A plant is only as good as the soil it grows in. When a plant's roots have to struggle, the entire plant is affected. It is a lot easier for roots to move through porous soil than through soil that is too dense and tightly packed. Not all soil is naturally porous, but steps can be taken to improve it and make it a welcoming environment for plants.
Soil is rated on several factors, including porosity, which reflects its looseness and the distance between its individual particles. This distance can also be thought of as the spaces, or pores, between the soil particles, which varies depending upon the type of soil and its composition. The lower the porosity rating, the more difficult it is for plant roots to move between the particles. Porous soil is recognizable by how efficiently water moves through it and how quickly it crumbles when compressed and released.
Porous soil allows for the passage of water, air and nutrients, which all plants need for proper development. Plant roots depend on the open spaces between soil particles for the delivery of substances such as nitrogen, phosphorous and potassium, all of which are found in soil. Porous soil makes it easier for roots to absorb them, as well as the water that all plants need for hydration and food production. There are two types of pores -- small micropores and large macropores -- that either hold water or conduct water and air, and the amount of each is determined by a soil's makeup.
There are three basic types of soil: clay, silt and sand. In heavy clay soil, the particles are very small and very close together, allowing for little to no space between them. Because of these micropores, clay soil stays wet longer than sandy soil, because the water has no place to go. Plants growing in clay soil must deal not only with limited root growth but excessive moisture, which is a death sentence for all but the most moisture-tolerant species. To make matters worse, clay soil dries to the consistency of concrete during droughts, giving plants little chance to survive. In soil that is too sandy, the particles are much larger -- macropores -- and the soil can't cling together to retain water or nutrients. Silt falls between clay and sand in terms of particle size and its pore makeup. It is fine-grained, but has large pore spaces.
The ideal soil is called loam, a combination of clay, sand and silt, with the inclusion of large amounts of organic materials that holds the whole system together without compromising drainage, air movement and water retention. While it would seem to make sense that adding sand or silt to clay would lighten heavy non-porous soil, it's is a recipe for disaster without the addition of something to bridge the gap between the tiny particles in clay and the large ones in sand. Clay mixed with sand generally produces an unworkable substance that is not much of an improvement over pure clay. Organic material in the form of compost or aged manure is necessary to bind all types of soil particles together and produce a loose, workable medium that all plants do best in. Living organisms such as bacteria, earthworms, ants and other insects act as natural earth movers that help keep soil porous by opening it up as they move through it.