COMPOST IS NOT A SOIL SUBSTITUTE
Compost can be mixed with soil as an amendment, used as a top dressing in perennial gardens and around trees and shrubs, and even sprinkled lightly as a top dressing on lawns. But compost by itself should never be considered a planting medium. For raised bed gardening it is recommended to blend 1 part compost to 3 parts soil.
COMPOST AND NITROGEN
Compost is noted for a number of positive benefits when mixed with soil including its moisture holding capacity, beneficial microbes, disease suppression, high organic matter, major nutrients (NPK), Calcium, Magnesium, micronutrients and enhanced cation exchange capacity. However, it is not always practical to use only compost for 100% of the nutritional needs of plants – particularly nitrogen loving plants like lettuces, spinach etc. That’s because only a small proportion of the N will be available to crops the first year. If the compost is finished, one can usually assume that about 10% of the total N will become available during the year of application. Under these circumstances, it will be beneficial to side dress greens and other nitrogen loving plants with some form or organic nitrogen such as blood meal, cottonseed meal, fish emulsion, Pro Gro etc.
COMPOST AND MOISTURE RETENTION
One of the major but little known benefits of utilizing compost as a soil amendment is its ability to retain moisture. One cubic yard of compost will typically hold 100 gallons of water. When worked into the soil for vegetable gardens, ornamental, tree and shrub plantings, new lawns etc. BPC compost will dramatically help plantings thru dry periods as the growing season progresses. Unlike peat moss (a product mined from Canadian peat bogs) compost is a local product made from recycled materials and holds as much of more water as peat moss in addition to its numerous other benefits.
SOIL TESTING IS EXTREMELY IMPORTANT
Without knowing the nutrient and microbial condition of your soil, it’s impossible to consistently grow plants that meet your expectations. All successful farmers frequently test the soil from their various fields. Most farmers will even have the soil from a field or a section of a field tested for a specific crop such as tomatoes, sweet corn, winter squash etc. The results of these crop specific tests help the grower fine tune their nutrient management program.
Home gardeners should do the same, but do not have to be so crop specific. Knowing the nutrient and organic matter needs of your garden soils and amending these soils as needed with lime, compost and/or other nutrients is extremely important for long term growing success. Just adding lots of compost year after year and nothing else may not result in consistent plant growth and production. It’s recommended that all gardeners at some point collect samples of their soil for analysis by the UMass Soil Lab http://soiltest.umass.edu/ or some other experienced testing laboratory. This type of testing is far superior to home test kits in terms of both accuracy and professional recommendations and service.
Compost and pH
The pH of many commercially available composts tend to be slightly alkaline. Although a goal of compost production is to have finished compost as close as possible to a neutral pH of 7.0, this is rarely the case given the variety of raw materials that most compost operations process annually. The compost produced by Bear Path Compost consistently has a pH above 7.5 and has averaged about 7.9 over the last 16 years. The desirable soil pH for most vegetables is around 6.8. However, discussions with UMass Extension have confirmed that slightly alkaline compost when thoroughly mixed in soil will not have any noticeable effect upon soil pH. Further discussions with several other sizable compost operations including The Vermont Compost Company, Vermont Natural Ag (Moo Doo) and Green Mountain Compost have verified that their composts, made from raw materials similar to those used at Bear Path Compost, are all in the alkaline range 7.5 to 7.9.
When routinely using a top dressing of compost around acid loving food plants such as blueberries or ornamentals such as azaleas and rhododendrums. it is recommended to periodically test the pH of the soil around these plants to determine if an acidifying soil amendment may be needed.