Do plant roots always grow downward?
Why do plant roots grow in this direction?
It's where the nutrient is.
Why do plant roots grow for this reason?
Because plants are amoral.
An example of tree roots not growing downward is J rooted seedlings.
This is generally caused by an over or under supply of water. Sometimes having a nursery heat bed (used to increase the growing rate) set too high for that particular plant type will have that effect as well.
Tree roots generally grow toward moisture until it has access to the required amount of water. This is usually downward but not always. If a hard clay pan or rock shelf is encountered tree roots may grow sideways to access the volume of water required. In poorly drained soils tree roots will activley avoid stagnant water which might contain pathogens damaging to trees.
Trees have adapted to variouse environments in variouse ways, for example in an area with low, light rainfalls the soil moisture can be less in the subsoil than the top soil. In this case trees can adapt in several different ways, deep roots to access subteranian moisture or shallow roots to take immediate advantage of any minor rainfall or even dew. sometimes they will store exess water in special cell structures see Australian baobab trees. Nutrients do play a role but are usually dissolved in water before trees have access.