MOP , or potassium chloride , is the most commonly used potash fertilizer and can be used to grow many foods, especially chloride-loving vegetables such as beets, celery, Swiss chard. and other plants that are resistant to chloride. Its chloride content can benefit soils with less chloride, making them more resistant to disease. There are several downsides to this potash fertilizer. For example, if high levels of chloride are present in the soil or irrigation water is being used for growing crops, adding MOP can create toxicity. That means fertilizer levels must be carefully managed and MOPs must only be used on selected crops.