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 related either directly or indirectly to an improper match between these cultural elements. A basic understanding of the factors that mediate this relationship can be 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 of 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% to 80% total pore space. A portion of these pores is occupied by air. Plant roots require oxygen for growth so 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 reenter 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% to 30% air following irrigation.

Water is also held in the pore space of a growing medium. The availability of this water for plant growth 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 the 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 more slowly 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. This frequently results in 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% to 70% water after drainage.

Putting Them All Together

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 over-watering may be much greater.

Aeration is an essential 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.

A 2:1 v/v bark:sand medium was used for this example.