In addition to available moisture and adequate drainage, the most important property of a soil for horticultural applications is its pH level or soil reaction. The range of a soil's acidity or alkalinity is expressed in pH values. A reading of 7 is considered neutral.

Most important nutrients become available for plant growth in a slightly acid soil with a pH of about 6.5. If a soil is quite acidic (below 5.5) then nitrogen, phosphorus, and potassium become virtually unavailable to plant roots while other nutrients such as iron, manganese, and boron become readily available. Acid-loving plants such as Rhododendron and Azalea can tolerate and thrive in this type of soil. Many plants are also intolerant of alkaline soil; this can be more of limiting factor to growth than soil acidity.

Since all plants have specific tolerances for pH, a soil test is advisable to indicate the pH level. With this knowledge you can determine if a soil must be modified in order to grow desired species. A testing service is offered by Cornell University. When you receive the results of your soil test, you will also receive recommended additives to improve the soil. 

Generally, a limestone product is added to raise a soil's pH or alkalinity, and a sulfur product is added to lower pH or increase acidity.  The amount to add will depend on the area you are covering, the texture of the soil and the amount of adjustment in pH required.  Application is best made in the fall or winter months to give the soil a chance to stabilize before planting.

For additional soil considerations to address when planning a garden you can refer to our guide Site Inventory and Analysis for Landscape Design.

Courtesy of the NYBG Plant Information Service