Q. What is soil pH and how do I test soil?
For most home gardens, soil needs to be tested every 2 to 3 years. Although many factors such as type and size of particles and amount of organic matter determine a soil's nutrient holding capacity, drainage and suitability for plant growth, soil pH will indicate whether a soil is acidic, neutral or alkaline. The pH is expressed in a scale from 0-14 where 0 is acid, 7 neutral and 14 is alkaline. Most garden plants in our area do well in a slightly acidic soil with a pH range from 5.5 to 7, although certain plants will have a particular growing preference. A pH reading below 5.5 is quite acidic and is more suitable for growing acid-loving, ericaceous plants such as Rhododendron and Azalea, or blueberries. If a pH is over 7 it indicates that the soil is alkaline and more suitable for lawns and certain vegetables, or herbs.
To determine the pH reading of your garden soil, obtain a home soil-testing kit and follow the instructions to take a soil sample and test it. Since soils vary considerably on a site, it is best to take random soil samples and mix them together to get an average reading. For lawns, take a sample from the top 2 to 4 inches of soil. If the area to be sampled is for trees and shrubs, a sample taken to the depth of 8 to 12 inches is recommended. If the soil from your sample is found to be too acidic for the type of plants you want to grow, you will need to adjust the pH by adding lime; if the pH is too high, it can be lowered with the addition of sulfur.
A complete soil analysis will test for nutrients in addition to pH. The most important nutrients for plant growth include nitrogen, phosphorus, and potassium (N-P-K) although the presence of other minerals, called trace elements or micronutrients, is necessary for plant growth in lower proportions. If an area in your garden has been used over and over again to grow the same types of plants, i.e. vegetable crops, the soil may become depleted in certain trace elements over time. Contact your local Cooperative Extension Service for information on complete soil testing services available to you.
It is also advisable to test your soil for composition, as the types and distribution of particles within a soil will influence its drainage capacity and the availability of nutrients to plant roots. Take a handful of soil, if it feeds gritty, it is mostly sand; if it feels slippery, clay; if crumbly, loam. If there is organic matter present, a black coating will remain on your hands after moistening them and squeezing a handful of soil. Count the number of earthworms in a soil sample, 1' x 1' x 6"; if there are at least ten, your soil content is sufficiently organic.
Courtesy of the NYBG Plant Information Service