Fast Tracking Soil Organic Matter

Leaves. We like using un-composted municipal collected leaf mulch to improve soil organic matter. There are different ways to get it done. You can soil incorporate them, and then fallow the field in long rotations. It’s economically challenging. You can surface apply leaves between rows of some standing row crops like potato and pumpkin cash crops. At the end of the season, incorporation of the decomposing leaves improves soil over years. It’s labor intensive. But this is Jersey, where everything has to be done faster and fields have to pay their way. Last fall a grower from South Jersey asked,

Can I speed up the process of increasing my soil organic matter using incorporated un-composted leaves, yet avoid the crop deficiencies that result from nutrient tie-up?

He’s talking about the temporary (one or two seasons) soil carbon:nitrogen ratio imbalance that occurs after adding 10-20 tons/acre of leaves that have a carbon:nitrogen ratio of 50:1 to a production field where the ideal ratio would be 10 or 12:1. What he doesn’t say is that he’s got enough to do as a grower without having to become an on-farm compost manager, too.

There’s another way to harness the power of un-composted leaves to increase soil cation exchange capacity and organic matter: soil attributes which buffer nutrients against leaching, increase water-holding capacity, help reduce soilborne diseases, and contribute to higher yields of healthier crops. Using aged municipal leaves, i.e. windrowed leaves that have not been composted, but allowed to decompose for one year, this grower improved his Coastal Plains soils faster than a decade of cover cropping or years of fallowing.

Another Method

A decomposing pile of leaf mulch was applied about 4-inches deep and chisel plow incorporated in Fall 2013 to a field at our Rutgers Research and Extension Farm. The field had been cover cropped regularly with cereal rye, yet measurements of organic matter remained hard to budge on Aura gravelly sandy loam soil, typical for the area in our experience. Soybeans were drilled in 2014, and soil samples pulled in summer 2014 from the standing soybean crop. Six months after one leaf mulch application, there was a 28% increase in cation exchange capacity and an 80% increase in soil organic matter (see table).

Aura Gravelly Sandy Loam Soil Cation Exchange Capacity (meq/100g) Organic Matter (%)
Un-amended 6.9 1.0
Aged Leaf Mulch Amended 8.8 1.8
Change +28% +80%

With this experience, we told the grower to give it a try. We sampled a large, intensively farmed (frequent double cropping) vegetable field of Sassafras sandy loam soil. Half the field received a 4 to 6-inch layer of aged municipal-collected leaf mulch in Winter 2014-2015 that was incorporated. The other half was not amended with leaves. Both were seeded in greens and herbs. In April, two soil samples (each sample a composite of six subsamples) were pulled from each half of the field. After one leaf mulch amendment, there was a 31% increase in soil cation exchange capacity and an 83% increase in percent organic matter (see table).

Sassafras Sandy Loam Soil Cation Exchange Capacity (meq/100g) Organic Matter (%)
Un-amended 11.6 1.2
Aged Leaf Mulch Amended 15.2 2.2
Change +31% +83%

No crop nutrient deficiencies were observed in these fields.

Year-Old Leaf Mulch Gives the Best of Both Worlds

Here’s our thoughts on why we’ve had good results from using aged leaf mulch. Stockpiling leaf windrows on-farm for a season after they are received from municipalities, allows time for partial decomposition. It is not necessary to actually compost leaf windrows in this situation. Breakdown via decomposition appears sufficient to avoid crop nutrient deficiencies from microbial inhibition of soil nitrogen.

Farmers can apply for USDA NRCS NJ EQIP cost-sharing assistance when adopting leaf mulch practices.

-Acceptance of holding on-farm leaf windrows, varies among NJ communities and their municipal code inspectors, so check locally.
-A Rutgers survey in 2007 indicated NJ municipalities collect a staggering 289,000 dry tons of waste leaves annually. Six North Jersey counties of Bergen, Essex, Mercer, Monmouth, Morris, and Union combined collect 172,000 dry tons. Connecting with municipal works departments might pay big dividends on your soil productivity.

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