Life in the Soil: Who Cares?

By: Tom Loynachan, Professor of Agronomy, Iowa State University

Getting Into Soil & Water 2012

We all should care! Without life in the soil, there is no life above soil. Yes, the soil is composed of many, many living, breathing organisms. Just how much depends on the soil but numbers can be staggering in our fertile, high organic-matter soils of the Midwest. It is often said that a handful of soil contains more living organisms than people on planet Earth. The only reason this can be true is because most are so small we must use a microscope to see them. In a sphere the size of the period at the end of this sentence (approximately 500 μm in diameter), about 125 million bacteria would fit into the volume of the sphere. That’s approximately 1/3 of the US human population! 

So if they are so small, how can they have an impact? It is because of their numbers and distribution. They are everywhere in nature and are active in decomposition of plant residues and animal wastes, recycling carbon, providing nutrients to plants (including nitrogen through biological fixation), forming soil structure to provide air and water for plants, and maintaining a sustainable environment in the soil. 

Based on genetic evidence, we are finding there are many soil microbes that we can’t culture in the laboratory. Without culturing them, it is difficult to determine what they do. Estimates of the number of microbial species range from 10,000 to 100,000 per gram of soil. Clearly there is much yet to be learned about soil microbes. 

Figure 1. Freshly turned soil gets its ‘earthy’ odor from soil actinomycetes shown here (500x magnification, image credit T. Loynachan).  

Carbon and Nutrient Cycling

Organic matter from plants and animals fuels the soil food web (Fig. 3). Producing organisms use sunlight and carbon dioxide in the atmosphere to form plant tissue. As the tissue falls to the soil, a whole series of events occur in a food web. The smaller organisms decompose the waste materials and do two things: 1) release carbon dioxide back to the atmosphere, and 2) incorporate a portion of the carbon into their biomass (living tissue). This biomass serves as a food source for higher-level predators, and the process is repeated. Some larger organisms such as earthworms and higher animals directly consume the plant materials but their wastes and, yes, eventually their bodies are returned to the soil. The soil is the biological ‘incinerator’ that converts all these materials back to starting components. The carbon is recycled back to carbon dioxide in the atmosphere and the minerals are released in such form that they can be reused for new plant growth. In a burial service: “ashes to ashes, dust to dust” is often said, and this appropriately describes the activity of soil microorganisms in recycling. 

Soil Humus  

A small portion of the decomposing plant and animal residues is converted into soil humus. Soil humus appears not to 

be just plant and animal components that fail to decompose. The process of humus formation is complex and results in an organic material that remains after prolonged microbial decomposition. Microbes are thought to synthesize end products that polymerize to form the amorphous humic structures. Research shows that some of these compounds are stable for hundreds of years. As the organic matter becomes oxidized, it turns dark brown to black. Humus provides nutrient-holding capacity and stores moisture, both of which are needed for plant growth. 

Soil Structure 

Soil structure is the grouping together of sand, silt, and clay particles. It is important for the soil-water-air relationships required for plant growth. Two processes are involved in structure formation. First, there must be forces that join the particles together (such as root pressure, freeze/thaw or wet/dry cycles, etc.) and then the particles must be ‘glued’ to bind together. One of the important glues is soil humus. Another glue is fungal hyphae (strands) that surround soil particles binding them together. It is estimated that 10-to-100 meters of fungal hyphae can occur per gram of soil. 

Nutrient Availability 

Microbes impact nutrient availability for plant growth in three main ways: a) they release the nutrients contained in plant and animal tissue as the tissue decomposes, 

b) they increase phosphorus and other immobile nutrient availabilities in the soil through mycorrhizae, and 

c) they provide nitrogen through nitrogen fixation. 

Mycorrhizae are symbiotic associations between fungi and higher plants that extend the soil volume through which plant roots can take up nutrients and water. Phosphorus is particularly important because it is immobile in the soil. The fungus increases the absorptive area of the plant roots. 

Nitrogen is the most deficient nutrient for plant growth in many soils of the world. The cereals (corn, wheat, rice, and barley) are very responsive to nitrogen fertilization. Prior to human-made fertilizers, bacteria were the main source of nitrogen for plant growth. They can take nitrogen gas from the atmosphere and make it into a form available to plants. Some plants such as legumes develop special structures called nodules to house the bacteria. The plant feeds the bacteria and the bacteria provide nitrogen to the plant. Thus with mycorrhizae and nitrogen fixation, there are microbial methods of providing or enhancing the availability of two of the three primary macronutrients commonly used in fertilizers. Long term we need to better understand these natural, biological systems to maximize food production while minimizing environment impact. 

So, the next time you walk across a field, watch your step. Life itself above ground is sustained by the living organisms below ground. 

To see soil organisms on the move, visit: http://www.agron.iastate.edu/~loynachan/mov