Salt build up in the soil – yes. You may read how unused fertilizer turns to salt. It’s more accurate to say that unused fertilizer salts build up in the soil, increasing the salinity level. A short lesson in soil and plant chemistry should make this clear. Plant roots absorb minerals from the soil; they don’t need much, some only in microscopic amounts, but they are a necessity. The minerals need to be in a form the roots can absorb, and that form is what is called in chemistry a “salt.” The chemical activity in the soil transforms the minerals into the salts used by the plants. We fertilize when the soil is deficient in some minerals, or has been depleted by over-farming or by things like wind and water. Obviously a plant in a pot of soil will eventually use up all the minerals unless they are replaced somehow. People fertilize their plants to replace the lost minerals, so the plant will continue to grow and be healthy and beautiful.
However, if you add too many minerals, fertilize to often, the plant can’t use all those minerals, and they sit around in the soil – remember, the minerals are in the form of salts – and the salt (salinity)level of the soil just continues to rise. As you can probably imagine, this becomes very unhealthy for the plant, and eventually lethal. The two main things to do to correct the situation are to wash out the salts – that’s what is called “leaching” the soil, and to take the plant out of the pot, wash off as much of the old soil as possible and repot in new fresh soil. After that, to keep the salinity level under control, it’s important to not over-fertilize. Most houseplants don’t need fertilizer more than twice a year; those in high light may take it once a month. A good idea is to always cut the recommended strength of any fertilizer by 1/4 or even 1/2. Also, many people like to make sure there is a good runoff whenever they water, then they empty that water to remove the excess salt. That’s not necessary however. You can learn more about watering from my YouTube channel