Design and Maintenance of Green House 3(2+1)

Lesson 22 Watering, Fertilization, Root substrate and Pasteurization

22.2 WATERING

1. Judge watering requirements by substrate look, feel, and weight
2. Plant symptoms of underwatering:

• Wilting

• Slowed  growth

• Smaller leaves

• Possible leaf burn

3. Plant symptoms of overwatering:

• Excess growth

• Soft growth

• Possible root damage

• Wilts  easily  under strong light

22.2.1 WATERING SYSTEMS

Water is primarily supplied to the system through water mains installed underground or overhead. Two-inch PVC pipes are commonly used in 20,000 sq. ft. greenhouses, and 3-inch pipes are used in 50,000 sq. ft. greenhouses.  Double water mains may need to be installed for fertilizer application. Water efficiency can be improved with use of pulse watering system, where plants receive maximum water without runoff (i.e., boom watering).

22.2.1.1 Hand watering

• Not  economical due  to labour costs

• Beneficial for spot watering

• Water supplied through hand-held field  hose

• Water breaker should be installed on end of hose

22.2.1.2 Perimeter Watering

• Plastic polyethylene or PVC pipe run along bench edges

• Water is sprayed under foliage through nozzles that are staggered along the pipe

• Nozzles can spray 180°, 90°, or 45°

• Water is projected farther into bed by 90° and 45° nozzles

• Nozzles are attached by holes punched

Fig 22.2.1 Perimeter Watering

22.2.1.3 Twin-Wall Watering

• Good for long or sloping benches

• Constant water pressure along tube

• Tube  consists of two  sections:

-     Outer chamber

-     Inner chamber

• Water first enters the tube in the outer chamber through a special pipe  fitting connected to the  water supply

• Water moves down the length of the tube until it reaches the end, where it begins to enter small pores along the tube leading to the  inner chamber

Fig 22.2.2 Twin Wall Watering

22.2.1.4 Tube Watering

• Polyethylene micro-tubes run from water supply to each individual pot.

• Emitters are attached to the end of the tube.

• Water is supplied by ¾-inch polyethylene or PVC pipes run along the centre of the bench

• Tubes are attached to the pipe through drilled holes.

• Consistent tube length is required.

• Benches should be level to insure even watering.

• Method can also be used for hanging plants.

Fig 22.2.3 Tube Watering

22.2.1.5 Overhead Watering

• Water is applied through 360° nozzles attached to top of riser pipes

• Nozzles may  be designed to rotate 360°

• Riser pipes are periodically attached to a pipe run along the centre of the  bed

• Riser pipes reach well above plant tops

22.2.1.6 Boom Watering

• Boom runs along rails attached down centre of greenhouse

• Boom is propelled by an electric motor

• Can be programmed to water one side only or to skip sections of the greenhouse

• Good example of pulse watering

22.2.1.7 Mat Watering

• Good for several pot  sizes

• Polyethylene sheets are placed on benches

• A 3/16 to ½ inch thick moist mat is placed on  top  of the  sheets

• Pots are placed on the mat, then take up water through holes on the bottom through capillary action

• Very important that pots have bottom holes

• Once pot is lifted from mat, capillary action is broken and it becomes necessary to re-water pot from top to re-establish capillary action

• Benches should be level to insure even watering

• To prevent algae, perforated polyethylene may be placed on top of mat for pots  to sit on

• Watering tubes placed 2 feet apart run down the length of the bench to supply water to the  mat

22.2.1.8 Ebb-and-Flood Watering System

• Pots are placed in a level, watertight bench

• The bench has channels in the bottom and a hole in the centre for the water to enter and exit

• A filter and a tee valve are installed in the hole

• Water is pumped into bench to a level of ¾ to 1 inch over 10 minutes

• Pots are allowed to sit in water for 10-15 minutes

• Water is drained out over 10 minutes

• Easy to change pot sizes

• High humidity may cause problems

22.2.1.9 Flood Floor Watering System

• Greenhouse floor is paved with a slight slope toward the centre on either side or a lip that runs along the perimeter

• A drain hole is installed in the centre

• Hot-water heating pipes are installed to speed up the time needed to dry the floor to lower relative humidity

• Flood greenhouse with water

• Time required to flood greenhouse will vary

22.2.1.10 Trough Watering

• Troughs containing one row of plants are placed parallel down the greenhouse with spaces in between

-      Reduces humidity

-      Promotes dryer foliage

• Troughs are slightly sloped for the water to drain into a gutter where it is returned to a holding tank

Fig 22.2.4 Trough Watering

22.3 FERTILIZATION

In protected cultivation, the  cost  of fertilization is small in  relation to the  vegetable production costs, so fertilization has  been   usually high, as  growers have   no  incentives to  save  fertilizers and pretend, mistakenly in many cases, that  the crop did not suffer any kind of nutrient deficiency. Nevertheless, nowadays, the trend to minimize the environmental impact has resulted in the   adoption of the   so called ‘Good agricultural practices (GAP) code’.

22.3.1 The nutrients cycle (soil cultivation)

In horticulture, and  especially in greenhouses, there is  more   leaching of  nitrates than in  other agricultural systems, due  to the   high   supplies,  the   high   contents   of organic matter in  the  soil  and  the  surplus irrigation in  relation to  the   ETc  (Dasberg,1999b). Its environmental impact can be notable, mainly, in the surface and under­ground aquifers.

The applications of nitrogen fertilizers in   the   greenhouses normally exceed the crop’s requirements, increasing the risks of nitrate leaching to the aquifers (Thompson et al., 2002).

Phosphorus does not usually cause pollution problems, except in exceptional cases of soils with low phosphorus fixation capacity   or   when large   quantities of   animal manure are applied over many years.

The   potassium leachate is, normally, limited and does not cause important problems   of   environmental impact, as   it   is retained in the soil in high proportions.

Other macronutrients, such as calcium and magnesium, do not cause environmental problems, as they   are natural components of the soil, which retains them in large quantities.

22.3.2 Nutrient Extraction

It is necessary to know the fertility characteristics and nutrient levels in the soil, making the pertinent soil analysis, to schedule fertilization. Normally, if the  nutrient level  is good, fertilization in practice is based on supplying  the  crop’s uptake, corrected for the  use efficiency, which  allows  for  maintaining, after  the  crop  cycle, a proper fertility and nutrient level. If the levels of any nutrient are high, or if the irrigation water contains it in sufficient amount, the inputs must be consequently corrected.

As a guide, Table 22.3.1 summarizes the approximate nutrient uptakes of some horticultural crops.

It is important to know the   nutrient absorption dynamics, to adapt the inputs to the extraction rates, which vary through the cycle and are influenced by the climate conditions, especially by soil temperature and radiation.

When  the   availability  in   the   soil   of some   nutrients is  high, over consumption may occur (in  potassium) or  it  may  negatively affect   the   quality  (nitrogen) of  the fruits, in  extreme cases   being   possible to induce salinity, or even  phytotoxicity. The availability of nutrients must be balanced and adapted to the plant requirements, to avoid antagonisms and possible restrictions to the nutrient absorption, which allow for optimum fertilization. Therefore, it is fre­quent to maintain predetermined relations between all or some of the nutrients

Table 22.3.1 Approximate nutrient uptake of some horticultural crops (compiled from very diverse sources).

Crop	Yield (t/ha)	Plant Uptake (Kg/ha)
		N	P2O5	K2O	CaO	MgO
Tomato	80	250	80	500	300	70
Pepper	40	180	60	180	160	50
Aubergine	50	250	40	300	150	25
Melon	60	230	80	400	300	70
Cucumber	200	320	160	600	250	100
Squash	40	70	70	390	-	-
Lettuce	40	100	50	250	50	12
Green bean	45	150	15	60	30	6

22.3.3 Tolerance to salinity

The tolerance to salinity of the crops may be assessed in several ways.  The  most  extensively  used  method  (Ayers   and   Westcot,1976)  quantifies the  tolerance (Table  22.3.2) by the percentage of the maximum yield that would be obtained for a certain level  of electric  conductivity of the  saturated extract of the  soil  (ECe) or  the  irrigation water (ECw) used. For greenhouse crops the tolerance to salinity can be quantified (Sonneveld, 1988) by means of the irrigation water salinity threshold below which there is no problem, and the percentage of yield decrease experienced by   the   crop   per   unit increase of salinity in the irrigation water, above   the threshold value. This method is more useful for substrate crops. Table 22.3.3 summarizes the data in this respect. The specific growing conditions (cultivar, evaporative demand, management, and microclimate) may    affect these threshold values (Cohen, 2003). In Mediterranean greenhouses, Magan (2003) estimated the salinity threshold value of the nutrient solution to decrease the fresh weight tomato harvest between 4 and 5 dS m-1.

The importance of the water quality to minimize the leaching fraction is enormous, influencing its environmental impact potential.  Poor quality water will require considerable leaching and, as a consequence, will generate more   negative impact than good quality water.

Table 22.3.2 Tolerance level of some crops to salts (dS m−1), expressed as the expected yield (in percentage of the maximum yield).

(Source: Ayers and Westcot, 1976.)

Percentage of maximum yield
Crop	100%		90%		80%		50%
	ECw	ECe	ECw	ECe	ECw	ECe	ECw	ECe	Max ECea
Climbing bean	0.7	1.0	1.0	1.5	1.5	2.3	2.4	3.6	6.5
Broccoli	1.9	2.8	2.6	3.9	3.7	5.5	5.5	8.2	13.5
Melon	1.5	2.2	2.4	3.6	3.8	5.7	6.1	9.1	16.0
cucumber	1.7	2.5	2.2	3.3	2.9	4.4	4.2	6.3	10.0
Potato	1.1	1.7	1.7	2.5	2.5	3.8	3.9	5.9	10.0
Lettuce	0.9	1.3	1.4	2.1	2.1	3.2	3.4	5.2	9.0
Onion	0.8	1.2	1.2	1.8	1.8	3.2	2.9	4.3	8.0
Pepper	1.0	1.5	1.5	2.2	2.2	3.3	3.4	5.1	8.5
Spinach	1.3	2.0	2.2	3.3	3.5	4.9	5.7	8.6	15.0
Strawberry	0.7	1.0	0.9	1.3	1.2	1.8	1.7	2.3	4.0
Tomato	1.7	2.5	2.3	3.5	3.4	5.0	5.0	7.6	12.5

Table 22.3.3 Tolerance of some vegetables to salinity in greenhouses

	Threshold value ECw (ds m-1)	Yield decrease by salinity (%)
Tomato	1.8	9
Pepper	0.5	17
Cucumber	1.5	15
Green Bean	0.5	20
Lettuce	0.6	5

 

22.3.4 Fertigation

This practice of joint application of irrigation and fertilization is known as fertigation. The control centre of a localized irrigation facility must have the necessary equipment to fertigate. This involves the use of soluble or liquid fertilizers, allowing for adjustable dosing and   fractioning of the inputs which optimizes their use.

22.3.4.1 Criteria of fertigation

Traditionally, the fertigation criterion of supplying the nutrients as a function of the expected uptake by the plants prevailed.

Nowadays, the   criterion of providing nutrients based on an ionically balanced physiological solution, used in soilless crops, is extending to conventional soil cultivation, when a suitable automated irrigation head is available.

In soilless cultivation the correction of the nutrient solution is performed based on its analysis. In  soil  cultivation, the  classic method  of   analysing  the   saturated  soil extract is being  replaced by the  use  of suction  probes, with which a sample of the soil solution is extracted for analysis. However, information on the ideal nutrient levels to use with this method is still scarce.

22.3.4.2 A practical example: a soil-grown tomato crop

Depending on each   case’s specific conditions (soil fertility, climate and irrigation type), there is notable variation in tomato fertilization (Castilla, 1995). Preliminary analysis of the soil is necessary. In general, fertilizers are applied depending on the crop’s estimated nutrient uptake. Although the variability in nutrient uptake is enormous, values that refer to harvest unit are in general lower.

• Between  2.1   and   3.8   kg  of  N  t-1 of harvest;

• Between  0.3   and   0.7   kg  of  P  t-1 of harvest;

• Between  4.4  and 7.0 kg  of  K t-1 of harvest;

• Between 1.2 and  3.2 kg of Ca t-1   of harvest; and

• Between 0.3 and 1.1 kg of Mg t-1of harvest.

The differences in nutrient uptakes are influenced by the type of pruning and, especially, by the timing of the removal of the axillary shoot. It is advisable to prune shoots as soon as possible to minimize the wasteful uptake of nutrients by the   crop.

The   scheduling of fertilizer application   must rely   on   the   type   of fertilizer used, on the irrigation technique and on the soil conditions, among other factors. In sandy soils,  with low  water storage capacity,   supplies  must  be  frequent with  the irrigation (conventional), whereas in heavy soils  it  is  only  necessary to  apply part  of the  nitrogen as a top  dressing (Geisenberg and  Stewart, 1986).

With surface irrigation, the most common practice is to apply the phosphorus with the pre-­planting fertilization, for example when applying manure (around 30 t ha-1), and at a time when half of the potassium is applied. The rest of the potassium and  nitrogen are applied in alternate weeks  after   transplanting until  1  month before  the  end  of  the  cycle  (Nisen et  al., 1988).  With   drip irrigation, all fertilizers can be applied by fertigation, although it is common that at least part of the phosphorus is applied with the manure.

In drip irrigation, it is essential to know the absorption rhythm of the mineral elements in order to schedule fertilization (Zuang, 1982).  In Mediterranean unheated greenhouse crops for autumn-spring cycles, fertilization rates higher than 0.3 g N m-2 day-1 do not seem advisable (Castilla, 1985). The   fertilizer’s content of   the   irrigation water is, in some cases, notable and must be taken into account for the fertilization schedule.

Nitrogen excesses negatively affect  fruit quality, and  maintaining an N:K ratio  at 1:2 (or  even   1:3)  during the  fruit   enlargement stage,   with  drip  irrigation,  favours  their quality (Geisenberg and  Stewart, 1986). Equally, the balance between other nutrients, especially between calcium (when its supply is required) and potassium, and magnesium is necessary, as well as between the different forms of nitrogen (nitric/ammoniacal).

In drip irrigation, the amount of salts in the water must be limited, if possible, to 2 g l-1 (which is not feasible, in some cases, when saline water is used), to decrease possible dripper blockage problems.  When using good  quality water, it is a  usual practice to  add   sodium chloride (common salt)  to the  water, up  to the  indicated limit, to improve tomato quality, because     the soluble  solids content increases with salinity which contributes to the  improvement of its internal quality, although the  fruit  size  is reduced.

A good irrigation efficiency is, logically, required for efficient fertigation and also contributes to a significant reduction in the environmental impact of fertilizer (nitrogen, especially) residues.

Foliar fertilization, in tomato crops, is usually limited to microelements, when deficiencies are   forecasted or   observed. Leaf  analysis (of  the  limb, petiole or  the whole leaf)  is  a  good  auxiliary index on which to  base  the  scheduling of  fertilization, being  more  common than sap  analysis, as the latter displays a wider variability and  requires more  thorough sampling (Chapman, 1973;  Van  Eysinga and  Snilde, 1981; Morard, 1984).

In greenhouse crops, low soil temperatures (15°C) in winter may limit absorption of nutrients, especially phosphorus (Wittwer, 1969) and   nitrates (Cornillon, 1977). On the other hand, high   temperatures favour nutrient absorption, although the nutrient uptake per harvest unit is not affected, as previously   thought   (Nisen et al., 1988).

22.3.4.3 Fertigation of soilless crops

22.3.4.3.1 The nutrient cycle in soilless crops

Soilless crops, with free drainage, have similar problems to soil cultivation regarding the nutrient cycles. The management conditions  (leaching percentage, characteristics of the  nutrient solution) of the  soilless crop (open system) will  determine its  environmental impact, which will  be similar to that of crops grown in  soil  if the  leachates are similar. If leachates are recirculated (closed system) the salinity and   pathology problems must be considered and the nutrient con­ centrations must be well   monitored and controlled.

22.3.4.3.2 Preparation of the nutrient solution

In an ideal soilless growing system there are no  mineral inputs from  the  substrate, and therefore,  all  nutrients  must be  supplied together with  the   water,  in   the   nutrient solution. The  preparation of  the  nutrient solution  requires prior analysis of the irrigation water, to  allow for  the  formulation of the best   nutrient  solution  depending  On  the crop  to  be  grown. The  preparation of this nutrient solution will  also  depend on  the technical characteristics of the  available fertigation hardware (and,           possibly, software).In the  simplest case,  one  concentrated solution tank  is available and  another tank for the  acid. Most facilities have two tanks for solutions (A and B), one of which already contains the acid. In  this  case,  tank  A has most  of the  acid  to correct the  pH (usually, nitric or  phosphoric, and   rarely sulphuric), the  phosphates and  the  sulphates, as well  as the  microelements, except for  iron.  In this tank A, part of the potassium nitrate can be incorporated, but no calcium salts must be added, to avoid precipitates. Tank  B contains the  calcium nitrate and  the  potassium nitrate (all  or only  a part),  as well  as some nitric acid  to regulate the  pH  and  the  iron chelates. The magnesium nitrate is usually added in tank   B, but neither sulphates nor phosphates must ever be added, to avoid precipitates.

When three tanks are  available, one  of them is  destined only  for  the  acid  that  is usually nitrous  acid, although sulphuric or phosphoric acids can  be used. In sophisticated facilities, managed by means of a computer, several tanks are usually available, containing solutions of individual fertilizers.

The injection systems of concentrated solutions in the irrigation water flux are of such complexity or simplicity in agreement with the type of tanks used.

In mixing the fertilizers their solubility and    compatibility must   be   taken into account, not forgetting that it depends on temperature (Tables 22.3.4 and 22.3.5). The literature on the preparation of simple solutions is extensive (e.g. Cadahía, 1998).

Table 22.3.4 Solid fertilizers most commonly used in fertigation: analysis and solubility at 20°C.

Fertilizer	Analysis of N-P2O5-K2O-othersa (%)	Solubility (gl−1)
Calcium nitrate 4H2O Ammonium nitrate	15.5-0-0-26.6 (CaO) 33.5-0-0	1200 1700b
Ammonium sulfate	21-0-0-22 (S)	500
Urea	46-0-0	500
Potassium nitrate	13-0-46	100–150
Potassium sulfate	0-0-50-18 (S)	110
Mono potassium phosphate	0-52-33	200
Mono ammonium phosphate	12-60-0	200
Magnesium sulfate 7H2O	16 (MgO)-13 (S)	700
Urea phosphate	17-44-0	150
Magnesium nitrate 6H2O	11-0-0-9.5 (Mg)	500

 

The first three values in each entry refer to N-P2O5-K2O. Where there is a fourth entry this refers to other compounds. The exception is the entry for magnesium sulphate which has no N-P2O5-K2O and contains16 (MgO)-13 (S) as indicated. b Steep water temperature decrease for concentrations above 250 g l−1.

Table 22.3.5 Chemical compatibility of the mixture of some common fertilizers in fertigation: I, incompatible; C, compatible. (Source: Cadahía, 1998.)

	NO3NH4	Urea	(NH4)2SO4	(NH4)2HPO4	(NH4)H2PO4	KCL	K2SO4	KNO3	(CaNo3)2
NO3NH4	-
Urea	C	-
(NH4)2SO4	C	C	-
(NH4)2HPO4	C	C	C	-
(NH4)H2PO4	C	C	C	C	-
KCL	C	C	C	C	C	-
K2SO4	C	C	C	C	C	C	-
KNO3	C	C	C	C	C	C	C	-
(CaNo3)2	C	C	C	C	C	C	C	C	-

 

22.3.4.3.3Parameters of fertigation with soilless crops

The proper management of fertigation requires the periodic analysis of the nutrient solution to assess its goodness of fit to the requirements of the crop and to perform necessary adjustments to its composition. In addition, it is necessary to frequently monitor (automated or manual) the pH and EC (electrical conductivity) of the nutrient solution and the leachate, to prevent any anomaly.

In practice, when a computer is available,  a certain threshold of EC of the  nutrient solution is fixed,  for instance 2.5 dS m-1, modulated as  a function of the  solar  radiation, decreasing it  by  0.1  dS  m-1  for each 30  W m-2   of  solar   radiation that   exceeds 400 Wm-2 (Urban, 1997b).  Obviously, these rules must adapt to the specific conditions of each operation.

Normally, the pH is not regulated but a certain value is fixed.  The EC and pH sensors must be duplicated, at least, to prevent an eventual failure. In addition, the system must be fitted with alarms. Other complementary analyses are carried out on the substrate solution (extracted with a syringe) and the drainage, to correct the nutrient solution.

Analysis of vegetable tissue and   sap provide information on the nutrients that are really absorbed by the plants (Cadahia, 1998).  The analysis of the conducting tissues   is usually preferable to that   of the leaves, whose composition varies slowly. In these analyses, the time variations are more relevant than the absolute values (Morard et al., 1991).

22.3.4.3.4 Pathogens in the drainage waters

The recirculation of the drainage water requires the use of good quality water and its disinfection, to suppress pathogens (bacteria, fungi and virus) in the recirculation water.

The use of ozone, UV sterilization, thermal treatment and ultrafiltration are effective to a varying extent, but the last technique has the drawback that only 70–80% of the drainage water is recovered (Dasberg, 1999b). The use of bleach in recirculation water dis­ infection gives good results. Treatment with UV radiation is effective, but it is necessary to pre-filter the water so the radiation penetrates well (Dasberg, 1999b).

22.3.4.3.5 Automation

The use of computers, with several degrees of automation to manage the fertigation, is growing among greenhouse   growers.  It must be expected, in the  future, that  these systems will become integrated with climate    control  systems,  in   those  greenhouses in  which the  technological level allows for  it,  for  combined optimization.

22.4 ROOT SUBSTRATE

22.4.1 Functions of substrates

• Serves as a reservoir for plant nutrients

• Serves as a reservoir for water available for plants

• Must provide gas exchange between roots and the atmosphere outside the root substrate

• Provides anchorage or support for the plant

22.4.2 Limitations of materials

• Sand : excellent support and excellent gas exchange but poor water and nutrient-holding capacity

• Clay: high nutrient- and water-holding capacity plus excellent support but poor gas exchange

• Water : water and nutrients; can even supply gas exchange, but offers no support; if plants are given support

• Field soils (when placed in a pot) : excellent support, nutrient-holding capacity, and water-holding capacity, but poor gas exchange

22.4.3 Desirable properties of a substrate

• Stability of organic matter

  • decomposition of organic components = smaller particle size = finer texture= smaller pores = reduced gas exchange and reduced aeration also means a loss of substrate volume

  • straw and saw dust (excluding some like redwood) are examples of materials with poor stability

• Carbon-to-nitrogen ratio

  • Organic materials are broken down by microbes

  • Microbes require N for decomposition

  • If C : N ratio > 30 C : 1 N, & substrate contains

  • If organic materials decomposes rapidly, the microbes will utilize N

  • The C : N of sawdust is about 1000 : 1

  • Pine bark has a C : N of about 300 : 1, yet is still suitable to use…

• Dry bulk density

  • oven-dry weight of substrate particles ÷ volume given in lb/ft³

  • useful for predicting materials handling

  • if too low, as the substrate dries out, top-heavy pots topple over

• Wet bulk density

  • weight of substrate at container capacity ÷ volume reported in lb/ft³

    • container capacity is moisture content of substrate just after complete saturation and loss of gravitational water

    • ([volume needed to saturate - drainage volume] ÷ total volume of container) × 100

    • usually reported as % of total volume

• Moisture retention and aeration

  • goal is a substrate with adequate available water +sufficient aeration + acceptable wet and dry bulk densities

  • substrate at container capacity composed of :  solid particles

  • pores filled with : unavailable water & available water (micropores),  air (macropores)

• Unavailable water (hygroscopic water)

  • held by solid particles so “tightly” that it is unavailable to roots common.

  • roots would have to create a suction > 15 bars to separate water from the particles

• Available water

  • volume of water at container capacity – volume of water remaining at 15 bars pressure

• Cation exchange capacity

  • many substrate components have fixed negative electrical charges

  • will attract and hold positive-charged cations

  • CEC = milliequivalents per 100 cc of dry substrate

  • 6 to 15 me/100 cc is desirable

  • clay, peat moss, and coir have higher CEC’s

• pH

  • most greenhouse crops = 6.2 to 6.8 in soil-based substrates (20% or more soil) and 5.4 to 6.0 in soilless substrates

22.5 ROOT MEDIA PASTEURIZATION

Sterilization is the killing of all living organisms on or in a material. Pasteurization is the killing of most living organisms on or in a substance. In the past, the general dogma is that pasteurization kills the harmful organisms (i.e. disease-causing fungi) in a substrate but allows most of the beneficial organisms to live. However, pasteurization does kill many of the beneficial organisms (i.e. Trichoderma, Gliocladium, nitrifying bacteria) in the substrate (or substantially population) and may actually increase the incidence of soil-borne diseases. Unless field soil is added to a substrate, or there is some reason to believe the substrate may be contaminated with weed seed, nematodes or a high level of disease-causing organisms (i.e. with repeated monoculture in ground beds), substrates should not generally be pasteurized.

22.5.1 Methods of Substrate Pasteurization

22.5.1.1 Steam:

Steaming is the best methods for pasteurization of substrates. The basic objective is to apply the steam until the coldest area of the substrate reaches 160° F (71° C) for 30 minutes. Of course, other areas of the substrate will be at a higher temperature. The substrate should be moist and loose prior to steaming and a system that allows the steam to move into the substrate should be used.

Lime, superphosphate, inorganic fertilizers and microelements can be incorporated prior to steam pasteurization. However, slow-release fertilizers (unless otherwise indicated), manures and urea-based fertilizers should not be incorporated prior to pasteurization.

Several problems can occur as a result of steam pasteurization of substrates. Manganese toxicity can occur if a field soil containing large amounts of manganese is included in the substrate. Toxic levels of ammonium can occur when substrates with high volumes of organic matter, manures or ammoniacal nitrogen are pasteurized. This often occurs because the microorganisms involved in converting organic nitrogen to nitrate are killed by pasteurization. The ammonifying bacteria recover first followed 3- 6 weeks later by the nitrifying bacteria. During this time ammonium can build up to toxic levels. If slow release fertilizers are added prior to steam pasteurization, the coating may be damaged and fertilizer salts rapidly released. This can result in a high E.C. levels and subsequent damage to the plant root system.

22.5.1.2 Chemicals:

• Methyl Bromide is scheduled to be removed from the market. It is highly toxic and is most often used for field situations. When being treated with methyl bromide, the soil should be loose and moist and must be covered tightly prior to the injection of the gas. Since methyl bromide is odourless, chloropicrin is usually added as a safety precaution. Methyl bromide effectively kills soil-borne fungal and bacterial pathogens, nematodes and weed seed.

• Chloropicrin is also known as tear gas. It is used in a manner similar to methyl bromide.

• Vapam is similar to chloropicrin. It is often used in the nursery industry to pasteurize soil beds. It is most effective against weed seed.

Growers should always exercise caution when using chemicals and follow the label for the specific chemical being used. After chemical sterilization, the medium should be allowed to sit for up to 14 days before planting to allow any phytotoxic chemical residue to dissipate.

22.5.2 Important facts of Root media Pasteurization

1. Greenhouse root media should be pasteurized at least once per year and more often as required to rid them to harmful disease organisms, nematodes, insects and weed seed.

2. Numerous microorganisms develop in root media which are not harmful. These can be beneficial by providing competition for harmful microorganisms, which might otherwise proliferate. For this reason root media are pasteurized and not sterilized i.e. only some organisms are killed.

3. Root medium may be pasteurized with steam by raising it to a temperature of 140° - 160°F for 30 minutes.

4. Volatile chemicals are also used for pasteurizing root media.

5. Both steam and chemical pasteurization require that the root medium be loose and of moisture content suitable for planting amendments such as peat moss, manure and bark should be incorporated prior to pasteurization to prevent introduction of diseases or pests.

6. Pasteurization can result in ammonium and manganese toxicities in certain situations. If the root medium contains organic matter rich in nitrogen, such as manure, steam and chemical pasteurization can result in an excessive release of ammonium, particularly in the period of two six weeks  after pasteurization. Either these materials should be avoided or an adjustment should be made in the watering practice to ensure adequate leaching of ammonium. Many soils contain large levels of manganese, most of which is unavailable. Steam pasteurization causes conversion unavailable manganese to an available form. A toxic level is sometimes reached. This is another reason for pasteurizing root media at low temperature and for only the necessary length of time.

7. Pasteurization of root media is designed to eliminate harmful organisms. It does not protect against future infestations. Good sanitation practices must be employed to maintain clean conditions. Some considerations include disease free seeds and plants, sterilization of containers and tools, a pesticide program, foot baths, a clean working area, sanitation outside the greenhouse, and proper control of temperature and humidity.

REFERENCES

1. Thomas M. Blessington et.al. “Methods of watering Greenhouse crops.” Fact sheet 867, Maryland Cooperative Extension, University of Maryland

2. Nicolas Castilla, 2013. “Greenhouse Technology and Management.” Ediciones Mundi-Prensa, Madrid (Spain) and Mexico.PP.179-201

3. www.faculty.caes.uga.edu

4. http://faculty.yc.edu