Hydroponic growing Media
Perlite in it's raw form is a naturally occurring form of volcanic glass that is porous and has some inherent water content.
It is unusual in that when it is heated to a high temperature it greatly expands. This occurs when it is heated to around 850-900 deg C. When heated the water inside vaporises and due to the escaping vapour the perlite expands to up to 16 times it's original size, creating a very porous granule with good water retention properties.
Expanded perlite is relatively cheap and it has been used for many purposes such as filter media, aggregate for cement based building materials and building blocks etc
In conventional horticulture it is often used as an additive in compost to aerate the soil and stop it from binding or clumping.
In Hydroponics it is used in it's pure form. The advantages are that it is cheap, sterile, free draining, provides plant support and has some water holding properties.
Rock wool is manufactured by heating stone to a melting temperature of approximately 1600 deg C. It is then spun at high speed to create filaments of rock or stone, a process not dissimilar to the process used to make candy floss. This results in a thick mass of fibers of minerals, some forms will contain additives or resins to make it bind and repel water.
In industry/construction it is valued for it heat insulation properties and resistance to fire.
In hydroponics Mineral wool or Rock wool is used for its considerable water and air holding properties. It is ideal for the raising of seeds and provides a good supporting structure for larger plants and seedlings.
Seeds are often raised in small cubes of rock wool. Then the cube and seedling are moved into another type of hydroponic system such as NFT or to larger beds of rock wool.
The advantages of Rock wool in hydroponics is that it's cheap, holds water well, supports the plant and is light in weight.
In it's raw form mineral wool has a high pH, this makes it unsuitable for plant growth. The pH must be lowered. conditioned mineral wool is used in hydroponics, this has a very stable pH. For that reason and because it often contains the aforementioned resins, using rock wool intended for use as insulation etc is not really a good idea.
Vermiculite is a naturally occurring mineral that expands when heat is applied to it. The expansion process is called exfoliation (peeling off in flakes or scales) and is performed in specially designed furnaces.
In it's natural form Vermiculite is formed by the hydration of certain volcanic minerals. deposits exist in the USA, South Africa, China, Brazil, and many other countries in less commercial quantities.
The commercial uses include refractory insulation, fireproofing of steel and pipes etc, soil conditioner, lightweight cement aggregate and loose fill roof insulation etc.
In general horticulture it is used in potting mix to aid in water retention. In Hydroponics it is used as an aditive to Perlite to improve it's water retention properties.
Also known as grow rocks etc are small round inert spheres of baked clay. It is manufactured by forming clay in to small round pellets and fired in rotating kilns at around 1200°C. This causes the clay to expand, in a similar way to popcorn, and in the process become porous.
Expanded clay is light in weight, and in normal use does not compact over time. The shape of the individual pellets depends on the quality and brand of the material.
Grow rocks or expanded clay are widely used in hydroponics as they are pH neutral, can be sterilised, reused, provide support of the plant and are relatively light in weight.
While sand is cheap and easily available, it is heavy, it doesn't drain very well, and is hard to clean sufficiently. It can also contain impurites that affect the pH of your nutrient. It has been used in Hydroponics, but few continue to use it after discovering the many more satisfactory alternatives.
