Horticulture Principles and Practices

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.