Rising soil salinity is a serious, climate change-related issue that growers are increasingly struggling with.
Why is soil salinity an issue?
There are several factors that can drive salinization in soil: the use of certain high-salt fertilizers; saltwater intrusion in coastal regions where seawater from the ocean seeps into groundwater reserves; drought, particularly in arid regions, when lack of rain means there is less diluting effect from surface water; and most of all, irrigation. Irrigation water contains salts that accumulate in soils and shallow groundwater gradually building up, until over time that accumulates to a level that’s not suitable for most plants.
Increasing soil salinity is a problem of growing concern because many of our economically important crops (think almonds, onion, and carrots) are mostly or highly salt-sensitive.
Why is salt bad for plants?
Salt impacts plants in two ways. First, it reduces the plant’s ability to take up water, and two, plants will actually take up some of these salts which will then damage tissues and other issues, such as leaf burn or defoliation. In severe cases it will cause the plants to die.
As soils become more saline, plants become unable to draw as much water from the soil. This is because the plant roots contain varying concentrations of ions (salts) that create a natural flow of water from the soil into the plant roots. As the level of salinity in the soil nears that of the roots, water no longer enters the root. When the soil salinity levels are high enough, the water in the roots is pulled back into the soil. The plants then are unable to take in enough water to grow.
What is sodicity in soil?
Sodicity is amount of sodium present in irrigation water. The chief characteristic of sodic soils from an agricultural standpoint is that they contain sufficient exchangeable sodium to adversely affect the growth of crop plants. According to the FAO, for the purpose of definition, sodic soils are those which have an exchangeable sodium percentage (ESP) of more than 15.
What crops are affected the most?
The most extremely salt sensitive crops are deciduous fruits, nuts, citrus and avocado. According to the FAO, sodium toxicity symptoms occur even at low exchangeable sodium (ESP) levels. See more on this FAO.org chart. Beans are considered sensitive so you should pay close attention to soil salinity when growing them.
Salinization and drought impact around the globe
“Irrigation inevitably leads to the salinization of soils and waters. … In many countries irrigated agriculture has caused environmental disturbances such as waterlogging, salinization, and depletion and pollution of water supplies. Concern is mounting about the sustainability of irrigated agriculture,” according to a report by the U.S. Department of Agriculture. It suggests crop yield reductions due to salinity occur on an estimated 30% of all 56 million acres of U.S. irrigated land. Other drought-impacted areas such as Australia are similarly affected. And as we mentioned at the start, poor drainage and tidal seepage is a problem as well, commonly seen in Asian countries such as Vietnam where seepage is causing soil to become increasingly salty.
Soil salinity and EC
One management tool that farmers can utilize is to measure the salinity of their soil. As rain and irrigation occurs, this may change. The most common measures of salt concentration is estimated based on the electrical conductivity or EC of the soil. Measuring the salt over time can show if salinity is increasing or not. There are a number of sampling techniques for measuring the soluble salts in soil in the field or laboratory.
How to monitor soil salinity
AquaSpy intelligence soil and crop health monitoring sensors and algorithms give growers better insights into the their soil salinization and how well the crop roots are absorbing available moisture. Being able to see if crop roots are not absorbing life-giving water and nutrients provides an early indicator of waterlogging or stress.
The electrical conductivity measurement (EC) can indirectly be used for determining plant health. EC is a proxy for how much salt is present in the soil. As we have demonstrated, high mineral content irrigation water can be quite harmful to plant health and success. By carefully monitoring conductivity changes between multiple layers of the soil, the grower can observe application of fertilizer, match fertilizer to the active root zone, and see if there is leaching of fertilizer or unwanted mineral buildup. Different crops have different sensitivities to salinity which could impede the plant’s ability to take up nutrients. Both the moisture and conductivity (EC) are influenced by the temperature of the soil. Therefore, a reliable system such as AquaSpy uses the temperature input by layer to compensate the moisture and conductivity reading appropriately. It is not enough to just measure a single temperature point in the top layer of soil. Your ideal system should have a temperature measurement for each sensor as the temperature difference from top to bottom can be well over 10 degrees.