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Soil-less culture is the method of growing agricultural crops without the use of soil. Instead of soil, various inert growing media, also called substrates, are used. These media provide plant support and moisture retention. Irrigation systems are integrated within these media, thereby introducing a nutrient solution to the plants' root zones. This solution provides all of the necessary nutrients for plant growth. The most common method of soil-less culture is hydroponics, which includes growing plants either on a substrate or in an aqueous medium with bare roots. There are many designs of hydroponic systems, each serving a different purpose, but all systems share these basic characteristics (Figure 1.3).
Soil-less agriculture has been used to reduce pests and soil-borne diseases affecting monoculture crops. Hydroponics can in fact control soil-borne pests and diseases by avoiding the contact between plants and soil, and because soil-less media can be sterilized and reused between crops. This reuse of substrates meets the particular demands of intensive production. Some substrates are far better than soil, particularly in terms of water-holding capacity and oxygen supply at the root zone. Farmers have also improved plant performance through increased control over several crucial factors of plant growth. Nutrient availability at plant roots is better manipulated, monitored and real-time controlled, leading to higher quantitative and qualitative productions. Moreover, most soil-less culture methods use a fraction of the water necessary for traditional soil-based production because the nutrient solution is recycled.
Soil-less agriculture is one aspect of the major scientific, economic and technological developments in the general field of agriculture over the last 200 years. In general, but redominately in developed nations in temperate climates, there has been an increasing demand for out-of-season, high-value crops. Partly, this is a result of widespread improvements in living standards. This increase in demand has led to the expansion of many types of protected cultivation systems to boost production capacity and prolong the supply of crops throughout the year. Within these protected systems, crops can be grown in soil. However, in order to stay competitive with open-field agriculture production, intensity has had to increase in order to offset the higher production costs associated with controlled environment agriculture. As a result, there has been a shift from soil production to soil-less culture to address the changing needs of agriculture. This approach provides alternatives to toxic soil sterilization to control pests and pathogens, and can help to overcome the soil-tiredness problems that monoculture practices have brought.
Beyond its significantly higher yields compared with traditional agriculture, soil-less agriculture is also important because of its higher water- and fertilizer-use efficiency, which makes hydroponics the most suitable farming technique in arid regions or wherever nutrient dispersal is an issue for both environmental and economic reasons. The offset of soil makes hydroponics an indispensable solution in areas where arable land is not available. Soil-less agriculture can instead be developed in arid lands, in saline-prone areas, as well as in urban and suburban environments or wherever the competition for land and water or unfavourable climatic conditions require the adoption of intensive production systems. The high productivity for the small space required makes soil-less agriculture an interesting method for food security or for the development of micro-scale farming with zero food miles.
To summarize, the four main reasons why soil-less culture is an expanding agricultural practice are: decreased presence of soil-borne diseases and pathogens because of sterile conditions; improved growing conditions that can be manipulated to meet optimal plant requirements leading to increased yields; increased water- and fertilizer-use efficiency; and the possibility to develop agriculture where suitable land is not available. In addition with the rising in demand for chemical- and pesticide-free produce and more sustainable agricultural practices, there has been extensive research into organic and soil- less methods. Section 6.1 discusses these differences in more detail.
A major concern regarding the sustainability of modern agriculture is the complete reliance on manufactured, chemical fertilizers to produce food. These nutrients can be expensive and hard to source, and often come from environmentally harsh practices accounting for a substantial contribution of all carbon dioxide (CO2) emissions from agriculture. The supply of many of these crucial nutrients is being depleted at a rapid pace, with projections of global shortages within the next few decades. Hydroponics is much more efficient in terms of water and nutrient use than is soil-based agriculture, but its management is more complicated and requires a different set of inputs, especially during installation. Electricity is generally required to circulate or oxygenate the water. However, it does not require fuel to plough soil, it does not require additional energy to pump much higher volumes of water for irrigation or to carry out weeding control, and it does not disrupt soil organic matter through intensive agricultural practices. The initial costs, building materials, and reliance on electricity and inputs will also be important limitations to aquaponics, but in this case the need for chemical fertilizers is completely removed.
Source: Food and Agriculture Organization of the United Nations, 2014, Christopher Somerville, Moti Cohen, Edoardo Pantanella, Austin Stankus and Alessandro Lovatelli, Small-scale aquaponic food production, http://www.fao.org/3/a-i4021e.pdf. Reproduced with permission.