It is very raw for you to find eggs laid by spider mites in the soil. This is not to say that they cannot lay eggs in the soil too. But when we talk about where spider mites lay their eggs more frequently, we are referring to undersides of plant leaves and on shrubs. It is very common to find eggs laid by spider mites on the undersides of plant leaves, especially in a plant field that is dry and warm. If you do not control the breeding of spider mites at the initial stages, you would find them laying eggs all over the undersides of your plant fields and wreak havoc. So, always makes sure to spray the undersides of your plant leaves very well. Also, do well to touch the root and soil of your plant field, you might just be killing eggs of spider mites.