If I have been adding manure and compost to my vegetable garden for years can I assume that the nutrient levels are adequate, making a soil test unnecessary?
A. Manures can add salts, which can build up over time and actually stress out your plants. At moderate levels, salts add fertility to the soil but at high levels they can become unbalanced and toxic. Plant roots have a more difficult time extracting water from the soil water when the Electrical Conductivity, a measure of total salt content, is above 4.0 dS/m or mmhos/cm. Mushroom compost, in particular, can have excessive potassium, sodium and chloride levels and should be regarded more as a fertilizer than a traditional compost. Unlike compost, it is a poor choice for the addition of organic matter because it will reduce its volume by about half within a year because it is still in a state of decomposition. If you choose to use mushroom compost, use no more than a one inch layer and make sure that it is well mixed into the soil. Also, do not use it as a potting soil. Many gardening books recommend the use of wood ashes to supply potassium. Too much ash can elevate soil alkalinity and
