Roots cannot make oxygen out of carbon dioxide as
do the leaves. That's why it is so important to maintain a good
supply of soil air and for the soil to remain loose enough to allow
rapid root expansion.
When roots are cramped, top growth slows or ceases, health and
disease resistance drops, and plants may become stressed despite
applications of nutrients or watering. Other plants that do not seem
to be competing for light above ground may have ramified (filled
with roots) far wider expanses soil than a person might think. Once
soil is saturated with the roots and the exudates from one plant,
the same space may be closed off to the roots of another. Gardeners
who use close plantings and intensive raised beds often unknowingly
bump up against this limiting factor and are disappointed at the
small size of their vegetables despite heavy fertilization, despite
loosening the earth two feet deep with double digging, and despite
regular watering. Thought about in this way, it should be obvious
why double digging improves growth on crowded beds by increasing the
depth to which plants can root.
The roots of plants have no way to aggressively breakdown rock
particles or organic matter, nor to sort out one nutrient from
another. They uptake everything that is in solution, no more, no
less while replacing water evaporated from their leaves. However,
soil fungi are able to aggressively attack organic matter and even
mineral rock particles and extract the nutrition they want.
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