The Growing media is one of the most important bur less discussed topic.
Water holding capacity is one of the most important feature. While Particle size is controlling factor for both the water-holding capacity and aeration. High water-holding capacity and humid air spaces in the mix are important for germination and seedling and cutting growth, while good aeration and moderate water-holding capacity are essential for long-term plantings.
The pH balance is one of the considerable factors for substrate. The ideal substrate should be inert for nutrient salt and should not alter pH micro environment of system. The ph of substrate mix should not exceed a water pH of 5.5, with the optimum pH range between 4.5 and 5.5.
Adding nutrient to substrate is another good idea but a grower must keep the ratio as per his/her practical experience. From a practical standpoint and for short-term cropping (growth period of less than 8 weeks), nutrient elements and the other essential elements would be added to the mix when constituted.
Characteristics of some substrate components
Sand A common component it is added to provide porosity in fine mixes or weight when needed to keep plant containers upright in either fine or coarse mixes. However, sand should not constitute more than 20 to 25% of the mix. If more than 50% of a mix is sand, weight and reduced water-holding capacity become a problem. The recommended grade of sand is “builder’s” sand, which is a coarse-particle sand; 100% passes a 10-mesh sieve but only 30% a 40-mesh sieve.
Rockwool or Stonewool Clean, (can cause skin irritation), sterile, lightweight when dry, reusable, high water holding-capacity (80%), good aeration (17% air-holding), no cation exchange or buffering capacity, provides ideal root environment for seed germination and long-term plant growth. Rockwool is probably the most widely used hydroponic growing medium in use in the world today for the production of tomato, cucumber, and pepper). Rockwool has excellent water-holding capacity, is relatively inert, and has proven to be an excellent substrate for plant growth but the disposal of used rock wool is becoming a major problem. In India availability of material with right density and costing is issue.
Vermiculite is Porous, sponge like, sterile material, lightweight, high water absorption capacity (five times its own weight), but excellent with 10 to 20 % in substrate mix, easily becomes waterlogged, relatively high cation exchange capacity.
Perlite is Siliceous, sterile, sponge like, very light, free-draining, no cation exchange or buffer capacity, good germination medium when mixed with vermiculite, but the problem associated is dust can cause respiratory irritation.
Expanded clay Sterile, inert, range in pebble size of 1 to 18 mm, free draining, and physical structure can allow for accumulation of water and nutrient elements, reusable if sterilized, commonly used in pot hydroponic systems
Coconut fiber Made into fine (for seed germination) and fiber forms (coco peat, palm peat, and coir), useful in capillary systems, high ability to hold water a
nd nutrients, can be mixed with perlite to form medium that has varying water-holding capacities, products can vary in particle size and possible sodium contamination.
Peat Used in seed raising mixes and potting media, can become waterlogged and is normally mixed with other materials to obtain varying physical and chemical properties.
Sawdust Fresh, un-composted sawdust of medium to coarse texture good for short-term uses, has reasonable water-holding capacity and aeration, easily decomposes which poses problems for long term use, source of sawdust can significantly affect its acceptability.
Rice hulls Lesser known and used, has properties similar to perlite, freed raining, low to moderate water-holding capacity, depending on source can contain residue chemicals, may require sterilization before use