“Hermanos en el camino” y su praxis social

Soil can be sterilized by using certain fumigants. Chemical sterilization is less effec- tive than steam sterilization and is toxic. The soil cannot be planted immediately after the process, unlike after steaming. Depending on the chemical used, a waiting time of about 24 hours to several weeks may be needed. Further, during the application, the room must be vacated to prevent worker poisoning. To effectively permeate the soil, it must not be cold but a desirable warm temperature that differs according to the type of chemical used. Chemicals used include chloropicrin (or tear gas). Because of the ef- fect on humans, precautions must be taken to control the gas, such as watering the area after application to provide a water seal or providing the treatment under airtight cover. Further, chemical sterilization should be used only when the greenhouse is vacant (no plants).

Other chemicals used in sterilization include formalin and vapam. Vapam is not toxic to humans but must not be used in their presence. Treated growing media are ready for use only after several weeks. Some plants are sensitive to the residual effects of certain chemical treatments. For example, carnations are damaged when grown in media treated with Dowfume MC-2 (a mixture of methyl bromide and 2 percent chloropicrin). This chemical is extremely toxic to humans and must be used with great caution.

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As indicated earlier, soil is the most popular medium for the cultivation of crops. Soil, or any solid medium, provides physical support for plants to grow. Today, plants are culti- vated on a large scale under controlled-environment conditions indoors. A variety of

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media have been developed to facilitate such an undertaking. Perhaps the most revolu- tionary is growing plants in liquid media (water fortified with nutrients). The science and methods of water culture, or hydroponics, are presented in detail in Chapters 4–13.

One of the earliest and still widely used liquid media in plant culture is Hoagland’s

nutrient solution, an all-nitrate solution. Various nutriculture systems have their own

unique liquid media recipes. Crop needs and cultural conditions call for special nutrient systems to be used for the successful production of specific crops.

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Plants require appropriate temperature, moisture, light, and air for good growth and development. These conditions occur both above- and belowground. In addition, plants need a medium in which to grow, the common one being soil, even though modern tech- nology enables plants to be grown in soilless media. Air is required for respiration (for energy); water is needed for germination, photosynthesis, and translocation of solutes within the plant, among other functions. Light is needed for photosynthesis and also af- fects reproductive activities in some species through photoperiodism. Light affects plants through its intensity, duration, and quality. Long-day plants require more than twelve hours of daylight to flower. Short-day and day-neutral plants have varying day- light requirements. Temperature is the intensity factor of heat energy and regulates the rate of chemical reaction. Soil has layers called horizons that make up a soil profile. The topsoil is of most importance to horticulture. Soils vary in texture (particle size distri- bution) and structure (particle arrangement), as well as in chemical characteristics such as soil reaction (pH) and nutrient level. The nutrients used by plants in large amounts, called major nutrients, or macronutrients, are nitrogen, phosphorus, and potassium. Those required in small or minute amounts are called micronutrients, minor nutrients, or trace elements and include copper, molybdenum, zinc, chlorine, manganese, magne- sium, and calcium. Nutrient elements can be artificially supplied by fertilizers. The soil is home to a large number of organisms, some of which are beneficial to plants.

Soil is the primary medium in which plants are grown. Plants may be grown directly in the soil in the field or in soil placed in containers. Sometimes the medium used for potting may be entirely or partially nonsoil or artificial. Before use, the soil has to be prepared. This preparation is called tillage. Primary tillage (rough) precedes sec- ondary tillage (finer tilth). Several concepts are important in tillage. In conventional tillage, the field or plot is deeply plowed using primary or secondary operations before use. In minimum tillage, only the spot where the seed is to be placed is disturbed. Conservation tillage includes practices that conserve soil moisture and prevent erosion. Different implements, including plows, harrows, listers, and tillers, are available for var- ious tillage operations. Potting soil frequently includes artificial ingredients such as per- lite and vermiculite. Organic materials used include peat moss, sphagnum moss, and wood products; inorganic ingredients include sand and limestone.

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Boodley, J. W. 1998. The commercial greenhouse, 2d ed. Albany, N.Y.: Delmar. Bunt, A. C. 1976. Modern potting composts: A manual on the preparation and use of

growing media for pot plants. University Park: Pennsylvania State University Press.

Janick, J. 1986. Horticultural science, 4th ed. San Francisco: W. H. Freeman. Kramer, J. 1974. Plants under lights. New York: Simon & Schuster.

Practical Experience

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Maracher, H. 1986. Mineral nutrition in higher plants. New York: Academic Press. McMahon, R. W. 1992. An introduction to greenhouse production. Columbus Ohio

Agricultural Education Curriculum Materials Services.

Nelson, P. V. 1985. Greenhouse operation and management, 3d ed. Reston, Va.: Reston Publishing Co.

Rice, L. W., and R. P. Rice, Jr. 1993. Practical horticulture. Englewood Cliffs, N.J.: Prentice Hall.

Singer, M. J., and D. N. Munns. 1987. Soils: An introduction. New York: Macmillan. Excellent and well-illustrated site on plant nutrient deficiency symptoms

http://www.ces.ncsu.edu/depts/hort/floriculture/def/

General discussion of fertilizers

http://www.answers.com/topic/fertilizer?cat=technology

Soil test

http://www.agr.state.nc.us/cyber/kidswrld/plant/soiltest.htm

Soilless mixes

http://hgic.clemson.edu/factsheets/hgic1456.htm

USDA plant hardiness zone interactive map

http://www.usna.usda.gov/Hardzone/ushzmap.html?

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1. Plant sixteen pots of a selected plant. Group pots into sets of four. Select a source of nitrogen fertilizer and four different levels (0, 1, 2, 3). The amount chosen depends on the size of the pot and the type of plant. Your instructor will give you guidance. Apply one level to each set of pots. Observe the differences in plant growth at the end of the course by measuring a selected number of plant characteristics.

2. Repeat exercise 1 using other nutrients (phosphorus and potassium) and different plant species. You may increase the number of pots per set so that you can measure plant weight and other characteristics at periodic intervals. These intermediate measurements are destructive, requiring the samples to be uprooted or cut and dried. You may also take measurements at periodic intervals without destructive sampling.

3. Collect samples of soil from various areas—a garden, a flower bed, and a lawn. Test for soil reaction (pH) and other macroelements by using a simple garden soil test- ing kit.

4. Place a set of seedlings of corn and one set of beans under bright light. Place sim- ilar sets of plants in darkness. Observe the differences in plant growth and devel- opment after three weeks (or more) and explain the results.

5. Visit a farm equipment rental company or dealership to see the various available tillage implements and learn about their functions.

6. Obtain samples of the various components of soilless mixes—vermiculite, perlite, peat moss, and sphagnum moss—and create your own mixes. Test the physical properties of the mixes, including weight per unit volume, texture, and drainage.

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1. Distinguish between climate and weather as it relates to plant production.

2. Discuss how horticulturalists protect field-grown plants against adverse tempera- ture conditions.

3. Give the importance of the USDA hardiness zone map in the horticultural industry. In what zones does your state fall?

4. Distinguish between the role of light in plant growth and development based on its intensity and duration.

5. Discuss light-induced abnormalities in plants and how horticulturalists manage light in plant production for high-quality products.

6. Discuss the role and importance of soil pH in plant nutrient management.

7. What is a soil test and how is it used in crop production?

8. Discuss the factors to consider in selecting and applying a method of supplemental moisture application in a crop production enterprise.

9. What is the importance of soilless mixes in modern horticulture? What is the ra- tionale of the formulation of a soilless mix?

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