Written by Dr. Lois Berg Stack, Extension Professor (2011). Revised by Dr. Lois Berg Stack, Extension Professor, and Mark Hutchinson, Extension Professor (2012). Revised by Dr. Lois Berg Stack, Extension Professor (2016)
Note to readers: This document contains many common soil science terms. Understanding these terms, which are italicized in the text, will help you understand soils as you read gardening books.
Soil is a dynamic three-dimensional substance that covers some of the world’s land surface. It varies from place to place, in response to the five factors that form it: climate, topography, organisms, the parent rock below surface, and time. Our Maine soils developed since the last glacier moved across the region, largely in response to the parent rock (largely granite) and topography. Most Maine soils are acidic, and have a somewhat depressed ability to hold and exchange nutrients used by plants. Our native plants evolved in this system, and are well adapted to Maine soils. However, we often amend Maine soils by adding organic matter, lime and/or fertilizer, in order to increase the productivity of our food and landscape plants.
Soil performs four major functions:
It provides habitat for fungi, bacteria, insects, burrowing mammals and other organisms;
It recycles raw materials and filters water;
It provides the foundation for engineering projects such as buildings, roads and bridges; and
It is a medium for plant growth. This text focuses on this last function.
What does soil do for plants?
Soil supports plant growth by providing:
Anchorage: root systems extend outward and/or downward through soil, thereby stabilizing plants.
Oxygen: the spaces among soil particles contain air that provides oxygen, which living cells (including root cells) use to break down sugars and release the energy needed to live and grow.
Water: the spaces among soil particles also contain water, which moves upward through plants. This water cools plants as it evaporates off the leaves and other tissues; carries essential nutrients into plants; helps maintain cell size so that plants don’t wilt; and serves as a raw material for photosynthesis, the process by which plants capture light energy and store it in sugars for later use.
Temperature modification: soil insulates roots from drastic fluctuations in temperature. This is especially important during excessively hot or cold times of year.
Nutrients: soil supplies nutrients, and also holds the nutrients that we add in the form of fertilizer.