Growing Media

Pea gravel

We did start with sand, as that's the media that was being used in the university study we read about. We found, however, that the sand compacts too quickly to maintain good aeration for the roots, and also restricted the water flow.

We changed to "pea gravel" and are satisfied with the results. Although over time it will compact, normal crop harvests/rotation give us opportunity to cultivate and loosen any sections that are settling. We did find that our good Missouri creek gravel is not available everywhere - I think you and Peggy were the ones to describe southern pea gravel as round and very smooth not what you need for "bacterial housing".

Paula

Rockwool

If you want to maintain "organic" status be advised that Florida Organic Growers Certification Standards prohibit rockwool.

Quote:
b)Prohibited:

1)Rockwool is prohibited because of the use of phenol resins during manufacturing used to reduce surface tension and because of problems with disposal.

2)Synthetic acids are prohibited, including use as a pH corrector.

Emmett

Perlite

Have you considered PERLITE? It is being used all over the world in hydroponics and soilless applications.

Perlite is not a trade name but a generic term for naturally occurring silicous rock. The distinguishing feature which sets perlite apart from other volcanic glasses is that when heated to a suitable point in its softening range, it expands from four to twenty times its original volume.

This expansion is due to the presence of two to six percent combined water in the crude perlite rock. When quickly heated to above 1600°F (871°C), the crude rock pops in a manner similar to popcorn as the combined water vaporizes and creates countless tiny bubbles which account for the amazing light weight and other exceptional physical properties of expanded perlite.

This expansion process also creates one of perlite's most distinguishing characteristics: its white color. While the crude rock may range from transparent light gray to glossy black, the color of expanded perlite ranges from snowy white to grayish white.

Expanded perlite can be manufactured to weigh as little as 2 pounds per cubic foot (32kg/m3) making it adaptable for numerous applications.

Since perlite is a form of natural glass, it is classified as chemically inert and has a pH of approximately 7.

Horticultural Applications

In horticultural applications, perlite is used throughout the world as a component of soilless growing mixes where it provides aeration and optimum moisture retention for superior plant growth. For rooting cuttings, 100% perlite is used. Studies have shown that outstanding yields are achieved with perlite hydroponic systems. Other benefits of horticultural perlite are its neutral pH and the fact that it is sterile and weed-free. In addition, its light weight makes it ideal for use in container growing. Other horticultural applications for perlite are as a carrier for fertilizer, herbicides and pesticides and for pelletizing seed.
Horticultural perliteis as useful to the home gardener as it is to the commercial grower. It is used with equal success in greenhouse growing, landscaping applications and
in the home in house plants.

Pine bark

As far as growing media are concerned, there is a lot written about differentypes but most have disadvantages that outweigh the advantages. For example, vermiculite collapses after a while and becomes a soggy mass, perlite is not cost effective and provides too much drainage and too little water-retentive capacity, gravel has too high a cation exchange capacity, and so on. The only growiing medium that we recommend is composted pine bark as it combines an optimum water-retentive capacity together with good drainage and if you use the medium size you will not need to mix it with anything else. It is cost effective and it works.

Attie

Floating system

Gordon Creaser, at the Aquaponics conference in NC last November, showed us a floating system for growing leaf crops. As I recall about 3/4" to 1" holes were punched in the Styrofoam, about 7" apart. The plants were held in place by a piece of paper with a keyhole slit in it. This paper will disappear over the next few weeks. the plants are added at the 4 leaf stage, no media is needed , the roots hang down into the water. If you are growing talapia, you will need to protect the roots from the fish. If you need higher nutrient content, the fish can be acclimated to hydroponic nutrients. Cuttings will readily root when stuck through the Styrofoam into the water.

hope this helps,
David

Soilless medium

Gordon recommends a soilless medium (Sunshine Mix, Promix, Fafard - or make your own - 1/3 perliote, 1/3 peat moss, 1/3 vermiculite) seeded in 280 count cell trays. These guys are very small cells - about 3/4". Then they get transplanted to the rafts with the paper collars.

Adriana

2 parts perlite and 1 part vermiculite

I've had good luck for over 20 years using a starting mix that is 2 parts perlite and 1 part vermiculite. This works well for seeds or for a cloning mix.

kris

Sawdust

In the 1950s and 1960s, the Saanichton and Agassiz research stations developed a method of cropping greenhouse cucumbers in sawdust (Plate IV c). This method received general commercial acceptance in British Columbia, and to some extent in
Alberta, in the 1970s and 1980s, but it is now being replaced by rock wool. Some advantages of sawdust culture are its low cost, light weight, and its wide availability. Although rock wool also claims some of these qualities, sawdust could again receive renewed attention because it is easier to dispose of than rock wool. The sawdust used as a growing medium in the past was derived from Douglas-fir and western hemlock. Sawdust from western red cedar proved toxic, especially when fresh, so avoid its use. Other organic or inorganic media can be mixed with sawdust to improve its chemical and physical
properties.

However, the various substrate mixtures must be formulated and tested on a small scale under well-controlled
conditions. The uncontrolled distribution of a many organic media mixtures with diverse chemical and physical
characteristics confuses growers and detracts from the profitable use of this valuable Canadian resource. Use only the
horticultural-grade sawdust and ensure that it is free from contaminants that are toxic to plants (e.g., antifungal
chemicals used by the lumber industry). If in doubt, have a sample of the sawdust analyzed and ensure that neither the EC nor any particular element (e.g., manganese) are outside normal levels. Place the sawdust in troughs, beds, upright bags, bolsters, or even large pots. Regardless of the container, use a minimum of 10 L of medium for each plant. The practices followed in sawdust culture are similar to those described earlier for peat. Apply fertilizer in two ways: either, supply
all nutrients in solution at each irrigation (Table 22P and Table 22aP); or, incorporate some of the fertilizer into the
growth medium before planting and deliver the rest through the irrigation system (Table 23P and Table 23aP). The fertilizer rates described in Table 22P are recommended for cucumber production in unfertilized sawdust. Those described in Table 23P
are recommended for cucumber production in sawdust enriched with superphosphate (0-19-0) at 2.4 kg/m#179 and dolomitic
limestone at 4 kg/m#179. To ensure the long-term availability of calcium and magnesium, supply half the limestone ground
coarse and half ground fine. You can prepare the solutions described in Table 22aP and Table 23aP in two ways. One is to
dissolve the fertilizers at the prescribed rates in water and apply the resulting nutrient solution directly to the crop. The other is to prepare concentrated stock solutions (e.g., 100 times the prescribed rates) and incorporate these solutions
into the irrigation water using a fertilizer injector with a 1:100 mixing ratio. Table 24P outlines revised quantities both for making stock solutions for dilution at a mixing ratio of 1:100 before application, and for preparing the final solution
directly. Always remember that calcium and sulfates cannot be mixed together at high concentrations without some
precipitation of calcium sulfate, and therefore at least two stock solutions must be prepared.

Gravel culture

Kevin C. writes:

Maybe Dr. Howard Rush can shed some light, or some fertilizer, on this...maybe.
On page 273 and 274 of "Hydroponic Food Production" he writes:

"Gravel culture initially is blessed with many advantages, but over time some of these advantages are lost.

Here are the advantages:
1. Uniformwatering and feeding of plants. 2. Can be fully automated.
3. Gives good plant root aeration.
4. Adaptable to many types of crops.
5. Has proven to be sucessful on many commercial crops grown both outdoors and in greenhouses.
6. Can be used in nonarable areas where only gravel is available. 7. Efficient use of water and nutrients by use of a recycle system.

Here are the disadvantages:
1. Costly to construct, maintain, and repair. 2. With automatic valves, etc., failures occur often. 3. One of the biggest problems is the root buildup in the gravel, which plugs drainage pipes. Each crop leaves some roots behind and the moisture-holding capacity of the medium increases. Consequently, watering frequency may be reduced each year. Watering and aeration stresses occur. Over the years this root buildup results in a graveled soil and the advantages over a soil system will be lost. Eventually the gravel will have to be cleaned of roots, if not completely changed. Sterilization between crops by use of chlorine alone becomes ineffective.
4. Some diseases such as Fusarium and Verticillium wilts can be spread through a cyclic system very rapidly."

He also says on pg. 243 that "The best choice of gravel for a subirrigation system is crushed granite of irregular shape, free of fine particles less than 1/16 inch (1.6 mm) in diameter, and coarse particles more than 3/4 inch 1.9 cm) in diameter. Over half of the total volume of particles should be about 1/2 inch (1.3 cm) in diameter. The particles must be hard enough so they do not break down, able to retain moisture in their void spaces, and drain well to allow root aeration."

Then he says on pg. 247: (still talking about gravel systems) that "A 10- to 15- minute period for filling and draining respectively, or a total time of 20 to 30 minutes, is generally acceptable. In the removal of the free solution from the bed, nothing less than complete drainage is recommended. Only a film of moisture on the gravel particles is desired. If puddles of solution remain in the bottom on the plant bed, poor plant growth results. This rapid filling and draining of the growing beds can be achieved by use of large pipes in the bottom of the beds.

In summary, proper irrigation
cycles should:
(a) fill the bed rapidly;
(b) drain the bed rapidly and
(c) get all the solution out."

Now, he goes on to discuss the advantages and disadvantages of sand culture on pg. 325 and 326, but I think we pretty much agree that fine sand in a grow bed is perhaps not the best way to go with aquaponics systems that use direct pumping of "frappe'd" fish waste onto the sand bed, unless the cumulative feed burden or fish waste per cubic foot of sand per day is pretty low, relatively speaking, so I won't do the M'uadib Sandworm thing right now.

Recycled plastics

I did a brief search on recycled plastics like lexan, etc and these might offer a light weight alternate to heavier gravel beds especially when you consider task of wrangling the material during maintainance. Several plastics are recycled into pellets for injection molding and available in drums and though more expensive than most natural materials. This might offer an interesting way again to show another benifit to aquaponics by incorporating recycled materials into the system... just my two cents before my wife chases me out of the office for the day...

Arlos