Root development

Lateral roots are initiated from primordial cells in epidermal cells close to the root tip of secondary roots.  Not all primordia cells, however, further develop into lateral roots.  Environment factors such as soil moisture and genetics influence triggering the primordia’s cell division and setting up lateral root tips.  The more lateral roots, the less carbohydrate available for growth for the secondary roots.   Each lateral roots requires energy for growth, resulting in shorter roots.

​There is some evidence that more lateral roots favor phosphorus uptake because this mineral tends to not be mobile and that nitrate uptake tends to be favored by fewer laterals because this mineral is more mobile. This becomes especially significant when either mineral has a short supply in the soil.
(http://www.plantphysiol.org/content/166/2/590)
 
Root tips from the laterals and main secondary produce the auxins that attract the carbohydrates to the roots, allowing root development. Absorption and movement of water and mineral to the stem and leaves allows larger leaves and consequently more carbohydrate production.  Being underground we still do not know everything about root development but it obviously depends upon lots of variables, some of which we control.