Air and water balance for maximum crop yields

Growers know that a good Growing Media must hold water and gradually release it to crops, and also allow air infiltration, which is important for healthy roots.

The physical properties of the substrate, such as porosity, pore composition and water holding capacity have a significant effect on plant growth and quality. By ensuring sufficient air, water and proper balance of available nutrients for plant roots, crop welfare is promoted. Stressed plants are more susceptible to diseases and pests, which increases maintenance costs and has a negative effect on profits.

Pore Space

The pore space is the volume available for air and water in a substrate. This means –  the volume of the substrate minus solids. Coarse particles in the growing medium create wide pores, which provide space for air exchange and  drainage. Fine particles create narrow pores that help  hold water.

Back-scattered electron (BSE) images of the pore structure of Sphagnum peat. The micrographs obtained with a scanning electron microscope (SEM) at 10kV. Photo: ResearchGate

The recommended range of air porosity for the growing medium varies depending on the crop and the stage of crop production. For germination or propagation medium, air porosity is recommended from 10-15%, while for most greenhouse plant growing media with coarse components, air porosity is recommended to be 15-25%, depending on the culture. Therefore, selecting the right growing medium for the crop type , it is very important for growers to maintain a balance between air and water in the substrate for plant health and root development.

Sphagnum peat has a very high porosity: pores filled with water and air make up more than 90% of its volume. Its structure is very stable over time to keep this porosity.

Pore Size

Substrate pores are of two types:

  • Macropores are wide pores that allow water infiltration and aeration. Most of the water drains out from the macropores, thus lowering the water level of the substrate from saturation to water holding capacity.
  • Mesopores or Micropores are narrow pores that allow the substrate to retain water. These types of pores retain water that is easily available for plants.

Water Holding Capacity

Water holding capacity (% ) is the percentage of total pore space that water occupies after excess water has been drained from the substrate. After draining the excess water, air fills the space the drained water once occupied. The substrate can hold water inside the narrow pores against gravity by capillary forces. Water drains out of the large pores because the force of gravity is greater than the capillary force in these pores.

The unique properties of sphagnum cells to retain and release water make peat products an indispensable component in soilless substrates.

The amount of water that the substrate is able to hold is the water holding capacity.

The water holding capacity of the substrate is not the same as the volume of water required to saturate the substrate. This is because at water holding capacity, both air and water space is at maximum in the substrate. Whereas at saturation all pores are filled only with water, this is not a good situation, because air is necessary for root growth. Total pore space affects the water holding capacity. Water holding capacity is considered to be a dynamic property of the substrate. It can be affected by substrate type, root growth and compaction.

Effect of container height on the distribution of air and water space. Photo: Purdue University
The height of the container

The height of the container is one of the properties that affects the water holding capacity. Water holding capacity of the substrate decreases as the container gets taller. This is caused by the increased effect of gravity on the water column. Substrate’s water holding capacity increases with diameter of the container.


The roots of the plant must needs the right balance of air and water. It is important that the substrate has the appropriate proportion of coarse and fine particles for the needs of the crop to be grown.

Coarse substrate particles create macropores that quickly release water, decrease water holding capacity and increase air porosity.

Fine substrate particles create micropores that increase water holding capacity and decrease air porosity.

Therefore, a substrate with a good ratio of coarse and fine particles may have a high water holding capacity, but it can be released evenly as root suction increases, creating appropriate air porosity as the substrate dries out between waterings.

Particle size affects the capillarity of the substrate, which indicates the distance water can be wicked by a substrate. The length of the capillaries in the coarse substrate is short and with a lower capacity to absorb water. In turn, the length of capillaries in small particles is long and able to easily wick water a longer distance within the substrate.

The tray cells used for germination substrates are small, so finer substrates are used for this purpose, creating many micropores and limited air porosity. As the roots absorb the small water reserve inside the cell, it is replaced by air, thus ensuring the necessary air porosity.

Coarse substrates are used, for example, for container production. The coarse mixture provides a suitable ratio of air, water and solids. After saturation, most of the water will drain due to the large number of macropores, while part of the water will be retained by the micropores. Coarse fraction peat provides the same level of air exchange and drainage that can be achieved, for example, by the presence of perlite and vermiculite.

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