Spring 2012 - Buckeye District
Many rosarians have experience using mulch to cover their rose beds. There are many types of mulches that can be used and results are not always as expected. Strong opinions can be made for and against the use of mulches but do we really understand the effects of spreading a layer of organic material around our rose plants? Let’s examine some of the more common mulches and how they impact the soil. Popular mulches in our area include: • Leaves � Used extensively in natural woodland areas and in areas where trees are abundant. Leaves are the least expensive mulch available but make a better mulch if composted. • Compost from yard waste � An excellent mulch and soil conditioner that you can make at home by composting various types of yard wastes such as grass clippings, leaves, and plant tops from vegetables and fl owers. This partially decomposed material rates as one of the best organic mulches. • Mushroom Compost � This material is available in garden centers and in areas where commercial mushrooms are grown. It is inexpensive and has good color for use in the landscape. • Hardwood Bark � Shredded hardwood bark is one of the most popular mulches used in landscape plantings. TO MULCH OR NOT TO MULCH? THAT IS THE QUESTION. 16 BY MIKE MITCHELL It is a byproduct of the paper and lumber industries that can be recycled as mulch. Its pH is slightly alkaline but this problem can be managed by adding 3 lbs of elemental sulfur per cubic yard of bulk bark or per 100 square feet of bed area (Williams, 1997). Some common benefi ts of all organic mulches are: • Mulches conserve moisture by reducing the amount of soil water lost through evaporation. • Mulches help maintain a uniform soil temperature. They act as insulators, keeping the soil warmer during cool weather and cooler during the warm months of the year. • Mulches minimize soil erosion and compaction from heavy rains and aid in water penetration. • Mulches help with weed problems. If the mulch material is weed-free to begin with, and if it is applied correctly, weed seeds in the soil won’t germinate. Or if the mulch layer is deep, seedlings that do germinate can’t push up through it. • Mulches often give a neater and more fi nished appearance to a fl ower bed, evergreen or shrub plantings, or the vegetable garden (Williams, 1997). • Effects of Organic Mulches in Soil • Organic mulches, derived from plant material, will decompose in time and enrich and improve the soil. This results in increased aeration of silt or clay loam soils and added water-holding capacity of sandy loam soils. Mulching improves
and stabilizes soil structure (arrangement of soil particles) by reducing the impact of rain, weight (people walking on the soil) and cultivation of soils, especially when wet. In short, compaction of the soil is reduced. • The pH (acidity or alkalinity) of soil can be changed depending on the mulch selected. For example, most composts will be slightly alkaline (pH greater than 7) and excellent for use in acidic soil regions, while continuous use of oak leaves, pine needles, pine bark and sphagnum peat moss will increase acidity. The breakdown products of leaves, including oak leaves, will be alkaline, but continuous use of oak, pine and sphagnum peat moss products will keep the soil surface acidic (pH less than 7). • Organic mulches contain both major and minor mineral elements essential for plant growth, but should not be considered substitutes for fertilizer. In fact, materials such as straw, wood chips and sawdust have high carbon to nitrogen ratios; and therefore, nitrogen must be added to the soil at the time of mulching. The high carbon to nitrogen ratio materials are easily decomposed by microorganisms. The microorganisms require nitrogen to multiply and survive. As a guide, 1 to 2 pounds of actual nitrogen per 1,000 square feet should be applied. For example, if using a 10% nitrogen fertilizer, such as a 10-6-4, applying 1 pound nitrogen to 1000 square feet would require 10 pounds of the fertilizer product. If additional nitrogen is not supplied, the nitrogen in the soil will be used by the microorganisms in the root zone of desired landscape plants, and nitrogen defi ciency will likely occur (Dr. Elton Smith, 1996). 17 Let’s consider for a moment the effect of organic mulch on available Nitrogen. As shown in Figure 1, during decomposition decaying plant material is broken down and incorporated in the soil as particulate organic matter (POM). Nitrogen is released from POM as soluble organic N. Then, soil microorganisms may mineralize the dissolved organic N to ammonia (NH4+), which may be further oxidized to nitrate (NO3-). Ammonia and nitrate are considered the main sources of N used by plants in most ecosystems (Chapin, 1995). The rate of decomposition varies depending upon the type of mulch as shown in table 1. The proportion of Carbon to Nitrogen in the mulch has been found to be a consistent predictor of the rate of decomposition. The ratio of C:N also determines whether the decomposition process is initially a net Nitrogen producer or consumer. For mulches with a C:N ration of less than 20:1, the process of decomposition will provide net Nitrogen to the soil, any ratio higher than that will consume Nitrogen during the decomposition process. This can be proven by the metabolic process. Overall, soil microorganisms require one atom of N for the decomposition of 20 atoms of C (Davet, 2004). Therefore, for mulches having C:N greater than 20:1, microorganisms have to supplement their N demand by absorbing N from the soil. Ah Hah! Just as you have always suspected, mulching is bad for the rose garden because it takes Nitrogen from the soil! Nitrogen atoms that we are working so hard to provide for our vigorous plant growth! But wait there is more to the story. cont.