Properly irrigating agricultural land is a delicate balance. More water does not always mean a more abundant harvest. In arid climates, supplying more water than needed causes more trouble instead of more productivity. The principles of balance and efficiency, observed at all levels of the universe, are also apparent in irrigation.
Salt is a critical compound for life. Salt that dissolves in water becomes ionized, and plays a crucial role in maintaining the ionic equilibrium of the human body. It allows the sodium-potassium pump to run. Salt is also necessary for plant and animal life, but the amount of salt must be balanced here, too. When this balance is disrupted, land exposed to excessive salt becomes barren. This is a real risk for arable land. In areas where temperatures are high and there is little precipitation, over-irrigation with the intent of boosting productivity might backfire, causing over-salinization of the land. Over-salinization happens when water travelling underground from the earth’s surface encounters a layer of soil with little permeability. As such, the water starts accumulating on this layer. If the irrigation of such land continues, the level of water accumulated underground rises.
When humidity on the surface of the land decreases due to evaporation and plants consuming water, the accumulated underground water begins travelling upwards to the surface through capillary channels in the soil. This is called “capillary movement.” During this phenomenon, salt already in the soil and in the underground water is also transferred to the upper layers of the soil. Yet, this upward movement of salt in geological structures is not a main determinant of soil salinization; what is critical is the amount of salt in the irrigation water. The rate of salt in irrigation water should not exceed 1000 ppm. Even the Euphrates River, which is considered to be high-quality irrigation water, carries approximately one ton of soluble salt to one hectare (10,000 m2 or 2.47 acres) of land every year.
Salt carried by irrigation water does not directly cause over-salinization. There are mediating factors, such as the height of the underground water table, the natural hydrogeological characteristics of the land, over-irrigation, and excessive and erroneous use of fertilizers.
The evaporation of water from the earth’s surface accelerates during the hot and dry summer months. In return, the rate of salt accumulated in soil increases. When land is irrigated every year, the amount of accumulated salt in the upper layers of soil increases. This might cause fertile land to gradually become salty and barren. Such over-salinization does not happen in areas with high precipitation, since salt that exists naturally in soil in such areas is first carried to rivers and underground water sources, and then to lakes and seas.
Over-salinization is a huge problem. Every year, approximately 10 million hectares of land are becoming barren because of over-salinization. Due to wrong irrigation methods and extremely dry seasons, over-salinization is common in many developed countries where fertile agricultural land is faced with desertification. Such a loss of fertile land means a loss of billions of dollars in these countries’ budgets.
Another reason for over-salinization, which then leads to desertification, is the destruction of forestland for use as agricultural land. With the disappearance of trees, the ecological balance of many environments has been slowly destroyed. This has been followed by improper irrigation, eventually causing over-salinization. As a result, attempts to turn forestland into agricultural land have generated vast but useless fields. Australia is losing 130 million dollars worth of agricultural land every year – and this number keeps increasing.
There are historical records documenting that vast fertile areas turned into deserts due to over-salinization. The region called Mesopotamia, surrounded by Iraq, Eastern Syria, and Southeastern Anatolia, was the cradle for many important civilizations due to its highly fertile land. Unfortunately, this region has largely gone through a mass desertification. Researchers have determined that this desertification was predominantly due to the over-salinization of the soil.
What is the solution?
Traditionally, the most important measure taken against over-salinization has been to establish a drainage system for fields in arid or semiarid climates. This system can be in the form of surface or subsurface drainage through which excess irrigation water, along with the salt in the soil, are removed via canals or pipes.
Different kinds of crops require different amounts of water. Diversifying the kinds of crops planted in the same field can also prevent desertification. Some years, crops that do not need much water should be planted to help reduce the amount of salt in the soil. Using a drip or sprinkler irrigation system can further reduce salinization.
The time of day irrigation is performed also has an effect on the soil. Irrigation should be performed after sunset, especially in hot climates, to minimize the water evaporation that triggers salinization. Another way to reduce evaporation is by planting crops that have thick vegetation during the summer months. They can shield the soil from the hot summer sun.
Increasingly, some farmers are combatting over-salinization by planting salt-tolerant crops. Grain crops – like wheat, corn, or barley – are not salt-tolerant, but plants like sugar beets, cotton, grapes, sunflowers, and alfalfa can help to decrease the amount of salt in the soil.
The final but most important step in reducing over-salinization is educating people in the agricultural sector. They should be informed about all aspects of proper irrigation. This would not only decrease over-salinization, but also maximize productivity.
If we can put all these measures into practice, we will have the chance to avoid the “white death” of much of the Earth’s soil. We have to learn to look at the ecosystem holistically, realizing that all bounties given to us, including water and salt, are only useful if they are consumed in a balanced way. Only then can we leave the world entrusted to us in a greener, more productive and livable condition for future generations.