Let’s say soon that “photographic” needs certainly impose an unnatural concentration of the plants for a pure aesthetic goal, but you will pardon us for sure: the most beautiful top models are made up to be even more beautiful than they really are!
Said this, your kind question open an interesting topic on which it is right to write some words .
First of all: how much soil does a plant need to grow up? A few, very few, less that we think because the role of the soil is more “ strategic” than functional.
In order for you not to worry about this statement, we invite you to consider the sizes of certain trees in high mountain: pines and firs of 10-20 meters that live on a very limited quantities and depths of soil. The soil that covers the mountain rocks is so few that it would make us give up building our own vegetable garden.
The soil, the top soil play a different role than the one we are used to think of: it is a wonderful 24 hours and 365 days operative chemical laboratory, able to transform the vegetable material into such small components that they can be spread by water to be absorbed by the roots.
A second question raises as obvious: but how much does a plant consume? Few, very few. Let’s think back to our fir on high mountain to have an idea.
The sixteen elements that are necessary for the life of a plant ( Nitrogen, Potassium, Phosphorus etc) are “ absorbed” and used by the plant in minimum quantities: we talk about milligrams and, in many cases of micrograms. Some of these elements are not either consumed: they are necessary only like catalysts to produce the sugars the plant needs.
In the hydroponic cultivation ( hydroculture to make it simpler) the soil does not exist: plants grow and grow up well in a solution where all the necessary elements are. The roots cling to lifeless material, like clay or gravel, only for a sole function of sustenance.
Going back to us, apart from the “photographic” needs to create a beautiful image, there is no reason why a plant can’t grow up beautiful, sane and productive in a minimum space of soil. I have recently bumper into a dracena of more than 2 meters with leaves longer than 80 cm ( what we will surely define a beautiful plant) that have grown for more than 20 years in a pot with a diameter of 30 cm, where, considering the space taken by the roots, only 2/3 glasses of soil can stay. (By the way, I have seen it at a gas station, in a space that, for the present vapors, is even very toxic – wonders of Nature).
The necessity to give normally a correct space to the plants of the vegetable garden and to the ornamental ones is due to 2 reasons: to avoid that the plants cover each other limiting the exposure and grant a sufficient space among the plants for a correct ventilation. The lacking ventilation increases the level of humidity and causes, for examples, fungal diseases.
Both problems can be however solved: we can improve the exposure by using some simple reflecting panels so that the plants can receive the light from all sides and a minimal forced ventilation that can keep the microclimate and the caused humidity under the critical threshold. All devices that are usually used in greenhouses. For example the melons, which would require many square meters of soil, are grown today vertically in green house, by fastening the branches and sustaining the fruits in hanging wire gauzes.
Why are not your plants so beautiful and fertile? Difficult to say at distance. Maybe – therefore you console yourself a little bit . for the same reason why my basil is this year so poor, while my neighbor’s one is wonderful ( but my roses are more beautiful). We can not understand all, but we like to talk with people who, like you and us, would like to understand and do better and better.