TABLE 4–11 Selected Standard Mixes 1. Cornell Peat-Lite Mix A Quantity Materials Per Cubic Yard Per Cubic Meter Sphagnum peat moss 0.5 yd 3 0.5 m 3 Vermiculite 0.5 yd 3 0.5 m 3 Ground dolomitic limestone 5 lb 3 kg Single superphosphate 1–2 lb 0.6–1.2 kg Calcium of potassium nitrate 1 lb 0.6 kg Fritted trace elements 2 oz 74 g Wetting agent 3 oz 111 g 2. Cornell Peat-Lite Mix B Substitute horticultural perlite for vermiculite 3. The University of California Mix Quantity Materials Per Cubic Yard (lb) Per Cubic Meter (kg) Hoof and horn or blood meal (13%) 2.5 1.47 Potassium nitrate 0.25 0.15 Potassium sulfate 0.25 0.15 Single superphosphate 2.5 1.47 Dolomitic lime 7.5 4.42 Calcium carbonate 2.5 1.47 The University of California Mix, as constituted above, should be used fresh; if storage is needed, the mix should exclude the hoof and horn or blood meal. 2. Good drainage and aeration. The mix should be well drained to reduce the danger of overwatering, which can cause a perched water table. Most plant roots are intolerant of anaerobic conditions. Materials such as perlite and sand are used for good aeration. Good soil drainage guards against the buildup of harmful salts. 3. Proper pH. Improper soil reaction interferes with nutrient availability for plants. Certain organic materials used in mixes produce acidity or alkalinity in the medium. The use of lime is necessary to correct soil acidity. 4. Fertility. Most soil mix ingredients do not contribute appreciably to the fertility of the growing medium. Additional materials such as fertilizers are needed to provide the required nutrition for proper plant growth and development. Advantages of Soilless Mixes The major advantages of soilless mixes include the following: 1. Uniformity of mix. The physical and chemical properties of a mix are uniform throughout the mix, a condition not found in field soil. The homogeneity of the medium makes it possible for plants to grow and develop uniformly, provided other growth factors are also consistent. 2. Ease of handling. Mixes are lightweight and easy to transport. Ingredients are easy to scoop and move around during manual preparation. 3. Versatility. Mixes can be made to order (custom mixed) for specific needs; can be used to amend soils in the field by mixing into flower beds, lawns, or garden soil; and are convenient to use in container plants. 4.10 Potting Media 147
4. Sterility. Mixes, initially at least, are free from diseases and pests. Seedling germination is less prone to diseases such as damping-off. 5. Good drainage and moisture retention. A mix can be custom-made to provide the appropriate degree of drainage and moisture retention. 6. Convenience of use. Mixes are ready to use when purchased. Disadvantages of Soilless Mixes Disadvantages of soilless mixes include the following: 1. Light weight. Some mixes are very light, especially when dry. When used in potted plant production, they are easily toppled over by even a gentle wind or push. 2. Limited nutritional supply. Mixes that incorporate fertilizers provide nutrition for a limited period of time. The component ingredients of mixes are generally void of any appreciable amount of plant growth nutrients. Micronutrients are especially lacking and should be supplemented with an appropriate fertilizer program when growing a crop. 3. Lack of field correspondence. Mixes are constituted to provide minimal problems to germinating seeds and rooting plants. As such, the physical conditions of mixes are different than field soil conditions. Roots grow rapidly and ball up in pots. During transplanting into the field, care should be taken to create good root contact with the soil. Some plants may not establish quickly if root contact with the soil is poor. 4. Cost. Some mixes are expensive (but worth the investment). 4.11.1 FIELD STERILIZATION 4.11 SOIL STERILIZATION Sterilizing field soil can be accomplished by harnessing the energy of the sun. Solar pasteurization, or soil solarization, is a method of pasteurizing the soil by using solar energy. The goal of pasteurization is to rid the soil of harmful bacteria, fungi, nematodes, and weeds. The area to be treated is first cleared of grass and weeds. The soil is then cultivated, raked, and watered uniformly to a depth of about 12 inches (30 centimeters). A clear plastic is stretched tightly over the area and tucked under a border of soil. The plastic cover should be left in place for about four to eight weeks for effective solarization. Black plastic reflects heat, even though it controls weeds much better. Beds with northsouth orientation receive sunlight most of the day, and the effect is better. Field temperature under solarization can exceed 120°F. Plant diseases known to be controlled by this treatment include fusarium wilt, verticilium wilt, pinkroot, and southern blight. Weeds effectively controlled include cheeseweed (Malva neglecta), annual bluegrass (Poa annua), and pigweed (Amaranthus spp.). Plants grown in recently solar-pasteurized soil grow faster and larger than those in nonpasteurized soil. The soluble nutrient levels in soil with reference to nitrogen, calcium, and magnesium have been found to increase after solarization. Solarization is used by organic gardeners. 148 Chapter 4 Plant Growth Environment 4.11.2 STERILIZATION OF GREENHOUSE SOIL Growing media for indoor use can be sterilized by a variety of methods. The goals are identical to sterilization of field soil.