Welcome to our ongoing series highlighting our Aquaponics Curriculum!
Over the coming weeks, we will be posting a series of articles covering the basics of each Curriculum Topic. Today, we are covering Topic 1, Lesson 3, Substrates for Hydroponic and Aquaponics. Each of these blog posts is aimed to give an overview of the subject and provide one or more critical thinking questions. You can find our first post in this series here. Enjoy!
A substrate, also called a medium or media, is a supporting material or base on which a plant can grow. The most commonly used substrates are: Rockwool, lightweight expanded clay aggregate, coco coir, coco chips, perlite, vermiculite, peat moss, lava rock, river rock, and sand. Some uncommonly used substrates include: oasis cubes, floral foam, growstones, pine shavings, pine bark, polyurethane foam, water-absorbing polymers and rice hulls. A good substrate supports the plant, allows for air flow, and is porous.
Buffering capacity is a measure of how efficient a substrate resists changes in pH. A high buffering capacity will neutralize a nutrient solution if an acid or base is added. Porosity is the amount of open space that allows for air exchange in a substance. A high porosity material will allow for more air exchange. Water holding capacity is the amount of water a substance can retain. A substrate with a low water holding capacity does not retain water well. Cation-exchange capacity is the amount of salts or ions it a particular substance can store. A high cation-exchange capacity would allow for a high salt content in the material that may be used at a later time when the liquid solution may be devoid of salt content.
Rockwool (stonewool) is spun wool from basalt rock, a type of lava rock, and is similar to fiberglass and carbon fiber. Because it is a mineral, it does not degrade. Rockwool is sterile and therefore does not have any contamination issues. Although Rockwool has a pH 8.0 or more, it has a low buffering capacity and will not affect the pH of the nutrient solution or other substrates it comes in contact with. Because the water holding capacity of Rockwool is high, susceptible plants may get stem or root rot.
Figure 1: Rockwool sheet buried in light weight expanded clay aggregate with seedlings.
Light weight expanded clay aggregate (LECA) is produced by super heating clay to create a porous texture. It is lightweight but can provide ample support for many plants. LECA is sterile and does not degrade so it can be reused after sterilization. LECA has a neutral pH and has a high porosity and a mid-to-high water holding capacity.
Coco coir (coconut fiber) and coco chips come from the coconut husks that are largely thrown away as a waste product. Coco coir decomposes very slowly and is pH neutral. The substrate has a high water holding capacity and a high porosity. Coco coir is smaller and more fibrous than coco chips. These substrates are compressed into blocks. To prepare the coco coir for use, it is soaked in water which expands and loosens the substrate.
Perlite is amorphous volcanic glass that is heated at high temperatures until the material expands. Perlite is porous and has a high holding capacity. Perlite is light weight and floats. Using perlite for support may be difficult if the system is flooded at any time. Because perlite floats so easily, it is usually used in conjunction with other substrates.
Vermiculite is very similar to perlite. Vermiculite is phyllosillicate or mica that has been heated at high temperatures until the material expands. Vermiculite is very light weight and floats as well. Therefore vermiculite does not provide good support in float systems. Vermiculite has a high cation-exchange capacity.
Peat moss (sphagnum) is made from a group of more than 100 different moss species. Peat moss is dried and because it is biological matter, peat moss decomposes as well. However, peat moss naturally grows in acidic environments and the dried decomposing substrate reflects the acidity. Peat moss has high porosity, high water holding capacity and also has a high cation-exchange capacity.
Lava rocks are porous, lightweight and abundant. Lava rock has a pH of approximately 8.0 and a low buffering capacity. Lava rock is chemically inert. However, some lava rock may be jagged and can damage plant roots or be hard to work with without gloves or tools. Additionally some lava rock may contain aluminum that can leach into the nutrient solution.
River rock can be made commercially or naturally. These rocks are easy to find and inexpensive. However, river rock is heavy and not porous so it cannot retain water. Because river rock does not have water holding capacity, it is recommended to use the rock in combination with other substrates that have some water holding capacity.
Sand is similar to rock but has some water holding capacity as the particles are quite a bit smaller and the surface area is greater. Sand is often used with other substrates that have a better water holding capacity for moisture retention. On the contrary, sand is heavy and has a lower porosity depending on the particle size.
Critical Thinking Question:
What are the advantages and disadvantages of a low water holding capacity?