World Soil Day 2021; December 5th

FORUM: “Halt soil salinization, boost soil productivity” World Soil Day 2021

The theme for 2021 focus our attention on salt-affected soils and their implication to food security, climate change and the SDGs. FAO has launched the Global Map of Salt-Affected Soils. Most of them can be found in naturally arid or semi-arid environments in Africa, Asia and Latin America. The map allows experts to identify where sustainable soil management practices should be adopted to prevent salinization.

Salinization of soil negatively impacts plant development and induces land degradation. Saline earths show lower agricultural productivity, worsen farmers’ wellbeing, and the economic situation in the region.
Managing soil salinity at early stages helps to reverse it. However, heavy contamination leads to complete loss of farmlands and desertification due to the negative effect of salinity on soil properties. The United Nations University states that about 5,000 acres have been lost daily all over the world because of salinization since the 1990s, as of 2014. In this regard, the problem requires immediate addressing.

What Is Soil Salinization?

Salinization of soil is an excessive accumulation of water-soluble salts. Typically, it is table salt NaCl. The list is far more extensive and includes various compounds of sodium, potassium, calcium, magnesium, sulfates, chlorides, carbohydrates, and bicarbonates. In general, salt-affected earths are categorized as saline, sodic and saline-sodic, depending on the content.

The major soil salinity effect on plant growth is tampering with water absorption. Even with sufficient soil moisture, crops wade and die due to the inability to take up enough water. At a global span, according to the report of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) of 2018:
° about 190 million acres are completely lost;
° 150 million acres are damaged;
° 2.5 billion acres are impacted by salinization.

What Causes Soil Salinization?

Soil salinization occurs when soluble salts are retained in the earth. It happens either naturally or because of improper anthropogenic activities, particularly farming practices. Besides, some earths are initially saline due to low salt dissolution and removal. Soil salinization causes include:
° dry climates and low precipitations when excessive salts are not flushed from the earth;
° high evaporation rate, which adds salts to the ground surface;
° poor drainage or waterlogging when salts are not washed due to a lack of water transportation;
° irrigation with salt-rich water, which amplifies salt content in earths;
° removal of deep-rooted vegetation and a raised water table as a consequence;
° leakage from geological deposits and penetration into groundwater;
° sea-level rise when sea salts seep into lower lands;
° breezes in the coastal areas that blow salty air masses to the nearby territories;
° seawater submergence followed by salt evaporation;
° inappropriate application of fertilizers when excess nitrification accelerates soil salinization.

Indicators Of Soil Salinity

Salinization can be noticed visually by analyzing the soil surface, speed of water infiltration, and vegetation state. As salinization proceeds, signs get more severe. For example, slight whitening on the surface changes into distinct salt crystals. Apart from visual changes, there are indirect indicators of extra salt concentration. These are poorer water quality or animal behavior when livestock refuses to drink water due to its salty taste.

- Surface Changes Because Of Salinization
- damp areas and waterlogging;
- ground whitening at early stages and salt crystals at late ones;
- increased water level in furrows;
- bare soils (where plants fail to grow due to salinization);
- deterioration of roads, buildings, etc.;
- white or dark circles around water bodies.
- Salinization-Related Indicators In Vegetation
- plant withering;
- crop loss;
- reduction of biodiversity;
- appearance of salt-tolerant plants in the area and their further dominance.

Soil Salinity Measurement

Apart from visual estimation, there are more reliable methods to measure soil salinity, e.g., to assess the earth’s electric conductivity with specific devices. With a rise of salt concentration in the solution, its conductivity rises, too. Other types of soil salinity tests find out the exchangeable sodium percentage or sodium absorption ratio.

Besides laboratory tests, salinization is estimated on farmlands or remotely. Salinity sensors are used for monitoring surface soil salinity in the field. Satellites retrieve data with SWIR and NIR bands for remote sensing analytics of water absorption and its possible correlation with salinity.
Adverse Effects Of Salinization

Soil salinization consequences are far from positive, covering multiple aspects of ecology and human life. It affects crop production and water supply industries, induces the risk of floods and soil erosion, and decreases biodiversity.
Agricultural Production

Water saturation in plants depends on the level of salts in groundwater and the plant itself. Water is absorbed in the process of osmosis and flows from less salt-concentrated areas to more concentrated ones. When the salt concentration is too high, it means that the soil’s osmotic potential is essentially negative. Plants suffer from osmotic stress when they fail to take up water, even when it is present in the soil. Basically, the process is similar to drought stress due to a lack of moisture in the ground. As a result, vegetation dies. Salinization tampers with nitrogen uptake too, which slows plant development and causes a yield loss.

Another soil salinity effect on agriculture is ionic stress due to harmful ions in soil salts, e.g., chloride or sodium. Apart from their toxic impact as such, these positively charged ions impede the acquisition of other positively charged ions vital for crop growth (particularly potassium and calcium). The result is the same as with osmotic stress due to salinization – vegetation dies