Irrigation is one of the most critical of all production practices. And yet it is frequently overlooked and taken for granted. To provide conditions for optimum plant growth it is essential to become familiar with the factors that influence soil moisture.

Types of Irrigation:

In most cases, water is applied to the upper surface of the media. This water may be applied by means of an overhead sprinkler, a drip or trickle irrigation system, by hand using a hose or similar device, or some combination of these delivery systems. Overhead sprinklers and hand watering have a tendency to “waste” water and also wet the foliage, which increases the potential for diseases and injury. Drip or trickle systems are the most efficient and provide greater control over the amount of water applied. Also, since the foliage does not become wet there is a reduced potential for diseases and injury.

Water may also be applied to greenhouse crops using subirrigation or capillary mats. However, in areas where soluble salts are a problem, mats do not provide for leaching, thereby increasing the risk of salt injury.

By far the most commonly used type of irrigation in Texas is the drip or trickle system. Several types of emitters are available commercially which provide a wide range of capacities. The most common are generally in the 1-3 gallon/minute range.

Frequency of Irrigation:

Frequency of irrigation is largely determined by existing environmental conditions. During the months of March – September most Texas growers must irrigate their crops at least once a day and often two to three times. This frequency of irrigation means that growers must consider the physical characteristics of their growing media (i.e. water holding capacity and drainage) very carefully. Particularly where soluble salts are a problem. Often nutritional problems, such as magnesium and micronutrient deficiencies, arise as the result of excess leaching. In these cases special attention must be given to media amendments and nutritional regimes to provide for optimum plant growth.

Irrigation Volume:

The amount of irrigation water to apply is perhaps more important than how and when to irrigate. A general rule of thumb to follow in irrigating greenhouse crops is to apply 10- 15% more water than the container will hold. This facilitates leaching at each irrigation and reduces the potential for the accumulation of soluble salts. Of course the rate of irrigation must be low enough to allow the water to percolate through the growing media as opposed to over flowing the top of the container. When using soluble fertilizers in the irrigation water it is especially important to allow at least 10-15% for leaching to avoid salt build ups.

Condition of the Media:

The condition of the growing media is very important in determining irrigation efficiency. Most of the peat moss, bark and other organic constituents used in soilless growing media have hydrophobic or water repelling characteristics. When excessively dry, these materials have a tendency to be difficult to “wet” and therefore require careful attention during irrigation. In some cases a “wetting agent” may be required to provide adequate absorption. The key to avoiding problems associated with wetting is not to pot plants in excessively dry media or allow media to dry out between irrigations. Again, these problems may become more acute in the presence of soluble salts.

Porosity & Water Holding Capacity:

Porosity and water holding capacity of the growing media is another factor which influences irrigation practices. Optimum combinations of these two characteristics provides enough large pores to allow for adequate leaching and aeration as well as a water holding capacity which minimizes irrigation frequency. Another consideration in this area are root diseases associated with wet, poorly drained media. Therefore it is important to maintain moisture levels which are not conducive to root diseases.

Water Quality:

Water quality largely influences irrigation practices. As previously mentioned, the presence of soluble salts requires that growing media be well drained and that at least 10- 15% more water than the container will hold be applied at each irrigation. It is also advisable to reduce the wetting and drying of the media between irrigations to avoid increases in relative concentrations of soluble salts. Be sure to have your water quality tested on a regular basis to monitor these conditions.

Putting it All Together…

The relationship between growing medium, air, and water is one of the least understood aspects in the production of greenhouse and nursery crops. As a result, a significant amount of plant loss may be either directly or indirectly related to an improper match between these cultural elements. A basic understanding of the factors that mediate this relationship can be extremely valuable in developing sound management practices.

Pore Space:

Growing medium consists of solids (i.e., peat moss, bark, perlite) and pore space. Pores are created by the spaces between the solid components of the medium. Therefore, a mix which contains coarse aggregate has fewer but larger pores than one made up from fine aggregate. The size and distribution of pores is one of the most critical factors in developing a growing medium with optimum physical characteristics.

Most soilless growing media contains 60-80% total pore space. A portion of these pores is occupied by air. Plant roots require oxygen for growth and adequate aeration of the medium is necessary. As roots take up oxygen they also give off carbon dioxide. This exchange of gases is primarily by diffusion through the pores of the growing medium.

Although total pore space is a measure of a growing medium’s ability to hold air and water, pore size determines the rate of drainage and gas exchange. Large pores permit air to re-enter the medium following irrigation. Since growing medium in containers holds a relatively large quantity of water, the percentage of pore space filled with air is reduced. Therefore, an adequate distribution of large and small pores is essential. On average, most mixes contain 10-30% air following irrigation.

Water is also held in the pore space of a growing medium. The availability of this water is largely determined by how tightly it is held by the solid component of the medium. The closer a water molecule is to a solid the more tightly it is held through the forces of adhesion and cohesion. Therefore, a fine mix may hold more water than a coarse mix, but less of it is available to the plant. In general, the amount of unavailable water is relatively high in soilless growing medium.

Drainage:

Drainage is affected by pore size and shape of the container. Water occupying large pores is held less tightly because the molecules are not as close to the solids in the medium. As a result, this water is more available to the plant and also drains at a faster rate than water occupying smaller pores.

The length of the soil column also influences the rate of drainage. The taller the container, the greater the force of gravity on the water occupying the pore space. This results in increased drainage. Shorter columns, of an equal volume of medium, hold more water, drain slower and contain less air.

Compaction is another factor that affects drainage. Packing growing medium into a container can significantly reduce the number of large pores. When this occurs, it creates less available water to the plant, reduces aeration and gas exchange, increases water holding and decreases drainage. In smaller containers the effect of compaction can be even greater.

Water Holding Capacity:

Establishing a balance between the water holding capacity of a medium, aeration and drainage is a key to optimum plant growth. The cost of irrigation often forces growers to utilize a medium that holds excessive amounts of water. However, the results are frequently increased crop time, reduced root growth, and poor plant quality.

The amount of water a medium holds is dependent on the components used, pore size and distribution, as well as the shape of the container. Most soilless mixes should hold approximately 60-70% water after drainage.

Developing an effective plan for irrigation management requires a careful match between the needs of the plant and the growing medium. Although growers are most frequently concerned about the lack of water in the medium, research now indicates that the potential hazards from overwatering may be much greater.

Aeration is a factor for optimum plant growth, and requires careful consideration when developing a growing medium. However, the pressure to conserve water is forcing growers to use mixes that hold large volumes of water. An understanding of the relationship between growing medium, air, and water can be extremely helpful in developing sound management practices.

Table 1. Flow rates for low volume irrigation systems.

Conversion Factors:

1 gpm = 0.134 cu. ft. per min. = 500 lb. per hr. X sp. gr.
1 cfm = 448.8 gal. per hr.