The water found in the pores of the soil is called the soil solution and carries dissolved nutrients as well as microbial life. Water is necessary to transfer soluble nutrients to all parts of the plant and must be present in sufficient quantities for the elongation of plant cells that contribute to growth. In waterlogged soils, water molecules displace the soil air that contains oxygen. This causes lower respiration rates in the roots and can actually decrease the ability of the plant to take up water from the soil, which causes the plant to wilt.
Rainwater is slightly acidic because of the carbon dioxide naturally present in the atmosphere and may become even more acidic as a result of pollution. Rain is also a transport vehicle for microbes that are swept into the atmosphere. If there is insufficient rainfall for healthy plants, a supplemental source is required. Commercial growers may have access to surface water, such as ponds and rivers with irrigation ditches used to divert water to the crops. They may also use a sprinkler system. Alternatively, wells are often used to supply irrigation water and are preferred because the water is less likely to transmit disease.
Water quality should be tested prior to planting. If the water supply is from surface water or a well, this can be done by sending a water sample to a laboratory. You want to check for pH, mineral content, and soluble salts, as well as toxic chemicals that could injure your plants. If you are using municipal tap water to water plants, you should ask the county or municipality for a water quality report, which they should provide to you at no charge. High levels of chlorine or fluoride in tap water may injure your plants. If your water is not good, you can invest in equipment to purify it.
Overhead water washes off leaves and may help deter foliage pests, but water can be lost to evaporation and may not reach the roots, especially in hot, dry weather. Overhead rinses with organic amendments such as compost tea or kelp extracts have been reported to increase plant vigor. Pivot irrigation is an overhead sprinkler system that projects water out from a central source in a circular motion moving like the hands on a clock, which results in a circular patch of irrigated land that looks like big green circles when viewed from overhead (Figure 4.3).
When plants are exposed to a frost, an early morning rinse with water to wash frost off the leaves will prevent damage as long as the leaves have not yet been exposed to sunlight. Plants can be protected from frost by covering them with cloth or plastic or by moving potted plants to a sheltered area. Your local weather report will give you notice if a frost is expected overnight.
Water applied directly to the root zone by drip irrigation concentrates the water by the roots where the plant can access it. Drip irrigation delivers water through a soaker hose, which is perforated. This method is more easily applied in the small garden than in large commercial fields, which may have problems with clogging. The hose is laid on the ground close to the stems of your plants and can be covered with mulch to reduce evaporation.
Water that is applied too late in the day, close to dusk, or after nightfall may create problems because of conditions that favor the growth of fungi. Many types of fungi thrive in cool, moist conditions and reproduce with spores. The spores are transported in water. It is important to avoid working in a wet garden, as this can spread microbial diseases that are in water droplets. The spores can become attached to your shoes, gloves, or tools and cause you to infect other plants. It is not a good idea to transfer garden tools from one place to another unless they are sterilized between uses, as residue left on tools can spread microbial diseases. Wet soil can also be easily compacted by footsteps or heavy machinery, which makes it harder for plants to grow.
Plant Water Requirements
How much water does a plant need? That depends upon the plant, the climate, and the rate of transpiration. Transpiration is the passage of water as a vapor from the leaf to the atmosphere. Water molecules move from areas of higher concentration to areas of lower concentration. High temperatures, low humidity, and wind can increase the rate of transpiration because they remove water from the air. If there is less water in the air than in the plant leaf, water will move from the plant into the air.
Plants that do not receive adequate water intake through the roots during times of increased transpiration may wilt, and if the conditions persist, they will die. The amount of water required varies among species that have evolved in climates with different amounts of rainfall. Plants that are native to arid and semiarid regions have modified leaves and stems that conserve water and reduce transpiration. These plants are sometimes referred to as drought tolerant or xeric.
Plants that are native to tropical regions with warm, moist air tend to have broad leaves that shed water and may transpire at a higher rate than xeric plants when exposed to hot, dry weather. High transpiration combined with evaporation dries out the soil. Plant water requirements are often listed as low, moderate, or high, with xeric plants at the low end and tropical plants at the high end. Plants with shallow lateral roots are usually not drought tolerant and require more frequent water than plants with deep taproots, which will do well even if the soil dries out between watering.
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