Biodiversity in the orchard

The term biodiversity, although coined less than thirty years ago (first published by the American biologist Edward O. Wilson in 1988), is deeply rooted in our society and is now part of our cultural heritage. It is enough for us to stop for a few moments to look closely at the leaves of some of our vegetables, or the soil of the terraces, to realize that we are accompanied in the horticultural adventure by hosts of small animals. With the help of suitable instruments, such as magnifying glasses or microscopes, we would discover that in addition to the visible creatures, there are also, in overwhelming quantities, tiny and invisible to the human eye.

The orchard, in short, is alive, like any natural habitat. In fact, our orchard is a fully-fledged habitat, with all its flows of energy, nutrients and its complex network of ecological relationships. Several current trends are based on this holistic vision of the orchard (or agricultural production spaces) and consequently bet on the natural balance as a guarantee of good health and productivity of our crops: organic agriculture, biodynamic or permaculture, to name the best known.

The practice of organic horticulture is based largely on understanding the natural flows of nutrients and energy that are established in our small productive habitat, hence the practical importance of knowing a little of what elements make up the complex puzzle and how they interact between them.

This approach is diametrically opposed to that of conventional horticulture, which, following the guidelines of the market and the intellectual currents that have led us to consider nature as a productive resource alien to man, is characterised by trying to control the natural processes of productive plants from outside. Through the entry of products, often of synthetic origin, to regulate imbalances (e.g. plant weakness, infections, pests, lack of nutrients) that can undermine production and even threaten the integrity of the crop. The multiple undesirable effects of chemical treatments, as well as the dependence they generate, are the main arguments of the detractors of these conventional practices.

The subject is very topical: an introductory book to the currents of organic horticulture will give us examples to illustrate the above. For the time being, we will not dwell on it, and we will stick to the subject at hand.

The inhabitants of the soil

It should come as no surprise that most of the animals in our garden are below ground level. It is there where the best conditions exist to prosper: a dense substrate with a vertical profile that offers an important volume of habitat through which to move; humidity, essential for any form of life, and available in different forms; minerals that gather the necessary elements for the formation of organic molecules, which are the fundamental pieces to build living beings;

in various forms, the result of the decomposition of plant matter and of the soil animals themselves; and environmental stability much higher than that found only a few centimetres above, where temperature and humidity oscillate with greater amplitude and speed.

The biomass of the inhabitants of the soil (i.e. the accumulated weight of all organisms) may seem exaggerated: up to 2.5 kilos per square metre when we gather all its inhabitants. To better understand the magnitude, density and complexity of the subterranean ecosystem of our orchard, we must bear in mind that most of these inhabitants are in the first twenty centimetres of depth. In natural habitats, in the absence of the mechanical work with which we remove the soil in our crops, the inhabitants of the soil are concentrated in a horizon (this term describes the different layers of the soil) even narrower, approximately 5 centimetres.

Among the microscopic inhabitants of the soil of our orchard are mainly bacteria and fungi, accompanied by protozoa and nematodes. Other larger decomposers, such as earthworms, termites or millipedes, complement the extensive family photo.

The health and quality of the soil depend on its chemical composition and its small inhabitants since they are in charge of decomposing the organic matter (remains of the vegetables above all) so that it becomes available again so that the plants can absorb it through their roots and thus start the cycle again. They also carry out the essential task of making available to the plants the mineral elements present in the soil and in the bedrock that gave rise to it. This last task is carried out almost exclusively by bacteria, among whose overwhelming diversity there are species capable of disintegrating any mineral, thus releasing the ions of which it is composed.

Among the elements necessary for plants that are generally mineralized, and therefore can only be absorbed by plants after disintegration by bacteria, are for example sodium and potassium. These, together with nitrogen, are the main components of any chemical fertilizer, the application of which is always aimed at balancing the presence of these elements in the soil. Nitrogen, on the other hand, is not found in soil minerals. The essential bacteria are in charge of providing it to the plants, which need it for their development. The atmospheric nitrogen-fixing bacteria, known by farmers for their strategic importance for the proper functioning of any productive terrain, are Azotobacter (which fix nitrogen alone), and Rhizobium (alone they cannot fix nitrogen, only when they join in symbiosis to the roots of legumes to form so-called fixation nodules). Although the complexity of the biochemical processes that make possible the collaboration between legumes and bacteria has been understood for relatively few years, the virtues of legumes as soil fertilizers have been exploited since ancient times, with fallows, rotations and planting of legumes accompanying other crops.

Thus, without the precious ecological service provided by the tiniest inhabitants of the soil, it would become sterile and could not grow on any plant. Despite their small size and the fact that they are generally unnoticed, we owe the existence of all terrestrial ecosystems, and therefore our own, to bacteria. It is worth understanding their needs and taking care of them to the extent of our possibilities.

Surface Inhabitants

Although the interior of the soil is the most populated and harbours the greatest number of living organisms, it is costly to travel through it. That is why most pests reach our orchard from the surface, either by land or by air. And in the same way, an endless number of auxiliary fauna do it, that if they find the right conditions will remain contributing balance to our ecosystem and avoiding that there is an overpopulation of any of its inhabitants (either harmful or beneficial).

Thus, on the ground congregate another multitude of small animals, fungi and plants, which although they do not enjoy the stable conditions of the interior of the soil, they manage to survive. In many cases, they are protected from the elements using more or less hard external coverings, which above all avoid the loss of humidity. These extremes are very evident in insects: those that we will find on the ground or on the plants of our garden almost invariably show a hardened and protected body, while those that reside under the ground, like for example termites, have a softer and less isolated abdomen.

On the surface, one of the main objectives of maintaining good biodiversity is precisely the control and prevention of pests. The other great service provided by the auxiliary fauna is the pollination of our vegetables, which is necessary for most of them to bear fruit.

Among the most feared inhabitants of our garden are invertebrates such as aphids and snails, which can damage our plants, or fungi such as downy mildew or powdery mildew. Our orchard will never be exempt from its threat, but it will undoubtedly have a better chance of absorbing the blow if it has a well-balanced ecosystem, where the potential predators of these pests are present in sufficient number. To the already popular capacity of the ladybirds to control aphids must be added so many other species that exert an excellent control of the populations of invertebrates (larvae of chrysoprase, Hymenoptera, and even hedgehogs and insectivorous birds, which will not hesitate to control the levels of prey such as slugs and snails).

Controlling fungal infections will usually require direct intervention, which can always be carried out by applying natural products (e.g. herbal teas or slurry prepared from local wild plants). Although we do not have specific species to combat these pathologies, the maintenance of a healthy and biodiverse ecosystem will also help us in this case, since the propensity of our vegetables to develop infections decreases with the quality of the horticultural ecosystem. And here we must remember, once again, that the agricultural ecosystem we must take care of starts at the very roots of our plants, and reaches to the top of their stems.

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