Black Gold: Using organic matter in horticulture

Linda Benedict, Constanin, Roysell J.  |  8/19/2009 7:49:08 PM

R.J. Constantin

To many gardeners and horticultural operations, organic matter is considered “black gold.” Since ancient times, it has been used both as a mulch on top of the soil and as an amendment incorporated into the soil.

Using organic matter mulches or soil amendments can drastically alter the physical properties of most soils. Organic matter will loosen clay soils and increase the moisture-holding capacity of sandy soils. Organic matter lowers soil temperature, decreases soil bulk density and improves soil structure. Organic matter deters erosion because it reduces the splashing caused by rain. The improvement in soil structure and tilth usually results in more root growth and better yields. Organic matter also can prevent crusting, and this aids emergence of small vegetable seedlings.

Organic matter, which is a good source of nitrogen, phosphorus and sulfur, affects the nutritional properties of soil including nutrient content and pH. Microorganisms that gain nutrients from the organic matter may influence nutrients from other sources. Several organic materials such as pine bark and pine straw may lower soil pH. Oak leaves will be acidic when fresh; upon decomposition, they will increase soil pH or alkalinity slightly.

Biological effects

Biological effects are numerous because organic matter is a source of energy for microorganisms that promote soil granulation. Organic matter can influence the incidence of diseases by directly or indirectly influencing the growth of parasitic organisms. Certain chemicals, such as antibiotics, phenolic acids or auxins that can enhance plant growth, also can be released by organic matter. Organic mulches can reduce or help control undesirable weeds, too.

Living mulches have many advantages in horticultural production. Plants, such as rye and wheat, can be grown in row middles of crops like strawberries to help control erosion, keep plastic mulch in place, reduce weeds, hold moisture and add organic matter to the soil.

Several Louisiana industries generate large quantities of organic byproducts including sewage sludge, pine bark, pine straw, cotton gin trash, crab and seafood meal, hardwood bark, sugarcane bagasse, yard waste such as ground-up limbs and grass clippings, straw, chicken litter, stable chips, animal manures and tree chips. Horticultural industries, especially the ornamental industry which includes growing of shrubs and trees, routinely use these byproducts. 

Pine bark success

One of the most successful uses involves pine bark. Once considered a waste product, bark has become a valuable product in the ornamental industry – so successful that shortages have occurred in recent years. Milled pine bark is used not only as a landscape mulch but also almost exclusively for growing containerized woody ornamentals and fruit trees. The product is an excellent growing medium. Its light weight helps hold down shipping costs.

Pine straw is used in the Southeast as a landscape mulch, too. Equipment has been developed that allows largescale use on horticultural crops.

Sugarcane bagasse has been used as a growing medium for vegetable transplants. In tests at the LSU Agricultural Center’s Hammond Research Station, bagasse, after composting, proved to be a good growing medium for ornamentals. In containers, however, it deteriorated rapidly, leaving a large head space. Ammoniated rice hulls are used as a growing medium, too. Peat moss, an organic material, is widely used in the bedding plant industry, alone and mixed with other organic materials.

Peat moss is lightweight, and bags of it are easy to use in filling trays. Most landscapers add peat moss when planting trees, shrubs and bulbs.

Blueberry industry

Organic material is widely used in the blueberry industry in the southeastern United States. Most state extension services recommend adding organic material, such as peat moss, to the planting hole before planting blueberries. This not only helps in the establishment of the plants but also helps lower or maintain soil pH at acidic conditions since blueberries prefer acid soils. Pine bark can be used in establishment of blueberry plants, but research at the Hammond Research Station has shown that banding pine bark underneath the plant resulted in more growth and production of blueberries than when the bark was incorporated throughout the planting soil. Blueberries, especially the newer southern highbush varieties, also respond to organic mulches. Research at the U.S. Department of Agriculture’s Small Fruit Station at Poplarville, Miss., has shown that organic mulches are a requirement for adequate growth and production of southern highbush blueberries.

Azaleas also require acidic soils. Peat moss and pine bark are two amendments used in establishing azaleas.

In the production of vegetables and fruit crops, the response to organic matter varies depending on soil type, species, cultivar, fertilizer rates, age of compost and time of application. Response has usually been higher in sandy soils having low productivity, but these responses have been somewhat erratic. Adding organic matter to soils used for horticultural production will normally be beneficial to soil tilth and structure, usually resulting in better production.

As valuable as organic matter can be, growers of horticultural crops face a few disadvantages:

  • During microbial decomposition of the organic materials, it is possible to release phytotoxic materials that may injure seedlings of some species.
  • Whether used as mulches or soil amendments, some organic matter such as corn cobs, wood chips, straw and sawdust can result in nitrogen deficiencies. The microorganisms use the nitrogen to break down the organic materials, forcing the need for extra nitrogen fertilizers from other sources.
  • Some sources of organic matter, such as fresh animal manures, are very high in nitrogen and, if applied at excessive rates, can cause burning of plants from ammonia fumes.
  • Undesirable microorganisms can be added to the soil with the organic matter.
  • Mold, fungi and bacteria may develop on organic matter in the soil or when used as mulches.
Organic mulches may work as well as or better than some herbicides, but they may have disadvantages. Herbicide applications are usually cheaper. Weed seeds can be introduced in mulches, such as straw and grass clippings. Mulches also can harbor insect pests and attract varmints such as voles. When used in large quantities, high water-retention organic materials such as peat moss can hold too much water and may damage the roots of plants. Dried organic matter used as mulch can be a fire hazard. A few of these products such as sewage sludge can add heavy metals, which have to be monitored. Some organic materials must be composted before use to obtain maximum benefits. Certain organic materials that decompose rapidly are readily available. Others that are high in lignin, such as plant stalks, wood chips and shredded tree limbs, may take years to decompose.

Organic mulches do not work well for all plants. For example, large quantities of organic mulches placed around citrus trees and other species in areas of high rainfall and high humidity, as in Louisiana, can promote the growth of Phytophthora rot. This can kill the trees, especially when the mulch touches the trunk area. In research studies at the Hammond and Citrus research stations, scientists found that adding up to 30 tons of five different organic materials to soil over a two-year period did not improve yields for tomatoes. Results for strawberries were inconsistent. Tillage reduces the organic matter content of soils, so less tillage or no-till systems of production can help preserve the organic matter content of soils, especially in vegetable production.

Organic matter continues to be extensively used in horticultural operations where advantages usually outweigh disadvantages. Using organic matter helps promote tilth and soil structure and, at the same time, reduces waste.

R.J. Constantin, Resident Director, Hammond Research Station, Hammond, La.

(This article was published in the spring 1999 issue of Louisiana Agriculture.)

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