POTTING MEDIA PRIMER

The first thing to recognize is that regular field soil is unacceptable for use with pots. Traditionally, for potting purposes such soils were amended with well decomposed manure which improves the physical characteristics but this approach has many problems.

ADVANTAGES OF MODERN POTTING MIXES (OR: WHY CHANGE?)

Modern potting mixes are developed to overcome some of the short falls of traditional soil based mixes:

  1. Non availability of standardized top soil and manure: too often we now get subsoil and low grade, sticky manure.
  2. Media containing a large proportion of soil are heavy.
  3. They are aesthetically unacceptable: everything: plants, pots, floors and buildings etc are stained by the soil and drainage water.
  4. Plant growth is relatively slow: this is due to poor aeration causing relatively sparse root growth. This may be actually desirable once the plant is full grown but at the production stage it is too wasteful.

At the same time, we must keep in mind the advantages of our traditional mixes and try and incorporate these into new media:

  1. Years of experience with traditional media means that everyone in the production-marketing-customer chain is comfortable using it. This is one of the biggest stumbling blocks to using new and improved soil less media; since we have been using the soil less media from 20 years, we have long experience with it and confidence that it can be used successfully in India.
  2. Low cost: soil and manure mixes are cheap. However, as my calculations show, they are not as cheap as we would think. New media using coir, thermocole etc are expensive but again, not as much as we may assume. If seen in the correct perspective, the benefits far outweigh the added costs.
  3. Easy availability: though it would seem that both are easy to get, actually the situation is that it is getting increasingly difficult to get good top soil and well rotted, fibrous manure even if one is willing to pay. Alternate media components are not easy to source either- we have had to develop most of them in-house. We hope to offer superior grade materials at honest and competitive rates soon.
  4. Traditional soil based media gives slow but compact, hard growth- this is an important benefit because of the way even ornamental house plants are handled (or mishandled) by our postproduction systems. However, by using the correct components in new soil free media and implementing correct fertilization procedure, it is possible to have good growth control in these new soilless media too.

The bottom line is that the only way to compete in today's tight market is to innovate, use new marketing tools and generally give a better product than the others can. The current situation is that the gardener who starts up on his own has rock bottom overheads and because he is hands-on and totally in touch with the plants, does a better job than the old time nurseryman who employs his growers. The only way to compete with such low cost producers is to learn and adopt new and more modern technology: clean, light weight soiless media is corner stone of modern pot plant culture and needs to be adopted by all forward thinking and ambitious nurserymen.

The greatest difficulty in implementing a soiless media program for your nursery is that it requires a systems change at various levels. Just as it is not possible to jump a 10 feet gap in 1 foot increments, it is difficult to introduce soiless media in a small way in your nursery. Understanding the concepts, developing a suitable media and then using it on a substantial scale is the only way to bridge this gap.

MEDIA CONCEPTS

  1. WATER HOLDING CAPACITY (WHC): this is the amount of available water held by the media after watering and drainage. It depends on the choice of components used and can be adjusted to suit your needs. Whether your media needs higher or lower WHC really depends on: a) Size of the plant in relation to the pot- a large, established plant will absorb water rapidly while a recently planted, small plant in a large pot will tend to need much less water. b) Watering regimen: if you tend to water freely, a media with lower water holding capacity is better. The reverse is true if there is a chronic shortage of water. c) Season and environment: plants potted up during wet months will do better in a medium with lower water holding capacity because of less risk of over watering. Similarly, if the environment is generally hot and dry, you may want to have higher WHC.
  2. AERATION: as long as plants are getting enough water (they usually do because we tend to over water and plants will droop when dry), the most critical component in the media is air. There is an inverse relationship between WHC and aeration- increasing one will decrease the other and vice versa. It is better to have a media with higher aeration and lower WHC because watering more is generally easier and economical but air cannot be added once the plants are potted and with time the aeration of the media will decrease.
  3. A critical concept to understand for aeration is the difference between structure and texture; Structure refers to the way individual particles of a component (usually fine clay particles), are associated together (with humus) to form small clumps- because these clumps act is relatively large particles, they give an aerated medium. However, if the structure of these clumps breaks down (for e.g. by adding Sodium from Sodium nitrate containing fertilisers), the individual clay particles are so fine as to give a media with very poor aeration indeed. Texture, on the other hand, refers to the size of individual particles of any component. A course texture means larger particles and so larger empty spaces between particles for air to occupy. Thus fine sand will have less air and so poorer aeration compared to course sand. In addition to size of individual particles, the more uniform the particle sizes in the component, the better the aeration. If different particle sizes are mixed, the fine particles occupy the space between the larger particles, at the expense of air and so reduce media aeration markedly. Thus adding fine sand to a media will usually decrease aeration because the sand particles fill the gaps or voids between the larger media particles. (That is the principle of concrete: the large spaces between stone chips are occupied by sand and the space between the sand particles by cement, leaving no air spaces).

  4. pH and SALINITY: These are very important considerations. Traditional media are used primarily because they naturally have acceptable pH and salinity levels. When moving to soil less media raises two pH problems: the buffering capacity of soil is removed so that pH can swing more easily and availability of micronutrients in soil less media is better at lower pH so one needs a lower pH for these media. The pH to aim for is between 5 & 6 and luckily most of the components available locally fall in this range. Once you move out of this pH range, all sorts of micronutrient problems start to occur. If your water supply is alkaline, some acidification of the irrigation water may be useful. pH meters are available but are difficult to maintain and calibrate. It is better to use a good quality pH indicating solution as accuracy is not really required, just the confidence that pH is within the desired range.
  5. Salinity is often a problem with manures (chicken manure is particularly bad) and some coir supplies. Salinity is measured as the Electro Conductivity (EC) and is generally a measure of the salt (and roughly the fertilizer) content of the media.. It is always better to keep it low initially; once plants are growing it can be raised with liquid feeding. Good quality portable EC meters are now easily available and are a vital tool for all growers; make sure you buy one with the correct range and use it regularly to test salinity of media components, final mix and liquid fertilizers.

CLASSIFICATION of MEDIA COMPONENTS

There are two useful ways to classify media components; both systems are complementary and help in understanding media properties and aid in media selection & ratios to achive the desired objectives.

All media components can be classified based on their origin into Organic and Inorganic materials. They can also be classified based on their interaction with water as water holding or non water holding materials.

ORGANIC/ INORGANIC:

ORGANIC: material of plant origin and include coconut coir, sawdust, rice hulls, groundnut shells, manure etc. Points to keep in mind with organic materials are: the C/N ratio - if too high, bacteria ( which are much more efficient than plants) will quickly absorb all nitrogen, leaving plants starved salinity- especially when using manure and coir rapid decomposition leading to shrinkage and deleterious changes in physical properties toxicities: when using some sawdust's or bark pests & weeds: from manure, groundnut shells etc inconsistent quality: most items will vary from lot to lot

INORGANIC: material often though not always of mineral origin. Include sand, perlite, vermiculite, stone chips, thermocole granules, etc. Points to keep in mind here are: They can suffer from all of the above problems except poor C/N ratio Though they do not decompose, they can collapse like vermiculite to greatly reduced aeration. Pests or weeds are generally not an issue but if stored poorly they can pick up weed seeds or pathogens. Toxicity can often occur depending on the mineral origin of the product e.g. some stone chips may cause micronutrient toxicities or pH issues. Vermiculite available locally is for industrial use and often contaminated by oil residue etc.

ABSORBENT/ NON-ABSORBANT MATERIALS: Another way of looking at materials would be to classify them as water absorbing or non-absorbing. Strongly absorbing materials include coir, vermiculite and manure while totally non absorbing materials include stone chips and thermocole granules. Some materials will fall somewhere in the middle, e.g. groundnut shells. Once understood, intelligent use of this concept will allow minor modifications of a basic media to increase or decrease WHC and aeration to suit various crops and conditions.

AVAILABLE COMPONENTS

The following materials are available in Western India and have been or are being used by us. It is certainly possible that other materials can be used to give similar or better results.

ORGANIC MATERIALS:

  1. Manure: well rotted cattle manure is a traditional media component. The best quality manure from Aarey Milk Colony- several years old, fluffy and fibrous, is an excellent material but difficult to get. Often you are stuck with young, filthy, sticky material from the gutters off the highway- avoid this at all costs.
  2. Coir: easily the most important material for us here in India, it must form the basis of all soil less media in the future. There are now many suppliers in South India but prices are still uneconomical for less than truck loads and quality extremely erratic. We treat all our supplies and test before using. Excellent quality treated coir can be purchased from us. When buying compressed coir check EC and wash with 2-3 changes of water prior to use. We like to place 10-15 kilos of compressed coir into a 200 liter plastic drum, fill with water and leave overnight. Tie a piece of shade cloth over the drum and pour off the excess water and repeat the process. If your water is good and low in salts, this will do the trick.
  3. Another thing to look out for is dark, fine coir- this is material which is many years old and decomposed- it may be OK to raise vegetable seedlings but is totally unsuitable for pot plants- this type of coir will collapse into a thick, air-less mush within months, killing or severely retarding plant growth. Always buy young, light brown fibrous coir.

  4. Sphagnum Peat: mentioned here only to say that it is not available in India. Imported material is too expensive. Peat also decomposes rapidly in our warm conditions. Many articles in Indian books and magazines suggest its use. It shows that the authors are impractical and ignorant at best and plagiarists at worst. Sphagnum peat is neither practical nor economical for us in India and is being replaced by coir in many parts of the world.
  5. Rice Hulls: usually abundant, we have tried them with very mixed results. Adding a token 5-10% in the media will do very little good. When burnt in a special way to give rice husk charcoal it is widely used as a propagation and rooting material in Thailand. In this form it is like fine sand and would be virtually sterile. This rice husk charcoal definitely gives great results for plant propagation and needs further investigation and trials.
  6. Groundnut shells: these need to be allowed to undergo rapid, controlled composting- this decomposes the soft inner lining, leaving the tougher outer tissue. The heat generated also probably helps kill some pathogens. The shells are then hammer milled and sieved to get a uniform particle size. Using direct shells is counter productive- they act like the stone chips in concrete to reduce media aeration.
  7. Sawdust and other wood products: rarely work- we are unable to get sawdust from a single type of wood and the high C/N ratio is a problem. Wood chips 3mm or larger would be a useful additive but we have never found suitable supplies.
  8. Moss: the green moss available from Mahabaleshwar is epiphytic and rots when used as a media, even for orchids. Also it is environmentally catastrophic to rip it out of the Moss Forests. Never use it as it is no good. Even for hanging baskets and moss poles, there are better options: fully plastic baskets that don't require moss or wire baskets lines with coir matting. Coir rope works fine for poles. White sphagnum moss is available in North India in very limited quantities and can be used for special plants like orchids.

INORGANIC MATERIALS:

  1. Top Soil: Traditionally top soil is used as the basic component of potting media. At its best the results are good though growth is usually slow due to relatively poor aeration and subsequent sparse root growth. Main problem is availability of topsoil- if subsoil is used plants will often stop growing or show complex deficiencies. Other issues include weed seed load, poor aesthetics as the soil colors pot, leaves and surroundings (check out most Bombay balconies), environmental degradation and weight.
  2. Sand: good quality sharp river sand, well graded, is an excellent and cheap addition. It does increase the bulk density of the media, making it heavy and difficult to handle. Unfortunately it is extremely difficult to get this type of sand here and so we use small amounts of quartz sand for rare succulents only.
  3. Stone chips: basically a substitute for sand. We have a basalt stone locally which gave a lot of problems initially. We are now able to identify suitable stone and grade it so that there is no toxicity. We may even be getting some slow release nutrients like magnesium and potassium through this but have no proof of it. (Crushed stone powder is a recommended organic fertilizer in fields).
  4. Vermiculite: generally available as an industrial grade material, we have worked with a manufacturer for several years to get material suitable for horticultural use- it is available by volume. Vermiculite is mildly alkaline and has good levels of potassium and magnesium.
  5. Perlite: Several reasons why we don't like this material: the grade available is industrial, being imported it is too expensive; it can lead to fluoride toxicity for sensitive plants and is one cause of leaf chlorosis. In the mix it works just like sand and can be replaced by any number of cheaper and better substitutes.
  6. Thermocole or Styrofoam beads: these act like light weight sand, have no nutrients and do not affect the medium in any other way. Beads bought from industrial manufacturers are too expensive and being round, tend to move out from the media during mixing. After years of trials we have managed to develop a low cost horticultural grade with somewhat angular sides that mix better with the rest of the mix. It is still difficult to handle because it tends to fly away- we may sell it mixed with some coir to make it easier to use. It is ideal for hanging basket mixes and for media containing some sand or stone chips to offset their weight.
  7. Refractory Grog: these are graded small broken pieces of used fire brick. They give exceptionally good root growth, and are especially valuable for cacti & succulents as well as other plants needing good drainage. The growth of most plants with grog added at 10-15% by volume definitely exceeds the improvement in drainage it provides. I can only conclude that it is providing phosphorous and micronutrients in a slow release form ( this is not as wild as it sound- one would expect such fusible elements to get deposited on the furnace wall; also the alumina in the grog would bind these elements and provide slow release). The problem is availability and cost- large quantities are never available as the process of breaking and grading has to be done by hand. For the same reason the price is also high but it is worth using for speciality or rare plants.

USING COMPONENTS TO FORMULATE POTTING MEDIA

A wide range of materials can be used to make potting media. Ideally the number of components should as small as possible: I have seen European growers using just pure peat to get excellent results. However, this is only possible because of the availability of very young, chunky fibrous peat and the controlled conditions under which plants are grown.

We have to mix many items to get the right combination of WHC, aeration, pH and salinity; the components are decided upon depending on:

  1. Availability & Cost: unless one is growing speciality or rare plants, economic considerations are foremost for all growers- we must develop the best media for the least cost. Can soil less media compete with red soil/manure mixtures economically? First one must know the cost of this traditional mix. The calculations are as follows:
  2. We buy both materials by the truckload, paying more or less depending on the truck size. Almost always you will get less material than is promised. For a basis of calculation, let us take the cost of a two brass truck of either material as Rs.2000/- .

    One brass is 100 cubic feet of material (the length of the truck multiplied by the breadth and that figure multiplied by the depth of the material, all in feet). Thus the material is costs Rs. 10 per cubic foot by volume.

    It is better to move to a metric system for all calculations. One cubic foot is 27 liters of material. Thus each liter costs approximately 37 paisa. The actual cost is substantially more because you never get what you pay for and also manure rapidly shrinks in volume once left in the open.

    Once you have the materials at the nursery, there are further losses: since the material is not packaged, some is just wasted and lost. For pots you need to sieve at least the manure- this results in 10-15% unusable material. Finally, when mixing the two material, you get some shrinkage- one liter of each when mixed will always give less than 2 liters of the mix because some soil fills the large pores between the manure particles.

    Taking all the above into consideration, I reckon the minimum cost per liter of mix (if a 2 brass truck costs Rs.2000/-) at 75 paisa. We pay Rs.5000/- for a 3 brass truck so our cost would be more.

    Cost of filling a pot: check the volume of any pot size in our catalogue to get this figure. As a rule of thumb, a 5" pot is 1 liter.

    Costs of alternate media: we are trying to sell all media components by volume: this means you know exactly how much you are paying to fill a pot of any size. When you buy by weight, it is simple for the seller to add water to the component so that you are paying for water!! This is usually the case when buying coir by weight in loose bags.

    Compare cost with all the advantages of soil less media: faster growth, light weight, clean and non staining etc before deciding. Also keep in mind that changing over to any new media will require considerable changes in all the systems at the nursery and that there will certainly be problems initially. Unless you are committed, you will quickly abandon the effort and revert back to the traditional mixes. However, once you succeed in implementing a soil less media system in your nursery, you will never want to go back to soil.

  3. Environment and Cultivation regime: this will determine your basic mix. For example, if you are growing under poly, you may use a high WHC mix by adding coir because plants won't be exposed to rain and there is more controlled watering. Plants exposed to rain, especially those potted just before or during the monsoon will require excellent drainage. Plants under shade cloth will require a mulching layer of stone chips or peanut shells to prevent dripping from breaking from up the soil particles. The cultivation regime will also determine the media formulation: how and how much do you water, how often do you use liquid fertilizer (weekly, daily, alternate watering...) etc.
  4. Plants cultivated: surprisingly, it is possible to formulate one basic mix that is used for the majority of the plants you grow. This mix should take into account the above considerations as well as your environment and growing systems. After trials show that a mix works well, it may be modified to better suit specific plant groups. For example, you may add more non absorbent materials like stone chips & thermocole for succulents or plants liable to rotting. More manure for plants needing a rich media (such as Poinsettia), more thermocole for hanging baskets. Reduce manure and increase groundnut shells for plants you want to remain compact (the shells lock up some nitrogen) and so on.

By first understanding the principles governing soiless media, then identifying suitable components and them mixing these in suitable ratios it is possible for any grower to get a low cost media that will grow most plants very well and give a light weight, aesthetic and clean pot plant.

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