Step 1: Materials List
Step 2: Tool List
Please note that this project was completed with a few expensive tools that are common for home projects. The first is a #80 drill bit; the smallest commercial bit available, which has a diameter of 0.3 mm. These bits are very easy to break and a CNC machine was used for higher stability and control when using them. Most people do not have access to a CNC machine, but you may be able to use a drill press, as long as it is stable enough and you are careful. Before you start this project, please make sure you can complete this step. Also, independent of your method, please purchase several of these bits; changes are high that you will break a few.
The second tool that is not common for most home projects is a heat gun capable of softening Polyethylene tubing. This was the tool used for this project; however, any controllable heat source would also work (preferably non-fuming), such as a propane camping stove (though this was not attempted). A simple open flame from a lighter is not recommended.
Step 3: Container Construction - Prepare the Boards
While cedar was the originally planned building material, it was not available at the time of construction. The most ideal redwood boards found locally had dimensions of 8’ x 11.25” x 1.5”. Two boards were purchased and both were cut in half into 4’ segments. Three segments were used for the base and the two long sides of the planter, with the remaining two short ends of the planter being cut from the remaining 4’ segment. These segments were cut with a length of 14.25”, leaving 33.75” remaining.
Step 4: Container Construction - Screw Into Place
Once the initial shape was formed and held steady with the brackets, pilot holes were drilled along the parameter of the container at the joints. Along both long sides of the planter, pilot holes were drilled using a 1/8” drill bit every 6”, starting 3” in from the side. If you are using a size of screw other than a #10, you may need to use a different sized drill bit for the pilot holes. As a rule of them, the drill bit should never be larger than the inner diameter of the screw. Nine additional pilot holes were drilled in each of the two shorter sides of the container as well; three along each joint. They should be somewhat equally spaced. The only major concerns are that you do not hit any of the existing screws you already placed or drill too close to the edge of each board. Once all the pilot holes have been drilled, fill in each with a 3” wood screw. When dealing with screws this long, pilot holes are a necessity.
Step 5: Container Construction - Shelf (1)
Step 6: Container Construction - Shelf (2)
Once you have the three remaining pieces, start by attaching the horizontal shelf by itself. It is important that you do not make the shelf level with the top of the box, unless you plan on filling the box completely with soil (with no lip of wood remaining around your plants). The height of this intended lip is how low the top of the shelf should be. Ideally, you will want the bottom of the bucket to be level with the soil. During construction, I chose to place it 1” lower from the top of the box to make sure the plants will have enough soil. (However, I would recommend installing it 2-3" below the top of the planter, now that my planter is complete)
Carefully measure this distance below the height of the container and mark it. Align the top of the shelf with this mark and offset it to the back of the planter using the dimensions in the figure above. Hold the shelf still and drill one of three pilot holes, equally spaced along the shelf and immediately fill it with a 3” #10 wood screw. This should hold the shelf, for the most part, but you will still need to hold it when drilling and inserting the remaining two screws.
Next, turn the container vertically so the shelf is raised. Set one of the wood braces even with the side of the shelf and drill a pilot hole at a 45° angle into the end so that the 2” screw will come into contact with the supporting structure. Careful placement of these screws is necessary: too far towards the center of the brace and the screw will not come in contact with the support, two close to the edge and the wood may split. Once the first screw is in place, repeat the procedure for the second screw, then the entire procedure for the second brace.
Step 7: Container Construction - Drainage Holes
Step 8: Irrigation Design
If you are content with knowing this, please skip to the next section, otherwise read on.
There is really only one equation you need to know to design a gravity irrigation system: Bernoulli’s equation, as shown below.
This equation is basically a restatement of the principle of conservation of energy, only applied to an incompressible continuum. Water can be assumed as such for a majority of applications, including this one. The equation states that for any location, or node, within a closed system, the dynamic pressure plus the gravitational field effect plus the static pressure will be equal to an unchanging system constant. The dynamic pressure is equal to the density of the water multiplied by the velocity at the node, divided by 2. The gravitational field effect is the density of the water, multiplied by the acceleration due to gravity (9.8 m/s2), multiplied by the height of the node, taken from an arbitrary, but constant, reference point. The static pressure is simply the force divided by the area at the node. Finally, the system constant is a potential constant offset of the entire system, which can be zero.
As a simple example of how to use this equation, let the elevation of the soil be our vertical height reference and let the system be built so that the bottom of the water reservoir is level with the soil. When the bucket it filled with 5 inches of water, what will be the instantaneous velocity of the water at the emitters? By examining a node at the top of the water level in the bucket, the velocity the height is moving down is negligible, so there is zero dynamic pressure. For the static pressure, we know that Earth has an innate pressure level of about 1 atmosphere, and at the top of the water in the reservoir, this is the only pressure acting on that node. However, because we know that the final state of the water will be to return to atmosphere in the planter, we can choose to ignore this value, as it will simply be a static offset for the entire system. This leaves the gravitation field effect equal to the system constant at this node, and defines the system constant for the entire system and is equal to the density of the water, multiplied by the acceleration due to gravity, multiplied by 5 inches.
We can now examine the node of one of the emitters at the location the water is released into the planter. Here, we are interested in knowing the velocity of the water, therefore we will be solving for the dynamic pressure. For the gravitational field effect, we defined this height as the reference point, therefore the height and gravitational field effect is zero. The static pressure is once again the atmospheric pressure of Earth, which we already canceled out from earlier; therefore we must do the same here. And finally, we already calculated the system constant from the first node. This leaves the dynamic pressure at node 2 equal to the gravitational field effect at node 1. The velocity at node 2 can then be solved, as shown below.
To discover the total leak rate of the system, you then just need to multiply this velocity by the cross-sectional area of the emitter and number of emitters in the system. But keep in mind this is only the instantaneous leak rate at the current height of water in the reservoir; as the water height decreases as time goes on, the leak rate will also decrease. If you with to know exactly how long your planter will be watered and at what rate, you will need to use a write a simple script with the actual dimensions of your system. This can be accomplished by discretizing time (with each step being on the order of seconds) and continually calculating the leak rate and volume loss at each step. This was done in MATLAB for the system shown here so you can see an example of the expected results.
One factor that has been ignored above is resistance in the system. Because the supply lines are short and relatively wide, we can calculate the resistance to be relatively low. This is also a more complicated equation and somewhat more difficult to calculate. Because of the difficulty and that my experimental observations are that it is negligible, resistance will not be discussed further.
Step 9: Irrigation Construction - Water Reservoir (1)
Step 10: Irrigation Construction - Water Reservoir (2)
Step 11: Irrigation Construction - Water Reservoir (3)
Step 12: PVC Emitters (1)
Next, you will need to measure and mark the locations of the emitters. The system shown here was constructed with eight emitters, each placed 6 inches from each other. Measure 3 inches from one side, mark the location, then continue measuring and marking every 6 inches. There should be 3 inches left from the far side when you are complete.
With the locations marked, it is time to drill the holes. If you are using a micro drill bit (gauge #80 through #61), do not use a hand drill. Because the bits are so small, you can not use them to hold the weight of the drill and you will snap the bits easily. You can probably use a drill press, as long as it stabile enough. I attempted this on a cheap drill press, but it continually broke the bit. I finally used a CNC machine with a controlled feed rate and it worked perfectly. Once you have found a system that works, drill the eight emitters along the length of pipe at the marked location.
Step 13: PVC Emitters (2)
The above experiment was carried out with this system, with water volume measured every 20 minutes. The results are shown above and support Bernoulli’s equation with negligible friction. In the second figure, the red lines represent each individual emitter and the blue lines represent the compete system. The final figure shows the distribution in the water volume measured from each emitter. While emitter 2 is unusually low (perhaps due to a partial clog), the other emitters appear to be somewhat uniform. If you are not satisfied with the uniformity test, you can attempt to unclog any potentially blocked emitters by flushing water through the PVC pipe and attempted to force air through the emitters with your mouth (it may look strange, but it works). If you test it again and the uniformity has remained unchanged, you can potentially drill additional emitters.
Step 14: PVC Emitters (3)
Both ends of the pipe need to be open to atmosphere in order to easily purge the air from the system after water is added. On both of these open ends, cut another square of the 50 micron filter paper and attach with a rubber band to prevent particles from clogging the emitters.
Step 15: Soil Layering and Plant Placement
Once the layer of rocks has been placed, fill with soil and fertilize as recommended on the packages.
With the limited space available in the planter, do not be afraid to plant a few types of varieties of plants together. You will have to research to make sure the plants require similar soil conditions and are not detrimental to each other, but when done correctly, it is an efficient use of space. I am not an expert in this, but for this planter, we are planting strawberries and spinach. Both require slightly acidic soil (strawberries: 5.5 to 6.5 ph, spinach: 6.4 to 6.8 ph) and the small spinach plants can be placed around the larger strawberry plants easily.
There are plenty of websites on the internet to help you determine excellent pairings of produce, as well as the correct spacing of the plants.
Step 16: Conclusion
Applying to the Bernoulli equation to the dimensions of the planter, we can see an excellent agreement from measuring the height of the water in the bucket as shown in the figures below. An interesting note, though, is that while the model agrees exceedingly well, the model was completed with an emitter diameter of 200 microns (0.2 mm), instead of the 300 micron diameter of the #80 drill bit actually used to from the holes. This could be due to a number of factors: 1) The force due to drilling stretched the PVC pipe as it drilled, and then bounced back once the bit was removed, 2) The PVC pipe slightly swells with contact with water, or 3) The resistance in the emitters is not negligible. None of these explanations sound completely believable to me, but I am also not too concerned about it. Besides, smaller emitters without a noticeable reduction in uniformity is ideal. If I was building a larger system, I would definitely want to determine the cause of this observation, but for this small scale example, it is actually a positive effect, independent of the cause.
And there you have it! A passive, gravity-irrigated planter capable of keeping the soil moist for hours after watering.