Soil provides the main source of water and nutrition for plants and the physical support to keep plants upright. More than 1,000 types of soil exist in the United States, composed of varying proportions of weathered geologic material, air and water. Different types of soil hold moisture and nutrients in varying amounts for use by growing plants. To encourage healthy, productive plants, a gardener needs detailed knowledge about the soil on his property and a plan to provide amendments and conditions that promote growth.
Sand, silt and clay particles compose basic soil texture. Sandy soil, with the largest particle size, loses moisture and nutrients quickly. Although clay soils hold nutrients and moisture well, the clay soils compact easil,y making it difficult for plants to access moisture and nutrients. Gardeners prefer sandy-loam soil with mostly sandy and small portions of clay and silt. Macronutrients, such as nitrogen, potassium and phosphorus, may exist in the core soil or be added through fertilisation.
A soil's pH represents the acidity of the soil on a scale from 0 to 14. The best soil for growing plants ranges from 6.5 to 7.5 pH, although some fruits and berries need soil with lower pH values, in the 5.5 to 6.0 range. The pH of the soil affects the availability of nutrients to plants. For example, plants have difficulty obtaining macronutrients, calcium and magnesium from soils with a pH below 5.5. On the high end of soil pH values, plant have trouble accessing micronutrients such as calcium, magnesium, iron, zinc, copper, cobalt and boron when soil pH exceeds 8.5.
Organic material is a component of surface soil. Organic material in the soil provides plants a source of trace or micronutrients. Organic material improves soil pH by helping lower the pH in alkaline or high pH soils and raising the pH in acidic soils. Soils rich with organic materials are better able to retain moisture. Undecomposed organic material attracts earthworms, which further enrich the soil with castings and create air pockets for root growth. Undecomposed organic material takes nitrogen from the soil, however, which means the soil will need amendments before planting.
Most plants do poorly in soil that retains too much water. Soggy soils have small particles of predominantly clay. Sandy soils, on the other hand, lose water too quickly through drainage allowed by the large sand particles. Soils with about 70 per cent sand and some silt and clay retain moisture well for plants. Adding decomposed organic materials improves soil drainage.