Fallen Leaves and C:N ratios

Fall leaves not only provide a contrast of beautiful colors from reds to burgundy to oranges, but what we do with them once they fall to the ground. Traveling through neighborhoods you see homeowners raking the leaves to the street so that the city can pick them up and dispose of them, or piled up and burned. In contrast some homeowners are returning leaves back into the turf through multiple mowing with a rotary or a mulching mower.

The advantages for removing leaves is that the lawn looks clean and aesthetically pleasing; and in the case of sports like golf removing leaves makes finding your golf ball much easier. Additionally, removing leaves reduces the shading to the turf and the possibility of moist conditions developing that may enhance fall and winter disease development.

The advantages of returning or mulching leaves back into the lawn or turf include 1) reducing the cost of bagging, raking, and disposing of leaves, 2) nutrients primarily nitrogen are recycled; and 3) potential benefits. Regarding nutrients specifically nitrogen being released and returned for turf growth depends on the carbon:nitrogen (C:N) ratio. Soil microorganisms use carbon for energy and nitrogen for maintenance and growth. Within the microbes a C:N ratio of 8:1 is ideal for living. To acquire the necessary carbon and nitrogen in the soil to maintain an internal 8:1 ratio the microorganisms are ideally at a C:N ratio of 24:1.

Now if we add an organic material to be degraded by these soil organisms whether nitrogen is released or captured depends on the C:N ratio. For example, we add wheat straw with a C:N ratio is 80:1 to our turf the soil microorganisms will consume the carbon but will need to find extra nitrogen in the soil. The result is the consuming of extra nitrogen is known as immobilization. Immobilization ties up extra nitrogen soil that is now unavailable for plant growth, which may potentially lead to a temporary deficiency in nitrogen. Eventually over a relatively long period of time decomposition occurs and with the dying and decomposition of soil microorganisms the nitrogen would be released.

In contrast, if we were to add grass clippings with a C:N ratio of 19:1 the soil microorganisms would consume the leaves and releasing extra nitrogen (mineralization) resulting in a temporary soil nitrogen surplus. With grass clippings having a C:N ratio close to 24:1, the amount of excess nitrogen immediately released might be minimal (although the nitrogen would eventually be released). However, because of the C:N ratio, grass clippings are readily broken down in a short period of time.

In general, if the C:N ratio > 30:1, immobilization of nitrogen occurs as maximum biological activity is reached. With C:N ratio < 20:1, mineralization of nitrogen is reached with maximum biological activity.

Regarding leaves, the C:N ratio is around 50:1. The breakdown of leaves would be considered intermediate between grass clippings and wheat straw. Leaves range from sugars (quickly broken down) to cellulose (intermediate) to lignin (slowly broken down) is the reason for an intermediate type of breakdown rate. However, given that many turfs are fertilized the breaking down of leaves especially mulched or repeatedly chopped up with a mower, can break down rather quickly.

A few examples of C:N ratios of organic matter.

MaterialC:N ratio
grass clippings19:1
Rotted manure20:1
Oak leaves26:1
Leaves, variety of35:1 - 85:1
Corn stalks60:1
Tree bark200:1 - 491:1
Sawdust (weathered 2 months)625:1