"When we tug at a single thing in nature, we find it attached to the rest of the world." -- John Muir
The roots of pine trees are similar to most other trees. The seedling starts with a primary root from which soon branch lateral roots. The primary root may extend deeply as a "tap" root although this is not particularly characteristic of pines. The roots normally continue downward to a level where decreasing oxygen content limits their further growth and this fluctuates with water availability. When the soil is very wet the roots may recede and then when it dries out and there is more space available for oxygen, the roots resume growth to a deeper level. This may be a seasonal pattern. Loose sandy soils permit (and require) deeper root growth than clay soils because the larger sand particles (0.02 -2 mm.) are separated by greater space which holds more oxygen but less water (due to rapid drainage), in contrast to the smaller clay particles (less than 0.002 mm.) which are more compact and hold more water and less oxygen (less well drained). The intermediate particle size is silt (0.002 -0.02mm.). Generally pines prefer sandy or silty soils and combinations, e.g., loamy soils. Most pine roots extend down to about 3 feet, but can be deeper in sandy dry soils.
The function of the roots is (1) support the above-ground part of the tree and (2) extract water and nutrients from the soil. The absorption of nutrients is usually aided by an in intimate association of the root with a fungus called mycorrhiza ("root fungus"). The mycorrhizae extend out into the soil from the roots and more efficiently absorb water and nutrients for the roots in exchange for sugars produced by the pine. Most pine mycorrhizae are ectropic (external, forming a sheath over the root surface). Mycorrhizal associations are more important in poor soils than in rich soils.
The cross-section of a root has epidermis on the surface, a woody cortex and a central conducting vessel. "Hairs" extend from the surface epidermis of younger roots or root branches. In mature pines the root may constitute only about 10% of the total mass of the tree, but in some species on dry sites, the root may be larger than the upper portion of the pine. In seedlings the root is proportionately larger, usually about 50% of the total mass.
"Marshall's Generalized Iceberg Theorem: "Seven-eighths of everything can't be seen." -- Anonymous