Mulching Principles and Calculations

TECHNICAL NOTE

USDA NATURAL RESOURCES CONSERVATION SERVICE PACIFIC ISLANDS AREA

Agronomy Technical Note No. 8

MULCHING PRINCIPLES AND CALCULATIONS

Purpose

This technical note is designed to give guidance on the installation of the Mulching (484) practice and to share which mulching materials work best for different mulching purposes. The mulching materials listed and discussed in this technical note are not comprehensive. Therefore, if other mulches are being used, please contact the Pacific Islands Area (PIA) State Agronomist for additional guidance.

Specifications for the installation, operation and maintenance of the practice shall be prepared for each treatment unit in accordance with the requirements in the Conservation Practice Standard and the guidance in this technical note. The site specifications shall be recorded in the Pacific Islands Area Mulching (484) Jobsheet and given to the client.

Organic Mulches

Organic Mulch Nitrogen Concerns

Whenever organic mulches are being applied to the land, you should consider what that will do to the mineralization/immobilization of Nitrogen (N). Immobilization is when N is taken up by soil biota and made not available for plant uptake. This occurs when there is an abundance of organic carbon in the soil. Generally, when mulches have a Carbon to Nitrogen (C:N) ratio greater than 25:1 there will be immobilization of N. However, when the organic material has a C:N ratio of less than 25:1 there is more than adequate N. As a result, additional N will be mineralized and made available for plant uptake. By understanding the C:N ratio, better decisions can be made about if additional N should be supplemented to prevent N deficiency problems. Below is a calculation to determine the estimated amount of N immobilization or mineralized:

If the above equation results in a positive number then N is being immobilized, if the results are negative then there will be mineralization of N.

Immobilization Calculation Example:

Example Data: / Calculations:
4 inches of straw mulch applied (2 ton/ac)
C:N Ratio 80:1
Desired C:N Ratio 25:1
Straw is 45% C / 4000 lbs x 45% C = 1800 lbs C
1800 lbs C ÷ 80 C/N = 22.5 lbs N in the straw
1800 lbs C ÷ 25 C/N = 72 lbs N needed
72 lbs N – 22.5 lbs N = 49.5 lbs N immobilized

As this example shows, a significant amount of N can be “robbed” from the soil for the decomposition of the added organic mulch. Greater N immobilization will occur as more mulch is applied and the higher the C:N ratio. For instance, if 6 tons of wood chips with a C:N ratio of 400:1 were applied per acre and everything else remained the same from the above example, over 180 lbs of N would be immobilized.

For more information about C:N ratios for a wide variety of organic materials consult the National Engineering Handbook, Chapter 2, pages 2A-5 to 2A-7.

Manufactured Mulch

Plastic Film

The plastic film used as mulch is made of polyethylene, is nonporous, and usually comes in long sheets on a roll. The nonporous nature of the mulch may contribute to more runoff and potentially more erosion. Therefore, the plastic film mulches should be installed on the contour cross slope. The lifespan of the plastic film should be noted when selecting the type to use. Photo-resistant types are available, but cost more.

Black plastic film mulch is commonly used by vegetable and pineapple growers for weed control. Non-black colors like green and brown are better colors for keeping the soil cool, but are more expensive. This type of mulch should be installed according to the manufacturer’s recommendations and/or installation instructions. On large farms this type of mulch is usually mechanically laid on the soil surface. A disk on both sides of the roll opens a furrow in the soil and a packer wheel in the back of the roll anchors the plastic mulch.

Clear plastic mulch can cause the soil to heat to temperatures capable of killing some soil diseases and weed seeds. The plastic acts as a greenhouse, allowing short wave sunlight to pass through, but trapping the long wave heat coming off of the soil. If crops are to be planting into clear plastic, it should be shaded by covering with straw or wood shavings until the plants will provide adequate coverage.

Erosion Control Mats

Erosion control mats or blankets can be made of coconut fiber, polypropylene, straw, and burlap. These types of mats are usually porous and commonly used for erosion control on slopes, especially after construction projects while grass is starting to grow on the disturbed soil.

A major cause of failure for erosion control mats occurs when water flows between the soil surface and the mat. Thus, the contact between the mat and the soil surface must be maximized. The soil must be graded to create an interface between the mat and the soil surface that is generally free of ruts and rills.

The mat must be anchored onto the soil at the very top and bottom slopes of the treatment area. The anchoring trenches are typically 6 inches wide and 12 inches deep. The edges of the mat are placed in the trenches and covered with soil. Intermediate anchoring trenches or staples should be installed cross-slope at intervals per the manufacturer’s installation recommendations and/or instructions.

Porous Synthetic Weed Barriers

Porous, synthetic weed barriers are usually made from plastics like polypropylene or polyester and will photodegrade if exposed to the sunshine. For this reason, weed barriers are usually covered with an organic mulch layer like wood chips or dried vegetation to stop the sun from destroying it. When selecting the type of synthetic weed barrier to use, consider the product lifespan and the specific need. Woven weed barrier is woven with a texture like burlap and is more expensive, but can tolerate being walked upon. Spun-bonded weed barrier is thinner and cheaper than woven. It is the most common type sold in garden shops, but wrinkles easily if walked upon.

These types of weed barriers will allow infiltration of water, but will not allow sunlight through to the soil, preventing most weeds from growing. Compared with plastic film, porous weed barriers are commonly believed to have less problems of root disease because the soil is able to dry.

Weed barriers typically come in rolls of 300 to 750 feet in length. The most common roll width is 6 feet. The longer and wider the roll, the more cumbersome it will be to install. Weed barriers are also available in squares, which can be use on small jobs. Garden shops commonly sell weed barriers in 3 to 4 feet widths that are often cut into 3 to 4 feet squares.

Mow or herbicide weeds to allow the weed barrier to lay flat on the soil. If needed, prepare the soil by chiseling, disking and rototilling to remove clods or sod. Bury (with soil) or staple the edges to prevent the weed barrier from blowing. If using staples, follow the manufacturer’s recommendations. Rocks and logs about 4 inches diameter are often used to hold down the edges of weed barriers before covering them with an organic layer like wood chips.

Cover the beginning of the roll with 6-10 inches of soil, usually digging a small trench and burying the fabric will suffice. Someone may have to stand on the edge of the fabric to act as an anchor during the initial installation.

Mulch Selection

Mulch selection will probably be determined by availability of material; however, to help you plan and make decisions on which mulch to select, please consult Tables 1 and 2. Table 1 provides information specific to several common mulching materials in the PIA, while Table 2 gives guidance as to the effectiveness of different mulches for differing purposes.

Site Selection

Fine mulch, such as saw dust or wood shavings should only be applied to well-drained soils. Mulching with fine organic material on wet or poorly-drained soils can result in making the soil too wet for the intended purpose. These mulches can settle together resulting in less water infiltration of water and oxygen exchange. The additional oxygen demand to break down the organic mulch on wet soils may result in anaerobic zones or areas within the soil profile.

Mulch should be anchored to the soil on land that is subject to erosion due to steep slopes. Typically, long, fibrous mulch such as hay or straw works best. Anchoring can be accomplished by spreading the mulch on a loosely tilled field. After the mulch is spread, a disc set to pull straight should be slowly pulled across the slope of the field. The disc will push the straw or hay into the loose soil which will help it stay in place. On smaller sites or in locations where field equipment is not available, the same process can be accomplished with a dull spade.

PI Agronomy Technical Note 8Page 1 of 6April 2012

Mulching Principles and Calculations

Table 1. Common Mulching Materials, Rates, Expected Lifespan, and C:N Ratio in the PIA

Mulching Material / Typical Application Thickness/Layers / Typical Lifespan of Material in the PIA / Typical C:N Ratio / Notes for Consideration:
Chipper Materials (mix of bark, limbs, leaves) / 4 inches / 6-9 months / 100:1 / Avoid weed seeds.
Cinder/Gravel / 4 inches / 3-5 years / N/A / Crushed coral can increase soil pH over time.
Compost / 4 inches / 3-6 months / 10:1 / Best used as a soil conditioner rather than mulch.
Hay / 4 inches / 3-4 months / 40:1 / Should be anchored.
Lawn Clippings / 2 inches / 1-3 months / 20:1 / Dry to kill stolons before using. May contain weed seeds and pesticide residue.
Leaves / 4 inches / 3-4 months / 40:1 / May blow easily.
Macadamia Nut Husks and Shells / 4 inches / 8-10 months / 50:1 / Shells may last years.
Plastic Film / 1 layer / 1-3 years / N/A / Check lifespan of product. Follow manufacture recommendations.
Paper - Newspaper / 6 layers / 2-6 months / 50:1 / Avoid glossy and colored prints. May contain heavy metals.
Paper - Office Paper / 6 layers / 2-6 months / 130:1 / Avoid glossy and colored prints. May contain heavy metals.
Paper - Cardboard / 1 layer / 2-6 months / 375:1 / Avoid glossy and colored prints. May contain heavy metals.
Straw / 4 inches / 6 months / 80:1 / Should be anchored.
Woven Weed Barrier / 1 layer / 3-5 years / N/A / Check lifespan of product. Follow manufacture recommendations.
Wood - Sawdust / 2 inches / 4-6 months / 400:1 / Nitrogen immobilization is likely.
Wood - Shavings / 2 inches / 4-6 months / 400:1 / Nitrogen immobilization is likely.
Wood - Chips / 4 inches / 6-9 months / 400:1 / Nitrogen immobilization is likely.

Table 2. Evaluation of the effectiveness of different mulches for the different purposes of this practice

Mulching Material / Water/Energy Conservation / Moderate Soil Temperatures / Erosion Control / Suppress Weeds / Vegetative Establishment / Improve Soil Quality / Reduce Airborne Matter and Wind Erosion
Chipper Materials (mix of bark, limbs, leaves) / Good / Good / Fair / Good / Fair / Fair / Good
Cinder/Gravel / Fair / Fair / Fair / Good / Poor / Poor / Good
Compost / Good / Good / Poor / Good / Good / Good / Fair
Hay / Good / Good / Good / Good / Good / Good / Good
Lawn Clippings / Good / Good / Fair / Fair / Good / Good / Good
Leaves / Good / Good / Fair / Good / Good / Good / Poor
Macadamia Nut Husks and Shells / Good / Good / Fair / Good / Fair / Good / Good
Plastic Film / Good / Poor / Fair / Good / Poor / Poor / Good
Paper - Newspaper / Fair / Fair / Fair / Good / Poor / Poor / Fair
Paper - Office Paper / Fair / Fair / Fair / Good / Poor / Poor / Fair
Paper - Cardboard / Good / Fair / Good / Good / Poor / Poor / Good
Straw / Good / Good / Good / Good / Fair / Poor / Good
Woven Weed Barrier / Fair / Fair / Good / Good / Poor / Poor / Good
Wood - Saw Dust / Good / Good / Poor / Fair / Fair / Poor / Poor
Wood - Shavings / Good / Good / Fair / Fair / Fair / Poor / Fair
Wood - Chips / Good / Good / Good / Good / Poor / Poor / Good

PI Agronomy Technical Note 8Page 1 of 5April 2012

Mulching Principles and Calculations

Mulching Rates

Mulching rates depend upon the purpose for which the mulch is being applied. Generally, mulching to assist in the establishment of vegetation will be the lightest mulching rate to allow for plenty of sunlight for seedling growth, and will be the heaviest for weed growth suppression. Mulching rates for the reduction of water or wind erosion and for the improvement of soil quality will be determined using the Revised Universal Soil Loss Equation, Version 2 (RUSLE2) or Wind Erosion Prediction System (WEPS). All other application rates will developed by the planner based upon the information in the standard and this technical note or other resources. Figure 1 shows how much mulch is needed to achieve different coverage for selected mulches. As the graph shows, there is not a linear relationship between pounds of mulch applied and percent coverage. As the percent of desired coverage is increased the pounds of mulch required also increases, but at a faster rate. As a result, planning for 100% coverage may not be practical.

Mulching Calculations

Convert inches of mulch to cubic yards:

Convert cubic yards of mulch into tons:

Typical Mulch Densities:

  • Wood Chips 0.30 – 0.5 ton/yd3
  • Cinders0.75 – 1.0 ton/yd3
  • Gravel 1.20 – 1.4 ton/yd3

Grazing Technical Note No. 4Page 1 of 7Pacific Islands Area, November 2011

Pasture Condition Scoring