Lowlifes: Life in the Soil
By Erik Biksa
We often work very hard to make sure that our favourite plants are getting what they need in their diets to offer us the yields we desire. Have you ever just stuck a plant in the ground outdoors in the full sun only to return in a few months to find it has reached a stunning maturity? So, in terms of nutrients and the soil, what allows this to happen? The answers could be infinite, as there are billions of microscopic lives in a simple handful of good soil. On the other hand, the answer is simple in itself. The most productive soils are generally the most alive soils.
Now before we go any further, this does not necessarily mean that soil grown plants are more productive than plants grown by other methods including hydro and aeroponics. In fact, some studies suggest that the growth rates of strawberries are at their highest the couple of weeks following soil pasteurization (sterilization). Luckily for the length of this article though, this “sterile” period only lasts a couple of weeks before life finds its way back into the soil. This being the case, we can help to control which organisms exist and at what levels in our grow media by cultivating our soil(s) with biological products and organisms.
Since many of us are dealing with soilless mix such as coco or peat-based grow media, there are some long-term benefits to be had by increasing the “life” in the soil. The reason the term “long-term” is used is that for short-term crops you may not realize the full benefit of increased microbial activity because it takes a period to develop a complex “universe” in your soil. This brings us to longer vegetative periods with fewer plants, or re-using the grow media over several crops. DO NOT consider reusing your grow media if you are a novice grower. Even veteran growers will need to get a couple of “recycled” crops under their belts before they can gain a working understanding of their “living” medium. Remember that what you do to one crop, you are doing to the next to one on some level or another.
If you intend to reuse the grow media, raised beds are easy and inexpensive to construct and can be made to any size. However, I would not want to have to move during mid crop with several 4′ X 8′ X 12″ deep raised beds. The following details of raised bed construction have been borrowed from “Large Scale Indoor Gardening” by William Walker:
- The bed(s) should be constructed of two inch by fourteen inch rough cut cedar or two inch by twelve inch cedar.
- Every six to twelve inches along the bottom of the board make a quarter inch cut through the depth of the material with a circular or hand saw. The bottom of the board is now grooved, allowing for drainage when it sits flat against a floor.
- After all four sides have been joined and the bed is in it’s final position, fill the lower half inch to one inch with grow rocks for drainage.
- Layer one half inch to one inch of washed river sand onto the layer of grow rocks. Smooth and flatten, taking care not to over-compact the drainage layer.*
- Install hardware cloth (stainless screen) on top of the drainage layer. Make sure that where sheets join, there is about six inches of overlap. Also ensure that the hardware cloth is secured an inch or two up the sides of the boards.
If you intend to add heating cables, the layer of sand is the place to do it. Usually the medium warms up during the light cycle and retains some of the heat through dark hours. A word of caution: only use heating cables in very cool conditions as they can quickly heat up and dry out the root system.
Now, you have a raised bed with one to two inches of drainage, leaving near a one foot depth for the planting medium. If you intend to keep re-using the media for a period, it is best to avoid mediums containing perlite and vermiculite, as they can lead to drainage problems (clogging in the hardware cloth/screen). For improved drainage you can mix river sand into the medium if you desire. I would recommend using a high quality coco coir as the base. It tends to compact less and retain its structure much longer than peat can. It also holds a lot more airspace and is closer to neutral pH. If you opt to use sphagnum peat moss as your base, add five to six cups of dolomite lime to the mix to help buffer pH. This also provides a slow release of calcium and magnesium while helping to reduce nutrient toxicity. With either, you should use a high quality wetting agent for the initial wetting of the media. Coco is much easier to re-wet than peat if it has dried out.
When you’re medium becomes very dry, it is wise to add a wetting agent when re-wetting. It helps break the surface tension of the water coming into contact with the medium reducing “dry pockets”.
In addition to the peat or coco base, you may consider further amending the grow media. A natural first choice is premium granular humic acid. This material is highly oxidized granular leonardite, the same material that some high quality Fulvic acids are extracted from. This will help to improve macro pore space while providing your plants with a slow release of fulvic acid(s) and feeding the soil with a slow release of humic acid(s). It is 100% organic and there are numerous studies to support the benefits. Pyrophyllitic clay or paramagnetic soils can also be added, providing essential micro and sub micro elements and possibly silicate, cation exchange, etc. Greensand is also a source of trace minerals.
Diatomaceous earth is a fossil powder of sorts that is excellent in controlling soil-born insects. You will need to add smaller and smaller amounts of these with each successive crop.
To keep macro-nutrients consistent from crop to crop, it is recommended that you only supply the bulk of the plant’s nutrient requirements via fertigation. If you have too much in the way of slow release nutrients in the soil (such as bone meal, kelp, rock phosphate), it will be too difficult to know and control the level of nutrients available to the plant in following crops.
If you want to grow 100% organic, just purchase or brew organic tea solutions that can be watered into the crop manually or by drip irrigation. Very few organic solutions will not clog drippers, so you may want to run spaghetti lines open-ended, but with very precise control over flow rates, and the frequency and duration of applications.
When re-using the medium in the raised bed, remove as much root debris, leaf matter, etc by hand from the soil without disturbing the hardware cloth. The cloth should keep most of the roots out of the drainage layer. To further re-loosen the soil and speed decomposition of old roots, run a small roto-tiller up and own the bed(s). Be very careful that the tiller blade(s) are set to the correct depth, and do not ensnare the hardware cloth. Leech the medium heavily with warm to hot water. If possible, hook up your lines to a hot water tank, and let it run for 48 hours through the system. A floor drain that doesn’t clog is a must for this application. Do not consider using raised beds for several crops if you are unable to apply large volumes of water to flush through the raised beds to wash away salts, residues, etc. Before replanting, you will need to re-buffer the medium for pH. If using peat-moss add another five to six cups of dolomite lime and loosely till into the soil. For coco, check the pH of the medium before planting and make any necessary adjustments although few, if any, should be required. After a healthy crop or two you should have a pretty good population of microbes working for you in the soil. Most microbes require a source of carbon, so it’s not a bad idea to add several liters of worm castings to the medium each planting. Worm teas are said to be high in carbonic acid. One veteran grower remarked on re-using their raised beds: “. . .each crop just gets better and better. . .”
To create a complex life system in your beds, you might consider some of the below as additions or to learn more about how you can make them work for you.
Bacillus – several varieties of Bacillus (i.e Bacillus megaterium) have been found to play a role in the conversion of unavailable forms of phosphates into plant available forms. In natural settings they can provide near ten percent of the available phosphorous in the soil solution. With increased levels of plant available phosphorous, Bacillus strains become less effective. However, if the Bacillus can sustain as a back up it may continue to provide hungry blooms with phosphorous if it should become otherwise unavailable or “locked out”. This bacterium is of special interest to organic farmers who incorporate rock phosphate into the grow medium or if introduced through fertilizer teas, preparations, etc. Rock phosphate tends to be mostly unavailable, breaking down into plant available forms over time.
Mychorhizae – This fungus forms a symbiotic relationship with the plants. The mychorhizae fungi penetrate the plant and grow around the plant’s root system, effectively increasing the root surface area available for nutrient absorption. The fungi are able to transfer phosphorous into the plant, the product of their digestion of soil materials (nutrients).
Nitrosomonas – Converts plant available ammonium (NH4) to unavailable nitrite (NO2). In doing so, it acidifies the soil, which in nature is a means of storing it in the soil for later use.
Nitrobacter – These bacteria convert the nitrite resulting from the nitrification into nitrate (NO3-), a potentially available form of nitrogen.
Rhizobium – A bacteria associated with nitrogen fixation, usually in legumes. The bacteria form a symbiotic relationship with the plants, increasing the plant root surface area available for nutrient absorption. Much of the air we breathe contains a percentage of inert nitrogen gas (N2). These bacteria digest the atmospheric nitrogen and feed the plant nitrogen in a plant available form. In some agricultural field crops, this is the plant’s principal provider of nitrogen. Additions of nitrogen fertilizer can actually inhibit the performance of these bacteria.
Pseudomonas – This is another PGRB (plant growth regulating bacteria) which is associated with the solublization of unavailable forms of phosphate in the soil.
Streptomyces – Bacteria that secrete a variety of compounds including antibiotics that prevent and control root zone pathogens. A closely related species of Streptomyces produces the antibiotic that we use, streptomycin. Many studies demonstrate the bacteria’s effectiveness at controlling root diseases and select foliar diseases. An interesting consideration noted in one study is that they will also reduce levels of some nitrogen fixing bacteria in the soil.
Trichoderma – Strains of Trichoderma bacteria are found naturally occurring in many soils can play a role in the prevention and control of root pathogens, ultimately providing a healthier soil environment which can lead to higher yields. Some research suggests that the proteins in Trichoderma can degrade chitin, a structural component found in pathogenic fungi such as powdery mildew and in insects. Some innovative propagating plugs, growing blocks and materials are inoculated with strains of Trichoderma. If a healthy soil environment is maintained, the bacteria will continue to colonize the roots and multiply in the growing media. The Trichoderma help to form a protective layer around the root system, helping to fend off invading pathogens, etc.
Urease – an enzyme that plays a role in the conversion of urea CO(NH2)2 into ammonia which is able to pick up a free hydrogen ion to become ammonium (NH4), a plant available form of nitrogen which may be absorbed by the plant, stored, or nitrified. Some organic standards recognize certain grades of urea, as it is a carbon based source of nitrogen.
For most indoor applications, additions of urea are not recommended unless they are prepared specifically (usually as a blend) for your applications. Urea is usually very unstable in the soil solution, disrupting pH levels and at worst burning plants and toxifying the soil.