Principles of Irrigation Management Pt. 1

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. A Charlotte irrigation specialist can help you to decide what amount of irrigation your plants need.


Crop Factors

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.


Water Factors

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. As mentioned, a Charlotte irrigation company can help you to figure out what type of irrigation system is right for you.


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 105^oC. 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. Contact a Charlotte irrigation company to discuss all of your irrigation options.

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.

source: Texas A&M