Soil Organic Matter (SOM): How to measure ?

I often encounter a common question on measuring soil organic matter (SOM) while discussing regenerative agriculture. It’s followed by another query on the relationship between soil organic carbon (SOC) and SOM. let me try to clarify such confusion in this article.

Soil organic matter contains plant and animal tissues at different stages of decomposition. It comprises carbon, hydrogen, oxygen and small amounts of nitrogen, phosphorus, potassium and other elements. Depending on how old the organic matter is it is categorized into passive and active types. Passive SOM consists of materials that are several decades old while residues in active SOM are few years old.

Broadly SOM fractions can be divided into 3 types. The interaction of plants and microorganisms produce byproducts during decomposition plus root exudates and sugars referred to as dissolved organic matter. It’s a continuous process that keeps happening every moment. Dissolved organic matter contributes to less than 5% of the total soil organic matter composition. The second type is fresh, decomposing plant and animal tissues called particulate organic matter. Depending on the soils it contributes up to 25% of SOM. Humus is the next category that is dark black or brown organic matter that is formed in the soil due to the decaying plant and animal residues. In fertile soils humus constitutes up to 50% of SOM and is the stable organic matter.

Soils with high organic matter offer several benefits for farmers and ranchers like,

-  Improves aeration, water holding capacity, infiltration and prevents runoff.

- Nutrient storehouse for crops and provides essential nutrients over time (cation exchange capacity CEC)

- Enhances soil microbial activity and diversity and provides nutrition for billions of diverse microorganisms.

 Soil carbon is often mentioned during discussions on carbon sequestration, carbon offsets, regenerative agriculture and climate smart agriculture. Interestingly carbon is called the ‘king of elements’. The term carbon is derived from Latin ‘carbo’ meaning coal. In the human body carbon is the second most abundant element after oxygen. Soil carbon is that carbon component of organic compounds that is measurable. It is difficult to measure soil organic matter directly in the lab, hence laboratories measure and report soil organic carbon. In other words, soil organic carbon is the measurable part of SOM.

Soil Organic Carbon (SOC) plays a vital role in sequestering carbon dioxide from the atmosphere and addressing the climate crisis.  Farm management that facilitates building soil organic carbon in agricultural and pastoral lands over a period helps to significantly reduce atmospheric carbon dioxide. It's important to know how to measure SOC and estimate SOM, without measurement it would be difficult to identify the gaps for scientifically improving farm management. 

Let me discuss how to measure SOC and estimate SOM in a hectare of farmland. 

The first step is to collect the soil samples following the standard procedures using a soil probe or auger and dispatch it to a nearby lab for analysis. From the lab report check on the total organic carbon percentage that will be used for calculating soil organic matter. On an average it has been found that about 58% of the mass of organic matter in soil is carbon. While estimating the percentage of Soil organic matter (SOM) from Soil organic carbon (SOC) a conversion factor 1.72 is used (100/58 =1.72).

 Let’s presume SOC is 2.5% of a farm. SOM is calculated as follows;

SOM (%) = SOC (%) x1.72

               = 2.5x1.72

               = 4.3%

In this example the soil organic matter is 4.3%. 

The SOM (%) can be converted into weight for a given depth and area. It helps to estimate organic matter in tons per hectare of soil. From the soil analysis data, bulk density and soil organic carbon values are required for estimation. The calculation is simple.

SOC in tons of carbon per hectare = SOC x bulk density (tons per cubic meter) x depth (meters) 

Let’s consider a hypothetical soil analysis data comprising SOC (2.5%), bulk density 1.1 grams per cubic centimeter and soil depth 10 centimeters. It’s important to convert SOC% to decimals(0.025), bulk density into tons per cubic meter (1.1 tons/cubic meter) and depth in meters (0.1 meters) per hectare (10,000 square meters) 

SOC tons/hectare = (0.025) x (1.1 x 0.1 x 10,000)

                           = 27.5 tons of carbon/hectare (t C/ha)

Further SOM per hectare can be deduced using the above conversion factor of 1.72. The amount of Soil organic matter would be 27.5 x 1.72 = 47.3 tons of organic matter per hectare.

Estimating SOM every year or at least once every 2 years to assess the management practices adopted in the farm is vital in regenerative agriculture. It helps to course correct some of the practices that are contributing to the loss of soil organic matter from farms and ranches. Soil organic matter is the lifeline of the soil, farmers need to take utmost care in preventing its loss. Growing leguminous cover crops, crop rotation, cultivating perennial forage crops, application of compost, agroforestry and silvi-pastoral systems, reduced tillage, contour planting and several regenerative agriculture practices that are appropriate to the region help in building soil organic matter and sequester carbon.

Low-cost solutions to tackle climate crisis? Let's start with our own kitchen wastes

When we think of climate change, some of the solutions that come to our mind are shifting to green technologies in industry, agriculture, transportation, energy coupled with legal binding climate enabling policies at national and international level. These macrolevel solutions no doubt are vital to address climate change. However, the moot question is what you and I can do to mitigate the current crisis. Some of the steps that we can take is to reduce the wastage of water, food, drive electric car, use energy efficient gadgets and more. But it’s not enough to reduce the carbon dioxide levels in the atmosphere.

We, the human species are responsible to a large extent to the current climate crisis. The way we live, eat and act matters a lot. Rehumanizing the connection between humans and nature is becoming more important in the current context. No animal produces as much wastes as human beings generate.  Lets take the example of household wastes. What do we do with them? Conveniently we dispose our wastes into a trash can, which gets into a larger trash can that gets picked up by the city municipality which further gets into landfill, a much larger trash can. Landfills have a limited capacity to hold wastes, eventually it gets filled, after few years new landfill sites are identified and the same process continues.  

If you have visited landfills, they are not pleasant sites. Whenever we drive pass landfills immediately we turn on the air recirculation button to avoid the stench getting inside the car. Landfills liberate several gases and the liquid that comes out of degrading wastes (leachate) is toxic to environment and pollutes water bodies. In addition, landfills are the breeding ground from rodents and animals. Most of the problems in the landfills are due to the biodegradable wastes that we throw in the trash cans. The common household biodegradable wastes are food wastes, vegetable and fruit peels and garden wastes. Instead of throwing these wastes they can be transformed into sweet smelling compost. It doesn’t require compost turners or special bins. If you have a small garden you can bury these wastes in the garden during spring and in winter you can compost in any container in your garage or outside.

Personally, I manage my household kitchen wastes and try to motivate my friends to manage their wastes. I would like to share a story of my friend Mr. Shreyas Nayak who lives close to my residence in Fairfield, Iowa. Shreyas and his wife Reena got interested into turning wastes to wealth and they share their experience.

Household Kitchen Waste composting journey- Experiences of Nayak family in Fairfield, Iowa

Our journey on composting our kitchen waste started in May of 2020.This was after the first COVID-19 lockdowns and gatherings of up to 10 people were allowed. We invited Dr. Thimmaiah to a barbeque on our deck wherein we decided to try making rice pancakes (dosa) on the grill.  It was a super duper successful effort and we enjoyed the gathering.

Shreyas and Thimmaiah

Making rice pan cakes (dosa) on the grill

      

In between our fun, frolic and experimentations, Dr. Thimmaiah walked over to our vegetable garden patch to inspect the quality of our soil. He immediately identified the lack of organic material in our soil as the reason for our poor crops the previous 2 years.  Shrey – you need to increase the organic content of this soil, he said.  How do we go about that, we asked.  For the next 1 to 2 years, you have to put all your kitchen waste in this patch of soil, he responded.

And that is how our endeavor to prevent our kitchen waste from entering the Iowa landfills started and it has now become an obsession.

Collecting kitchen wastes in a container 

We selected a couple of containers, with tight lids on  them, to store all our kitchen waste of a few days (usually a week or so).  These included all vegetable and fruit peels, cut off unusable pieces of bread and other foods gone bad. It really included everything you can literally think of including coffee and tea grounds, juices and milk products gone bad, paper napkins, etc , but excluded plastic, metals, aluminum and other non-biodegradable products.

Once we’ve collected enough waste, say in about a week or so, we would go to our vegetable garden patch and dig a hole about a feet or so deep.  Drop in all the kitchen waste and cover it back up with soil completely (the one that came from digging the hole). 

Digging a shallow pit (1 feet deep)

Kitchen wastes in the pit.

                                                           

                                                        

Covering the wastes with soil

Wooden planks to prevent rodents

       

 

This felt very fulfilling and mentally rewarding when we began and the joy lasted for about 2 to 3 weeks. Then the trouble started. Living in Iowa which is full of critters like ground hog, gophers, squirrels, raccoons, etc, one day when we went to work on our patch, we realized that the critters had sniffed out our waste and started digging out our composting waste. So we had to think about reinforcements to prevent the kitchen waste getting raided by the critters.

Fortunately we had some wood planks lying in the backyard.  We figured that strong / hard cardboard pieces would also do the trick.  We started covering our composting waste with wooden planks and laying some bricks on top of them to prevent the critters from getting into the waste. 

Vegetables grown from using wastes

We were diligent in the effort from May 2020 to November 2020 and could really tell the difference it was making to our soil and our plants.  The plants started having thicker stems and providing us a lot more produce, not to forget a much healthier produce than the past.  Plus the land and the plants did not need as much watering as the plants got a lot of their food/nutrients from the kitchen waste buried in the soil.

Middle of November or so, the ground started freezing up as temps went below 32F here in Iowa. We thought that it was the end of our composting of kitchen waste till next spring. The next batch of our kitchen waste that we had collected was about to give into our garbage can and then onto the landfill but before doing that, we decided to check with Dr. Thimmaiah if there were any other options available to us. 

As usual, Dr. Thimmaiah’s brilliance shone thru.  He said that we could compost our waste in buckets inside our garage.  The temperature in our garage ranges at about 45F and Dr. Thimmaiah said that at that temp there should still be some microbial activity to allow for composting of the kitchen waste. At these cold temperatures breakdown of wastes is very minimum. 

He shared the following pic with us on how he was doing it and that is all we needed to embark on this journey. Dr. Thimmaiah explained - Use a burlap bag in a bucket.  Start with 1 to 2 inches of soil or compost.  Add your kitchen waste on top and cover it up with 1 to 2 inches of soil or compost or potting mixture. Keep repeating to the top of the bucket.

Composting in a burlap bag during winter

Covering the wastes with compost or soil

 

So, we used one of our garbage cans for this effort.

Composting in a garbage can

Wastes covered with soil or compost

   

We are glad and surprised that because the waste is covered with the soil, there is absolutely no stench at all.

So here we are at the middle of February 2021.  Since May of 2020 to present – there has been no kitchen waste going into the landfills of Iowa from the Nayak household.

This has now become such an obsession that we look forward to burying our kitchen waste into the soil every week.  We collect our waste for the entire week and bury it in our soil on the weekends. When we get back into spring, March / April timeframe, we look forward to taking this fertile soil and spreading it on our vegetable garden patch for another awesome crop in 2021.

Myself and my wife both have full-time jobs that require about 9 to 10 hours of commitment each day. Apart from that we also spend an hour or so meditating each day and then another hour or so working out.  We also cook and eat at home most if not all the time. If with such a schedule, we are able to accommodate composting kitchen waste in our soil, we think most others should also be able to do it.

Do take on this journey.  It will be one of the most fulfilling endeavors of your life.