The Economics of Mulching - 2002
There has been quite a bit of discussion and research done on the mulching of citrus and avocado trees. Mulching is the practice of applying to the soil surface any material such as paper, plastic, rocks, chipped yardwaste, etc. that will affect the soil beneficially. In citrus and avocado we most commonly think of mulching with agricultural or urban organic wastes, such as manure, yard trimmings, or packing house residue. Some of the research dating back to the 1920's shows favorable responses to mulching in both avocado and citrus. Some of the manifold benefits that have been ascribed to organic mulches have been weed control, nutrient fertilization, nematode control, root rot control, and improved water management. One problem with having so many affects, is that it is hard to give a dollar value to a mulch. How much is it worth as an herbicide, a fertilizer, a soil conditioner? Work is currently underway making this economic analysis of mulch, but will take some time to give some definitive results.
On the other hand we often know what these materials cost. Anywhere from free to $30 or $40 a ton. What sort of investment are we talking about when engaging in a mulching program? Since this is the time of year when rains come and help settle and leach out any salts that might be present in a mulch, this is a good time to think about the economics of mulches.
First of all it is important to recognize the large volume of material entailed. Mulch applied at the rate of one inch to an acre is equivalent to 135 cubic yards. Three inches deep is 405 cubic yards, and if you want to have a real effect, try 6 inches at 810 yards per acre. At four yards of mulch to the ton, those rates would be 34 tons, 101 tons and 202 tons per acre. Of course, the amount of mulch per ton varies tremendously on the density of the product. Also, this is not a recommendations for these rates, only to point out the huge amounts needed.
The limiting rate for application might not be the sizable volumes, but the nitrogen loading rate. A common nitrogen content of yardwaste is 1%. At the one-inch application rate this would be 680 pounds of elemental nitrogen. This is in an organic form and would not all be released at once, but some accounting would need to be made of it.
Or rather, the limiting factor may be the cost of the material to produce, haul and spread. In Ventura it costs $3.50 to chip a ton of yardwaste, $2 a ton to haul within a 10 mile radius of the production facility, and $2 a ton to spread it on flat ground. That is, the material to your field costs $7.50 a ton. Applying the one-inch rate of 34 tons to the acre costs $255. The three-inch rate would be $765 per acre.
It is clear that this stuff is not cheap. Even if it were free, there are still the attendant costs of shipping and handling. One way to reduce the costs would be to apply the materials only to the tree rows, three feet to either side of the tree. This would reduce the total application requirement to about a third of what it would take to treat the full acre. At some point, there might be subsidies given to growers from trash haulers, in order to remove these materials from the waste stream into landfills. In the future, though, we will have some good economic data on which to base a mulching program, so that we can say how much a mulch is worth for its various benefits.