Not all dirt is created equal. Soil can be categorized into a variety of types that determine the fertility and suitability of the area for planting. Soil types are composed of organic materials and particles. Depending on the composition of these materials, a soil may be hard or soft, gritty, well draining or compacted. Knowing soil types helps determine whether a soil is good for growing.
According to the University of Illinois Extension, soil type is determined by how much sand, silt or clay is present in the soil. This is broken down into texture. Sand has a gritty feel, while silt is smooth and powdery to the touch. Clay has the smallest particles and feels smooth to the touch, failing to break apart when dropped.
Soils that are high in sand content have large pores that allow water to drain quickly. Silty soils drain soil slower, while clay soils tend to retain or reject water depending on how tightly compacted the soil is. Adding sand to clay often makes the condition worse. A mixture of equal parts silt, sand and clay makes a loamy soil that has the best qualities of all three.
Soil amendments improve the conditions of poorly draining soils such as clay. Organic materials such as compost, animal manure or garden debris are tilled into poor soil over a period of several years to improve conditions. Organic material provides nutrients and microorganisms, which break down the organic material into humus. A good soil full of organic material is rich and brown.
Soil structure also plays a large part in how suitable an area is for planting. Granular soil has small pieces that are found in the surface horizons of the soil where roots have grown, says NASA. Single-grained soil does not stick together and is often found in sandy soil. Rocky soils that are platy (made up of layers) or blocky are usually unsuitable as a growing material.
Although soil type plays a big part in growing foods, the pH and nutrient levels play another big part. Alkaline and acidic soils are determined by a pH reading. Soils that are 7.0 or lower on the pH scale are considered acidic, while soils above 7.0 are alkaline. Plants require a fairly specific pH range to grow in. Alkaline and acidic soil types require regular testing to determine their suitability for planting.