Organic matter is critical for the function and health of soils. So what exactly does organic matter do that makes it so important and so special?
Soil is mostly crushed up rocks and pore space. Grind those rocks into small pieces (sand), even smaller pieces (silt) and tiny pieces (clay) and you have the mineral component of soils. About 40 to 50 percent of an average soil is made up of this fraction. And it is heavy. The bulk density or weight per unit volume of the mineral fraction is about 2.6 grams per cubic centimeter (g cm3). Pick up a 5-gallon bucket of rocks and you will see what I mean.
Most of the rest of the soil is made up of pore space, typically filled with a combination of air and water (40-50%). Now if you just mixed the sand, silt and clay together you would end up with some pore space as those oddly shaped particles don’t stack too neatly. However, that space wouldn’t total anywhere near 40 to 50 percent. It’s likely to be 20 to 30 percent. The bulk density of a compacted poor soil. And while it wouldn’t be as heavy as solid rock, the bulk density would be over 2 g cm3. Contrast that value with the bulk density of a good, well-structured soil that is typically under 1.2 g cm3. The difference between the two is the organic matter.
Most soils have between one and five percent organic matter by weight — a very small fraction with some critical jobs. One of the first and most important functions of organic matter in soil is to provide structure to the sand, silt and clay that make up the majority of the soil. Soil organic matter (SOM) is the glue or joint compound that holds soil particles together, forming aggregates. That is what is meant when people talk about the “tilth” of the soil. That is what makes it easy for plants to develop extensive root systems, for water to infiltrate into the soil and for a large portion of that water to stay in the soil for plants while simultaneously leaving enough pore space for air so that the plants can breath.
Second thing is that organic matter is more than just carbon. Those who work with composts or biosolids know the slow nutrient release mantra. The nutrients in organic matter include both the major and minor nutrients needed by plants and animals who live in the soil. These are in organic matter in the form of proteins and other compounds. Recent studies have also shown that the nutrient rich organic matter stays longer in the soil than the pure carbon stuff. Pretty much the same reason that you have eggs for breakfast instead of just popcorn. The nutrients and other compounds keep you full longer. Well-balanced SOM is like a well-balanced diet. Those nutrients are feeding more than the plants — the microbes and the soil food web are responsible for referring to soils as the “living skin of the earth.” If you thought that your intestinal fauna were abundant, try measuring a similar quantity of soil.

Sourcing Organic Matter

Hopefully you are convinced that soils need organic matter. So where is the best place to get it? In the most basic sense the only source of organic matter for soils is the sun. Photosynthesis, the plant (yes I know some algae do it too) based process, turns carbon dioxide (CO2) from the atmosphere into fixed carbon. Way back before agriculture when most of the soils we farm were being formed, that is exactly where they got it. In natural systems, the plants provide the organic matter.
This is a cycle. The plant grows and dies, and a portion of the organic matter from the plant returns to the soil. The soil gets more productive as a result of the increased carbon and so the next season the plant grows even bigger and leaves more organic matter. Before agriculture, with the exception of the occasional dinosaur or other now extinct creature, the plants that grew were perennials and all of the carbon that they fixed stayed put.
Then we started planting seeds, in many ways a noble act. Except from the soils perspective. We planted annuals and took most of the above ground carbon away to eat. Adding insult to injury, we typically turned the soil to make a neat planting bed. This both took away a portion of the carbon that had previously been returned to the soil and exposed the carbon that was in the soil to erosion (think about the Dust Bowl) and/or rapid mineralization, turning back into CO2. As a result, the vast majority of agricultural soils in the world have lost a huge amount of organic matter. Average concentrations in some of the traditionally rich black organic matter soils are down from 4 to 6 percent to under 2 percent. That is a huge decrease. In many of the poorer soils total organic matter is less than one percent.
Take that back to the start of the column and that gives you both reduced aggregation (and poor soil structure) as well as reduced nutrient reserves. This hurts both the plants and the bugs. So instead of the plants getting bigger and leaving more carbon, yields are starting to decrease. In addition, because we have taken many of the nutrients away with the harvested crops, the micronutrient reserves in the soil are increasingly depleted. We may add nitrogen, phosphorus and potassium but we rarely add the copper, zinc, iron, manganese, magnesium and so on.
The end result of all of this is not too good. But there are ways to fix this and figuring out the best ways or the range of tools that we have available will be the topic of next month’s column.
Sally Brown is a Research Associate Professor at the University of Washington in Seattle and a member of BioCycle’s Editorial Board.