© 1997 Henrik Nordström

Soil fertility is the single most important factor determining the success of a garden. Most topsoils in Western Washington are quite shallow, having been formed by the decaying parts from the coniferous trees which once covered most of the area mixed with the top layer of the inorganic glacial soil. Virgin soil is quite acidic, so most of its fertility is tied up and unavailable to plants. The gardener must therefore raise the soil's pH-value by liming to make it less acidic so that the nutrients will be available to the plants.

The soils's fertility is determined solely by its inorganic content, i.e. the relative portions of nitrogen (N), phosphorous (P) and potassium (K) in various soil compounds. These three elements are called macronutrients. In addition, there are several so-called micronutrients and trace elements which occur in very low concentrations but are nevertheless extremely important for optimum plant development. These include calcium, magnesium, sulphur, iron, boron and many more. It is important to note that while liming makes the macronutrients more available to plants, excessive use of lime will restrict the availability of some of the micronutrients, especially iron. As a rule of thumb, an acidic soil which has not been limed for several years can be brought to optimum level for most vegetables (pH 6.0 - 6.5) by applying approx. 4 lb. of lime per 100 square feet. Most lime sold these days is so-called dolomite lime, which means that it also contains magnesium. While magnesium is an important plant nutrient, it also increases the price of the lime, and if a soil test shows that the soil contains enough magnesium, it is advisable to use regular lime. Avoid liming your potato and carrot beds within 12 months of planting to prevent scab on the potatoes and distortion of the carrots.

Nutrients contained in organic matter are not directly available to plants, and must first be broken down by the soil's micro-organisms into inorganic components. Organic matter is nevertheless of crucial importance to plants, since it aerates the soil, making oxygen available to the plants' roots, and equalizes the moisture structure of the soil.

A few areas in Western Washington contain naturally fertile soils, where vegetables will grow well with little or no addition of fertilizer. Of our Seattle P-Patches, the Picardo Farm has a very fertile soil. However, in most locations, the soil's fertility must be artificially raised by adding fertilizer. Manures and many other organic fertilizers contain all the macronutrients and micronutrients that plants need. Organic fertilizers free of animal by-products can be mixed from e.g. cottonseed meal, rock phosphate and greensand. Many ready mixes are also available in organic gardening stores.

Even on naturally fertile soils, it is important to add organic matter in the form of compost to prevent nutrient depletion and maintain a good soil structure. Any plant parts which are not used by the gardener should be given back to the earth. Non-noxious weeds do not necessarily need to be composted first; just place the pulled weeds around the vegetable plants as an organic mulch. Weeds can also be composted on your plot in just a few weeks by placing them in a black plastic bag.

A word about soil test kits: Most of them do not give very reliable results. The best thing to do is to take several samples from various parts of your garden, mix them well together and take a sample of this mixture to send to a soil testing lab. There are several commercial and university labs which do soil tests, and it is also possible to test the soil for toxic contamination, e.g. lead. Please call the P-Patch office for more information.