Just as some yards lack adequate sunlight to grow roses, some lack soil hospitable to growing vegetables. Filling planters or containers with a soil-free, or soilless, growing mixture can expand your gardening options to include homegrown vegetables and herbs. While you can purchase prepared soilless mixtures, making your own is easy and inexpensive.
Garden Soil Problems
Challenging conditions in basic garden soil usually stem from texture or mineral content. Loamy soil is light, rich in organic material, high in nutrition and capable of the balanced moisture retention and good drainage that supports good growth for a wide range of garden plants. Sandy soil tends to drain quickly, letting plant roots dry and wither, while heavy clay obstructs root growth and can drown plants. Quick-growing annual vegetables rely on light-textured, porous soil for rapid root expansion. Soil pH, the measurement of hydrogen ions available to help plants process soil nutrition, is also critical to successful vegetable gardening. Natural soil pH can range from highly acidic to highly alkaline. Most vegetables require neutral to slightly acidic soil. Adding peat moss, ground lime, or compost needed to adjust natural soil chemistry is strenuous and repeatedly demanding.
Other Soil Issues
Existing soil can contain two varieties of pathogens that make vegetable growing difficult or impossible. One type is the allelopathic enzymes generated by existing plants. Walnut trees, especially black walnut, sunflowers and onions are well known for generating chemicals that are toxic to other plants. Although the exact interaction is unclear, experienced gardeners advise that planting beans and onions close together stunts the growth of both. Tomatoes, eggplants and peppers wither and die anywhere near the dripline of a black walnut tree. Soil organisms cause root rots and vascular wilts, while bacteria and nematodes can impede plant growth as well. Vegetable plants are vulnerable to these pathogens if weakened by excessive or inadequate watering, poor nutrition or harsh weather during a short, annual growing season.
Soilless Planting Mix
Soilless planting mixes are specifically formulated to address problems of soil texture, pH and pathogen content inherent in natural garden soil. A classic mix contains 1 part peat moss or other fibrous material for light texture, 1 part vermiculite, for porosity, and 1 part perlite, for moisture retention. Ingredients, whether prepared or purchased separately, are sterilized and pathogen-free. Peat moss, or coconut coir fiber, is slightly acidic, while vermiculite and perlite are both heat-treated inert minerals, creating an essentially pH-neutral mixture. Adding small quantities of composted pine bark can mildly increase acidity, while ground limestone increases alkalinity.
Because potting soil is low in nutrients, fertilizer supports the growth of fast-growing, heavy-feeding container vegetables. Fertilizers are described by their balance of nitrogen, phosphorus and potassium, abbreviated NPK. A 5-10-5 NPK, or a balanced 10-10-10 fertilizer plus a superphosphate, 0-20-0, can be added to your soilless mixture before planting. As one container-garden example, Texas A & M suggests supplementing 1 bushel each of peat moss and vermiculite with 10 tablespoons. ground limestone, 5 tablespoons superphosphate and 1 cup of 5-10-10 garden fertilizer; divide all ingredients equally to make smaller amounts. A supplemental solution of 14-14-14 fertilizer, heavily diluted with water, provides moisture for plants throughout growth Possible fertilizer accumulation is flushed once a week with plain water. Because potting mix also lacks adequate trace elements, like calcium, iron, manganese and boron, your fertilizer should contain those as well. Supplementing a good soilless potting mixture with a balanced fertilizer will support the healthy productive growth of vegetables. Read and follow directions carefully.