The acidity or alkalinity of soil, measured in pH units, is important because it affects the availability of some of the major nutrients needed by plants for growth. Some plants are much better at adapting to lower or higher pH levels, acidity or alkalinity, and knowing the pH of your own soil allows you to either plant what will grow well there, or change the pH by adding amendments.
Acidity and Alkalinity
The acidity or alkalinity of water or soil is measured in pH units. A pH of 7 is neutral. Numbers lower than that are acid and the lower they are, the more acid the soil or solution. Numbers higher than 7.0 are alkaline, and the higher they are, the more alkaline. Lemon juice and vinegar, for example, have a pH of about 2.2. Tomatoes are about 4.5. Baking soda is 8.2, ammonia is 11.0 and lye is 13.0.
Soil tests need to take an average of the conditions all over the garden, so it's important to take five to 10 samples from each area, at a depth of 6 to 8 inches, and mix them together. Use separate mixtures for vegetable gardens, flower gardens, lawns and shrub borders because each group of plants has different needs and may have been treated with different products in the past. Inexpensive home test kits are available at stores and are fairly reliable.
pH Effects on Nutrient Absorption
At a pH below 6.0, nitrogen, phosphorous and potassium become less available to plant roots. The soil will usually be low in calcium and magnesium also. In alkaline soils, many of the micronutrients, the elements needed in small quantities, become less available. Phosphorous becomes restricted above pH 7.5.
Some plants that are considered to be acid-loving, such as potatoes, are acid-tolerant and grow well in a more neutral soil but are grown at lower pH levels because of disease resistance or simply to have a crop that does well under those conditions. Others, such as rhododendrons and blueberries, require a pH lower than 6.0.
Soil Factors Affecting pH
Clay minerals in the soil and organic matter both act as buffers, agents that stabilize the pH of the soil and resist changes. This can be beneficial if the present pH is right for the plants you want to grow, but when you wish to change it, you will need to use more of the amendment than you would in sandy, organic-poor soils. Organic matter also tends to make the soil somewhat more acid.
Adjusting The pH Of Soil
To raise the pH of an acid soil, add ground limestone. The more finely ground it is and the more thoroughly mixed it is into the ground, the more quickly it will act. A coarse grind may take a year or more to be effective. Lime is also an excellent source of calcium and magnesium, essential elements for plant growth.
To lower the pH of a soil, use elemental sulfur. Because it reacts slowly in the soil, it is best to dig it in at least 6 inches deep a year before planting.