After reading this article you will learn about the root media and its pasteurization for raising green house crops.
Root Media:
Healthy root media is very important for the successful raising of green-house crops. It is a reservoir of nutrients, hold water, gases and provides anchorage or support to plants. In nature perfect root media is not available. One of root media may be good for aeration or in water holding capacity or nutrients.
Therefore to make complete media two or three media are mixed in certain proportion so that desirable media can be obtained. Apart these, four main functions of media, there are other characteristics which should be kept in mind while selecting or making media especially for pot culture.
These are as follows:
Stability of organic matter:
Pot plants raised in green house are held for two to four months (flowering and foliage plants) or one to two years, (shrubs, trees, climbers, etc.) and then they are transported for market. If organic matter of media decomposes at faster speed, there will be reduction in significant volume which is highly harmful to the plant especially small plants.
Moreover, decomposition of organic matter will lead to the formation of finer texture which will reduce aeration. Addition of straw or saw dust is not desirable because they decompose rapidly; however, where volume of medium is large enough, it permits shrinkage. To avoid such problem addition of sphagnum peat moss or coarse powdered bark is desirable.
Bulk density:
It provides support to the plant. Medium should be sufficiently heavy to prevent a potted plant from falling over due to weight of the plant. A mixture of sphagnum, peat-moss and perlite is sufficiently heavy. But after some time when larger plants have taken up water from such media they topple very easily when are handled during transportation. An acceptable range for bulk density of potting media is 640-1200 g/d m3 just after watering at container capacity.
Carbon: N ratio:
The amount of N in root medium in relation to organic matter decides the availability of N to crop for its growth and ultimately productivity. Normally when ratios is 30 C: 1 N, decomposition of organic matter will be ideal whenever this ratio exceeds, N already present in root medium will be utilized by micro-organisms rather than by plant. Crop will be deficient in N thus N should be added. The C: N ratio for straw is 1000: 1, whereas for tree bark is 300: 1.
Moisture retention and aeration:
The primary function of media to make balance between moisture and aeration. This is also necessary to ensure suitable long interval between watering and to provide adequate aeration. A heavy clay soil results in a high available water content at the expense of aeration. A larger portion of sand is required to improve aeration.
Addition of vermiculite, peat or sphagnum is also good. In pot culture depth of pot also govern the retention of moisture. In shallow or deeper pots capillary force holding water may be equal but in large pots, gravitational force pulling water out of pot becomes greater as the depth of pot is increased. Thus well formulated root media with high aeration and water retention values are suitable for wide range of pot sizes.
pH:
The pH level of root medium controls the availability of nutrients to the plants. Most of green-house crops are grown in slightly acid pH range of 6.2 to 6.8 in soil based media and at 5.4 to 5.6 in soil less media. Some crops are acid loving and they grown happily in a strongly acid pH range of 4.5-5.8.
Acidic media are moss and pine bark. Sand, rock wool and perlite are neutral. Vermiculite and hard wood bark are alkaline. Field soil can range from acidic (3.5). to alkaline (8.5). So depending upon the crop requirement mixture should be made.
Cation exchange capacity (CEC):
It is a measurement of the magnitude of fixed negative electrical charges and is measured as multi-equivalent per 100 cubic centimeters (me/100 cc) of dry root medium. A level of 6-15 me/100 cc is considered most desirable for green house medium.
High levels are desirable because it will act as good reservoir of nutrient. Thus in preparing root medium it is desirable to include a component with a high C.E.C. Root media components and fertilizers have negative electrical charged which will attract and hold positive electrical charges.
Components of root media:
There are many natural and artificial materials available by mixing them in pertinent proportions, a desired root media with desirable characteristics can be prepared. Selection of components is based on the required functions, cost and availability.
Field soil:
Most desirable soil for green house crops is having high degree of structure and loamy texture. After deciding the location for setting up green-house range, availability of good soil texture is an important factor. Good structure is further improved by growing mixed corps desirable leguminous crops for 2.3 years which improves the soil fertility and improve the structure as well.
Decomposition of root residues builds up humus. These soils when used for growing the green house crops result in better production. Normally flower producers do not change the soil every year so it is important to make a good soil in the beginning; on the contrary, pot growers need good soil regularly. However, mostly pot growers use artificial media for raising the flower or pots plants.
Sand:
It is mixed in soil to improve drainage and aeration of media. Mostly river sand or coarse sand is used. It should be washed before use. Sand is mostly free of clay, silt and organic matter. .
Sphagnum peat moss:
There are different types of peat. It is light tan to brown colour and least decomposes. It is formed from Sphagnum or hyphnum moss. It decomposes slowly and contain about 0.1-1.4% N. It is good water holding capacity (holding up to 60%) and acidic in nature (3.0-4.0 pH) and needs the application of lime to bring pH up. But when it is used with vermiculite no liming is needed. In India, now-a-days coco peat is being used on large scale.
Vermiculite:
It is mica like silica mineral and is very light in weight and thus is highly desirable to use for pot plants. It contains good water holding capacity. Mostly expanded form is used and it contains about 16 times water holding capacity.
Drainage and aeration qualities are also good because of large pores between particles. It is charged negatively and contains potassium, magnesium and calcium. The reaction of various vermiculite varies from slightly alkaline to alkaline but there is no problems in mixing with acidic components like moss or pine bark but generally not mixed with soil.
Perlite:
It is siliceous volcanic rock when crushed and heated to 980°C expands to form white particles with numerous closed air filled cells. Water will adhere to the surface of perlite but it is not absorbed into perlite aggregates.
Perlite is sterile, chemically inert and has a negligible CEC (0.15 m e/100 cc) and is nearly neutral with a pH value 7.5. Thus, it does not appreciably affect the pH level of root media. It is good substitute for sand for providing aeration in root media but may be little more costly than sand. It is better than sand in root media because of its light weight.
Synthetic root media:
(i) Rock wool:
Horticultural rock wool is formulated to a prescribed density to provide air and water holding requirements of plants. Insulation rock wool is not good for plant growth. It is produced by burning coke, basalt lime stone and possibly slag from iron ore. The mixture is heated at 1600°C temperature and it melts in furnace. A steam flows onto a high speed rotor. Droplets from rotor when it falls on heated mixture it lengthen into long fiber.
The fibers are sprayed with binding agent in an air stream which cools down and carry them into conveyer belt and are deposited. The cubes are made of this wool of different size with holes or without holes in different sizes. Keeping the duration of crop in view, the lengths can be 75-100 cm whereas the width may vary from 15, 20, and 30 to 45 cm while the height is 7.5 cm.
Polystyrene foam:
Commercially it is known as Styrofoam, Styropor and styromull. It is very good for improving aeration, and light weight to root media. Further it does not absorb water and has no appreciable CEC. Being neutral it does not affect pH. It is a white synthetic product containing numerous enclosed cells which are filled with air and available in beads or in flakes about 1-13 cm size. Larger particles are used in benches or for epiphytic orchids.
Pasteurization:
Prevalence of mild temperature conditions in green house and raising of a crop for longer duration or raising of few crops repeatedly accentuate the disease problem by providing a continuous host on which disease organism can build up. Thus, necessitates the pasteurization of root media.
It has become a standard practice virtually for all green houses and is done on a yearly basis. For shorter duration crop it can be frequent. Generally summer time is good for pasteurization as there is relatively low production. Apart disease (fungus) control, pasteurization also controls nematodes, insects and weeds. Pasteurization is done by two methods viz. steam pasteurization and chemical pasteurization.
Steam pasteurization:
In green house where heating during winter is done through steam, same facility can also be used for root media pasteurization. However, during this process useful bacteria also get killed and develop later on after few weeks. Now-a-days, equipment is available that will mix air with steam and adjust the temperature to a desired level below 100°C. Normally, temperature of 71°C is kept for pasteurization.
Before pasteurization rooting media should be loosened and should be sufficiently wet. This means that moisture content should be similar to the planting time. In wet and loosened soil steam passes rapidly than dry soil.
Too wet soil also acts as a bad conductor of heat. Steam is distributed in fresh flower ground beds through buried perforated pipes. For beds measuring 0.9 m wide one row is buried and whereas for 1.2 m wide beds two rows are buried.
Raised benches filled with a root medium may be pasteurized with or without buried steam conduction pipes. Pipes are buried at half the depth of soil. Some growers instead of buried pipes inject steam between cover and medium. Once the cover is inflated, steam readily penetrates the loosened root media. This system is easier to set up but require long time for steam to penetrate the root media.
While during pasteurization, the desired temperature should reach of cold spot which is at the end of bench. Temperature should be measured at the point. This cold spot can be corrected by applying an extra quantity of steam at that point.
After steaming, it should be covered so that it does not escape quickly and lost in air and thus, efficiency will be reduced. Cover helps in holding the steam in close contact with the root medium and pasteurization is done effectively. There are three types of covers namely polyethylene, vinyl and neoprene coated nylon fabric. Out of these nylon has the shortest life but it is cheapest.
After steaming problem:
There are two types of toxicity which appear after steaming i.e. ammonium and manganese toxicity. The organic matter of root media releases ammonium after steaming and may continue to release up to few weeks more.
Ammonifying bacteria convert nitrogen of organic matter to ammonia and nitrifying to nitrate nitrogen. During pasteurization the populations of both ammonifying and nitrifying bacteria are reduced to low level and develop again after 3-6 weeks. During this period of conversion ammonia to nitrate, ammonia may result toxic to certain crop e.g. roses and poinsettias.
In certain soils where parental manganese is more but is in unavailable form which becomes in available form after steaming and becomes toxic to crop. The longer the soil is steamed the greater is build-up of manganese, and hence, the greater risk of manganese toxicity. To avoid this soil should be pasteurized for only the length of time necessary i.e. 30 minutes. Toxicity results in tip burn of older leaves. Iron deficiency is also commonly caused by high levels of available manganese.
Chemical pasteurization:
Where there are no arrangements of steam pasteurization, chemicals offer a good alternative. However, these chemicals being hazardous, injurious to human health and environment but still followed. There are number of chemicals like Methyl bromide, Chloropicrin, Basamid, Formaldehyde, etc. Out of these, most commonly preferred are Methyl bromide and Chloropicrin.
Methyl bromide:
It is liquid under pressure and turn to gas when released. It is available in cans of 454-680 g or in cylinder for tractor mounting. At the time of application root media should be loose for rapid penetration of gas and should have moisture content as desired at the time of planting. For benches or rooting media it should be applied at the rate 6 kg/m3.
It is applied through an applicator. A layer of polythene is placed over bench or pile of media. Methyl bromide is injurious to carnation plants so it should not be used. It can also be applied on larger area with the help of tractor over which cylinder and polyethylene is mounted. Methyl bromide is released by plastic tubes at the bottom of chiesel (10-15 cm deep) and immediately covered by polythene. The leading edge is buried by soil and subsequent sheets are glued.
Chloropicrin:
It is also called as tear gas and is popular choice for carnation plantation. It is mostly applied at the rate of 32 cc/m2 of bench or field surface. Chloropicrin is injected into media by a hand applicator. It is covered by polythene. On large area, chloropicrin can be applied by tractor as in case of Methyl bromide is done and field is covered with polythene. Planting of the crop should be done when injurious gas is below injury levels which can be checked by sowing presoaked seed of lettuce on limited quantity of media.
Basamid:
It is available in micro granular form having active ingredient dazomet (DMTT) 98-100%. It is considered good soil fumigant for the control of nematodes, root diseases soil insects and weeds. It is suitable for fumigation of open land, media for pot plants and all kind of seed beds, nurseries and green houses. It is applied at the rate of 30-50 g/sq. m. After application irrigate the field or cover with polythene.
Under Indian condition where summer is very hot and temperature is going as high as 40-44°C, by leaving the field fellow during summer limited pasteurization can be achieved. Higher temperature can also be obtained by covering with black polythene and solarization will pasteurize the root media to certain degree.
Fertilization:
Prior to invention of chemical fertilizers, nutrients were made available to the crops from the organic matter present in the soil. Annual application of organic manures provided the additional nutrients. In green houses, flower production requires more frequent application of inorganic fertilizers.
Instead of dry forms their application, along with water or water containing soluble fertilizers enhances the productivity and thus found highly profitable. Further automation in fertilizers solution mixing in irrigation and automated water application reduces much expenses and increases productivity.
Now-a-days it is a standard practice to dissolve high analysis fertilizers carriers into concentrated solution. The concentrate is then proportionated by means of fertilizer injector into the water line of the green house at the final concentration desisted for crop application. Fertilizers most commonly are applied on a seven day basis or with each watering which is called fertigation. This fertigation system is getting popular in the green houses.
Pre-plant fertilization:
Soil analysis becomes the basis to decide about type and quantity of each nutrient to be applied keeping the pH and amount of nutrients present in the root medium. Normally, N, P K, S, Ca, Mg and micro nutrient in desired quantity are applied. Essential micro nutrients like iron, manganese, zinc, copper, boron, molybdenum, etc. can be applied in a single application.
Several commercial pre-plant mixtures are available for incorporation in root medium. Liquid application of micro nutrients after planting provides an alternative to dry pre-plant application. Most of complete N-P-K green-house fertilizers contain a full complement of micro nutrients.
Fertilizer concentrations are expressed in terms of either ppm or g/liter. It is easy to convert from one expression to another.
The first method is to determine the amount of fertilizer carrier required by following equation:
OZ of fertilizer carrier per 100 gal
= Desired ppm/75/Decimal fraction of desired nutrient in fertilizer
Let us assume we need 200 pmm N from a 20-10-20 fertilizer grade thus with above equation it will 200 ÷ 75/0.20
Further similarly it can be worked out the amount
= 13.33 Oz/gal of water which can be converted in g/L equation.
ppm = Oz of fertilizer carrier per 100 gal × 75 × decimal fraction of desired nutrient in fertilizer carrier
Concentration = 13.33 × 75 × 20=199.95 ppm.
Continual fertilization:
All essential elements are retained by root media except N&K which are not retained long in root media and need their application regularly. These are most commonly applied as a solution continually through crop production. Thus, it should be applied weekly or with each irrigation. It has been found that for most of crops, concentration of 200 ppm works well with each irrigation whereas 240 ppm N&K to 720 ppm observed better for sensitive crops such as bedding plants and elatior begonia, respectively.
Specific recommendation:
Required concentration of fertilizer varies widely with the crop. However most commonly used fertilizer in the green house is 20: 20: 20. Due to ammonical injures in above a 20: 10: 20 fertilizer is more commonly used. This is also commonly available. Other combinations are also available in the market. Other formulations like 15-15-15, or 15-0-15, 25-0-25 are also available.
It has been observed that most crops grow well on equally balance N and K but there are few exceptions e.g. Elatior Begonia or Petunia grow faster and develop more side shoots when they are fertilized with a ratio of 2N: 1K. The requirement of azalea is 3: 1 whereas for carnation it is 2: 3.
Other forms of fertilizer application are through slow release fertilizers in the form of plastic encapsulated fertilizers, slow soluble fertilizers, urea formaldehyde, sulfur coated fertilizers, chelated micro nutrients, etc.