Horticulture Principles and Practices

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