Growing Media, Potting, and Labeling

The information in this Web page is presented with the understanding that no discrimination or endorsment of any of the information linked to from this Web page is implied.

The growing of greenhouse crops involves numerous cultural inputs, not the least of which is the type of growing medium utilized. A plant is only as good as its roots. Therefore, the rooting environment is as important, if not more important, as the above ground environment. Greenhouse crops face relatively shallow depth and limited volume of growing medium hence, the choice of growing medium is of great importance. Growers observed that watering practices significantly impact crop quality, but they also realized that understanding the relationship between water retention, air porosity, and growing medium is determinant to produce greenhouse crops.

Growing media can be defined as a three-phase system (solid, liquid, and gas), as a network of pores filled with either gas or water, separated by a solid matrix, which consists of mineral particles of various sizes and organic matter.

Adequate pore space, proper soil pH, adequate water supply, sunlight, and mineral nutrients are the requirements for a proper growing medium.

I. Growing Media

Soil characteristics can be either 1- chemical characteristics including pH, cation exchange capacity, electrical conductivity, and fertility, 2- physical characteristics including bulk volume, bulk density, texture, and structure. Texture and structure affect air porosity and water retention.

Ingredients of Growing Media

(Put in question here and at other question mark icons? Suggest using the links we have provided and any other **reputable** (define?) info they have access to in order to answer the question or develop some kind of strategy. Sort of interactive, not so passive?)

II. Aeration and Water Retention

John says on Aeration and water retention

"A media should be choose so that it holds water long enough but yet not too long.
One of the biggest challenges to a greenhouse manager is to balance aeration and water retention. Just after seeding, plugs are too wet, and may drown. But later on, as plant matures and increases in size, plugs dry out very quickly. These changing demands of growing plants in greenhouse can be overcome by varying the watering practices as the plugs mature."

Several factors account for aeration and water retention including 1- components and ratios of media mixes determine water and air content in the root zone, 2- the container (height, width, and shape), 3- the mixes handling procedures, and finally 4- watering practices.

Starting with the same media volume (for instance, the sponge), the effect of container height on media air space is dramatic, leading to the conclusion that the choice of containers is important in managing water-air relations in the root zone, especially in plugs.

The shape of the container can be also questioned: is a square or round plug cell better? The answer is that in general square cells are easier to manage because of their greater volume, however, as long as the height is the same, the air does not vary.


III. Types of Growing Media Admendments:

2- Coir
It originates from ground-waste coconut husks. After most of the fibers are removed, the remaining coir, or coir dust is used for marketing. Chemical and physical properties of coir are variable depending on the amount of fiber remaining in the material. Coir has a high water-holding capacity, higher than peat and can be easily rewet after drying.
The level of micronutrients is low, however, the level of phosphorus and potassium. Calcium and magnesium may need to be added.
Coir can contain high level of chloride, so may not be used if non-leaching subirrigation is used. Coir has been seen as a low cost replacement for sphagnum peat moss

3- Bark
A byproduct of saw mills, this media improves aeration and reduces the cost of media. Bark, usually pine bark, must be aged and composted before being used so that phytotoxic compounds are eliminated. During composting, the pH of bark increases, thus it is recommended to monitor the pH and nutrient changes in the media.

4- Others
Several organic materials are also available including manure, corn cobs, straw, peanut and pecan shells. Compost - is the end-product of the decomposition of organic matter. Composts vary in their soluble salts, nutrients, and contaminents so it is important that you know what material was the source of the compost. However, organic material such as manure should be taken very carrefully since many problems are associated with the use of manure, especially in greenhouse.
Many other organic media can be used such as cedar chips, sludge (a semi-solid residue from wastewater plants. They are fairly rich in plant nutrients and the pH is usually about 6.0 - 7.5. Sludge mixes should be checked for heavy metals and soluable salt levels),
and rice hulls (a byproduct of the rice industry which improves drainage).

Perlite is a gray-white silicaceous volcanic rock, crushed and heated at 1400F (~760 C). Sterile and light in weight (6 to 9lbs/cuft), this material is absorbant, increases aeration and drainage but possesses no pH buffering capacity. Perlite is availble in three grades.

Vermiculite is a micaceous mineral expanded by heating to 14000F. Sterile and light in weight (6 to 8 lbs/cuft), this material is a hydrated magnesium-aluminium-iron silicate, with high CEC and can be available commercially in four grades where #2 and #3 are considered as best.

3- Sand
Very heavy in weight, sand is constituted of soil particle between 0.05 and 2.0 mm in diameter from the weathering of rocks. Therefore, it may contain weed seeds and diseases, resulting in the necessity to sterilize - steam-pasteurized this media. It also contains very little nutrients, decreases water holding capacity of soil media, and has no buffering capacity, or CEC. When combined with organic media, it provides good drainage and aeration.

4- Soil
Most greenhouse growers do not use large quantities of soil in their operations, they generally use "soilless" media because soils vary in consistency, they are usually heavy, tend to compact, and need to be pasteurized. It may contain toxic materials (herbicides for example) so there is a necessity in conducting bioassays for herbicide residues before using any kind of soil in the greenhouse.
For instance, you can grow some tomatoes in media before using in large quantity soil, and you look carefully for injury.

5- Rock woll
Rock wool is a melted rock at about 1600 C (for instance basalt), available as pellets, cubes, blocks, or shredded. It is absorbant, and has the capacity to retain good exygen levels and provides support for plants.

A desireable media mix for greenhouse would be 1/2 to 3/4 organic materials, 1/4 to 1/2 inorganic matter as well as lime and starter fertilizer.
There are many premixed formulations available for specific crops and propagtion methods. Greenhouse supply catalogs are good places to look for more information.
Another option is to mix your own. The following formulation has been developed in the 1960's at Cornell University:

1- Peat-Lite Mix A is made of : (this recipe makes one cubic yard of media)

On the market, many soil mixers are available, but the most succesful is the modified ciment mixer (Langhans, 1984), because it is resistant enough to handle the weight of the soil and of the water. This equipment is usually available in different size so that it can accomodates different size of greenhouse production. Some mixer can be stationary which is advantageous when there is a need for conveyors, water, electricity, and shelter. Other mixers can be portable and mounted on a truck which presents the advantage to be glose to the area that needs the mix.
Media ingredients, including peat moss and vermiculite are thrown into the mixer with water so that dust, lime, and superphosphate are limited and fertilizers are slowly released. (Langhans 1984). If the media mix is overmixed, it becomes too fine whereas overwetting leads to the formation of clugs. Therefore the right mixing time and the right water quatity are very important to determine.

John Kumpf on Growing Media Mixing Tools

"Although premixed growing media are famous, some growers still prepare their own. At KPL greenhouse at Cornell University, this old concrete soil mixer was used for decades, can hold up to 10 bushels at a time. It has a tumbling motion which works best when incorporating straight soil into the mix. The mixing procedure is as follow: the media is thrown in this end and is mixed by the tumbling motion. After adding the fertilizer, the media is again mixed for an additional 5 other minutes and results is a nicely prepared media for growing plant material.

Another step in media preparation is the pasteurization process. To do this, the media is placed in this box where aeriated steam passes through it. To prevent weed seeds from germinating, the temperature of the steam can reach 175 F from the top down for about 45 minutes. The steam fills up the top box and then moves down to the media. The top down movement of the steam leads to a more uniform temperature throughout the mix, whereas if the steam has a bottom up mouvement, a chimney effect might happen causing blowouts. A non uniform steam and temperature may result in germination of residual weed seeds even after pasteurization.
One advantage of this setup is that once pasteurization of the media is finished, the steam can be turned off and cold air can continue to run, which provides a cooler media that can be used half an hour after pasteurization. Whitout this option of cooling the soil, several hours would be necessary for the mix to cool down enough so that it is ready to be used.

This soil mixer is primarily used for mixing artificial media. Adequate space is required around the mixer: a proper overhead clearance is indispensable to get the media into the top of the mixer. Any overhead obstruction would interfere with the loading of the mixer, especially if a conveyor belt is present in the set up. It is also important to make sure that the mix container that fits under the mixer can be removed easily and without breaking too many backs!"

Pot and flat filling systems usually are composed of : - a soil supply hopper, - a container support, - a metering, and - a leveling device. Other optional components can be included such as a soil return conveyor, a soil level control, a compactor, pot holders, pot and tray dispensers, dibble, and automatic pot take-off.

Those machines are adapted to various container sizes, and possess variable filling rates (between 10 to 30 flats /min or 20 to 50 pots/min). Those machines usually require the presence of two or three operators.

Those machine can increase by two or three times the production rate per man-hour. Another advantages of these machines is the higher uniformity of filling that results especially when filling flats.

1 bushel = 1 1/4 cu. ft
1 cu. yard = 27 cu. ft.
1 cu. yard = 22 bushels
100 sq. ft. of bench area (at 6 in. soil depth) = 50 cu. ft. or 40 bushels of soil mix (Ball)

During soil mix procedure, usually a 15 to 20% shrink can occur, depending on the random organization of the mix particules between each others.

Two methods are primarily used for sterilization of media. Numerous amendments (e.g., perlite, vermiculite) are sterile and, therefore, do not require further sterilization. Whereas composted pine bark and peat contain populations of weed seed and suppressive microbes that might be eliminated by sterilization techniques.

1- Steam Pasteurization is commonly found in greenhouse. The general recommendation is the exposure to steam (at least 180° F) for 30 to 45 minutes. To achieve uniform heating, it is better to avoid big piles of mixes; smaller one are recommanded. The presence of an appropriate thermometer is handy to effectively monitor the temperature at various places in the piles. It is recommanded to avoid over-steaming because beneficial organisms would be killed and toxic substances may be released, especially when organic components are involved.
Aerated steam, an alternative pasteurization process, involves blowing a mixture of steam and air through the media. Aerated steam (140° to 175° F) uses less energy and fewer beneficial organisms are likely to be harmed. Because the steam moves, it condenses creating heat that warms media particles. Recommanded temperature treatment and length of treatment depend on the crop and the media used. However, some plant pathologist and horticulturists recommanded to leave the media under temperature treatment for 30 minutes and let it cool for 30 minutes at 90F (Aldrich and Bartok, 1994).

2- Solarization is rarely used because the process may requiere up to one month even under summer conditions requiring tremendous planning for future media needs. Solarization is accomplished by covering with clear plastic sheet a moist media (to a depth of 6 to 10 inches on a clean surface). The edges are sealed to the surface in order to prevent the loss of heat and moisture.

(Put in question here and at other question mark icons? Suggest using the links we have provided and any other **reputable** (define?) info they have access to in order to answer the question or develop some kind of strategy. Sort of interactive, not so passive?)

IV. Growing Containers

Plants grown in greenhouse requiere containers at different stage of their life, from seed ing to selling. Some plants remain in the same pot for their whole life whereas otheres need to be transplanted one or few times into larger container before being sold. Since each plant and each stage of their growing life have special needs, many types and shapes of containers are available. Conatiners help also the greenhouse manager to transport plants, to make an afficient use of space.
Containers are made of wide variety of material which each presents advantages and disadvantages. They can be made of clay, plastic, wood fiber, styrofoam, wood, peat moss, or soil.
Shape and size are also variable. The most common are round. The size is determined by measuring the inside diameter across the top. If made of other material than clay, they can be squarred and those pots are measured by the inside wdth of the top edge of the pot.

John Kumpf on Growing Containers

"Before selecting the proper container, you need to think about what the plant material will look like when it will be mature because the shape of the mature plant will actually determine the size, the type of the container. We want to select a container that supports the plant from it's juvenile growth stage right up to it's maturity. If the plant is not going to be repotted, it is important to pick a container that will support all through it's growing period. An example of that might be the Easter lily, which is put into a certain size pot and grown to maturity in it and not repotted. Slow-growing plants, like woody plant material, will often need to be repotted more frequently than rapidly growing plant material such as a Ficus plant or a large dracena . When it grows from right out of the propagation bench into a pot it oftentimes needs to be repotted several times before it reaches full growth. The depth of a container is also something to think about, as well as the width of the container. Remember, the deeper a container is, the better the drainage is going to be. Be sure to pick a container that has the proper height, proper width, and proper drainage. Never have small plant in a big pot and inversely, a big plant in a small pot.

Choosing the wrong container size can lead you to two different problems: when a small plant is in a big pot, poor drainage will occur and on the contrary, a large plant in a small pot will not have enough water and will not be able to reach its full potential as far as growth is concerned.
Color of the container si also another consideration to have in mind. For example, when Coleus are grown in the greenhouse, because of its various shades of red, the color of the pot should be choosen so that it does not clash with the color of the plants. The color of tha container can also help in accentuating the natural beauty of the plant.
If plants are tall and thin, you want to choose a container that will not fall over.
There are many kinds of containers, we see them in magazines and books, of all shapes, compositions, and sizes: wire, fiber, rounded, square, green, white, red, etc... Know what they are and use the best container suited for the job for the plant material that your growing. Orchids need different container than cacti!

See the Soil Table for Pots, Bulb Pans, Hanging Baskets, Flats and Inserts, Cell-Paks, and Plug Trays (copied with permission from the Ball Company) in Appendix A, at the end of this chapter, for specific soil volumes for specific pot sizes.

Those geraniums are growing in heavy weight fiber pots. There are heavy and medium weight fiber pots. Heavy pots are preffered because when plant materials are moved once it's been growing for a period of time, the sides of the container could break on the lightweight containers. Fiber pots allow good aeration.
Also, the bottoms of those containers on benches get wet allowing the plants to root through the bottom which is undesireable. In that case, taking up the moisture from the bench rather than from the container results in growth not regulated by the amount of water put on, but by the amount of water the plant can access from the bottom of a bench.

A bag of media can be considered as a container. On this picture it is a MetroMix 560 bags are used as containers with mature tomato plants growing in it. The bag lays on the floor of the greenhouse with the center lifted to increase the depth of the media, which in turn improves the drainage. Small slits cut in the bottom of the bag allow the water to run out. The rule of thumb is to plant tomatoes plant into three to four inch diameter holes. This type of container can support tomato growth for seven to eight months. This is a very simple, neat, clean way of utilizing a different kind of container.

A greenhouse bench can also be considered as a container: in a hobby greenhouse, a small garden center, or small greenhouse operation, a small bench can serve this purpuse since it can support plant material. The bench is filled up with the media in which plants are planted and will grow to maturity. Because the deeper the media is, better drainage occurs, the depth of the bench is somthing to pay attention to. Because it is not deep, it is important to emphasize on proper drainage to avoid a situation where your media is going to become water-logged resulting in poor plant growth."

(Put in question here and at other question mark icons? Suggest using the links we have provided and any other **reputable** (define?) info they have access to in order to answer the question or develop some kind of strategy. Sort of interactive, not so passive?)

IV. Potting

John Kumpf on Potting

"I would like to emphasize on how it is important to fill up the pot or container with soil because the depth of that soil is going to have an important effect on the kind of drainage that will occurs.

Here is an explanation to show you how depth of soil is essential: if some cheesecloth are soaked in water and hold flat, very few drainage happens. This actually depicts the situation when a pot is half full. When the cheesecloth is rotated, the water will begin to drain out of that cheesecloth. This represents then a pot filled to within an inch of the top creating good drainage. Drainage is essential especially when plants are fertilized since water is added every day.

Potting is a critical step: an adequate amount of water need ot be added. For instance, peat-like mix acts just like a duck's back, ie when you put water on it it just runs off. It will not penetrate into the media, and it will need to be watered, watered, and watered. But if some water is added ahead before potting, the media will have a better drainage. But the media should not be too wet either. After mixing the water in, you don't want it clumpy so that it balls up. One of the nice things about MetroMix is that it is just about in the right state you need for proper potting. You can almost pot right out of the bag.

Remember what I said about the container size, you have to make a choice about what kind of pot- clay or plastic pots. There is a difference. A new clay pot- general rule of thumb- water once for the pot and once for the plant. The pot is dry it is goining to wick moisture out of the media. You will find that clay pots dry out a lot quicker than the plastic pots.

Remenber a plant will not grow properly in a pot that is only half full of media. Fill the pot right up to the top. Just give it a little bit of firming. Level it off, put your hole in there for the plant roots, stick the plant in. Push the media around it and firm down a little bit. What I like to do is take my thumb and push my thumb around the edge of the pot. Now what's that doing? Why do I do that? I do that to make sure that I have a reservoir from the media to the top of the pot so that when you go to water it, number one- you're not splashing mix all over the place and number two-there's enough of a reservoir there to give you enough water in that pot to give you drainage at the bottom.

Another point, when you first pot up your plants make sure that you give them a good watering the first time. And remember what I told you about new clay pots- you always want to water once for the pot and once for the plant. you want to water your newly potted plant until you have water running out of the bottom of the pot.

Your goal is to have a nice uniform surface in the pot. From one pot to another the soil levels should be the same.

Clay pots dry out faster than plastic pots but are more stable. This may have a bearing on what type of media you would use in each kind of pot. You might want to have a little heavier mix with plastic pots."

V. Repotting

John Kumpf on Repotting

"Thinking about repotting plant material that is in a container that is too small for it. This is an example of tomato production again. I think about the homeowner, someone's got a tomato plant and they're putting it outside in the spring. They have it out on their patio and suddenly they realize that this tomato plant is being deprived water because it is in a container that is not big enough to supply it with the amount of moisture that it needs to keep the plant growing in a healthy way. So, here we have a container that is obviously not supporting the plant. The plant was tipping over and drying out. It needs to be repotted so we go to a size container that going to do us some good. If you're going to repot something into a larger container, put it into a container that going to help it, one that has extra room on the bottom of the container, on the sides, as well as on the top. When we go from one size pot to another size, think bottom, think side, think top. You need more room all the way around and the most critical area is more room on the top."

VI. Labeling

Labeling is very important, especially when horticultural research is conducted; it helps to recognize plants that have been under such and such treatments. Because of greenhouse environment conditions (water, humidity, and pesticide applications), tags and labels used should be designed to withstand harsh environments.

John Kumpf on Labeling

"Last but not least, don't forget to put a label in your pot.
You always want to make sure that you have a label to put in the pot. This is extremely important. You want to use a pencil to write on the label because ink and markers can fade and generally speaking a good no. 2 pencil. In general you want to put genus, species, variety/cultivar, and common name on the label. If possible, put the date it was planted and then you can add any other information, for example: treatment numbers, your name, etc. If you are going to be using different treatments you might think about using different color labels."


VII. Summary

The information in this Web page is presented with the understanding that no discrimination or endorsment of any of the information linked to from this Web page is implied.



Appendix A

Soil volumes for pots, bulb pans, hanging baskets, flats and inserts, cell-paks, and plug trays (copied with permission from the Ball Company). These figures are approximate. Actual soil volumes may vary due to type of soil used.

Pot Size

Approximate Dimensions

Top x Depth x Bottom

Pots per cu. ft. of mix
Azalea Pots
4 in. 4 x 2 13/16 x 3 in. 56
4 1/2 in. 4 1/2 x 3 3/8 x 3 1/4 in. 44
5 in. 5 x 3 13/16 x 3 9/16 in. 28
5 1/2 in. 5 1/2 x 4 1/4 x 3 7/8 in 24
6 in. 6 x 4 5/8 x 4 1/4 in. 18
6 1/2 in. 6 1/2 x 5 x 4 9/16 in. 16
7 in. 6 7/8 x 5 3/8 x 5 1/16 in 12
7 1/2 in. 7 3/8 x 5 11/16 x 5 3/8 in. 10
8 in. 8 1/8 x 5 7/8 x 6 in. 7
8 1/2 in. 8 1/2 x 6 1/4 x 6 1/4 in. 6
10 in. 9 7/8 x 7 3/8 x 7 1/4 in. 4
Geranium Pots
3 1/2 in. 3 1/4 x 3 x 2 3/8 in. 96
4 in. 3 15/16 x 3 7/16 x 2 7/8 in. 56
4 1/4 in. 4 1/4 x 3 9/16 x 3 1/8 in. 50
4 1/2 in. 4 3/8 x 3 7/8 x 3 in. 44
Square Pots
2 1/4 in. 1 15/16 x 1 7/8 x 1 1/2 in. 357
2 1/2 in. 2 3/8 x 1 15/16 x 1 7/8 in. 224
3 in. 2 3/4 x 2 1/4 x 2 1/8 in. 128
3 1/2 in. 3 1/4 x 2 3/4 x 2 1/2 in. 96
4 in. 3 5/8 x 3 1/8 x 2 7/8 in. 56
4 1/2 in. 4 1/8 x 3 5/8 x 3 1/4 in. 40
Standard Round Pots
2 1/4 in. 2 1/4 x 2 1/16 x 1 3/4 in. 256
2 1/2 in. 2 3/8 x 2 1/4 x 2 in. 208
3 in. 3 x 2 13/16 x 2 1/4 in. 120
3 1/2 in. 3 3/8 x 3 3/16 x 2 3/8 in. 80
4 in. 4 x 3 7/8 x 2 3/4 in. 48
4 1/2 in. 4 3/8 x 4 3/8 x 3 in. 40
5 in. 5 x 3 1/2 x 4 15/16 in. 28
5 1/2 in. 5 1/2 x 5 3/8 x 3 13/16 in. 20
6 in. 6 x 5 3/4 x 4 1/16 in. 16
7 in. 6 3/4 x 7 3/4 x 4 11/16 in. 10
8 in. 7 5/8 x 7 3/4 x 5 3/8 in. 6
10 in. 9 7/8 x 9 3/4 x 6 7/8 in. 3
Violet Pots
3 3/4 x 2 3/4 in. 3 11/16 x 2 5/8 x 2 3/4 in. 80
Bulb Pans
6 in. 6 1/16 x 3 9/16 x 4 9/16 in. 24
7 in. 7 1/16 x 3 13/16 x 5 9/16 in. 16
8 in. 8 3/16 x 4 x 6 3/4 in. 9
10 in. 10 1/4 x 4 15/16 x 8 1/8 in. 5
12 in. 12 1/8 x 6 x 9 3/4 in. 3
Lockwood Hanging Baskets
8 in. 7 1/2 x 4 7/8 x 4 1/16 in. 11
10 in. 10 x 6 1/8 x 4in. 5
Wall Pot 10 3/16 x 4 7/8 x 6 1/16 in. 10
1020N, 1020R Series 11 1/2 x 21 1/4 x 2 1/4 in. 4.6
1020K (Germination) 11 1/2 x 21 1/4 x 1 1/8 in. 7
20-Row Seedling Tray 11 1/2 x 21 1/4 x 1 1/8 in. 11
Standard Cell-Paks
804 2 3/8 x 2 3/8 x 2 1/4 in. 5.6
806 1 1/2 x 2 3/8 x 2 1/4 in. 6
1006 1 3/4 x 1 1/2 x 2 1/4 in. 5.4
1206 1 1/2 x 1 1/2 x 2 1/4 in. 9.2
1804 1 1/2 x 1 1/2 x 2 1/4 in. 9.2
Pro-Tray Growing Trays
Six-Pak 3 1/8 x 3 1/8 x 3 1/2 in. 11.5
18 Pots (cut) 3 1/8 x 3 1/8 x 3 1/2 in. 69.1
24 Pots (cut) 3 1/8 x 2 1/4 x 3 in. 126.8
18 Liner Tray 3 1/8 x 3 1/8 x 3 1/2 in. 3.8
24 Liner Tray 3 1/8 x 2 3/16 x 3 in. 5.3
24 Pro-Tray 2 3/8 x 2 5/16 in. 10
38 Pro-Tray 2 3/16 x 2 5/16 in. 7.5
50 Pro-Tray 1 7/8 x 2 5/16 in. 8
72 Pro-Tray 1 1/2 x 2 5/16 in. 9.4
98 Pro-Tray 1 3/8 x 1 1/2 in. 11.2
162 Pro-Tray 1 x 1 1/2 in. 14.5
288 Round Pro-Tray 13/16 x 1 in. 19.1
5/10 1 7/8 x 2 5/16 in. 39.8
Plug Flats
50 sq. flat 1 7/8 x 2 1/4 in. 6.3
72 rd. flat 1 7/16 x 1 3/4 in. 12.8
72 sq. flat 1 9/16 x 2 3/16 in. 7.3
98 sq. flat 1 5/16 x 2 in. 7.9
128 sq. flat 1 3/16 x 1 13/16 in. 9
162 sq. flat 1 x 1 1/2 in. 9.2
200 sq. flat 5/16 x 1 1/2 in. 10.7
273 sq. flat 3/4 x 1 1/8 in. 18.6
288 rd. flat 13/16 x 1 in. 19.1
288 sq. flat 13/16 x 1 1/4 in. 15
392 sq. flat 5/8 x 7/8 in. 23.1
406 sq. flat 5/8 x 7/8 in. 22.2
512 sq. flat 9/16 x 3/4 in. 24.7