The Planter Bag Run to Waste Growing System
Tomato and other vegetable crops can readily be grown in black polythene planter bags filled with about 10 litres of fine grade pumice,sawdust, bark or coir. Root systems in the these media are subject to diseases which normally occur in the soil and it is essential that there is no cross contamination by soil borne disease organisms between the infected greenhouse soil and the pumice in the bags. This can be achieved by completely covering the greenhouse floor with black and white polythene sheeting, and standing the pumice bags on top of this sheet. Eventual infestation of the media by disease organism must be expected after some 3-5 crops, but the cost of replacing the bags and media is less than the cost of sterilising the soil.
Growing in bags offers the grower better control over the root environment in terms of water supply, nutrition, aeration and root temperature, with the potential to increase yield but at the cost of demanding stricter and more skilled crop management. There need be very little crop production down-time between crops and the turn around between tomato crops can be as short as one week, which is a considerable gain over the 6 to 12 weeks required for turn around with soil grown crops. This reduced turn around time generally results in about 25% more yield per year from tomato crops.
Each plant's roots are confined in about 10 litres of media, whereas soil grown tomato plants roots can occupy about 70 litres or more of soil. Confinement in the limited volume of media demands frequent and accurate watering, and it is usual to water from 3 to 12 times per day, with carefully controlled amounts of water at each time. The accuracy and reliability demanded can only be achieved by installing automatic control systems which measure solar radiation and apply the correct quantities of water to each greenhouse.
These media have a very limited ability to hold plant nutrients, unlike soil which has great nutrient retention characteristics, so that the crop nutrient requirements cannot be supplied as a dry base fertiliser dressing incorporated in the soil before planting and supplemented with liquid feeds supplying mainly nitrogen and potassium. Crops in pumice or sawdust need a complete balanced nutrient mixture to be supplied in every watering .
The water plus nutrients is essentially a hydroponic solution, prepared by automatically dispensing stock nutrient solutions through two or three precision injector pumps in to the water supply. Crop nutrient requirements change during the course of each growing season, and crop nutrition in pumice requires more skilled management than is required for soil growing. This can be achieved by regular plant and solution analyses.
Media based growing thus demands much more detailed crop management on a day to day basis, and although automation reduces the drudgery and does effectively release management time for other tasks, and management time requirements will be similar for both soil and pumice crops.
The soil in each greenhouse will require hoeing, and grading to levels to suit the pumice bag growing system after clearing the old crops. Firm level soil paths are required with shallow V shaped trenches for standing out the pumice bags. The trenches are designed to drain any surplus water away from the bags, and so the bottom of the trench must have a fall over its length, with the result that the depth of the vee varies from end to end of each row.
The soil should be firmed as much as possible after forming the trenches, and it is probably worth using a heavy roller on the paths. Black and white polythene mulch film is then laid to cover the whole floor surface. This polythene is commonly available in several widths (4,8, 10m for example), and using the wide film (where possible) gives a better finish and is more economical. The reflecting white floor increases photosynthesis by as much as 18% for tomatoes by reflecting light back up into the leaf canopy.
ROW DIRECTIONS AND PLANTING LAYOUTS
North - south row directions are best in the southern hemisphere for even crop lighting. Planting should be in double rows to allow for easy plant layering over long growing seasons. Double rows in houses with posts should be arranged so that the roof support post fall between double rows and not in the paths. Double rows on 600mm centres are suggested but 800mm double rows may be required in to fit the post spacings. Paths between the doublerows should be 1m wide. A central cross path is suggested for houses with rows running the length of the house to minimise fruit transport distances to the packing shed and to reduce labour use for spraying. The path in houses with cross rows should be moved adjacent to the South wall in order to increase the row length and make layering easier.
Typical requirements for Irrigation Control & Liquid Feeding
The items to be installed must include:
multi-station irrigation controller
A & B solution pumps
possibly an acid dosing pump,
a water meter is highly desirable and if electronic pumps are to be used then a water meter with electric pulse output is required,
in line CF meter
A & B solution tanks.
It is necessary to have a water analysis to ensure that the feed recipes can be constructed to neutralise any alkalinity in the water and to balance the feed recipes according to the amounts of calcium and magnesium and any trace elements in the water.
The trickle irrigation flow rate required for the pumice bags is faster than that used in soil growing and it is necessary to allow for a trickle flow rate of 6 litres/plant/ hour. Upgrading of the water supply may be necessary, if the existing systems are not capable of providing sufficient flow.
The irrigation controller will only water one station at a time and the control solenoid valves should be installed in each house, as this will reduce the amount of mains piping.
ASSEMBLY AND USE OF MICROTUBE TRICKLE IRRIGATION SYSTEMS
The advantage of microtube trickle irrigation systems is their great flexibility, allowing them to be used over a wide range of flow rates and in unusual situations such as greenhouses with sloping floors. Another great advantage is the low cost of the components of the system. However microtube trickle irrigation systems must be carefully designed, if they are to work well. It is also necessary for the user to know the operating specifications of the design, and to observe any limitations imposed by the design. Contrary to popular belief, they are no more prone to blocking than other trickle systems, provided that properly filtered water supplies are used.
Prices for trickle irrigation components vary widely between different suppliers and it pays to shop around for the best prices.
Microtubing is extremely difficult to manufacture to an exact bore, and suppliers only supply microtube of a nominal bore. Samples of the same nominal bore from different manufacturers are unlikely to have the same average bore, and hence microtubes cut from different samples will have different flow rates. Manufacturers have difficulty in producing a consistent bore from day to day within the same plant, and different batches from the same manufacturer can vary in average bore. All the microtubing needed for any installation should be purchased at one time, in the hope that the whole lot will have been made at one time. The amount purchased should allow for making spare microtubes for replacement of any that become blocked during the working life of the system.
It has been shown that the actual bore can vary by several thousands of inch over quite short distances within any length of Microtube. This is the reason why the best uniformity of watering that is usually achieved is for about 90% of the microtubes in any system to deliver within 10% of the mean output for all the microtubes in the system.
Cutting microtubes to length
This does not have to be a long and time consuming job, but it can be so if not done properly. Microtube is usually supplied in hanks or loose rolls, and some sort of home made simple spinning jenny is needed to unwind it without tangling. Cutting hundreds of microtubes to exactly the same length is easy and quick if the microtube can be wound onto a tube whose circumference is exactly the length required. A tube of the right size can be made from a sheet of 28 gauge flat galvanised iron about 600 mm square. Drill two holes close to one side of the sheet, with the holes being spaced as far apart as the required length of the microtube. Drill another pair of holes at the same spacing on the opposite side. Roll the sheet into a tube and put a sheet metal screw through each pair of holes.
Fasten the end of the microtubing to the tube with sticky tape, and then wind on one to two hundred turns of microtube, keeping the turns taut and square to the tube. Stick tape over the top of the microtubing from end to end of the tube in a spiral at about 45░ to the coils of microtube. Finally, with a sharp knife cut down the centre of the tape. Make sure that the cut is right through on the first cut. The sticky tape stops the microtubes from flying all over the place as they are cut. The 45░ spiral cuts the ends of all the microtubes at 45░ and this ensures that the microtube end is not blocked if touches the inside wall of the lateral pipe.
Polythene header pipes must not be smaller than 32 mm in diameter and often 40 or 50 mm pipe will be recommended, depending on flow rates. Joining lateral pipes to header pipes smaller than 32 mm is difficult and leaks with small bore pipes are common, but the larger bores are easier and result in leak free joints. It is important that the header pipe is free from twist when fitting lateral pipes, as any subsequent untwisting may strain the joints. So, unroll the header pipe and leave it lying more or less straight in the greenhouse, and preferably in the sun for an hour or more to allow it to untwist, before attempting to make any joints.
Lateral take-offs are available in single (straight) and double (tee shaped) forms. They are best used with rubber grommets. The grommets will not ensure leak proof joints if the header pipe diameter is too small. Holes 19 mm in diameter are cut in the header pipes for the grommets. An ordinary carpenters brace with a sharp winged cutting bit is best for making clean edged holes. The rubber grommet is placed in the hole and lateral take off pushed through the grommet.
Twist in lateral pipes is just as much a nuisance as twist in header pipes, as the natural untwisting may pull microtubes away from the plants they are required to water. Lateral pipe should be allowed to uncurl in the sun for an hour or two in the same way as header pipes.
Inserting microtubes into the lateral pipes
Leak proof joints between the microtubes and lateral pipes are obtained by fitting the microtubes in holes in the lateral pipe wall which are smaller than the outside diameter of the microtube. The natural elasticity of polythene pipe gives a tight fit and watertight joint. Use the proper microtube awl for making the holes. These awls have a flat tip which punches a hole in the lateral pipe much smaller than the outside diameter of the microtube. The shaft of the awl behind the tip is tapered at 7░, so the base of the shaft is a much greater diameter. When the awl is pushed into the pipe, the flat tip punches a small diameter hole which is stretched to a large diameter by the expanding taper of the shaft. The microtube will slip easily into this hole if inserted soon after withdrawing the awl. The lateral pipe wall shrinks back to it's natural size within a minute or so giving a very tight grip on the microtube. The proper microtube awl is quite inexpensive (about $6), and no substitute is as effective.
Positioning the microtube ends
When plants are transplanted from pots, it is most important that the microtubes drip directly into the soil from the pots. The microtube needs to be held in a position to do this for best results. One way to do this is by inserting the end of the microtube into a 25-40 mm length of lateral pipe. The weight of the lateral pipe stub may be enough to keep the microtube in position, or the stub can be pushed into the soil. The pipe stub keeps the end of the microtube clean, as sometime bare microtube ends lying on the soil can be blocked by roots or moulds growing into the ends. If the pipe stub lies on the ground more or less horizontally it can improve the wetted soil surface area on some soil types. Various types of inexpensive plastic spikes are now also available for holding the end of the microtubes.
Flush out before use
When the system is first assembled the ends of the header pipes and the ends of the lateral pipes should be left open. Water should be run through the system to clear out any pipe drillings, dirt or anything which might cause blockages. The header pipes can be closed as soon as clear water is running from the ends. The cheapest way of closing the end is to fold it over and tie it back to the pipe. Closing the header pipe ends will cause the laterals to be flushed strongly, and they can be closed as soon as they are clear. Ready made lateral pipe plugs or end caps are available if desired, but the method of closing the ends must allow the ends to be opened for flushing out the pipes at least annually.
Fit a pressure indicator
The microtube system is designed to give uniform watering over a limited range of pressure and flows. Exceeding the maximum design pressure will result in uneven watering of the crop. Using less than the minimum design pressure can sometimes result in air locks blocking microtubes. It is thus essential that the system has a built in pressure indicator. If the design operating pressures are in the range of 1.5 to 4m head of water, then a clear plastic sight tube joined to one of the lateral pipes and hung vertically from the greenhouse roof is the simplest and most reliable pressure indicator. If the design operating pressure is greater than this, then low range 'bourdon' type pressure gauges are available.
Using microtube trickle systems
Clients using our design services will be given a table of flow rates versus head pressures. Low flow rates are often desirable for newly planted crops in soil which only require ball watering. Low flow rates give only a small spread of water around the drip point and assist with growth control over the winter months. Later in the season, when crop water demand is heavy, maximum spread of water around the drip point can be obtained by using the maximum pressure and flow rate, but always working with no more than the maximum design flow. Higher flow rates are required for planter bag cultures, in order to obtain good spread in porous media. The table of heads and flow rates supplied will not be exact, as the average bore of the microtubes will usually be different from the nominal bore. The average flow rate per microtube can easily be checked for an operating system by noting the time taken to apply a known quantity of water over the whole house. For example, if 500 litres are applied through a metering valve on a system with 1000 microtubes in 30 minutes, then the mean flow rate is 1 litre/microtube/hour.
Crop root systems adapt themselves well to the typical cone of wetting in the soil provided by trickle irrigation. Some growers move the trickle outlets in the summer as a means of increasing the spread of water, but I cannot recommend this practice as it leaves the established root system with less water than usual. If sufficient spread cannot be obtained at maximum flow, then occasional hose watering may be required to wet the soil, and better spread from the trickle irrigation will result after hose watering.
The trickle system should be inspected regularly for blocked microtubes, but this should not be problem if the water is properly filtered. A 200 mesh filter (= 80 Ám) should be installed as part of all microtube trickle systems, even those on town water supplies. Blocked microtubes can sometimes be unblocked in place, but often it is easier to replace them with new microtubes. The system should be given a high pressure flush out between seasons. The ends of the headers and lateral pipes should be opened and flushed out, as well as using high pressure to flush out the microtubes.
Microtubes.Microtubes are available in several sizes, 1.27 mm bore microtubes will be best for crops in sawdust and pumice culture, 0.89 mm bore is more usual for soil grown crops. Microtube length is usually selected so that one lateral pipe with microtubes inserted on both sides of the pipe can water a double row of plants. The following table gives theoretical output from microtubes in litres per hour per microtube. Output should be checked on site as the average bore of any sample of microtube is not always the same as the nominal bore.
Thin wall polythene lateral pipes with nominal bores of 13,15/16, and 19/20 mm are available. The size selected for any installation should be such that the total flow to all the microtubes on any lateral is within the limit for laminar flow for that size pipe, as otherwise the pressure drop down the length of the pipe will result in uneven output from the microtubes.
Header pipes should not be less than 32 mm bore and may need to be larger depending on system flow rates and acceptable pressure drops.
Reference : Microtube trickle irrigation in greenhouses for tomatoes and similar crops. R.A.J.White & G.K.Burge. Aglink HPP25. Information Services,MAF. Wellington, NZ. 1978.
This document: 1982,revised 16/2/94,1/8/94,11/10/99 ,10/5/2007.