Soil

Nutrition and Soil Quality

Soil quality can impact the growth and health of plants. Nutrients such as potassium and nitrogen are essential to plants' well-being.
Plants convert sunlight into chemical energy (sugar) through the process of photosynthesis. While this provides plants with the metabolic power (ability to build up molecules) they need to grow and maintain themselves, they also need nutrients to perform various functions. These nutrients are called essential elements and are classified into two main categories:
  • macronutrient: a nutritional element, such as nitrogen and potassium, plants need in quantities greater than 1 milligram per gram of mass of the plant
  • micronutrient: a nutritional element, such as copper and iron, needed by plants in quantities less than 1 milligram per gram of mass of the plant

Many of these elements are components of the organic molecules needed by plants to carry out protein synthesis and photosynthesis. These include proteins, lipids, and carbohydrates. If the building blocks for these molecules are not present in a plant's "diet," the plant will not be able to carry out many functions.

Functions of Essential Elements in Plants

Essential Element Major Function Amount Found in Plants
Nitrogen Building blocks of proteins and nucleic acids 15 g
Phosphorus Found in ATP, nucleic acids, cell membranes 2 g
Sulfur Parts of proteins and coenzymes 1 g
Potassium Regulates water balance, stomata actions 10 g
Iron Electron carriers in redox reactions 0.1 g
Copper Electron carriers in redox reactions 0.006 g

These essential nutrients are found in soil. Others include calcium, zinc, manganese, and boron.

Nutrition and Soil Quality

Plants can suffer from deficiencies of these nutrients. For example, a plant not getting enough nitrogen will show yellowing leaves and may die prematurely. A plant with a zinc deficiency produces young leaves that are abnormally small and its existing leaves show dark, dead spots. If soil is poor in these nutrients, fertilizers can be added to the soil. Fertilizers contain many of the missing nutrients that, once added, can help the plants grow healthy. Many farmers rely on these fertilizers in order to grow crops. Wild plants will not grow where nutrient content in soil is poor, but agriculture causes the same plants to grow in the same soil again and again. Over time, the nutrients are depleted in the soil. Adding fertilizer restores these nutrients, as does leaving a field fallow, meaning that no crops are planted on it for one or more seasons, which allows the nutrients to build back up in the soil. The layer of dead plants and animals found in the upper levels of soil that provides nutrients to plants is called humus.

Plants obtain the nutrients they need from the soil by forming as much surface area as possible on their roots. Roots grow tiny hairs to ensure that even more root surface comes into contact with nutrients. Plant roots can grow very deep into soil in search of nutrients. Roots have been observed growing as deeply as 60 meters below the soil in desert plants such as species of acacia and eucalyptus.

Formation of Soil

Compounds, elements, and minerals in soil affect soil quality. Contaminants such as pollution decrease soil quality.
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.
Mechanical weathering breaks down rocks into smaller particles. The minerals and other nutrients available depend more on the type of parent rock as well as subsequent chemical weathering. The size of particles can determine whether the soil will be a sandy or clay type of soil.
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.

Layers of Soil

The top layer of soil, the surface horizon, consists of decaying organisms. Below that, the subsoil contains humus, the nutrient-rich soil made of decayed organisms. The substratum is the deepest layer of soil, and often contains the most water. Below soil lies bedrock, which is rock rather than soil. Plants cannot grow into bedrock.
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.