Cultivation in containers (whether they are cultivation tables, pots or planters), given the isolation of the soil in use, forces us to take special care of the structure and fertility of the soil, which can not be plugged by neighbouring substrates or by the deeper layers, as happens in traditional orchards.
The problems of dryness (or excess of humidity) that we detect in our substrate will be the best indicators on how to act, in the same way, that it will be the vigour of our vegetables, which will show if it is necessary to add fertilizers of any kind. That is where the experience with your own urban garden and its particularities (local climate, sunshine it receives during the day, protection from high – or low – temperatures, etc.) becomes your best guide.
Anyway, it’s worth taking into account some general tips to ensure that your urban garden doesn’t lose vigour.
Without fertility, there will be no harvest
And it is almost certain that after spring and summer production the soil will have been exhausted to a greater or lesser degree. The first step, therefore, to ensure that the next plants have nutrients is to fertilize the soil of cultivation.
There are excellent homemade ways to produce your own compost, which added in generous doses before growing again will guarantee a good harvest. You can do it with a composting or a vermicomposter (to get the precious earthworm humus), and thus make the most of the organic waste generated in your home. Another option is to fertilize plants such as comfrey or nettles (among many others), which will provide the soil with the nutrients it needs. These are undoubtedly the most ecological options.
However, if you still don’t have your own composter or your own liquid fertilizer, many fertilizers on the market will allow you to reinvigorate the soil. There are solid (to add to the substrate before cultivation) and liquid (which can be dosed with irrigation water throughout the entire cultivation process) versions. There are those of animal, vegetable or mixed origin, so pay attention to the label of the product before buying it, especially if the final consumer is vegetarian or vegan.
The substrate, the basis of your crop
Regardless of which fertilizers we add, the substrate on which they sit is equally important. Before approaching the new season it is good to make sure that the structure of our growing substrate is the right one: that it is soft and that it can retain water.
If after applying your own compost or a commercial compost you detect that the soil compacts or dries in a way that affects the growth of your vegetables, you can improve the soil structure using mineral substrates or coconut fibre.
Several mineral substrates will give us oxygenation and above all water retention to the growing substrate. Among them are perlite (of volcanic origin), Marita (expanded clay) or vermiculite (mica), all marketed after expanding them with heat treatments. This expansion is what gives them the capacity to aerate and retain water (they become porous), as well as an extreme lightness, the latter factor to take into account when we must move our growing table or our pots. Incorporating a proportion between 10 and 20% of these minerals in our growing medium should be sufficient to ensure its functionality. In extremely dry spaces, it may be a good idea to incorporate polymers. It is a non-toxic and non-polluting product that is capable of absorbing water at a rate of more than 200 times its dry volume so that it provides us with extra water retention that in some cases may be convenient.
Unless the manufacturer tells us otherwise, these substrates are usually inert, with no organic matter added. To provide the nutrients that our crops need, it will be necessary to mix the mineral substrates with other organic substrates, or it will be necessary to add natural fertilizers to the irrigation water.
The contribution of these mineral substrates to our growing medium is not always purely structural. Vermiculite, for example, contains several elements that our plants need, such as potassium, calcium or magnesium. On the other hand, the presence of elements absorbed by plants means that this substrate will wear out over time and can be reused less often than, for example, perlite or arlita, which will last for many seasons.
If we have problems with low winter temperatures, a superficial layer of arlita (half a centimetre will suffice) will help us to protect the roots from frost.
And now we will be able to face the appetizing winter crops!