Principles of Irrigation Management
Water Management Guidelines for Nursery/Floral Producers
Although most of the water plants remove from the soil or media is lost to the atmosphere through transpiration, the small portion utilized in photosynthesis is vital for growth. Therefore, applications of the right amount of good quality water to greenhouse crops, at the optimum time, is an important factor in production of quality plants.
Greenhouse crops use large amounts of water continuously, but the rate of use depends on plant species, size, temperature, and atmospheric conditions. The plants enormous requirement for water is demonstrated by the fact that 300 to 500 pounds of water are necessary to produce one pound of dry organic matter. Although water serves many functions in the plant , over 99% of the water absorbed is lost to the atmosphere.
The water requirements of container grown plants vary because of age, succulence of growth, and plant species. Young plants, with little top growth, require less water because they lose less by transpiration than larger plants. Plants with succulent growth usually wilt sooner, because more of the tissue is composed of water. And, plants with large thin leaves absorb and transpire water more rapidly than those with small thick leaves. Plants should be grouped in the greenhouse or nursery according to their species, container size and type, growth rate and moisture requirements.
The quality of irrigation water is very important in the production of greenhouse crops because of undesirable chemicals found in some water. When a new well or growing operation is being planned, water sources should be checked for quality. For water quality to be acceptable, soluble salt levels should be below 600 ppm, and generally water above 1,200 ppm should not be used (see section on monitoring water quality). Specific chemicals found in water that may cause plant injury include chlorine, fluorine, and boron. Chemicals that leave undesirable foliar residues include iron, calcium, and magnesium. Drainage ponds and pools are not generally desirable sources of irrigation water because of the possibility of disease organisms and weed seed being distributed over the plants, and because of algae and other organisms developing which may clog the irrigation system.
Growing Medium Factors
The importance of adequate water holding capacity and proper aeration and drainage of container mixes cannot be stressed too strongly. Water holding capacity, the ability of a soil or media to hold water against the pull of gravity, is important because it determines irrigation frequency and influences fertility. Of the total water held in a potting mixture, only part of it is available to the plant. Available water is the portion between the maximum capacity of the medium and permanent wilting point.
The most meaningful measurement of water present in a medium is the percent by volume, of moisture (water) held after all gravitational water has drained. Maximum substrate water content, in percent dry weight, may be obtained by weighing the total medium, with gravitational water removed, then oven drying at 105oC. The media is then reweighed and the difference divided by dry weight. The percentage of water held at fifteen atmospheres can be determined on a pressure membrane apparatus. The difference between maximum water content andthe percentage at fifteen atmospheres is considered available water. These figures can be converted to a volume basis by multiplying their percent dry weight by bulk density. Container media used for most greenhouse crops should have at least 10-15% available water holding capacity by volume and 40-50% maximum air space by volume.
Proper drainage and aeration of container mixtures is as important as water holding capacity. Media aeration directly influences plant growth by its effect on soil oxygen, which controls to a large extent the uptake of nutrients and water. The major factor affecting drainage and aeration is type of potting mixture, but the surface beneath containers as well as drainage holes and other factors near these drainage holes are also important.
Approximately 10% sould be easily drainable to provide adequate aeration and water holding capacity. A simple method of checking drainage is to add water to the top of a container to a depth of 1/2 inch. If this water moves into the potting mixture within 1 minute the percentage of pore space is adequate. The more time (beyond 1 minute) required for the water to enter the media, the less satisfactory the potting mixture. Mixtures requiring 3 minutes or more to drain are completely unsatisfactory for use in containers and should not be considered. The surface beneath a container also influences drainage. This turn, affects depth of media that remains saturated in the bottom of the container. Placement of containers on raised benches provides the best drainage. However, many growers place containers directly on the ground or hard surface This can result in poor drainage as water becomes trapped around the base of the container. This situation may be minimized by providing adequate surface drainage of these growing areas.
Experience and good judgement are essential in determining how much and when to irrigate under any particular set of conditions. Generally, plants should be watered prior to wilting, and the medium should not be allowed to dry out excessively between waterings. The amount of water required by container grown plants, and the frequency of irrigation depends on the water holding capacity of the potting medium, amount of water already present in the medium, size of plant and container, species of plant, temperature, humidity and wind speed.
Controlling irrigation is important because too little water will retard plant growth and reduce quality, while too much will leach fertilizers and reduce aeration. Typically, container grown plants, under full sunlight, require daily watering when mature. However, during cloudy and/or cool weather, less frequent watering is usually adequate. Growing media with lower water holding capacities will require more frequent irrigation. When 35% to 50% shade is supplied (by either polypropylene or lath) irrigation frequency can be significantly reduced. The amount of water to apply at any irrigation depends on the amount moisture required within the container, the amount deflected by the plant canopy which is lost between containers when overhead sprinklers are used, and the portion evaporated during application. Generally, small plants that do not cover the container receive most of the water applied with sprinklers, while up to 50% may be deflected by the canopy of large plants. Normally the amount of water applied at each irrigation should bring the water content of the growing medium back up to 70-80% of container capacity.
Key components of any irrigation system are the well, pump, and proper size main and lateral lines. Frequently these components are undersized for the area to be watered, and serious inefficiencies occur. Since proper engineering of a watering system is necessary, it is important to carefully determine the area to be irrigated with consideration towards increased capacity. Overhead sprinkling is one of the most commonly used methods of irrigating greenhouse crops. Other irrigation methods include drip or trickle and subirrigation. General information on each of these systems follows, with mention of some of the major advantages and disadvantages associated with each method.
This method of application has relatively low initial and maintenance costs. A major drawback, however, is uneven distribution of water. This becomes a much more serious problem when the system is also used for application of fertilizers, herbicides and pesticides. Another negative aspect of overhead irrigation is that it promotes the development and spread of foliar diseases. Efficiency of overhead sprinklers is influenced by type of spray head, spacing, and wind velocity. Fixed spray heads are best for smaller areas where they are protected from wind, and they are most satisfactory for smaller containers.
Rotating impact heads supply water with considerable force. This is desirable to obtain wide coverage, but the plant's top may deflect some of the water, causing it to fall outside the container rather than enter the growing medium. Rotating sprinkler heads are best for larger container sizes and provide satisfactory coverage in calm wind conditions. Whirling, rotating sprinklers, of various designs, provide excellent coverage but the small water droplets may not penetrate when plants have a canopy over the growing medium surface. This type sprinkler operates best in protected locations.
Drip and Trickle Irrigation
Recent advances in design of small low-cost plastic tubing provide an excellent method of watering individual containers. This system is expensive to install, but will provide irrigation to individual containers without wetting the foliage. Several types of irrigation are available which deliver various volumes of water. Most drip systems operating at pressures varying from 40-60 ppi. become clogged bacause of the high calcium and magnesium levels in most irrigation waters, as well as from growth of algae and/or bacteria within the tubes. When small diameter tubing is used, water treatment (i.e. filtering) is desirable. These systems can also be used for injecting fertilizer and application of systemic insecticides and eeefungicides.
This method is frequently used for field grown crops where hardpans exist near the soil surface, but only a few nurseries use subirrigation for containers. Although this method of irrigation provides desired amounts of water, construction of beds/benches to hold water can be expensive. Another disadvantage of subirrigation is the continuous sheet of water which can provide a carrier for root rot organisms. Continuous capillary watering may also lead to salt build up in the media.