We’ve all heard the fables of salting an enemy’s fields, and those who are the salt of the earth are often near and dear to us, but what does salt in garden soil do to the plants that live there?
What is salt?
Most people think of salt as the ‘and pepper’ variety. Table salt (NaCl) is the crystalized form of equal parts sodium (Na) and chlorine (Cl). Salt is necessary for human life and we’ve been processing it for over 8,000 years. In fact, as Roman Legions were sometimes paid a salt stipend (salarium), the word evolved into “salary”. But in the garden, salt is more of a chemistry class than a history class. Chemically, a salt is any compound that is formed when an acid reacts with a base, bumping all or part of the acid’s hydrogen atoms off and replacing them with a metal or other cation. This neutralizes the acid. [Don’t panic - keep reading]
Chemistry 101: anions & cations
In case it’s been too long (or never) since you had a chemistry class, everything is made up of atoms. Atoms have a nucleus with neutral neutron(s) and positively charged proton(s) in the center and negatively charged electrons flinging themselves in semi-orbit around the nucleus. If the number of protons and electrons are equal, the atom is stable. If an electron gets bumped out of orbit, the atom becomes positively charged and it is called a cation [KAT-i-on]. If an extra electron is lured into orbit, the atom becomes negatively charged and it is called an anion. The reason this is important to gardeners is because the electrical charge of the atoms and molecules that make up the soil determine how many nutrients are available to plants. This is called the Cation Exchange Capacity.
Salts in the soil
There are many ways salt can make its way into garden soil. Ocean breezes, de-icing and over-fertilizing can all cause salinity levels to rise. Drought is another major factor in soil salinity. As ground water reserves are depleted, the mineral content increases and that means more salt. Ammonium and nitrate are excellent sources of nitrogen and they are commonly found in fertilizers in salt form. Gypsum and lime also contain salts. Normally, salts are leached out of the soil by rain and carried below the level of most plant roots, but not always.
Plant reactions to salt
Normal healthy soil contains many dissolved salts. These include:
As plants absorb water and nutrients through their roots, the water extraction can increase salt levels in the soil. Salt in the soil can stress plants, reducing root growth. If too much salt is absorbed, the plant is unable to perform photosynthesis and it will die. This is because of osmosis.
Osmosis describes the process by which a liquid (water) flows through a semipermeable membrane. Minerals (solutes) held in that water, and their electrical charges, pull from areas of low mineral concentration to areas of high concentration. When this happens in plants, the sugars and organic compounds normally found in plant cells attract water from the soil and pull it into the plant, along with important nutrients. When there is too much salt in the soil, this process is reversed and plants are deprived of important nutrients and water. Too much salt in soil can also lead to other elements, such as boron (B) reaching toxic levels. Symptoms of this toxicity include burnt-looking leaf edges, especially in older leaves, yellow streaking, and wilting. [I learned that the hard way when I watered my house plants with a batch of brine shrimp that died overnight. I lost over 180 of my 210 plants.]
To prevent excess salt in the garden, minimize the use of chemical fertilizers during drought and use rain barrels to collect low-salt rain water. Water from rain barrels can help offset the higher salt levels found in tap water.
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