Soil is important to plants--it supplies them with water and nutrients, and it anchors their roots. Here's a primer to help you understand one of your garden's most basic ingredients.

Sand
Sandy soils are made of relatively large rock particles that fit loosely together. These soils tend to warm faster in the spring and drain quickly during wet periods. Unfortunately, they don't hold water well during drought and lose nutrients more quickly than other soil types. Sandy soils feel gritty to the touch.

Silt
Silty soils are made from medium-sized particles. They shed excess water more quickly than clay, but not as quickly as sand. Silty soils tend to feel slick to the touch when they're wet.

Clay
Soils with a high clay content are made of small particles that fit tightly together. Clay soils hold water and nutrients during times of drought, but stay wet longer during wet periods. They're more susceptible to winter heaving (moving around) during periods of freezing and thawing, which exposes and harms roots of perennial plants.

Organic matter
Organic matter helps eliminate the disadvantages of both sandy and clay soils. Organic materials such as compost, decomposed manure, and shredded leaves hold moisture when the soil is dry, but still let soils shed excess water. They reduce soil compaction, allowing plant roots to spread more easily.

Loam
Loamy soils are those rich with organic matter. In addition to regulating water better than both sandy and clay soils, loamy soils encourage beneficial microorganisms such as mycorrhizal fungi, which reportedly help plants absorb nutrients and resist disease.

Hardpan
This is a layer of soil so compacted that plant roots can't grow through it. In extreme cases, water won't permeate through the layer. Hardpan can occur when sand is mixed with clay-unless there's a very high percentage of sand, the small clay particles will cling to the larger sand particles and eliminate the spaces between the particles where water moves through. It can also occur by compaction, especially with heavy equipment.

Acidic soil
An acidic soil has a low pH (lower than 7.0 on a scale of 0 to 14). The pH of a soil determines, in part, what nutrients plants can take from the soil. For instance, plants absorb iron readily in acidic soils, but have trouble absorbing molybdenum. Some plants, such as blueberries and rhododendrons, have adapted to acidic conditions and require acidic soil to grow well. Sulfur and aluminum sulfate tend to increase a soil's acidity.

Alkaline soil
An alkaline soil has a high pH (higher than 7.0 on a scale of 0 to 14). The alkalinity of a soil also determines what nutrients plants can absorb. For example, plants absorb potassium more readily in alkaline soils, but have trouble absorbing manganese. Some plants, such as some dianthus, thrive in alkaline soils, but most garden plants prefer a neutral or slightly acidic soil. Lime and wood ashes are two materials that increase a soil's alkalinity. Mulch
While not actually a soil component, mulch (such as compost, shredded bark, and shredded leaves) relates closely with the soil. Mulch helps prevent soil erosion, holds moisture, and reduces drastic temperature changes in summer and winter. An organic mulch will break down, adding organic matter to the soil.