Ornamental Plants

Annual Reports and Research Reviews

Properties of Materials Available for Formulation of High-Quality Container Media

Introduction

Successful production of containerized nursery stock requires that the growth medium is formulated from ingredients with controlled and predictable properties. Inadequate physical properties of mixes cannot be corrected during production of the crop. Therefore, all ingredients must have been sized to a reproducible particle size before formulation to avoid problems.

It is all but impossible to adjust the pH and add lime during plant growth. Therefore, chemical properties of ingredients, in terms of needs for liming or acidification, must be known. It is important that growers balance lime addition against the "alkalinity" of irrigation water. This is most critical for crops produced over a period of a year or more in containers without transplanting. Where alkalinity exceeds 100 ppm as calcium carbonate, addition of lime to mixes may not be necessary. Growers need to submit the basic mix as well as the irrigation water for analysis so that an appropriate amount of lime can be added during preparation of the mix.

Plant nutrients released by various mix ingredients must be known. Biosolids (sludge) and most manure composts, as well as yard waste composts prepared with large quantities of grass clippings, release significant amounts of micronutrients, nitrogen, phosphorous, potash, and other materials. Bark composts and peat generally do not. Bark, in fact, may immobilize nitrogen. Hardwood and some softwood bark composts release high amounts of manganese. High manganese must be corrected through addition of iron sulfate to the mix or toxicity will develop on manganese-sensitive plants. All of these properties must be taken into consideration during mix formulation.

One factor largely taken for granted by the nursery industry is the potential for natural disease suppression supported by compost-amended mixes. Although fungicides are available that control root rots, drenching over a two-year period far exceeds the cost of natural suppression. The fungicide purchase cost for materials required to treat a cubic yard of mix in pots (Subdue 2E and Cleary's 3336F) represents approximately $8.00. Most growers treat crops such as plants in the Ericaceae three to four times per year. This is a significant total cost. Thus, disease suppressive properties are very important.

This article reviews properties of materials for use in container media readily available in Ohio. A short section on mix preparation is included. For more detailed discussions of various technical aspects, consult the "Ohio certified nursery technician grower training manual."

Sphagnum Peats

Nurserymen typically use reed-sedge or sphagnum peat. Fine, particulate peats fill in pores in container media, reduce air capacity, and increase the potential for root rots. For these reasons nurserymen should only use fibrous peats in container media. Reed-sedge and fine Sphagnum peats should be used as soil amendments.

Light fibrous Sphagnum peat, H₂ to H₃ on the von Post decomposition scale, harvested from the surface layers (2-4 ft. depth) in bogs, has the potential to reduce root rots. This suppressive effect lasts up to six months. This type of peat, therefore, is used most widely in floriculture. The von Post scale is described in Table 1.

Dark, more decomposed Sphagnum peat still can be fibrous. Peat producers are constantly improving peat harvesting equipment so that the fibrous nature of Sphagnum peat is destroyed as little as possible during harvesting.

The darker H₄ on the von Post decomposition scale level peat has high cation exchange and buffer capacities. Therefore, this type of peat is ideal for incorporation in container media because disease suppressive effects in nursery media typically are supplied by either bark or other composts.

Sphagnum peat is deficient in macronutrients as well as micronutrients. Lime, starter fertilizer, and micronutrients all must be added to peat mixes unless composts containing such nutrients are added to the mix as well.

Pine Bark Composts

Two general types of pine barks are available to Ohio growers - red pine bark from Michigan and a mixture of five species from the Carolinas through the southeastern states to Texas. All are high in lignin and wax contents, which resist decomposition. This makes pine bark an excellent peat substitute. Growers need to select sources of pine bark that are low in wood content and of consistent quality.

Pine bark should be composted before it is used in media to improve its wettability and avoid a short period of nitrogen immobilization. It can be "aged" six to eight months in tall windrows turned periodically, or composted 10 weeks in 12-ft. tall windrows with weekly turning after amendment with 1 lb. urea per cubic yard. The bark must be kept moist (50-60% moisture) in both processes to avoid fungi from causing problems during composting and immediately after potting. Fungi that grow on dry bark form a "crust" that repels water. Mixes prepared with dry bark that is colonized by fungi are difficult to wet, and plants have problems taking up water. This type of bark also stunts plant growth for a few weeks after potting.

Composted or aged pine bark can be used in media at volumetric ratios of up to 65% and even 100%. Lime, starter fertilizer, and micro nutrients must be added to pine bark mixes unless other composts releasing these nutrients are added.

Pine barks suppress root rots, Fusarium wilts, and some nematode diseases, but not as consistently as other composts. These beneficial effects are destroyed if the mix does not drain well. It is important, therefore, to check physical properties related to drainage of the final mix.

Composted Hardwood Barks

The wood content (total cellulose) of hardwood bark may range from as low as 50% to as high as 75%. Therefore, all hardwood barks must be composted at least six months after amendment with 3 lbs. urea per cubic yard to avoid serious nitrogen immobilization during plant growth. Spruce, Eastern hemlock, and fir bark fall in between hardwood and pine barks. Less nitrogen (1-2 lbs. urea per cubic yard) is required to avoid nitrogen immobilization in these types of barks.

Both spruce and hardwood barks release excessive quantities of manganese for some crops (not Ericaceae). Therefore, iron sulphate (1 lb. per cubic yard) must be added after composting while the compost is blended into mixes. Micronutrients typically are added to hardwood bark mixes. Hardwood bark releases more calcium than pine bark. Therefore, less lime needs to be added to mixes containing composted hardwood bark. Again, balance lime addition against irrigation water alkalinity.

Composted hardwood bark has the best disease-suppressive properties of all composts tested so far. Growers typically add 15% by volume of a hardwood bark compost to container media used for crops particularly susceptible to Phytophthora root rots (taxus, rhododendron, and others). In the field, controlled experiments have revealed that hardwood bark mulches suppress Phytophthora collar rot of apple and Verticillium wilt of maples.

Composted Biosolids (Sewage Sludge)

At least four municipalities in Ohio produce composted biosolids (municipal sewage sludges). Ohio now has 14 years of successful experience with these products in container media, in ground beds, and on turf. In many of these applications, composted biosolids provide superior plant growth over any other compost.

Composted biosolids are a potent source of mineralized plant nutrients. Generally, not more than 20% by volume should be added to media to avoid excessive fertility or even ammonium toxicity problems. Evergreen azaleas respond well to 10-15% composted biosolids (on a volume basis) added to mixes. Winter kill has not been observed on such plants even after the 1995 winter. The percent nitrogen in composted biosolids ranges from 1.5-2.0%. Approximately 25% of this is released in the first three months after potting. Slow release fertilizer does not need to be applied until a month or more after planting. The fines in these composts (smaller than 1/8" diameter) hold the bulk of the readily available plant nutrients. These fines produce superior fertility effects in turf and ornamentals. Disease suppressive effects observed on turf have caused it to become used more widely in recent years. Its use in container media is increasing also. Trace elements are supplied in adequate quantities at least through the first year of the crop. Lime typically does not need to be added in high alkalinity water regions.

Composted Yard Wastes

Yard wastes generally include leaves, grass clippings, brush, and, unfortunately, also logs and tree stumps. Leaf composts generally are too fine in particle size to be utilized with consistent success in container media. They are ideal amendments for the landscape and field soil industries, however.

Grass clippings/brush composts offer ideal opportunities for use in the landscape. The salinity can be high because grass clippings mineralize almost entirely during composting. The K₂O content of these composts can be as high as 1%. It is very easy, therefore, to prepare media with toxic levels of nutrients with these composts. This is avoided if low quantities (15-25%) are added to media.

Producers of composted yard wastes in Ohio are constantly improving the product quality. Composts of consistent quality are made available by some producers. Composted yard wastes with a total nitrogen content of 1.3-1.8 % do not cause nitrogen immobilization and have been used successfully in container media in several states as well as Ohio.

Wood wastes prepared from logs and stumps can be used as mulches or ground to finer particles and sold to biosolids composting plants where carbon sources are used in large quantities. This material also is "colored" with iron-containing and other dyes and utilized as mulches in mature landscape plantings. Generally, these materials are unsuitable for incorporation into container media. The total nitrogen content of these materials typically is less than 1% and, depending upon the particle size of the product, chronic nitrogen immobilization is possible.

Composted Rice Hulls

Composted rice hulls have been used successfully in container media for at least 15 years. Fresh rice hulls contain weed seeds and plant pathogens. They are destroyed as this product composts in tall turned windrows.

Rice hulls resist decomposition because the cellulosic substances on this part of the rice plant are covered with silica as the plant grows. The shape of the hull is retained for several years in container media. Therefore, it can be used predictably in container media. At some nurseries up to 40% of the volume of the mix consists of rice hulls. The ideal volume depends on the blending ratio with other materials. In Ohio tests, 20% on a volume basis with 50-60% pine bark and the remainder (20%) as composted biosolids, hardwood bark, or composted yard wastes has provided excellent growth for a broad spectrum of species.

Composted rice hulls do not release high rates of micronutrients. Micronutrients must be added to mixes, therefore, unless composted biosolids or other composts containing these nutrients are added to the mix.

Composted Manures

Composted manures are becoming more widely available to the green industry. Composted poultry manures may vary considerably in nitrogen concentration. Some pelletized products contain 3-4% nitrogen. These materials can be top-dressed on containers as slow-release sources of nitrogen and trace elements. In Australia, nurserymen have used this product with great success for crops such as foliage plants. Some sources of composted poultry manure contain as little as 2% nitrogen. These materials can be blended with container media with great success as long as the concentration of available nutrients is taken into consideration.

Composted cow manure is much lower in nitrogen (1-2%) content than poultry manure. It typically cannot be incorporated into container media at rates higher than 15% on a volume basis because of the high small particles content and the potential high salts content.

Composted hog manures are now becoming available. In general, all manures release adequate quantities of trace elements. Biological control of soil-borne disease typically is associated with incorporation of these compost types in container media.

Blending Process

The first step is to settle on the ingredients to be blended into a mix and to determine (1) the physical properties related to drainage (air capacity) and water retention and (2) chemical amendment needs. Be certain to pay attention to salinity. As mentioned previously, lime addition must be based on irrigation water analysis and records at a particular location.

The physical properties of a mix cannot be predicted from an analysis of the ingredients. The final mix must be analyzed for drainage, air capacity, and water retention before it is used in the nursery. The air capacity of media used in nursery containers should exceed 20% for most crops and 25% for crops sensitive to Phytophthora root rots.

Because the particle size of various pine barks, hardwood bark composts, composted rice hulls, and sludge composts differs from source to source, it would be unwise to provide ideal blending ratios of ingredients in this paper. It is possible to state, however, that materials containing small amounts of fines (particles <1 mm diameter) generally yield ideal physical properties. furthermore, highly biodegradable ingredients (composted hardwood barks, biosolids, manures) should not be used at incorporation rates over 15-20%.

Adding enough water during blending to raise the moisture content of the organic fraction in the mix to 50% (weight basis) is critical. Furthermore, the blended mix should not be stored in large piles in a closed bin. Mixes stored in large piles ferment, particularly if stored in bins where air exchange from all sides into the base of the pile is limited. Such fermented (sour) mixes cause significant problems (root injury) immediately after potting. This often results in stunted growth for woody plants for weeks thereafter. Air must be able to "draft" freely up through the mix. The temperature in the mix should remain below 45ƒC. Height and width of piles should be reduced if this temperature is exceeded. Low temperatures (less than 40ƒC) allow beneficial microorganisms to colonize the mix and avoid root injury immediately after planting.

Concrete mixers, although they blend well, should not be used because it takes too long to load and unload such systems. This causes unnecessary grinding of particles, generation of fines, and destruction of desirable physical properties of the final mix.

Slow release fertilizers should not be added to a mix if it is to be stored for a long period (a week or more) before potting. An exception is urea formaldehyde, which often is added at a very low rate (1-2 lbs. per cubic yard), to offset nitrogen immobilization observed in bark mixes early during production.