Growing nutrient-hungry, short-cycle plants using organic cultivation methods in containers can be quite a challenge. Can the changing needs of your plants be accommodated in a contained environment without resorting to extra NPK?
Growing in containers isn't like regular gardening. Your plants are ... wait for it ... contained! When they're out in the field they can send out exploratory roots to discover new territories and exploit new water and mineral resources. Plants pretty much take care of themselves. However, when plants are growing in containers, they are far more dependent on YOU (for regular watering) and their immediate surroundings—everything your plants feed on needs to be right there to hand, so to speak.
Things get even more challenging when you want to grow fast growing, heavy fruiting annuals — tomatoes, cucumbers, zucchini, that sort of thing.
And times this challenge by two if you want to grow organically.
Your plants are going to put heavy demands on a relatively small amount of growing media. There's no reaching for the NPK mineral-based fertilizers if they start looking hungry. Furthermore, short-cycle plants don't have endless months to wait until certain elements are mineralized and available. The nutrient availability needs to be steady as your plants grow. It can't be too strong at the beginning or too weak at the beginning!
So what are the practical steps that can growers take to meet the demands of their organically grown, short-cycle container plants?
1. The Seedling / Cutting Mix
Invest in a quality organic seedling mix. A seedling mix differs from a regular potting mix in a few ways. Seedling mixes tend to contain more sand which allows the young roots an easier passage through the media. However sand retains water relatively poorly, so vermiculite and perlite are often added to counteract with their high water and air holding abilities. Seedlings and cuttings are particularly prone to anaerobic conditions caused through over-watering or drought stress is moisture levels drop too low, so vermiculite and perlite are a propagator's best friend!
Another essential installment to get in right from the get go is mycorrhizae. This funghi comes in the form of a power or liquid and forms a special relationship with many plant species. Essentially your plants feed it carbohydrates exuded from the roots and, in return, the mycorrhizae spreads out a mycelial network getting into microscopic nooks and crannies that even the root hairs can't penetrate. The funghi then translocates minerals and moisture back to your plants!
However, if your short-cycle plants are going to have any hope whatsoever of benefiting from this amazing symbiotic relationship, you need to introduce them right at the beginning!
A trace of powdered humic acid will aid in seed germination rates too, but nothing can replace a warm, humid propagation environment!
2. The Main Potting Mix
For neutral soil…peat moss or coco coir are ideal components.
Be sure to mix your potting mix really well. Commercial soil mixers are available but lots of growers use a kiddie pool and a trowel. Mix well or you will have hot spots!
Preparing your potting mix is a huge opportunity to install some goodies for your plants. Some will take longer to break down than others. A typical potting mix contains extra ingredients like coco fiber and perlite to help with water retention—essential for container gardening!
For every large (50 liter) bag of potting mix, you could consider adding the following additional elements:
Worm Castings (around 5 lbs) Often growers are advised to purchase "pure castings" and not just "worm compost". Worm castings are worm poop - and are a fine source of organic nitrogen and beneficial biology. However, some worm castings are not as rich in nutrients and biology as some vermi-compost products. Everything depends on what the worms have be fed on and how they have been treated.
(Note: if you use potting mixes like Ocean Forest, JustRight Xtra, or Roots Organic you don't need to worry as these come pre-loaded with worm castings and many other goodies. You don't want to over-do the castings as it can make your mix too "muddy" and deplete oxygen levels.)
Bat Guano ( 5 - 10 ounces) You can get guanos that are nitrogen rich (grow) or phosphorus / potassium rich (bloom). Both are great and provide natural protection against nematodes in your soil (the non beneficial variety) and also helps to activate beneficial biology in your potting mix.
(Note: Some guanos are more soluble than others. Fossilized guanos are as hard as a rock and need time to break down properly. Some guanos are also stabilized with phosphoric acid and provide the instant gratification that some people seek in mineral-based fertilizers. Be careful not to over-use or you will burn your plants, especially young seedlings.)
Bone Meal (5 - 10 ounces) Ground up animal bones (usually bovine.) Not only adds a wonderful slow-release form of natural phosphorus to your mix but helps to temper the strength of castings and high-nitrogen guanos.
Blood Meal ( 5 - 10 ounces) Powdered (dried) animal blood (usually bovine.) A quick-fix nitrogen boost and lowers pH—do not use too much. If you'd rather not use animal by-products or are concerned about blood meal's tendency to damage younger plants in warm conditions then try feather meal or alfalfa meal instead.
Rock Phosphate (6 ounces) Slow release phosphorus—essential for a healthy bloom and overall plant vigor.
Fine Dolomite Lime (1 - 2 tablespoons) Additional calcium (if you select the fine variety) and pH stability.
Coco Pith / Fine Coco Coir Aids potting mix structure (increases aeration and moisture retaining properties)
Humic Acid (trace) Available in powder or liquid form, humic acid will help to increase root vitality, improve nutrient uptake, stimulate microbial activity and increase yields.
Kelp Meal is another excellent humic source. Arguably kelp can replace several of the above amendments and is especially well suited to potting mixes for indoor gardens and short-cycle crops.
Remember, the larger the container, the more margin for error you're going to give yourself. Don't skimp just so that you can fit in more plants. It's a false economy. Fabric and air-pruning / root trapping containers are awesome but they do require more regular irrigations.
4. Beneficial Biology
You absolutely need to add beneficial biology if you're going to have any hope of breaking down all the goodies in your potting mix within the lifespan of a short-cycle plant (sometimes as short as 10 - 12 weeks.) One of the easiest ways of adding biology is to brew your own compost tea.
5. Regular Watering boosted with Liquid Organic Feeds
Some organic gardeners look down on liquid organic feeds. They claim that their use evidences the fact the soil has not been prepared correctly. But when it comes to growing in containers, they really are indispensible - after all, your plants are contained and so the delivery of some extra nutrition at the right time is going to be a very welcome thing indeed! And there's more good news - liquid organic feeds have come on along way in recent years. They are quickly assimilated by plants and are as easy to use as their mineral-based counterparts. As such, take it easy with them. Follow the axiom "Apply weakly, weekly" and you won't go too far wrong. A week's wait helps you to see the plant's response to the feed, often not as immediate as when using mineral-based feeds.