Answer:
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.
Physical properties of soil
Texture: Soil is composed of both minerals (derived from the rock under the soil or transported through wind or water) and organic matter (from decomposing plants and animals). The mineral portion of soil is identified by its texture. Texture refers to the relative amounts of sand, silt and clay in the soil. These three terms refer only to particle size, not to the type of mineral that comprises them. Sand is familiar to most of us, and is the largest textural soil size. Sand grains can be seen with the naked eye or with a hand lens. Sand provides excellent aeration and drainage. It tills easily and warms up rapidly in spring. However, it erodes easily, and has a low capacity for holding water and nutrients. Clay particles are so small that they can only be seen through an electron microscope. Clay soils contain low amounts of air, and water drains slowly through them. Clay is difficult to till, and warms up slowly in spring. But, it tends to erode less quickly than sand, and it has a high capacity for holding water and nutrients. Silt is sized between sand and clay. Individual silt particles can be seen through a lower-power microscope. It has intermediate characteristics compared to sand and clay.
Most soils contain all three particle sizes (sand, silt, clay). Loam is a term that is often used generally to refer to soils that are a mixture of sand, silt and clay. Most of our topsoils are loams. However, “loam” can vary from a rather equal mixture of the three textural sizes, to a mixture dominated by sand or silt or clay. As a gardener, you should inspect loam before purchasing it, because these variations affect management practices.
Structure: Sand is often found as individual particles in a soil, but silt and clay are almost always clumped into larger units called aggregates. The manner of this aggregation defines a soil’s structure. Soil structure is described by terms such as blocky, platy, prismatic and angular. Productive topsoils often have a granular soil structure. The size and shape of aggregates is influenced by mineral type, particle size, wetting and drying, freeze/thaw cycles, and root and animal activity. Decomposed organic matter, plant sugars excreted from roots, waste products of soil microbes, and added soil conditioners all act to cement particles into aggregates. However, aggregates can break apart from tilling, compaction, and loss of organic matter in the soil. Soil structure is a very dynamic process. Good soil structure increases the pore space (see below) that supports root penetration, water availability and aeration.
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