form nutrients will take, as well as their availability for plant absorption.
Table 1 illustrates the relationship between soil pH and the availability of various soil nutrients. When soil is acidic, minerals such as zinc, aluminium, manganese, copper and cobalt are soluble and available for plant uptake; however they can also be excessive in presence and therefore, toxic to plants.
Alkaline soil, on the other hand, may contain a higher quantity of bicarbonate ions, and this can affect optimum growth in plants by interfering with the normal uptake of other ions.
The Preferred pH Soil Levels
The most common range of soil pH is 4 to 8 pH and the range for optima availability of plant nutrients for most crops is 6.5 to 7.0 pH. Here are the pH ranges preferred by some of the common garden plants:
|Vegetables||Ideal pH||Garden Plants||Ideal pH||Herbs||Ideal pH|
|Artichoke||6.5 – 7.5||Ageratum||6.0 – 7.5||Basil||5.5 – 6.5|
|Asparagus||6.0 – 8.0||Alyssum||6.0 – 7.5||Chives||6.0 – 7.0|
|Beans||6.0 – 7.5||Aster||5.5 – 7.5||Fennel||5.0 – 6.0|
|Beetroot||6.0 – 7.5||Azalea||4.5 – 6.0||Garlic||5.5 – 7.5|
|Broccoli||6.0 – 7.0||Calendula||5.5 – 7.0||Ginger||6.0 – 8.0|
|Brussel Sprouts||6.0 – 7.5||Candytuft||6.0 – 7.5||Marjoram||6.0 – 8.0|
|Cabbage||6.0 – 7.5||Carnation||6.0 – 7.5||Mint||7.0 – 8.0|
|Carrot||5.5 – 7.0||Celosia||6.0 – 7.0||Parsley||5.0 – 7.0|
|Cauliflower||5.5 – 7.5||Crysanthemum||6.0 – 7.0||Peppermint||6.0 – 7.5|
|Celery||6.0 – 7.0||Columbine||6.0 – 7.0||Rosemary||5.0 – 6.0|
|Chicory||5.0 – 6.5||Coreopsis||5.0 – 6.0||Sage||5.5 – 6.5|
|Chinese Cabbage||6.0 – 7.5||Cosmos||5.0 – 8.0||Spearmint||5.5 – 7.5|
|Corn||5.5 – 7.0||Crocus||6.0 – 8.0||Thyme||5.5 -7.0|
|Cress||6.0 – 7.0||Daffodil||6.0 – 6.5|
|Cucumber||5.5 – 7.5||Dahlia||6.0 – 7.5|
|Garlic||5.5 – 7.5||Day Lily||6.0 – 8.0|
|Horseradish||6.0 – 7.0||Delphinium||6.0 – 7.5|
|Kale||6.0 – 7.5||Dianthus||6.0 – 7.5|
|Kohlrabi||6.0 – 7.5||Forget-Me-Not||6.0 – 7.0|
|Leek||6.0 – 8.0||Forsythia||6.0 – 8.0|
|Lentil||5.5 – 7.0||Foxglove||6.0 – 7.5|
|Lettuce||6.0 – 7.0||Gladiola||6.0 – 7.0|
|Mushroom||6.5 – 7.5||Gypsophilia||6.0 – 7.5|
|Mustard||6.0 – 7.5||Holly||5.0 – 6.5|
|Onion||6.0 – 7.0||Hyacinth||6.5 – 7.5|
|Parsnip||5.5 – 7.5||Iris||5.0 – 6.5|
|Pea||6.0 – 7.5||Lavender||6.5 – 7.5|
|Peanut||5.0 – 6.5||Lilac||6.0 – 7.5|
|Pepper||5.5 – 7.0||Marigold||5.5 – 7.0|
|Potato||4.5 – 6.0||Morning Glory||6.0 – 7.5|
|Potato – sweet||5.5 – 6.0||Nasturtium||5.5 – 7.5|
|Pumpkin||5.5 – 7.5||Pansy||5.5 – 7.0|
|Radish||6.0 – 7.0||Petunia||6.0 – 7.5|
|Rhubarb||5.5 – 7.0||Pinks||6.0 – 7.5|
|Shallot||5.5 – 7.0||Poppy||6.0 – 7.5|
|Soybean||5.5 – 6.5||Portulaca||5.5 – 7.5|
|Spinach||6.0 – 7.5||Primrose||5.5 – 6.5|
|Tomato||5.5 – 7.5||Roses||5.5 – 7.0|
|Turnip||5.5 – 7.0||Salvia||6.0 – 7.5|
|Watercress||5.0 – 8.0||Snapdragon||5.5 – 7.0|
|Watermelon||5.5 – 6.5||Sunflower||5.0 – 7.0|
|Sweet Pea||6.0 – 7.5|
|Sweet William||6.0 – 7.5|
|Tulip||6.0 – 7.0|
|Viola||5.5 – 6.5|
|Zinnia||5.5 – 7.5|
Table 2. Ideal Soil pH for different plants (Source: pH levels in Garden Soils, The Gardener’s Network )
Testing Soil pH
One of the easiest ways to correct the pH of your soil (both acidic and alkaline) is by adding compost. The alternative is to add an alkaline source (such as good limestone) to acidic soil, or an acidic source (such as pine needles or a peat moss) to alkaline soil. But before you begin, you need to know the pH value of your soil and the only way to know is to test it.
Soil pH can change throughout the year due to factors including rain type and depletion of certain nutrients. You should therefore monitor your soil pH before planting, during the preparation of beds, and regularly in random areas of your plot throughout the growing season.
One of the most convenient and accurate ways to test soil pH is by using a simple electronic meter. Soil testing for many applications rarely requires accuracy of more than a few tenths of a pH, so a simple, compact pH tester like the Economy pH pocket tester kit easily meets requirements, and is very economical in instances where several samples require testing. pH accuracy is affected by temperature fluctuations, so auto-temperature compensation is an important feature if changes in temperature are experienced.
How to Test Soil with a pH Pocket Tester:
- Scoop up loose soil samples with a clean, dry plastic jar. Avoid touching the soil with your hands to prevent contaminating the sample.
- Remove any stones and crush any clumps of soil to prevent damage or breakage to the electrode bulb.
- Mix your soil sample with distilled water (1:1 ratio) in a clean jar to form an emulsion. Cap the jar tightly and shake vigorously a few times. Allow the mixed sample to stand for 5 – 10 minutes so the salts in the soil can dissolve in the distilled water.
- Remove the cap of your tester, switch it on and submerge the electrode bulb fully into the wet soil slurry.
- Take the reading when it stabilizes. Record the reading. Repeat for higher accuracies if desired.
(A longer lasting and more accurate, but costlier, alternative is a larger handheld meter with a special soil electrode which allows you to measure your soil pH by simply wetting the soil with some water and inserting the probe directly into the soil to get a reading.
NOTE: Sample soil should still be examined for small stones. Even though the spear tip is rugged, hard surfaces like stones and pebbles can cause damage or breakage.)
Caring for your pH Tester
An electronic pH Tester requires very little maintenance and will serve you well for a long time if treated right. Here are some tips on maintaining your pH tester:
- Calibrate your tester regularly, about once a week if you use the tester 2 or 3 times a week. Calibrating your pH tester is very easy. Simply dip your tester into a pH buffer solution and press CAL. The tester detects your buffer value and auto-calibrates.
- Rinse your tester after each use, and keep the electrode conditioned for next use by soaking it in tap water or electrode storage solution. NEVER store your electrode in deionised water, solvents, acids or pH10 buffer! This will kill your electrode.
- Do not use your tester in liquids at temperatures above 80°C, or with high concentrations of heavy metals, sulfides, proteins or oil.
Updated January 2018
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