1. Surface drainage. Surface drainage uses open ditches to remove surface running water before it infiltrates the soil. The land is prepared such that it slopes very gently toward the ditches. Large amounts of water can be drained by surface irrigation in a short period. These ditches require maintenance to keep them open; open drains occupy land that could otherwise have been cropped and also interrupt the continuity of the land. 2. Subsurface drainage. In subsurface, or underground, drainage systems, channels are provided in the zone of maximum water accumulation to drain the excess water. The system must be placed below the plow depth. In one system, that most commonly used, the channels consist of perforated plastic pipes laid at predetermined intervals across the field. Water enters the pipes through the perforations and is moved to an outlet ditch. Other less common systems are the mole drainage and the clay-tile drainage systems. Installation of the mole system is less expensive than clay tiles, which are very costly. Outlets of the subsurface systems of drainage must be protected from clogging by sediments, dead animals, bird nests, and other obstructions. Advantages of Drainage Drainage is beneficial to plants and horticultural operations in several ways. 1. Strong structural foundation. Greenhouses and other structures constructed for horticultural use are more stable when located on well-drained soils. Greenhouses typically use large amounts of water for a variety of activities— washing, irrigation, and the like. The site of the facility should be well drained to handle the excess water. 2. Aeration. By draining the root zone, plant roots grow in an environment with a good balance of air and moisture. Aerobic bacteria are able to function properly to decompose organic matter to release nutrients for plant use. 3. Warm soil. Drainage of excess water decreases the specific heat of soil, allowing it to warm up more quickly. In clay soils and poorly drained areas, plants should be grown in raised beds. Raised beds improve drainage and free up pore spaces for air to warm the soil. 4. Timely field planting. Seeds require warm soil to germinate. Seeds can be planted at the usual time in the season if cold soils are drained to make them warmer. 4.8.19 ENVIRONMENTAL CONCERNS Land is irrigated mostly in drought-prone areas. In these areas, leaching is minimal, leading to the accumulation of salts in irrigated soil. Excessive fertilization predisposes soil to salinity. Crops have different salt tolerance levels. The use of fertilizers and irrigation water, while desirable for high economic production, has both short- and long-term environmental consequences. Groundwater and surface waters are polluted as a result of this crop production activity. Excess water from irrigation, either as runoff or drainage, ends up in the groundwater or rivers. This water is loaded with salts and as such causes the salt concentration of rivers to rise. Phosphates are readily fixed in the soil (adsorbed to soil particles). Phosphate pollution occurs in surface water as a result of soil erosion, which moves soil into surface waters. Nitrates, on the other hand, are readily leached and hence end up in the groundwater. Nitrates may accumulate in some leafy vegetables beyond normal levels. Nitrate toxicity is a problem in ruminants. 4.8.20 GREENHOUSE IRRIGATION Greenhouse irrigation is discussed in Chapters 12 and 13. 4.8 Soil Water 139
4.9 FIELD GROWING MEDIA Most horticultural plants are grown in solid media, in the field, in containers outside or inside of the house, or in controlled-environment greenhouses. Soil properties have implications in the way they are managed for optimum productivity. Most plant production or display occurs in the field in real soil. Depending on the soil properties and the needs of the plant to be cultivated, soils are physically and sometimes chemically manipulated to provide the best medium for growth. Growing plants indoors either at home or in the greenhouse requires the growth medium to be placed in a container. This situation poses new challenges in managing the growth medium, which should drain freely while holding adequate moisture and nutrients for plant use. Potting media frequently consist either entirely or partly of artificial or soilless components. Soils are disturbed to varying degrees to prepare them for crop production. Tillage is the term used for the manual or mechanical manipulation of soil to prepare it for use in crop cultivation. Conventional soil preparation (conventional tillage) involves turning over all of the soil in an area of land. Other conservative systems such as the minimum tillage (zero tillage or no-till) disturb only the spots where seeds or plants are to be located. 4.9.1 PURPOSES OF TILLAGE How soil is tilled depends on its intended use. Pan A layer in the soil that is highly compacted or very high in clay content. 1. Seedbed preparation. A seedbed provides an environment in which a seed can germinate and grow. It is loose, well drained, deep enough, makes good contact with the seed, retains adequate moisture, and is free of weeds. If the seeds to be sown are tiny, such as those of carrot and lettuce, the tilth should be fine. A rough finish is sufficient for large seeds and transplants. 2. Level land. Land leveling may be required while preparing the land to make it amenable to a chosen method of irrigation. 3. Weed control. Weeds are a menace to crop production and compete with crops for plant environmental growth factors. They must be controlled before planting and during the growth of the crop by appropriate tillage methods. 4. Incorporation of organic matter and soil amendments. Green manures, crop residues, fertilizers, and other chemicals may be added to the soil by plowing them under or mixing them in at the time of preparation. 5. Improved physical properties of soil. Compaction of the field impedes drainage, rooting, and general crop growth. A pan caused by traffic or tillage may be broken up by tillage methods. 6. Erosion control. The soil surface after tillage may be such that it impedes runoff. Stubble may be incorporated into the topsoil or ridges constructed to curb erosion. On a slope, the ridges or direction of plowing should be across the slope to impede surface runoff. 140 Chapter 4 Plant Growth Environment 4.9.2 TYPES OF TILLAGE In terms of the depth to which a soil is tilled and the purpose of tillage, there are two general classes of tillage. Each requires different implements. Primary Tillage In primary tillage, the soil is tilled to a depth of about 6 to 14 inches (15 to 36 centimeters). The topsoil is turned over, burying the vegetation and other debris. Primary tillage may be used to incorporate fertilizer and also to aerate the soil. The end product is a rough, cloddy soil surface. Because of the depth of plowing, heavy machinery and implements are used in primary tillage operations. The end product is meant to be transitional and is not ready for planting seeds or seedlings.
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