Sheet Composting Worth the Effort

Nov 24, 2014 | Growing Tips

By John Zarola

The property I purchased in Rio Rancho three years ago had never had a vegetable garden bed. The soil is 90 percent sand with little organic matter in it. Sandy soil allows water and nutrients to drain down away from plant roots. My goal is to have at least 5 percent organic matter in my soil within five years.

Organic material in the form of topical mulches and compost added to sandy desert soil is gradually decomposed by soil microrganisms to humus, which contains carbon, nitrogen and plant nutrients. Regular additions of finished compost gradually improves soil structure and fertility.

The humus, acting like a sponge, absorbs rain and irrigation water, then releases it to the plant roots as necessary. So, by regularly adding organic matter to my desert vegetable garden, I begin to drought-proof the soil.

In the early fall when harvest is completed, I will do sheet composting in one of my raised beds which is 4-foot-by-4-foot-by- 6-inches. All healthy plant residue — aerial parts and roots — is left in place to decompose, thereby recycling residual plant nutrients back into the soil.

With a spade fork, I’ll poke 10-inch-deep holes throughout the bed, then sprinkle 3 inches of finished compost over the bed. Then, I’ll rake the bed so that some compost will fall into the holes. This allows the humus to deeply condition the soil. The whole bed will then be slowly irrigated so that the water soaks down to a depth of 12 inches.

Then, I’ll begin layering on top of the moist soil. First, a layer of three sheets of newspaper is spread over the entire bed. They are then saturated with water. In a wheelbarrow, I’ll mix equal parts of horse manure mixed with shredded leaves and shredded paper. Water is added to the mix so that all materials become moist.

I’ll shovel the mix into the bed to a depth of 4 inches. That layer is then covered with three sheets of newspaper, once again saturated with water. After that, the whole bed is covered with 4 inches of straw.

My particular choice is to then cover the whole operation with a tarp, which keeps the moisture in the bed and prevents wind from disturbing the ingredients. Moisture and air are essential to this composting process as both are required by the decomposing microorganisms to transform the added organic materials to humus.

My bed also has composting worms already in the soil. They do well in moist soil, where they ingest organic materials, which are in or on the soil. Worms deposit their castings right in the soil, thereby adding humus year-round. They, along with soil micro-organisms, will transform the added organic materials to humus, which will provide nutrition for my spring bedding plants.

I will check the bed monthly and, if necessary, irrigate it to maintain a good moisture level in the layers.

In another raised bed I planted (in August) summer oats as a cover green manure crop. The oat grass grows quickly, developing an extensive root system until the first frosts, at which time the grass dies and forms a mulch on top of the soil while the dead roots are decomposed by soil micro-organisms and red worms to humus during the cool months. After the frost, I prefer to cover this bed too with a 3-inch layer of straw, which helps prevent moisture evaporation from the soil surface.

I find that the time necessary to set up a fall sheet composting operation and planting a green manure crop is minimal and the rewards are great.