The Chemistry of Plants and Soil

How do soils develop a good tilth structure and why does it allow plant life to flourish? The answer has a lot to do with chemical bonds that occur at a molecular level. Soil particles, water molecules, mineral compounds, biological enzymes, and microbial organisms interact in a complex web of relationships that make plant life flourish.

It is not the sheer presence of nutrients in the soil that causes plants to grow, it is their precise chemical composition and degree of solubility. In a fertile loam soil, minerals are broken down in chemical reactions to become water soluble and available for plants to absorb. There are chemical bonds that attach water molecules and nutrients to particles of soil, as well, preventing the nutrients from leaching away.

Without delving fully into the science of soil chemistry, there are some basic points that are relevant to all gardeners – soil pH, the purpose of the essential macro- and micro-nutrients, and the carbon to nitrogen ratio.

Soil pH

Soil pH is the measure of its relative acidity or alkalinity, expressed on a scale from 1 to 14. Above 7 is considered alkaline; below 7 is acidic. There are many natural factors that affect the pH of soil but, in general, areas of high rainfall have more acidic soil and arid areas tend to have alkaline soils. A slightly acidic pH (between 6 and 7) is ideal for the widest range of plants. Other than desert species, few plants tolerate highly alkaline soils. However, quite a few species require acidic soil, including blueberries and many ornamental species – hydrangeas, rhododendrons, azaleas, gardenias, and camellias are notable examples.

In general, soil pH governs the uptake of specific nutrients by plants. Over time, the management of garden soils can either increase or decrease pH. This can be a good thing (and done intentionally) in cases where soils are excessively acidic or alkaline. Or, it can be the unintended consequence of poor practices and lead to the decline of fertility – see the best management practices below for some good ideas and some things to avoid. For a reference on testing soil pH and how to use soil amendments to increase or decrease pH as necessary, see: http://www.clemson.edu/extension/hgic/plants/other/soils/hgic1650.html

Nutrients

Nitrogen, phosphorus, and potassium are the essential nutrients that plants require in large quantities for their growth and maintenance. Iron, boron, manganese, copper, chlorine, molybdenum, zinc, are some of the nutrients that plants use in very small quantities, but they are equally essential to the health of plants. In fertilizers, nitrogen, phosphorus, and potassium are represented by the letters N, P, and K, respectively. The three numbers that appear on bags of fertilizer represent the percentage of each found in the fertilizer. Nitrogen is responsible for the green, vegetative growth of plants. Phosphorus has many roles in the physiology of plants, but gardeners apply sources of phosphorus to strengthen the flowering and fruiting cycle of plant life, as opposed to the vegetative stage stimulated by nitrogen. Potassium strengthens the root systems of plants and plays an important role in disease resistance. In reality, the transfer of nutrients from soil to plants is highly complex and is determined by a host of environmental factors such as pH, temperature, moisture level, and others.

Carbon to Nitrogen Ratio

An understanding of the ratio of carbon to nitrogen (C: N) in soils is critical knowledge for gardeners. Carbon forms the bulk of plant tissues and is largely what is left when a plant dies and decomposes. Nitrogen is found to a lesser extent in the tissues of plants and dissipates into the atmosphere quickly after a plant dies. There is a fundamental relationship between the two elements that needs to be balanced for sufficient nitrogen to be available in soils. The reason is that nitrogen is consumed by the microbes responsible for the breakdown of carbon-based compounds. Thus, nitrogen will become deficient when excessive sources of carbon are introduced to the soil. The optimum C:N ratio in compost piles is approximately 30:1. Animal manures have a very low C:N ratio, while dried leaves, newspaper, and sawdust have a very high ratio. These materials are often mixed to establish an optimum ratio in a compost pile. For a complete list of the C:N ratio of common garden materials, see: http://oregonbd.org/Class/CtoN.htm

Next week, we will continue the discussion of soils with a brief introduction to the living organisms of the soil food web, and the basic biological processes that can be cultivated by gardeners to achieve highly fertile soils.