Saturday, November 15, 2008

Fertilizer - Feeding the Soil - Part 3

by Richard Chew

As I uncover more research on microbes and soil science, I began to realise and more appreciative of the little things that God has blessed mankind with. These lowly creatures though invisible to our naked eyes, they were there at the very beginning providing the crucial link in our food chain long before we ever discovered its existence.
If we understand the functions of these creatures correctly and work with them, we allow God's creation to work wonders in our garden. It is unfortunate, perhaps due to ignorance or lack of knowledge that man chose a different route. Chemicals, pesticides and are other 'synthetic' methods are being used to get the maximum yield. Unfortunately I belonged to that category.
Nevertheless it is never too late to learn and change.
One of the important lesson that I learned was to understand the carbon and nitrogen relationship. It is generally accepted that the best ratio is 30:1, that means the fertilizer mixture should have 30 parts of carbon to 1 part of nitrogen.
Previously I usually use a pre-mixed fertilizer that contains all the necessary ingredients that includes large amount of saw dust, chicken manure, fish meal and newspaper cuttings. Not knowing the ratio, I applied on young and establised roses. I had mixture of results. Some works well and some not. I didn't understand, till I picked up materials that explains the carbon nitrogen relationship.

The most noticeable 'side effects' was the yellowing black spots leaves. It only happened on the roses that I applied with the pre-mixed fertilizer that contains large amount of saw dust.
After applying for a week, I noticed the top soil was quite tight as I tried to loosen it. It was later that I learned that the saw dust has very high carbon content (about 400:1 ratio). And that probably explains the depletion of nitrogen in the early stage of decomposition.

I had to quickly, replace the top soil by mixing with compost soil that has ratio of 10:1. It helps to decompose faster and at same time provide higher nitrogen to bring down the total carbon nitrogen ratio. The following week, I noticed the number of yellow black spots leaves remain the same (no increased). So far it is working well.

In the same period of mixed, I water the roses with Miracid regularly. I don't expect a dramatic turn of results, but would expect greener foliage in 2 weeks time.

As for my Nozomi rose, I noticed it has greener shoots since I started aggressive watering with Miracid. It was quite badly infected with chlorosis. The leaves turned to very light green, almost yellow. Fortunately it has improved with this recent new shoot.

The diagrams below illustrates the Ion exchange responsible for cation & anion exchange in soil solution. Rhizo bacteria assist in the absorption of essentials minerals into the roots system. This goes to show, how important microbes are in assisting plant growth.

Calcium and Magnesium uptake

Phosphorus, Zinc & Copper uptake

Just as I am writing this post, I faced with a challenge to recover one of my rose that got injured during transplanting. To make matter worse, the rose was infected with initial chlorosis. The older leaves had lighter green tone.

And to add on the disaster, the soil that I replaced with were not fully compost. After transplanting, the following day the rose was droopy. My initial thought was that I didn't water enough. It was the following day, my worst fear confirmed. It was still as droopy.

I realised the problem is more severe. There was high chance that I may had injured the roots. At this time I noticed dieback at one of the main stem.

I immediately engaged into aggressive recovery. Cut out affected stems. Replaced the soil. Applied compost soil, rotted manure and vermicompost. Regular water with Miracid and fork the soil to create better aeration.

Chances are pretty slim that it can fully recover. But it is worth a try. Hopefully this would be sufficient to turn it around.

Read continuation in Part 4

Thursday, November 13, 2008

Compost Fundamentals: Compost Needs - Carbon Nitrogen Relationships [EXT]

I extracted this from

This posting gives you an idea of using the right combination of fertilzer to achieve optimal results

There are some essential factors involved in determining what type of pile to build and how to manage the feedstocks. Organisms cannot decompose organic material as efficiently without certain requirements, such as air, water and appropriate particle size. Following are the most important considerations.

carbon-nitrogen relationships

The course of decomposition of organic matter is affected by the presence of carbon and nitrogen. The C:N ratio represents the relative proportion of the two elements. A material, for example, having 25 times as much carbon as nitrogen is said to have a C:N ratio of 25:1, or more simple, a C:N ratio of 25. Actually, the ratio of available carbon to available nitrogen is the important relationship because there may be some carbon present so resistant to biological attack that its presence is not significant.
Organisms that decompose organic matter use carbon as a source of energy and nitrogen for building cell structure. They need more carbon than nitrogen. If there is too much carbon, decomposition slows when the nitrogen is used up and some organisms die. Other organisms form new cell material using their stored nitrogen. In the process more carbon is burned. Thus the amount of carbon is reduced while nitrogen is recycled. Decomposition takes longer, however, when the initial C:N ratio is much above 30.

In the soil, using organic matter with excess carbon can create problems. To complete the nitrogen cycle and continue decomposition, the microbial cells will draw any available soil nitrogen in the proper proportion to make use of available carbon. This is known as "robbing" the soil of nitrogen, and delays availability of nitrogen as a fertilizer for growing plants until some later season when it is no longer being used in the life-cycles of soil bacteria.

When the energy source, carbon, is less than that required for converting available nitrogen into protein, organisms make full use of the available carbon and get rid of the excess nitrogen as ammonia. This release of ammonia to the atmosphere produces a loss of nitrogen from the compost pile and should be kept to a minimum.

A C:N ratio of 20, where C and N are the available quantities, is the upper limit at which there is no danger of robbing the soil of nitrogen. If a considerable amount of carbon is in the form of lignin or other resistant materials, the actual C:N ratio could be larger than 20. The C:N ratio is a critical factor in composting to prevent both nitrogen robbing from the soil and conserving maximum nitrogen in the compost..

Since organisms use about 30 parts carbon for each part of nitrogen, an initial C:N (available quantity) ratio of 30 promotes rapid composting and would provide some nitrogen in an immediately available form in the finished compost. Researchers report optimum values from 20 to 31. A majority of investigators believe that for C:N ratios above 30 there will be little loss of nitrogen. University of California studies on materials with a initial C:N ratio varying from 20 to 78 and nitrogen contents varying from 0.52% to 1.74% indicate that initial C:N ratio of 30 to 35 was optimum. These reported optimum C:N ratios may include some carbon which was not available. Composting time increases with the C:N ratio above 30 to 40. If unavailable carbon is small, the C:N ratio can be reduced by bacteria to as low a value as 10. Fourteen to 20 are common values depending upon the original material from which the humus was formed. These studies showed that composting a material with a higher C:N ratio would not be harmful to the soil, however, because the remaining carbon is so slowly available that nitrogen robbery would not be significant.


Plant residues are made up largely of the following:

1. sugar, starch, simple proteins (decompose rapidly)
2. crude protein (decompose slowly)
3. hemicellulose (decompose slowly)
4. cellulose (decompose slowly)
5. lignin, fat, wax, etc. (decompose slowly)

Rate of decay and release of nutrients to the soil vary greatly. Likewise, demands of living soil microorganisms vary as they "break down" plant residue. Sawdust (made primarily of lignin and cellulose) uses vast amounts of energy to maintain the lives of microorganisms digesting it. A major product of plant decay is nitrogen (N) while the undigested portion is primarily carbon (C).
The optimum ratio in soil organic matter is about 10 carbons to 1 nitrogen, or a C:N ratio of 10:1.
Following are some sample C:N ratios of organic matter:

Sandy loam (fine) 7:1
Humus 10:1
Food scraps 15:1
Alfalfa hay 18:1
Grass clippings 19:1
Rotted manure 20:1
Sandy loam (coarse) 25:1
Vegetable trimmings 25:1
Oak leaves 26:1
Leaves, varies from 35:1 to 85:1
Peat moss 58:1
Corn stalks 60:1
Straw 80:1
Pine needles 60:1 to 110:1
Farm manure 90:1
Alder sawdust 134:1
Sawdust weathered 3 years 142:1
Newspaper 170:1
Douglas fir bark 491:1
Sawdust weathered 2 months 625:1

Tuesday, November 11, 2008

Mycorrhiza [EXT]

I extracted this from

It has good explanations on the role of fungal in roots.

by Lon Rombough

Mycorrhizal Fungii

In farming lore of times past it was a practice of French (and other) farmers to "bless" a new field by sprinkling it with earth from an older field known to be productive. Modern research has now shown that such a ceremony may have more than spiritual value.

It has been found that many species of fungi, beyond just breaking down and feeding on dead plants and animals, associate themselves with the roots of living plants, establishing a symbiotic relationship. These mycorrhizal fungi produce fungal threads that coat the plant roots, even penetrating the cells in many cases, getting nutrients from the plant while providing benefits for it's host at the same time. Indeed, a large number of plants require this "invasion" for good health, and many others are much better off with than without the fungi. Orchids and pines, for example, can't even grow normally without the presence of the right species of fungi on their roots. Another plant requiring fungi on it's roots is the temperate fruit,the pawpaw (Asimina triloba).

Foresters have been aware that plants need fungi for some years. In the Pacific Northwest, the seedlings in Douglas fir nurseries did better when the soil they were growing in was given an inoculation of soil from an old stand of the same species. Inoculating the soil of the nursery with fungi from old trees helped the young trees get off to a much better start.

Home growers and farmers knew composting helped plants, but probably thought it was simply the nutrients, when it fact the compost also helped by introducing more fungi into the soil, and providing improved conditions for them to grow, so that they might form associations with the plant roots. Since then, the study of fungi and how they associate with the roots of plants has moved into many new areas.

There are four classifications of mycorrhizal fungi, based largely on how they associate with the plant roots.

The first are the "ectomycorrhizal fungi". "Ecto" is a prefix meaning"outside,", so ectomycorrizahal fungi are ones that wrap themselves around the outside of the roots, without actually penetrating the cells, though they may insert themselves between the cells, even if they don't actually penetrate the cell walls. Because they tend to form a sheath around the roots of the plant, these mycorrhizas can actually be seen with the naked eye, or under a lens. They tend to suppress the formation of root hairs, but stimulate the growth of many more short roots than normal, so the plant has much more root growth overall. Root tips can be seen to be covered with a white layer, and fungal threads, or strands, are visible on and around the root.

Second are the "endomycorrhizal fungi." "Endo" = "into," thus endomycorrhizal fungi actually invade the cells, sending their hyphae, or fungal threads, into the cells of the roots themselves. They are not always obviously visible to the naked eye, though their effects may be seen in many ways, however, as we'll see farther on. They are also known as Vesicular Arbuscular Mycorrhizal (or VAM) fungi.

The third group are an "in between" group, the "ectendomycorrhizal"types, which has characteristics of the other two groups. Some researchers now put these types into one or the other of the first two classifications.

The fourth group, mentioned only in passing because of it's oddity, is something of a maverick, the "ericoid" mycorrhizas. One plant family, the Ericaceae, which contains blueberries, cranberries, rhododendrons, heather and heath, manzanitas, and a few others, only form mycorrhizal associations with a certain few fungi that haven't been found to associate with other plants.

Many species of fungi form mycorrhizal relationships with plants, including a lot of the common forest mushrooms in the Basidiomycetes, and the Ascomycetes. A classic example of a mycorrhizal fungi is the exalted truffle, which has to associate with the roots of oak and hazelnut trees to produce it's gourmet delicacy. Many of the species most people think of as"toadstools" are mycorrhizal types, so having mushrooms pop up in your yard might actually be a sign of healthy plants.

What exactly do mycorrhizal fungi do for the plants they associate with?

Increase the number of roots. As noted with the ectomycorrhizas, many of the mycorrhizal fungi stimulate root growth in plants. Dr.Robert Linderman of Oregon State University is a leader in mycorrhizal fungi research, and some of his work shows that the right strain of fungi can actually double the number of roots in potted nursery stock, making the plants more resistant to dryness, and allowing them to establish themselves faster in their new location when they are planted out. This applies to both annual and perennial plants.

Further, the fungi allow plants to take up phosphorus and other nutrients more efficiently. Indeed, Dr.Linderman's work showed that plants in phosphorus-poor soil with mycorrhizal fungi did better than fungi-less plants with adequate phosphorus.

Protect plants from disease. All the exact mechanisms aren't established, but plants with mycorrhizal fungi are less susceptible to diseases, both of the roots and the rest of the plant. Some of the effects must be due to improved nutrition of the plant, while others may be related to substances given off by the fungi, natural antibiotics which keep disease organisms at bay. The physical presence of the fungi may form a natural barrier to disease, as well.

Improve the soil. Dr.Linderman's work describes how many of the mycorrhizal fungi actually penetrate the cells of the roots, creating a sort of "leakage" or exuding of substances from the roots. These substances had various effects on the soil, one of the most important of which was causing soil particles to clump together into aggregates. This clumping effect opened up the soil structure to allow better movement of water and oxygen into the soil.

One particularly interesting aspect was Dr.Linderman's revelation that fungal threads or hyphae from one plant could actually reach out into the soil and connect with threads of other fungal species on other plants. In effect, this meant that as the network of hyphae becomes well established,all the plants in an area would be tied together into one giant community,presumably able to exchange substances and nutrition with each other. This could be at least one aspect of why mixed plant communities often seem to function better than stands of single species. With all plants tied together, different species might exchange with each other, balancing nutrition and moisture amongst the community.

While mycorrhizal fungi were first studied in trees, research now encompasses all types of plants, both annual and perennial. Many strains of mycorrhizal fungi have been isolated, many of which are adapted to specific plant species. More importantly for the consumer, much work is being done by commercial companies, who are culturing mycorrhizal fungi and making them available to growers. This eliminates the variable results of using compost, which didn't always contain the best strains for the plants it was used on. However there are also species of fungi that work with many species of plants.

Different companies have taken different approaches in working with mycorrhizal fungi. Buckman Laboratories sells the species Glomus intraradix as a treatment for turf and nursery plants. Plant Health Care, Inc. uses a second approach by selling mixtures of three Glomus species and a species from the Entrephospora family. They feel that such a mix insures that at least some of these VAM fungi will adapt to a wide range of soil conditions and pH. Bio-Organics is testing strains of many fungi specific for different species of plants, but also have mixes of strains and general purpose types for use on crops for which no specific types have been identified. Thus far, it appears that VAM types are the most useful commercially, with about 90% of the types in use being VAM.

Mycorrhizal fungi don't just affect the plant roots, either. The health of the entire plant is boosted when the right fungi associate with it. Don Chapman at Bio-Organics, one of the companies studying Mycorrhizal fungi, reported results at one grower's grounds in which all pots of test plants with mycorrhizae, accompanied with trace minerals and organic fertilizer, had little or no damage from insects. Plants treated with conventional chemical fertilizers, and lacking mycorrhizal fungi, had considerable insect damage to the leaves. One theory says that insects lack amino acids to digest plant matter and that only sick plants develop high levels of certain amino acids, which stimulates the insects to feed on them. If a leaf is healthy, it will lack the amino acids that stimulate insect feeding behavior and they leave after taking only a taste. Apparently even though the plants without fungi had all the correct nutrients supplied to them chemically, they weren't able to make good use of them without the presence of fungi on their roots.

One of Mr. Chapman's customers, a Master Gardener, reported the yield of his tomatoes doubled with fungi, while ripening time was as much as a week earlier. He also said the tomatoes tasted much better. Similar results have been reported with other crops elsewhere. One grower reported an increase of 6,000 pounds per acre on his sweet potatoes using mycorrhizal fungi.
In my own garden, I applied mycorrhizal fungi and minerals to part of a potato patch, and minerals without fungi to the rest of the patch. All the area with fungi came up faster, bloomed sooner, and there were fewer blanks in the rows. At this writing, the crop has yet to be harvested, so the overall yield isn't known, but the difference in the performance of the plants was striking enough that I'll be very surprised if there isn't some difference in yield.

In the case of perennial plants, one application of the fungi should probably treat the plant for life. With annuals, it's advisable to reapply it every year, to be sure there is enough in the soil to properly inoculate the plants.

The naturally known species of fungi occur in many different soil types and pH ranges, making it highly probable that the commercially available types should also work well in many conditions.
Dr. Linderman also mentioned that some mycorrhizal fungi are even effective in hydroponic situations.

The commercial products can be either liquid or powder, for various means of application, from soil drench to root dips. Bio-Organics has formulations that combine minerals, organic (usually fish) fertilizer and fungi all in one to give the plant an extra boost. The other companies deal mainly with commercial growers, but Dr. Mike Kern at Plant Health Labs says their flower bed drench should work well for home growers as well.

Not only growers, but the environment itself can benefit from using mycorrhizal fungi. With the fungi, much less fertilizer is needed, reducing the sort of over-fertilization that leads to runoff and contamination of ground water. In fact, extra fertilizers may not even have any added effect with the fungi on the job.

While mycorrhizal fungi may not be the absolute cure-all for agriculture, it is certainly the most promising tool to be developed in a very long time, promising to improve the health and yield of plants, reduce the need for fertilizer, improve the soil, and reduce the need for pesticides.
In cell biology, many of the "organelles" (literally, "little organs")of cells are thought to have originally been simple, small cellular organisms that took up residence inside larger ones. Perhaps they were parasites at first, but at some point the relationship became symbiotic, beneficial to both parties. When the large ones replicated, the small ones followed suit, so each new large cell had some of the small ones.

Eventually, they became so interdependent the small ones had become an integral part of the larger cells. An instance considered proof of this is the presence in some species of amoebae of a pair of rod-shaped organelles near the nucleus which look very much like bacteria. Treat the amoeba with a specific substance that kills only bacteria and the amoeba isn't hurt, but the two rod-shaped organelles disintegrate. Only after those organelles are gone does the amoeba die. This suggests the organelles were originally a bacteria-like life form that adapted themselves to living inside the amoeba, until they eventually took over production of a substance, or some other function the amoeba had previously been doing for itself.

It was energy efficient for the amoeba to stop duplicating the the symbiote's efforts, so the amoeba became dependent on the "bacteria," while the "bacteria" lost the ability to live outside the amoeba. Thus, the two organisms became parts of one new form.

It's a truism that patterns repeat in Nature, and this kind of interaction of life, where a species involves itself with another so closely that the two eventually become one, can be seen to repeat at other levels. In this case, we are considering the interaction of soil microflora with plants, and how it fits this pattern.

In the previous articles of this series, about mycorrhizal fungi, we saw that the fungi not only attach themselves to the plant roots, even to the extent of penetrating the cells, they can also send threads out into the soil and interconnect with other fungi attached to the roots of other plants. In essence, they combine entire plant communities into one big super-organism. As they do this, they both use substances from the plants, and provide others to the plants in return. They also translocate photosynthates between plants, as in the case where young trees in a mixed, established forest were found to be receiving a large percentage of their photosynthates from older trees, through the fungal connections. Obviously, this is a very intimately interconnected system, almost resembling the way blood vessels joint different parts of a body. If the fungi are indeed acting as "blood vessels", then it might follow that there would have to be "blood cells" of some sort.

I spent several hours with Dr. Robert Linderman (USDA, attached to Oregon State University) discussing this and related ideas. He pointed out that while the fungi are indeed important to the system, other microflora, such as bacteria, may have a greater role. First, while the fungi provide the connections, it now appears that bacteria travel on and even in the fungal threads, often moving into and out of the plants, both producing and acquiring substances and carrying them into the plants as they go.

But the bacteria almost certainly have a much more important role. Bacteria are commonly thought of as "simple" organisms, yet anyone who has studied them can tell you they are anything but simple. For instance, bacteria can ingest pieces of genetic material directly and incorporate it into their own genetic makeup. That is, they aren't just digesting it and using the amino acids to construct new RNA or DNA, they are taking in whole genes, intact, and incorporating them directly into their own makeup. One proof is in disease-causing bacteria, in which types with a cell wall not resistant to certain antibiotics could be mixed with genetic material from dead cells that had cell walls resistant to the antibiotic, and in short order they had picked up the genetic material and had developed the resistant type of cell wall. Bacteria have also been shown able to metabolize an incredible range of substances, including asphalt, crude oil, even metals. Bacteria can produce chemical compounds that human laboratories cannot, or could only do with great expense and labor.

Bacteria may be uncomplicated in appearance, but they are great metabolizers. Is it any wonder, then, that other life forms would find ways to make use of that ability to metabolize?

Dr. Linderman discussed the role of bacteria and other single celled organisms and concluded that they were as important, if not more so, to the plants than mycorrhizal fungi. Some types we recognize already, such as the rhizobium that colonize the roots of legumes and fix nitrogen. But the full range of what bacteria can do in connection with plants hasn't been worked out yet. In addition to fixing nitrogen, it is likely that they help make other elemental plant nutrients, such as phosphorus, potassium, etc., more readily available to plants, much as the mycorrhizal fungi do. But why couldn't they work with more complex substances?

Suppose you were told to build a house within a certain length of time and were given a choice of piles of materials to work from. The first pile contains only raw logs. You would have to cut them into lumber, finish the lumber and cut it to the needed lengths to be able to build the house.
The second pile, however, contains prefabricated sections of walls, pre-built ceiling joists, etc. A few extra pieces might need to be added or removed here and there, but mostly they can simply be put into their proper positions and be nailed together.
Unless you just like to do things the hard way, you would choose the second pile. With the first pile, you would probably exhaust yourself and might be able to build anything more than basic shelter. With the second pile, you could probably construct a much larger, more elaborate house with less effort.

Why should a plant be any different? Nature is conservative in the extreme, always choosing the route that uses the least energy, whenever possible. If a plant had a way to ingest pre-formed materials into itself, saving it the energy needed to assemble the compounds, why shouldn't it do that? We think of plants as natural synthesizers, taking in water, minerals and carbon dioxide and converting them into sugars, proteins, vitamins, etc...
That life is interconnected has been a common philosophical and spiritual theme down through the ages, but the study of mycorrhizal fungi has greatly strengthened it as a biological one as well.

Work on mycorrhizal fungi has been carried out more extensively and for a longer time in forestry than in other branches of horticulture, as was briefly noted in part one of this article. Work on the ecological niches of the fungi have shown that they may be far more vital to plant growth and the ecology in general than ever considered before. Indeed, their relationships to plants may actually be necessary for some species to be able grow at all.

Lon Rombough

The author of this article wrote a book titled The Grape Grower

Sunday, November 9, 2008

Iron Chlorosis Signals Problems [EXT]

I included this site in this post to help us understand Iron deficiencies.

Iron Chlorosis Signals Problems

Contact: Diane Relf, Extension Specialist, Environmental Horticulture Posted April 1997 Chlorosis means "becoming green." The term was first applied to a human condition in which the skin took on a greenish hue. Today it is more frequently used to describe plants whose normal color is green, but which have a condition which results in a lack of green color in some leaves. Chlorosis in plants results from nutrient deficiency, insufficient light, or certain virus diseases. The most frequent cause of chlorosis linked to nutrient deficiency is iron chlorosis. This develops when a plant is unable to absorb enough iron from the soil. Sometimes the soil does not contain enough iron; but most often iron is in the soil but in a chemical form which makes it unavailable to the plant. Soils with high pH (alkaline) often bind iron in a form plants cannot use. Most plants do not develop iron chlorosis in neutral (pH 7) or slightly acid soil, but begin to develop yellowing as soils become more and more alkaline (pH>7). Such plants include rose, iris, firethorn, spirea, apple and peach.

Some plants require very acid soil and develop iron chlorosis when they are grown in soils which are not acid enough. Such plants include azaleas, rhododendron, holly, hydrangea, blueberry and pin oak.

Iron chlorosis is different from other types of chlorosis; it affects young, new leaves first. Other forms of chlorosis often affect older foliage initially. Leaf color of plants with chlorosis ranges from yellow-green to almost white. Leaves showing iron chlorosis often retain green veins. In severe cases, leaf margins may be brown and die, and eventually entire plants may be killed. Even in mild cases where yellowing is slight, growth is reduced.

Iron chlorosis is usually associated with deficiency or unavailability of iron in the soil but it may also appear when roots are damaged by overwatering, poor drainage, or overfertilization. Any condition that kills roots or does not allow feeder root development can lead to iron chlorosis.Treat iron chlorosis by eliminating the cause. A soil test will determine soil pH. Alkaline soil, or soil which is not acid enough, may be treated with sulfur or iron sulfate to make it more acidic. Poorly drained soils should be improved and plants subject to iron chlorosis should be watered carefully. The color of plants several months after soil is amendmended will indicate need for future treatment.

To achieve temporary improvement in leaf color, there are materials available for providing iron quickly to plants. Iron chelates are organic compounds containing readily-absorbed iron. They can be placed in the soil for absorption by the roots or sprayed in dilute form directly on the leaves. Since the chemical concentration of the compound may vary with manufacturer, carefully follow directions given on the package.

When fertilizer is sprayed onto leaves the process is called foliar fertilization. It is a good way to supply some nutrients, including iron, to plants rapidly, but foliar feeding is not a substitute for good soil conditions. Foliar fertilizer can supplement soil nutrition at a critical time, but to eliminate iron chlorosis, correct the conditions causing it.

(Prepared by Virginia Nathan, Extension Technician, and Diane Relf, Extension Specialist, Environmental Horticulture, Virginia Tech, Blacksburg, VA 24061-0327.)