3 Types of Bottle Plant Watering Systems and the Science Behind Them

Watering plants in terracotta pots is a hugely water inefficient as well as time-consuming. Water is lost due to evaporation from the soil and through the terracotta as well as running out through the bottom of the pot.  There are better systems available and we will consider three types: You could just set up all three systems and use trial and error in order to optimise them.  They are, however, likely to be successful if we use the Scientific Design Thinking approach. Here we will look at the science behind them and how we can use this knowledge to make predictions and learn new concepts. Pressure in a Fluid – What might happen to the water if we had holes in a bottle? A bottle has three holes above one another in its side. How will the water flow out of the bottle when the lid is opened?  A, B or C? You may want to watch this video (or read below) to help you decide   Answer (C) A higher pressure would lead to the water squirting out further. You can see this by squirting water in your mouth out through a straw. The higher the pressure you exert, the further the water goes. The deeper you go in a fluid the higher the pressure gets due to the weight of all of the molecules above. Deep sea fish have adaptations in order to survive these massive pressures. Similarly, we live at the bottom of an ocean of air. Above you is about 100km of air molecules and the combined weight of all of these is about the same as having a small car on your head.  If you didnt also have air inside your body balancing these forces you would be crushed. Possible experiment: How does the rate of water flow change as the bottle empties? What might happen if the hole size was different but the pressure was the same? Would you see (A) (B) or (C)? Make a prediction and justify it Answer: Possible experiment: How does the diameter of the hole affect the rate of water flow from the bottle? So far we have two factors that affect the rate that plants would get water. The height of the water column and the size of the hole or holes. (1) Make a Drip Bottle Watering System So here is a bottle with a hole in the lid hanging upside down. What will happen?   What happens if you take the lid off?  Why? Possible experiment: What is the maximum size of hole in the lid so that water does not flow out of the bottle when turned upside down? Watch the video Challenge: Make your bottle take an hour to empty  How can you regulate the flow?  What might happen if …..
  • The hole in the lid is made bigger or smaller?
  • A hole is made at the top of the bottle? How might the size of this hole affect the flow rate?
  • Sand/cotton string is used to restrict the flow through the bottle?
  • A sports bottle top is used?
Alternative method. How does this work? (2) Make a Soak Bottle Watering System The same principles apply as to the drip bottle but multiple holes are now made in the bottle. This makes things more complicated. What did we learn before that may help us here? Volume marks can be added to the bottle to allow measurements What might happen if:
  • We changed the number of holes?
  • We changed the pattern of holes? Should there be more at the top and fewer at the bottom?
  • We changed the size of the holes?
Other Questions
  • Does the bottle drain at different rates in soil than in air?
  • Does it make a difference if the soil is wet or dry?
  • How can we use this to water plants more effectively?
Cutting the bottom off a large bottle allows us to have a soak bottle inside another bottle which is hugely water efficient and also requires less watering. Some questions:
  • What combination of bottle sizes works best?
  • How many plants can be grown in the big bottle?
  (3) Make a Water Bath – Hydroponics Watering System We can make a refilling bird or animal drinker from using a plastic bottle and a bowl (or bottom of another plastic bottle) The bottle needs to have a large enough hole in the cap or be mounted without a cap so that it empties without having to put another hole higher up. The bottle will fill the container up to the level that you put the hole. Why does it stop there? What have we learned already that will help us? Using this principle we can have several bottles filling a tray of water. We can add nutrients to these bottles in the form of Compost Tea or fertilizer. To prevent the plants being waterlogged, capillary matting can be used or layers of cloth that absorb moisture. Covering the tray with a waterproof lid will reduce water loss by evaporation.          
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