With grass cutting season upon us it’s a good time to start composting. If you’re like me and have reservoir of leaves that’s even better. Bringing the two together can result in mixture that produces a humus that is rich in nutrients and microbial organisms that will benefit your garden.
Compost was probably discovered when early settlers cleared land for cultivation, piling up brush and then realizing later that something was turning the plants and leaves into a rich humus. Today it’s become a science and we know very well how to build a perfect compost. What you should keep in mind is that there’s quite a bit of latitude in the process, and ways to compensate for a less than perfect pile.
Let’s Do Some Math
The perfect compost has a carbon to nitrogen ration of 30 to 1, a moisture level of between 40% and 60%, and air flow through the pile. So let’s break that down and see what’s entailed, and how we might get there.
Plants absorb carbon and use it to manufacture all of the structural components of the plant, such as leaves, stems, and roots. A living plant also contains nitrogen which is used to make chlorophyll and turn the leaves green. Because it is volatile and water-soluble, much of the nitrogen escapes when the leaves die and dry out leaving mostly the carbon.
The ratio of carbon to nitrogen in dried leaves can vary but we’ll use the ratio for oak leaves which is approximately 80 to 1 (or 80 : 1). To find the percentage of nitrogen we simply divide 100 by 80 and get 1.25% nitrogen remaining in the leaf.
Fresh cut grass has an approximate carbon to nitrogen ration of 20 to 1. Dividing 100 by 20 we get 5% nitrogen in the trimmings. Weeds we pull or hoe out of our garden may have more nitrogen content if there’s soil still attached to the roots. Fertile soil can have a carbon to nitrogen ration of 10 to 1, and include microorganisms that will jump start your compost.
If our perfect compost has a carbon to nitrogen ration of 30 to 1 then our percentage of nitrogen is 3.33%. Therefore it’s easy to see that leaves alone lack nitrogen, and the grass by itself has too much.
Just the same, dried leaves lack moisture and grass is too wet. And that’s another reason we want to combine them when we make compost.
To make the math easy I’m going to use 100 pounds of grass and 100 pounds of leaves for this example. If leaves have 1.25% nitrogen, then there’s 1.25 pounds of nitrogen in 100 pounds of leaves. Just the same, there’s 5 pounds of nitrogen in 100 pounds of fresh mowed grass. So let’s put the two together. (5+1.25=6.25 total nitrogen)
We now have 200 pounds of grass and leaves, and within that we have 6.25 pounds of nitrogen. Subtracting the nitrogen from the total weight we find that we have 193.75 pounds of carbon (200 – 6.25 = 193.75). As always to calculate the carbon to nitrogen ratio we divide 193.75 by 6.25 and find our ratio is 31. Pretty close to ideal.
This ratio can also be achieved with conventional or organic fertilizers which are rated by the percentage of composition. A fertilizer rated 10-10-10 contains 10% of nitrogen (first number), 10% phosphorus (second number), and 10% potassium (third number). While our primary concern is nitrogen the other elements simply enrich the result.
So going back to our example, we have 100 pounds of leaves with 1.25 pounds of nitrogen, and must add 5 pounds of nitrogen to achieve a 31:1 carbon nitrogen ration. If a fertilizer has 10% nitrogen we divide 5 pounds by 10% or 0.10 and find that we need 50 pounds of that fertilizer. (5 pounds/0.10 = 50 pounds) this formula holds true regardless of the source of nitrogen.
Building a Simple Compost
Here I’m layering leaves and weeds hoed from the garden to build a simple compost. In practice you probably won’t be weighing the ingredients for your compost, but following the “two browns and one green” method. You can use a bucket, a bag, or just guesstimate the amount to mix. In the photo above I’m layering leaves with weeds and grass that I’ve hoed out of my garden. Since it will eventually find its way back to the garden I didn’t bother to shake out the soil.
The fact is I have more hoeing to do and eventually the pile will be 3-4 foot high which is about right for a free-standing pile. Once I finish the pile I’ll let it set for 10 days or more, and then using my lawn mower (with the discharge blocked) I’ll grind it down and stack it back up. An active compost will generate heat in the range of 125-140 degrees Fahrenheit and kill most if not all of the seeds and rhizomes in the pile.
You can also use any carboniferous material as a substitute for leaves. Wheat straw has a carbon to nitrogen ratio of 100 to 1, but it can also be slower to break down because stems contain lignin. Sawdust ranges from 150 to 1 up to 600 to 1 depending on its age. So it’s a good idea to check an authoritative source if you’re using something besides leaves, and adjust the mix accordingly.
Okay, Nobody is Perfect
I’d rather a build compost in which the carbon to nitrogen ratio was double or even triple the ideal than a pile that was too wet (above 60% moisture) because a wet pile will quickly turn anaerobic and encourage an entirely different colony of bacteria, fauna, and fungi. One advantage to this strategy is that you can continue adding green material in the form of grass clippings or weeds from the garden as you turn it.
You also have to consider the effect of a heavy rain. A pile on the dry side will obviously weather the storm better, and yet it’s still a good idea to turn the pile or even spread it out to dry before restacking it after a rain.
But lets say you only have grass clippings. Your first priority is allow them to dry out for a few days before stacking them up, or turning the pile at least once a day. If you only have leaves then you obviously need to wet them before stacking. Further the rate of decomposition is inversely proportional to the particle size so you want to mulch those leaves to break them up.
A simple method for mulching leaves is to block the discharge opening on your lawn mower. Some mowers come with a separate piece that attaches to the housing and partially blocks the discharge. But I’ve also used a piece of cardboard duct taped to the housing to do the same thing.
Compost bins are easy enough to build. You can use fence stakes and wire mesh for the sides. Some people like to use pallets to form the sides and raise the pile to facilitate air flow but unless the interior surfaces are covered with wire mesh the compost will fall through the slats. Here’s a local source (West Plains) for 1/4″ mesh. You order online and pick up at Meeks.
For those who prefer something more formal there are any number of compost bins, generally made of durable plastic, on the market today.
The Soilsaver Classic Composter was awarded to me as a “field” prize during a composting demonstration by Brent Lidgard, Assistant supervisor, Sanitation and Recycling Dept., West Plains MO. Thanks Brent!
It’s pretty easy to put together, and as the compost ripens I just lift it up and scoop the pile into a wheelbarrow. The bin has “drawers” on the bottom level where the compost can be removed but it really doesn’t work that well for me.
You may have better results if the bin is placed on a hard surface.