Salt water can have a harmful effect on plant growth by hindering the plant's ability to absorb water due to the unfavorable osmotic pressure induced in the root system. This is referred to as the water-deficit effect of salinity and results in the plant condition known as physiological drought. An excessive amount of salt entering the plant through the transpiration stream will also reduce growth by injuring leaf cells in a condition referred to as the ion-excess effect of salinity.
Overall, plants can be injured when their foliage or roots are exposed to water containing salt. Water molecules are held tightly by salt ions. This makes it difficult for the plant to absorb water. When water containing salt enters the soil, the sodium ions also attach to soil particles, causing the ground to compact and become dense, reducing both drainage and aeration. The chloride component can mobilize heavy metals present in the soil and reduce plant vigor and growth.
Excessive amounts of sodium and chloride components in the root zone can cause the plant to suffer from phosphorous and potassium deficiency. Although plants require these mineral nutrients, the root system may bypass them in favor of absorbing the salt-derived ions. Chloride ions can also accumulate in toxic levels within plants and become concentrated in the plant's actively-growing tissues. This condition can cause twig die-back and leaf burn.
Because plants do not have the means to eliminate or excrete excessive salt from their tissues as animals can, their only defense is to remove the salt through dead leaves and needles. Plants that do not shed their leaves on a yearly basis, such as conifers, are particularly vulnerable to salt accumulations.