Soil type and basic ingredients

Four basic elements combine to make soil: minerals, organic matter, air and water. The unique make-up and relative amounts of each of these elements determines what soil type you have, and plays a significant role in determining what plants will grow well in your woods.

Mineral soil
Mineral soil makes up almost 50 percent of the volume of most soils. This soil element is categorized by particle type, from relatively large particles of sand, to slightly smaller particles of silt, to the smallest particles that make up clay. The predominance of one mineral type will effect your soil’s behavior. For instance, if your soil has a heavy clay content, it will probably not drain well and have a sticky consistency. If it has a generally sandy consistency, the soil will not hold water well and will probably be nutrient poor. With heavy silt, the soil will drain well, but have the ability to hold some moisture. If you have all three of these types of mineral soil in a mixture, it is referred to as “loam”.

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Organic soil
Organic soil consists of decomposing leaves and other organic matter, as well as very small invertebrate animals and other organisms. The organic layer forms the top of the forest floor, and everything of an organic nature that falls there gets processed by soil-dwelling creatures, from earthworms and millipedes to bacteria and fungi, which turn dead matter into nutrients that plants can use. A generous layer of organic matter tends to make for more productive soils. As small organisms break down organic matter, it mixes with the mineral soil below and forms the nutrient rich topsoil that plants depend on.

Topsoil: Where minerals and organic matter meet
Topsoil, the blending of organic soil and mineral soil, is a critical component of forest soil. It can take 100-600 years to form an inch of topsoil, making this soil layer a truly precious resource. And that same inch of topsoil that it took centuries to produce can erode in a single year if there are no plants and woody debris to hold it in place. The loss of topsoil to erosion not only undercuts the foundation of healthy trees and understory plant communities, it pollutes waterways if it washes away and into streams.

Good topsoil is rich in microorganisms that are essential to forest health. A thimble full of topsoil can contain 20,000 organisms, everything from earthworms, nematodes and bacteria, to algae and fungus. These creatures eat raw organic matter and produce humus, a substance rich in readily available plant nutrients. Without microbes, soil is essentially dead, and has little ability to sustain plants.

The surprising role of fungus
Fungi play a special role in forest soil by engaging in mutualistic relationships with many plant root systems. This plant-fungus relationship, referred to as mycorrhiza, gives trees added access to important nutrients, like nitrogen and phosphorus, and provides the fungi with sugars processed by plants. Without mycorrhizal relationships, many tree species would not be able to survive, especially in nutrient-poor environments, so protecting the soil structure that fungi inhabit can be critical to your forest health.

A large amount of the volume of soil can be taken up by water, up to 50 percent. For plants, water provides multiple essential services, from transporting nutrients to and within plants to sustaining organisms that facilitate decomposition and nutrient recycling. The water content in soil is determined in large part by soil texture – the smaller the particle of mineral soil, the greater the ability to hold water. But, in the case of clay, water can be held so tightly that roots cannot access it, so soils with a more balanced combination of water and air are actually more productive, like loams and silt-loams.

Though it is one of soil’s least visible components, air content is critical to productive soils. Oxygen, nitrogen and carbon dioxide are essential for plant growth and microbe respiration, and air spaces make it possible for root systems to expand. Compacted and waterlogged soils have low air content and can lead to plant death.

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