Soil

Good soil is essential for the proper growth of plants. This is because it provides not only a place for the plant to anchor itself but also a source of nutrients and water, as well as oxygen for root respiration. Many of the bacteria living in the soil are beneficial to the plants. They are called plant growth-promoting rhizobacteria (PGPR). Soil must have the correct amounts of minerals and elements for it to be useful to plants. For example, too much cadmium or lead can inhibit growth and degrade plant tissues.

Soil is formed as rock weathers. The continuous wetting, freezing, and drying of rock causes it to break apart. This is called mechanical weathering. Soil can also be formed through chemical weathering. In this process, the rocks are broken apart through chemical means, such as acid rain or oxidation (e.g., rusting) from oxygen in the air. How rocks are degraded into soil determines which minerals and elements they contain. For example, clay is often formed from chemical weathering. This soil type is loaded with negatively charged elements. These bind to elements with a positive charge, making it more difficult for plants to absorb them through their root membranes. To compensate for this, some plants have root hairs with special transport molecules on their surfaces that pump protons out of the cells. Soil may be composed of large particles (sand) or tiny ones (clay), neither of which are ideal for plant growth. Sandy soil contains a great deal of air and very little water, making it difficult for roots to take hold and obtain proper nutrients. Clay contains very little air and a great deal of water, and it is very dense, which also stifles root growth. Loam, made of particles larger than clay but smaller than sand, allows for the proper mixture of both air and water and is ideal for most plants. A soil's water content can also affect plant growth. In soil with very high water content, nutrients may be too dilute for many plants, whereas soil with too little water will cause plants to wilt. Soil exists in layers, with the newest soil at the top and the oldest soil at the bottom. Decaying organisms are high in nitrogen, an essential element plants need to grow. These decaying remains, called humus, can be found in the upper layers of the soil. The top layer is called the surface horizon. Although there is a great deal of organic matter here, much of it is not yet decayed enough to be useful to plants. The subsoil is the layer below the surface horizon and consists of rich humus. Below the subsoil is the substratum, which may also contain humus and frequently has higher water content than the upper layers. Plant roots grow into the subsoil and substratum to find water and nutrients. Bedrock, which is not soil, is found below soil layers. Plants cannot grow in bedrock because the particles of it are too close together and roots cannot penetrate it. Contaminants in soil affect both crops and wild plants. Contaminants resulting from human activity are called pollution. The most common pollutants are industrial and agricultural wastes improperly disposed of or spilled. For example, oil spills occurring during transport in pipelines release oil onto soil. Oil makes it difficult for the plants to obtain sufficient water, and it kills bacteria plants need to help them obtain nutrients. Other pollutants, such as heavy metals, can harm or kill plants. These pollutants can be taken up by plants used for human consumption and thus make their way into the food supply.