Building Bigger Root Systems

Without irrigation, most of the plant's water supply is obtained by
expansion into new earth that hasn't been desiccated by other
competing roots. Eliminating any obstacles to rapid growth of root
systems is the key to success. So, keep in mind a few facts about
how roots grow and prosper.
The air supply in soil limits or allows root growth. Unlike the
leaves, roots do not perform photosynthesis, breaking down carbon
dioxide gas into atmospheric oxygen and carbon. Yet root cells must
breathe oxygen. This is obtained from the air held in spaces between
soil particles. Many other soil-dwelling life forms from bacteria to
moles compete for this same oxygen. Consequently, soil oxygen levels
are lower than in the atmosphere. A slow exchange of gases does
occur between soil air and free atmosphere, but deeper in the soil
there will inevitably be less oxygen. Different plant species have
varying degrees of root tolerance for lack of oxygen, but they all
stop growing at some depth. Moisture reserves below the roots'
maximum depth beecome relatively inaccessible.
Soil compaction reduces the overall supply and exchange of soil air.
Compacted soil also acts as a mechanical barrier to root system
expansion. When gardening with unlimited irrigation or where rain
falls frequently, it is quite possible to have satisfactory growth
when only the surface 6 or 7 inches of soil facilitates root
development. When gardening with limited water, China's the limit,
because if soil conditions permit, many vegetable species are
capable of reaching 4, 5, and 8 eight feet down to find moisture and
nutrition.