One of the biggest problems of the 21st century is, perhaps, the need to provide water for the growing world population. The water supply must meet the demands for human, agricultural and industrial consumption. While the demand for water is increasing in relation to the available water resource, there is an excessive exploitation of water sources, in addition to its pollution, misuse and waste, caused by the use of inadequate and inefficient distribution systems.
Kammeyer (2017) pointed out that in addition to all the water challenges in the world are the imminent repercussions of climate change. Currently, there is a greater intensity of natural events related to water, such as droughts and floods, and this trend is expected to continue. It is projected that by 2050, 3.9 billion people will live in watersheds under severe water stress. The nature of these impacts will vary according to the region, changing the global dynamics of water stress.
Much of this new demand will be driven by agriculture, which already accounts for 70 percent of the world’s freshwater use. Food production must grow by 69 percent by 2035, which will increase water needs for agriculture. Therefore, the efficient use of water is essential, which has taken on greater importance in the agronomic field, coming to be defined as: production per unit of water. For example, in a tomato crop, to produce 1 kg of fruit under controlled substrate conditions, about 15 L are used with recirculation and use of drainage water, whereas 60 L are used in an open crop, and in greenhouse without reuse of drainage water, 32 L (Castilla et al., 1990).
The conventional cultivation in substrates, indistinctly of its container, was conceived originally for a management to solution lost, unlike other systems like the hydroponics or those of subirrigation, that were designed like closed systems by their intrinsic characteristics. However, due to the growing concern of society for the deterioration of the environment and, as a consequence, to the pressure that is being exerted on the different polluting human activities, including agriculture, these open systems are being adapted to the new requirements, thus allowing the collection and accumulation of leachates to be used later in the formulation of a new nutritive solution. These types of closed systems are usually called “systems with reuse of leachate”. Leachates are the nutrients that infiltrate deeper layers of the soil because of the flow of water. The lost solution systems involve the elimination to the environment of important volumes of leachates with a high polluting power, especially due to the presence of nitrates. This can be avoided to a large extent by means of the systems of cultivation without closed ground, since they allow to obtain a remarkable saving of water and fertilizers, generating a near total reduction of the environmental contamination.
The irrigation water that infiltrates in excess passes beyond the root zone, this water together with the deep percolation coming from the filtrations of the channels, dissolves additional salts of the underlying soils and substrates. Additionally, fertilizers and pesticides applied to soils can also be mobilized and discharged into drainage water, constituting an additional source of contaminants in receiving waters of better quality. For example, nitrate (NO3-) from organic and mineral nitrogenous fertilizers, as well as the mineralization of organic matter, can contaminate effluents and springs (Groeneveld et al., 2001), so although it exists naturally in soil and water, it is considered a compound that causes serious health problems when consumed from contaminated groundwater sources. Consumption of water contaminated by agriculture has been shown to be directly related to the disease methemoglobinemia, which occurs when nitrate (NO3-) is reduced to nitrite (NO2-) within the human body, transforming hemoglobin into methemoglobin, limiting its transport capacity of oxygen (Greer and Shannon 2006); in addition to a potential role in the development of cancer in the digestive tract due to the formation of N-nitroso compounds (Fewtrell, 2004).
From these needs is that the development of alternative cultivation has advanced with a desire to solve these environmental and health demands, generating among these new techniques, hydroponics (from the Greek hýdor ‘water’, and ponos ‘work’). Hydroponics is a set of techniques that replace the soil, where the crops are fed by a liquid solution of mineral nutrients (fertirriego). Importantly, the hydroponics can be a closed system, that is, the solution of nutrients after passing through the roots is recovered, and reused by a subsequent treatment to readjust pH and electrical conductivity through the use of injectors with stock solution.