I love soil. I didn't know I loved soil the way I do until I started studying soils academically. This probably goes back to my love of science in general. I find it fascinating the way natural systems work. They are so elegant and simple yet staggeringly complex as you begin to really examine them. This seemingly juxtaposed and even paradoxical dual nature just leaves me in awe. So considering that I do love soil and all the great science to be done when looking at soil I thought I would do a little primer on soil and why organic matter really matters. A lot.
First, what is soil? Really before you go on reading my answer, tell yourself what your definition of soil is. Do you find that it is difficult to succinctly state? Heck, even my soils reference textbook lists two definitions. From "The Nature and Properties of Soils" 14th edition by Brady and Weil; 1- a dynamic natural body composed of mineral and organic solids, gases, liquids and living organisms which can serve as a medium for plant growth. 2- The collection of natural bodies occupying parts of the earth's surface that is capable of supporting plant growth and that has properties resulting from the integrated effects of climate and living organisms action upon parent material, as conditioned by topography, over periods of time. I'm really going to focus on that first definition because it proves more useful for this post.
So if we want to build soil we ask what is it made out of? Well our definition says mineral and organic solids, gases, liquids, and living organisms. So to build soil we need proportions of each of those. In general your mineral bulk is going to be what you inherit from your site. That is it will be the sand, silt, and clay particles that are weathering products of the parent material of your site. If you are in Georgia you'll have lots of aluminosilicate clays, in Florida you get a lot of quartz sand. So for the most part we aren't going to be making huge changes to the mineral bulk of our soil though we can certainly modify the mineral portion to suit our needs. If you have a soil particularly high in carbonates you may need to adjust the pH and while this really isn't changing the mineral bulk that much we can consider it mineral portion modification, but I digress. So mineral you kinda get what you get, but that is OK because what we are really interested in in the organic portion.
Organic matter! This is the portion of soil that has a disproportionately huge effect on soil properties. So our best route to building better soil is to focus on the area where we see the best returns and that is by incorporating organic matter. Organic matter increase water retention, buffers pH, increases cation exchange capacity (CEC, essentially the ability of the soil to hold nutrients), and modifies the soil structure and texture. Soil organic matter is made up of humus which is jumbled collection of varying size recalcitrant organic polymers with lots of different functional groups. These polymers can be huge, like millions of grams/mole. Since they abound with functional groups like hydroxyls (-OH), and carboxylic acids (-COOH) among others they can adopt charges by protonation/deprotonation and can serve as hydrogen bonding sites. Crucial because if you have charged portion of the molecule then another oppositely charged molecule which happens to be floating along can bind to that charged portion and stick around until, say , your carrots happen to need it and suck it up. Hydrogen bonding also aids in water retention as water is polar (has charged ends) and will bond with other oppositely charged portions of molecules.
The ability of the functional groups on these molecule to protonate or deprotonate (gain or lose a hydrogen) helps them buffer the pH of the soil. pH= -log [H+]. That is the equation and simply put it means that pH changes depending on the amount of hydrogen in solution. So if we have a huge polymer chalked full of functional groups that can gain or lose a hydrogen we have a giant buffer ready to suck up any additional hydrogen or donate hydrogen as needed to maintain pH. The more organic matter we get into the soil the more the soil pH converges to the equilibrium pH of the organic matter. So if we get lots of organic matter into very alkaline soil the pH moves towards that of the organic matter bringing the pH down and in acid soil it works the same way to bring pH up. What pH problems ail you, organic matter can help fix.
Organic matter certainly helps where soil structure and texture are involved. In Florida we have very loose sandy soils. Drainage is great but water and nutrient retention can be tough. The huge organic polymers that make up humus are at the molecular level like little balls of yarn that are all jumbled up into knots. So if you mix in lots and lots of jumbled yarn knots then you get chunks of bigger balled up knots and when you mix that with the sand in the right proportion suddenly you have a soil that can hold together better than your sand due to all of these "knots" interacting with one another and tangling up sand and clay in them as well. Now our loose sandy soil has some structure and our plants can put down strong roots and be firmly, well, rooted for lack of a better term.
Organic matter helps in soils with very compact clay as well. Clay on the molecular level is like stacked sheets of paper so the molecules can stack very efficiently. Clay particles are small too so they also stack very efficiently (think of the difference between filling a vase with tennis balls (sand) vs filling the same vase with marbles (clay)). This efficient stacking leads to very hard packed soil. The interaction of the clay with itself on the molecular level is kinda cool too because just like two sheets of paper stacked together there is a huge proportion of surface area for interaction (now imagine touching the two tennis balls together and that small surface for interaction, ever tried to lift two stacked pieces of glass by lifting the top one straight up?). Keeping the knotted yarn ball analogy we had before, if we now mix in organic matter then those clay particles start to get dispersed into the knots, and with strands of yarn everywhere the sheets cant stack on top of one another so efficiently so the soil begins to loosen. See, isn't organic matter cool? It's almost like a cure-all.
A really interesting note is that living organisms are included in the components of soil. Microorganisms especially are so astoundingly important in soil formation, composition, and overall health that I will surely write several posts addressing the topic. For now though lets look at how organic matter affects soil organisms. It is as simple as this: organic matter is food for soil organisms. No food, no organism. Soil organisms use different organic molecules as energy, much in the same way that we use respiration to oxidize organic matter for energy, soil microorganisms oxidize organic matter for their energy needs. The diversity of organisms means that for just about every type of organic molecule that can end up in your soil there is an organism that can eat it. Most of the time this is happening very slowly or at a scale that you can't see. The most obvious exception to that is when composting. Your pile actually heats up and shrinks because enough microorganisms are metabolizing organic molecules that they can heat the bulk pile! Brilliant. Your pile shrinks of course because during aerobic decomposition organic molecules are oxidized to water and carbon dioxide, so a fair portion of the organic matter you put in your pile actually floats away in the form of carbon dioxide and water vapor. Trust me we will talk a lot more about composting and it's merits later. The key for now is organic matter feeds soil organisms. All types of soil organisms I must note since we didn't even mention fungi or macroorganisms like earth worms.
So all of this that I have written is to say that organic matter is extremely important in soil. Even though for most of us it will represent no more than 10% of our soil it will overwhelmingly control the soil properties. The properties endowed to soil by increased organic matter are all great so much of our focus in producing healthy soil should be geared towards increasing organic matter. I hope you learned something and have a new or renewed appreciation for organic matter in your soil. Now go compost something!
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