When leaves begin to drop in the fall, astute gardeners take time to improve the soil in their gardens by sheet mulching.
Sheet mulching, also known as lasagna gardening, is an organic, layering method that replenishes the soil while keeping the garden neat and attractive. The time frame between fall and the following spring allows several months for the organic matter in the layers to enrich the soil for fruitful spring planting.
The key ingredients in any sheet-mulching project are organic, nutrient-rich materials such as shredded leaves, fresh grass clippings, coffee grounds and vegetable peelings, most of which can be gathered from your own property. If needed, animal manures (chicken, rabbit, bat guano, etc.), straw and ground organic materials, such as bone meal, can be purchased from your local garden center. Avoid substituting hay for straw as hay can be laden with weed seeds.
Nutrient-rich sheet-mulching ingredients are any of a multitude of materials added to soil to improve water retention/infiltration, permeability, drainage, aeration or structure and, simultaneously, provide a better environment for root growth.
Marin Master Gardeners strongly advocate the inclusion of a layer of compost and of mulch to a sheet mulching project to nourish and protect plantings. Compost, either mixed into the soil or used as a top dressing, aerates the soil and adds nutrients. Mulch, as it is placed on the soil surface, is used to cover and shield the soil from the baking sun, drying wind and pounding rain, while also improving moisture retention and inhibiting the growth of weeds. Using mostly readily available materials, sheet mulching is an easy timesaving way to install and/or amend soil without having the hard work of digging or tilling.
The day before you start your sheet-mulching project, water the site well unless the ground is already wet with rain. The microbes that will turn your sheet-mulched plot into rich soil need water to do their work. After the water has soaked in overnight, cut down or weed whack any vegetation and remove any stumps/woody pieces. If thistles grow on the site, pull them out rather than weed whacking them, as their seeds are often able to mature and re-sprout into plants. To open up the ground, poke a spading fork across the site to promote better moisture and root penetration.
Choose a site that offers the proper setting as to exposure, drainage and pH for the intended plantings. To successfully feed the soil or smother weeds, it is much more effective to blanket a small area thoroughly than to spread the mulching layers too thin.
Sheet mulching can be as simple as a layer of newspapers topped by 8 to 12 inches of nearly any mulch material. However, some experienced sheet mulchers suggest that to increase the number of microbes acting on the pile that you put down 2 inches of aged compost over the dampened, weed-whacked area. Next, pile on a 1- to 2-inch thick layer of overlapping, light-blocking newspaper (dispose of colored and glossy pages) or cardboard (remove any staples and plastic tape). Wet the newspaper or cardboard layer thoroughly.
For the best outcome, test your soil using a soil test kit available from your local garden center or send a sample of your soil to a professional lab for more comprehensive testing. Depending on the test results or prior knowledge of your soil, consider adding a thin layer of the appropriate soil amendment(s) to your growing sheet mulching pile to improve the physical properties of your soil.
For example, agricultural lime adds calcium and magnesium that raises the pH of acidic soil and thus improves the uptake of nitrogen, phosphorus, and potassium in plants growing on acid soils. Rock phosphate, bat guano and/or bone meal are sources of trace minerals and boost phosphorus which promotes root growth, flower color and disease resistance. In soils where phosphorous is deficient, its addition can lead to large gains in fruit and seed production. Adding potassium-rich amendments such as kelp meal, greensand or rock dust also provides trace minerals. Nitrogen-rich amendments (alfalfa, blood, feather and bone meals, composted manure or rice hulls) are necessary to provide food for the microbes that will break down the materials in the pile and are vital to plants for leaf and stem growth.
Next, add 8 to 12 inches of loosely piled mulch (yard waste, leaves, finely ground bark or a mixture of these) or loose straw. On top of this layer, add an inch or two of compost and/or composted manure. Top off the layers with 2 inches of straw, leaves, bark, pine needles, or grain hulls. As you add layers, spray on enough water so that the pile remains in a "wrung out sponge" state.
Sheet mulching is a very forgiving process. If you can't find every item, don't worry. As long as you have enough newspaper or cardboard plus organic matter of almost any kind, your efforts will produce beautiful, nutrient-rich soil.