perlite

Soilless Cultures

Many methods of soilless culture are being used successfully. Some of the media used are peat, vermiculite, perlite, sand, pumice, rice hulls, and plastic Styrofoam. Often mixtures of these media are used in various proportions. Growing trials with various mixtures determine which proportions are most suitable to the plants in question. For example, flowering potted plants such as chrysanthemums, poinsettias, and Easter lilies and tropical foliage plants can be grown well in mixtures of peat-sand-pumice in a 2:1:2 ratio.

Peat: -

• Peat consists of partially decomposed aquatic, marsh, bog, or swamp vegetation. The composition of different peat deposits varies widely, depending on the vegetation from which it originated, the state of decomposition, mineral content, and degree of acidity
• There are three types of peats: moss peat, reed-sedge, and peat humus. Peat moss is the least decomposed and is derived from sphagnum, hypnum, or other mosses.
• It has a high moisture holding capacity (10 times its dry weight), high in acidity (PH 3.8 – 4.5), and contains a small amount of nitrogen (about 1.0%) but little or no phosphorus or potassium. Peat from hypnum and other kinds of mosses breaks down rapidly, as compared with sphagnum, and is not as desirable. Peat from sedges, reeds, and other swamp plants also decomposes rapidly.

Vermiculite: -  

• Vermiculite is a micaceous mineral, which is expanded when heated in furnaces at temperatures near 1093 degree Celsius. The water turns to steam, popping the layer apart, forming small, porous, sponge-like kernels. Heating to this temperature gives complete sterilization.
• Chemically, it is a hydrated magnesium-aluminium-iron silicate. When expanded, it is very light in weight (6-10lb/ft3) (96-160 kg/m3), neutral in reaction with good buffering properties, and insoluble in water, it is able to absorb large quantities of water, 3-4 gal/ft3 (0.4-0.5 mL/cm3).
• It has a relatively high cation exchange capacity and thus can hold nutrients in reserve and later release them. It contains some magnesium and potassium, which can be used by plants.
• Horticultural vermiculite is graded in four sizes:-
  • particles from 5 to 8 mm in diameter.
  • regular horticultural grade, from 2 to 3 mm.
  • particles from 1 to 2 mm
  • most useful as a seed-germinating medium, from 0.75 to 1 mm.

• Expanded vermiculite should not be pressed or compacted when wet, as this will destroy its desirable porous structure.

Perlite: -

• Perlite is a siliceous material of volcanic origin, mined from lava flows. The crude ore is crushed and screened, then heated in furnaces to about 760 degree Celsius, at which temperature the small amount of moisture in the particle’s changes to steam, expanding the particles too small, spongelike kernels, which are very light, weighing only 5-8 lb/ft3 (80-128 kg/m3).
• The high processing temperature gives a sterile product. A particle size of 0.063-0.13 in. (1.6 – 3.1 mm) in diameter is usually used in horticultural applications. Perlite will hold three to four times its weight of water.
• It is essentially neutral, with a pH of 6.0 -8.0, but with no buffering capacity; unlike vermiculite, it has no cation exchange capacity and contains no minor nutrients. It is most useful in increasing aeration in a mixture since it has a very rigid structure. While it does not decay, the particle size can become smaller by fracturing as it is handled.
• A fine grade is useful primarily for seed germination, while a coarser type of horticultural grade is best suited for mixing with peat, in equal parts, for propagation or with mixtures of peat and sand for growing plants.

Pumice: -

• Pumice, like perlite, is a siliceous material of volcanic origin. However, it is the crude ore that is obtained after crushing and screening without any heating process. It has essentially the same properties as perlite, but is heavier and does not absorb water as readily since it has not been hydrated. It is used in mixtures of peat and sand for the growing of potted plants.

Rice Hulls: -

• Rice Hulls are the outer husk or shell of the rice grain. After the rice grains are dried, the outer hulls are removed in the milling as a by-product. The rice hulls are thin, feather-light, and pointed in shape similar to rice grains.
• They do not decompose readily, lasting from 3 to 5 yr. They are neutral in pH and have no nutrients. Their smooth surface does not allow them to retain moisture. They are used in the raw state to free up heavy soils to help oxygenate the soils.
• They can also be used as a hydroponic substrate. They are mixed with peat or coco coir, usually at 20% of rice hulls. However, most soilless mixes using rice hulls prefer to use charcoaled rice hulls. This is done extensively in the greenhouse flower industry. Rice charcoal is created by burning (smouldering) the rice hulls very slowly. After burning, their structure becomes full of tiny pores, thus increasing their water-holding capacity and capillary action. Also, in this state with their large surface area, they provide sites for beneficial bacteria and other microorganisms and therefore are an excellent soil amendment.

Soilless mixtures: -

Most mixtures contain some combination of sand, peat, perlite, pumice, and vermiculite. The specific proportion of each component used depends on the plants grown. The following are some useful mixtures.

Peat: Perlite: Sand	2:2:1 for potted plants
Peat: Perlite	1:1 for the propagation of cuttings
Peat: Sand	1:1 for the propagation of cuttings and for potted plants
Peat: Sand	1:3 for bedding plants and nursery container-grown stocks
Peat: vermiculite	1:1 for the propagation of cuttings
Peat: sand	3:1 lightweight, excellent aeration, for pots and beds, good for azaleas, gardenias
Vermiculite: perlite	1:1 lightweight good for the propagation of cuttings
Peat: Pumice: Sand	2:2:1 for potted plants.