Michael J. Mulvaney,WFREC Cropping Systems Specialsits,  Cheryl Mackowiak, NFREC Soil Specialist, David Wright, NFREC Agonomy Specialist,  Barry Tillman, NFREC Peanut Breeder, Pat Minogue, NFREC Forestry Specialist

The path of Hurricane Michael left an estimated 2.8 million acres of damaged forest land in Florida and an additional 2.4 million acres in Georgia (Fig. 1). During the cleanup effort, many contractors have chipped or shredded those trees.  It has been reported that some of these contractors are offering to pay farmers to allow the materials to be spread on agricultural land for hundreds of dollars per acre. There are some issues that should be considered before allowing wood debris application on agricultural land.

Catastrphic Timber Damage

Fig. 1. Catastrophic timber damage in Gulf County, FL. Credit: Florida Forest Service

Woody debris mulch will tie up soil nitrogen (N) for a time

Spreading carbonaceous material will change the soil carbon (C) to Nitrogen (N) ratio (C:N), and is expected to tie up (or immobilize) N after application. The time required to reach a C:N ratio range of 20:1 to 30:1 (i.e., the equilibrium between N being tied up by microbial demand and N being available to plants) will depend on how much additional N is applied, and how quickly the C materials decompose. Smaller wood particle sizes or soil incorporation of wood will allow the material to decompose faster. The decomposition process will likely acidify the soil over the succeeding year or two, so some additional liming may also be needed.

Suppose wood chips are 50% C and 0.2% N (a 250 C:N ratio for pine species), and 1 ton/ac of chips are spread. That’s 1000 lbs C/ac and 4 lbs N/ac. Obviously, residual soil N will be immobilized as the chips decompose. To speed up that process and return to a 25:1 C:N ratio, you would need to apply an additional 40 lbs N/ac for every ton of woody material applied per acre, beyond the N required for your crop. Of course, the wood is not going to decompose quickly, so supplemental N applications may need repeating even after the first year. In other words, not all that carbon would be “mopped up” by a single mineral N application, but over time, that’s approximately what you’d need to apply to return to equilibrium.

Shredded wood debris pile

Fig. 2. Shredded wood debris stockpiled in large windrows, waiting to be land-applied directly, or as composted material at a later date. Credit: Cheryl Mackowiak

If urea is $400/ton, that equates to about $0.44/lb N, or about $18/ac in urea application per ton of woody material per acre. Although there is a cost to applying woody debris on agricultural land, it may make economic sense and it may increase soil organic matter in time, depending on how much is applied, if it is incorporated, how much additional N fertilizer is required, rainfall, etc. If 20 tons of woodchips were spread over an acre, approximately 800-1000 lbs N/ac is needed over the course of decomposition to get the C:N ratio back to equilibrium.

If renovation of pastures or row crops other than peanut were going to be planted over the next 2-3 years, woodchips can be worked into the soil and several applications of N applied during warmer, wetter periods to help decompose the woodchips.

Other Issues

Allelopathy

There could be allelopathic effects with crop plants, particularly early in the season. Allelopathy is when the chemicals from one plant inhibits the growth of another. Research conducted at the University of Florida found that wood chips from several tree species reduced radicle length of lettuce in the greenhouse (Ferguson et al., 2004; Rathinasabapathi et al., 2005). It is thought that polyphenol content is responsible for these effects. Allelopathic effects of fresh material is greater than aged material, so application of large amounts of fresh woody debris to agricultural land may be risky. How potential allelopathy from pine debris affects germination, emergence, and vigor of row crops is unknown. Smaller application rates are less risky. 

Foreign Matter Concerns

Depending on the particle size, there is the possibility that some woody material can end up as foreign matter during peanut harvest. If woody debris is laying on the soil surface, it may find its way into hay or silage bales.

Watch for shred or chip size

You will want to inspect the shredded or chipped debris to make sure that they are sufficiently processed and that there are few unprocessed limbs to hang up your equipment. There have been reports of limbs 2-3 feet in length, which would make for unpleasant strip tilling. You may want to be present to make sure they are spreading relatively uniformly and are not applying more than the agreed amount. In the case of other crops or land that will be established into pasture, the debris may impede seed drilling. If the material is spread too thick or not incorporated, it may cause dry pockets where seed will have trouble germinating or putting down roots. Large shreds or chips can also interfere with hay cutting operations.

Suggestions for Use

Mulching

Consider using the material as mulch for roads or walkways to prevent the issues mentioned previoulsy. Of course, as it decomposes, those roads may hold even more moisture and get even more muddy if it was not already a well-drained site. You can stockpile a large pile of material and use it as needed around the farm but keep it away from wood buildings, as woody debris tends to attract ants and termites, especially if stored over long periods, and large piles can build up heat and potentially catch fire.

Application to Pastures

If woodchips are applied to pasture land, no more than 20 tons/A should be applied with adequate N to keep grass growing while helping to decompose wood chips.  More tonnage can be applied but may shade out the grass causing lower production.  It is not advisable to apply woodchips on hay fields, as chips can end up in the hay if it is being sold.

Consider Composting First

Pile it up and let it decompose naturally over a few years or better yet, speed up the process significantly by layering it with manure and turning it a few times a year with a front-end loader. Keep the pile away from combustible material, as the pile will heat up during the composting process.

Windrowing and Burning

You may want to consider windrowing and burning, before spreading the resulting ash and charcoal as a fertilizer. This would make nutrients available much faster, but be advised that spreading charcoal on agricultural land may decrease the efficacy of preemergence herbicides. Check on burn restrictions in your county, and make sure to contact the Florida Forest Service for burn authorization before you start.

Summary

At a time when adding income to farming operations is critical, an offer of payment for wood chip application could be very attractive.  However, adding tons of any product to farm or pasture land is not without consequence.  Nitrogen is the key element for plant growth, so you want to ensure that crops and forages are not nitrogen deficient.   A large volume of debris chips could significantly reduce yields, if additional N is not supplied.  With so much income loss caused by the storm, you don’t want to harm the productivity of your fields in the near future.  If you are approached about considering this type of contract, ask lots of questions, know exactly at what is going to be applied and at what rate, and factor in additional N fertilizer costs.  If you want help determining the impact of a land application for your specific operation, contact your local county extension agent.  Like many other farming decisions, this all comes down to how much income will it produce versus the additional management it will require?

Michael Mulvaney

Cropping Systems Specialist, University of Florida, West Florida Research and Education Center, Jay, FL. Follow me @TheDirtDude