What it is and why it is one of the most important contributing factors to gardening success.
If you’ve ever looked into gardening or if you’re already into gardening and you keep up on gardening literature, I’m sure you’ve heard volumes of discussion with respect to organic matter. It seems like you can never escape the topic: organic matter this, organic matter that. Well, there’s actually good reason for that. Having lots of organic matter in your garden soil is one of the greatest keys to gardening success. What’s less often said or talked about is why. Why is organic matter so good for your garden? What is it about organic matter that makes it such an asset to the garden.
I love exploring the answers to why questions. I love to know why things work the way they do, particularly as it pertains to gardening. In this article I aim to explain why organic matter is so beneficial.
To start out, I’m going to briefly talk about soil, soil particles, and the different types of physical and chemical properties that result from the various combinations of soil particles. This will provide us with the necessary background to best understand why organic matter is such a champ in the garden.
Soil can be broken down into three major particles: sand, silt, and clay. (Ask any soil scientist and they will tell you that there’s a lot more to a soil than sand, silt, and clay, but for simplicity sake I’m going to leave at that). Sand is the largest of the three particles and is spherical in shape. Silt is next largest in size and is also spherical in shape. Clay is the smallest soil particle and unlike sand and silt, it is not spherical in shape. Clay is angular.
Soils can be composed of purely one of these particles, like a pure sand or a pure clay soil, or a combination of two or all three particles. A fairly equal 3-way combination of sand, silt, and clay is called a loam. Soil character, or the different physical and chemical attributes of a soil, is directly dependent on the various amounts of these three soil particles within the soil.
Sandy soils are great to work with; I mean they are wonderful to work with. The soil is nice and crumbly; it’s very easy to dig through. Sandy soils also resist soil compaction, which prevents plants’ roots from being hurt by lots foot traffic or by heavy equipment (which makes them excellent for sports fields that get used heavily). Sandy soils do have their drawbacks: they don’t hold plant nutrients very well at all and they also don’t hold water well either. So there definitely are some pros and cons to sandy soils.
Silty soils are also easy to work with and they also hold water a little bit better than sandy soils do. But silty soils also do not hold plant nutrients well. Silty soils are also very erodible because silt is not as course or large as sand so it is more easily moved by wind or water. Clay, even though it is even smaller than silt, is actually less erodible. This seems counterintuitive but the reason for this is because the clay particles are so small that they actually stick to themselves really well. This self-sticking ability does not hold true for silt particles.
Clayey soils are the best at holding plant nutrients which is a huge plus. Clayey soils can also hold the most water which can actually be a bit of a drawback: clayey soils hold water so well that they can easily become swampy. Unless you’re growing plants that are specifically adapted to living in wet soils, which most garden plants are not, swampy or boggy soils will suffocate plant roots and kill the plants. Remember that just because roots are underground doesn’t mean that they don’t need air to breath.
Another drawback of clayey soils is that they are terrible to work with. If you have a clayey soil, I’m sure you’ll agree with me that trying to work a clayey soil can be a job – a big job. As I mentioned earlier, clay particles are so small that they stick to each other and form clods of soil or large soil aggregates that interfere with cultivation.
Once more, this self-sticking ability is further enhanced by clay particle’s angular shapes, which, if you think about it, should make sense to us. For example, what would be easier to stack or fit together in a tight configuration: rectangular or square-ish shaped wood blocks or a bunch of ping-pong balls? Obviously the wood blocks are more stackable; and so it is with tiny clay particles.
Between the three soil types, you might be tempted to think that a combination of the soil particles, or a loam soil, would be the best soil type available. And it is true that loam soils are excellent soils. If you never amended your soil in anyway, a loam soil would be great. But there is something that you can amend your soil with that simultaneously amplifies the positive attributes of these different soil types while minimizing all of their individual weaknesses. . . . And what would that be? The answer? Organic matter!
Sounds too good to be true, right? But guess what: it is true.
Organic matter dramatically increases the nutrient holding capacity and water holding capacity of sandy soils without interfering with their nice crumbly workable characteristics. Organic matter also increases the nutrient holding capacity and water holding capacity of silty soils while also decreasing their erodibility. Clayey soils already have a fantastic nutrient holding capacity but adding organic matter increases it further still. Organic matter also breaks up clays so that they are not only a lot more workable, but it also helps clayey soils breath better by preventing soggy or boggy soil conditions.
It’s incredible how much of a positive impact organic matter has on soil. It really is the answer to every gardener’s prayers. In fact, organic matter’s remarkable ability to improve so many different soil types in so many different ways nearly makes any discussion about soil types irrelevant. I mean, who cares what your soil type is, just add organic matter and everything will be fine! That may seem like a real general or blanket statement but in reality there’s a lot truth to that. There are, of course, scenarios where soil texture is a very important subject that needs detailed consideration. For example, the professional sports field scenario that I talked about above, must have a sandy soil. But for the home garden, as long as you add a lot of organic matter to your garden, it almost really doesn’t matter what your soil is composed of: sand, silt, or clay.
I want to focus and really zero in on one of the most important chemical benefits that organic matter brings to soils, namely, nutrient holding capacity. One of the grand keys to gardening success is the ability of a soil to hold onto the necessary nutrients that plants require to grow and stay healthy. To help illustrate the sheer magnitude of nutrient holding capacity of soils with organic matter compared to soils without organic matter, I’ve put together a little graph from some of the data in one of my college soil fertility textbooks.
Take a look at this graph. On the y-axis we have Cation Exchange Capacity or what’s more commonly referred to as CEC. Basically explained, the CEC of a soil is simply a measurement that scientists use to determine how much capacity a soil has to hold nutrients. The scale on the y-axis goes from 0 to 80, and I didn’t bother to put in the units of measure because that kind of gets technical. Just know that the higher the number on the y-axis the greater the nutrient holding capacity of that soil; in other words, the higher a CEC the better.
On the x-axis we have our different soil types for comparison. Notice that, as I said before, the sandy soils hold very few nutrients. As we proceed down the x-axis to progressively finer soil types, the CEC increases at a fairly steady rate. But look at the CEC of organic soils. It’s not just a little bit higher in CEC, it’s double the next closest soil type in nutrient holding capacity. This is a massive benefit. Good gardeners are aware of this phenomenon. And therefore, continually amend their gardens with organic matter whenever possible.
Whenever I talk about the CEC of soils, sometimes I get asked, “What is it about organic matter that gives it such an enormous ability to hold nutrients?” That’s a really good question and it has to do with the chemical make-up of organic matter itself at the elemental level.
Without getting too technical, let me explain it like this. Organic matter is composed of a lot of different elements: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, to name only a few, all in different chemical arrangements or chemical compounds. The most dominate element, by far, in organic matter is carbon. Carbon is what creates the nutrient-holding capacity of organic matter and it stems from carbon’s own physical structure. Carbon has a very large surface area full of nooks and crannies that can hold nutrients. This is best illustrated by the two diagrams I have pictured below.
Looking at these two figures: they’re approximately equal in size but the diagram on the right has a much greater surface area (especially if you think of it in 3-D). This massive surface area simply has more space on its exterior to grab onto different elements that are floating around in the soil.
This is not true for sand or silt, which, by comparison with elemental carbon, are gigantic spheres with no deep or windy surfaces to trap nutrients. And we can see that reflected in the graph above. Clay particles, though still quite a bit larger than elemental carbon, are smaller than sand and silt, and they have angular structures which can create pockets or corners that trap or hold onto nutrients. That being said, notwithstanding, organic matter is far and away the single best nutrient holding substance in a soil.
(A cool example of this phenomenon in carbon is activated charcoal. Have you ever heard of someone ingesting charcoal when they think they’ve eaten something that might be toxic or poisonous? Why in the world would eating charcoal be beneficial for someone who may have something dangerous in their stomach? The answer is charcoal has an enormous absorptive ability and hence has a great potential to absorb or remove toxins from the gastrointestinal fluids in the gut and thus prevent illness. Any guesses as to what charcoal is made of or where this absorptive ability comes from? Carbon. Charcoal is nearly straight carbon.)
Another question I sometimes get when talking about carbon in organic matter is “Where does the organic matter get all its carbon?” The short answer to this is carbon dioxide from the atmosphere. The air is the reservoir for carbon in the form of carbon dioxide. Organic matter is nothing more than dead plant anatomy such as grass clippings from the lawn, fallen leaves in autumn, kitchen scraps like banana peels or orange rinds.
So organic matter comes from plants and it is the plants that, with the help of the sun’s light energy, pull carbon dioxide out of the air, take to carbon out of carbon dioxide, and use it in their bodies to perform the necessary biochemical reactions to live. Some have supposed that plants are able to reabsorb carbon from organic matter itself but this is false. All plants get their carbon from the air as carbon dioxide.
Carbon is the backbone element not only in plants but in all other living things since plants are the food source for the world. It’s interesting to think that all the carbon in all the living organisms on this planet comes from the carbon dioxide in the atmosphere, and the atmosphere only contains a tiny fraction of carbon dioxide. Current atmospheric carbon dioxide levels are only a mere 0.04%. The atmosphere is dominated by 79% nitrogen and 20% oxygen. Only 0.04% of the air is carbon dioxide and it is the source for all the organic matter in the world, including our food. It’s kind of weird to think of carbon dioxide as a food source but in reality, it is.
So there you have it! Organic matter is critical to gardening success. If you want to be able to produce delicious food for yourself and your family in abundance, never overlook the importance of getting organic matter into your garden’s soil.