Salinity refers to the dissolved salt content of a substance like soil or water. It may be measured in a number of ways; parts per thousand and parts per million are the two most common measurements, and it is sometimes expressed as a percentage as well. A number of devices are designed to be used in the assessment, as the salinity of a substance is a very important characteristic. Many people think of it in terms of salty water, but high salinity in soils is also a major issue.
In ocean water, salinity is more properly termed halinity, since a group of salts known as halides are dissolved in the ocean. Some people are surprised to learn that ocean halinity varies around the world, and that deeper water as a general rule tends to be saltier. The movement of water around the world's oceans is known as thermohaline circulation, a reference to the factors of temperature and halinity which lead to differing densities. Some scientists have expressed concerns about interruption of the thermohaline circulation system.
In other types of water, salinity is a perfectly accurate measure. Generally, when the level is less than 500 parts per million, the water is considered to the fresh water. Brackish water is somewhat saltier, with levels of up to 30,000 parts per million. Saline water has a salinity of between 30-50,000 parts per million, while even saltier water is considered brine. The dissolved salt content of water can be measured with a variety of tools, most of which can be used in the field by scientists.
Since salts have a profound impact on many living organisms, water salinity is an important concern for biologists. In an estuary, for example, a zone where salt and fresh water mix, the levels vary widely, supporting a wide range of flora and fauna. If this balance is disturbed by something like a storm surge or a flood of freshwater, it can have unpleasant results for some of the animals that call the estuary home.
In soils, salt can prevent crops from growing, a major concern in several countries where soil salinity is on the rise. Levels in soil are generally increased through poor land management, such as overfarming and excessive use of chemical fertilizers, compounded with extremely dry conditions. If the rise in salinity is not checked, the land can become useless for farming, and it may take decades to recover.