A soil test will help you to determine the 'plant available' nutrients in your soil, the soil pH, and the toxicity levels (if any) in your soil.
Soil testing is the process by which elements are chemically removed from the soil and measured for their 'plant available' content within that sample. These nutrients are phosphorus, potassium, calcium, magnesium, sodium, sulfur, manganese, copper and zinc, their quantity in the sample determines what kind and how much fertilizer is recommended.
Testing your soil will also help you to measure the soil pH, which will determine whether your soil is alkaline or acidic. This is important to your plants because at various soil pH levels, nutrients become more or less 'available'.
Finally, soil testing can help you determine whether youe soil is contaminated or contains potentially toxic materials. This is essential if your are going to plant food crops in your garden, since plants will absorb these toxins that you will eventually ingest.
Brooklyn College has an affordable soil testing service which is offered as part of the College's commitment to community. Listed below are the tests available from Brooklyn College, what they measure and their cost.
Toxic Metals - $20
Samples are analyzed for 5 toxic metals:
Standard Nutrients - $12
Samples are analyzed for major and micro nutrients including:
Soil pH - $5
Soluble Salts - $5
Organic Matter Content - $8
Soil Texture Analysis - $12
Visit the Brooklyn College website: http://www.brooklyn.cuny.edu/pub/departments/esac/1535.htm