DIY fertilizer applicator for a dripper system

The venturi and fertilizer bottle installed.
The venturi and fertilizer bottle installed.

Feeding your garden can be a joy but sometimes it can be hard work and who wants that? I’ve been pondering on how to get the dripper system to do the work for me. I like the venturi idea and an online search found some Venturi pieces that are perfect fro home irrigation.

Seasol™ and The Venturi Effect

For those of those not in the loop (yes, that was a vague attempt at a pun), the Venturi effect happens when you have two tubes, joined at rightish angles to each other and the second tube has a much smaller diameter opening than the first.

The venturin  in action
A close up of the venturi.

Passing a fluid through the first tube will create an area of low pressure that will suck a fluid from the second tube, mixing the fluids.

If you’re a seasoned gardener, you will already know the Venturi effect from using the type of liquid fertilizer applicator that click on to a garden hose. If you unscrew the applicator from the bottle you will see a small hole, attached to which is a fine tube.

You connect your hose and the water passes over a small hole and sucks the Seasol out of the container, mixing it with water and spraying it out over the garden. That’s the Venturi effect in action! I first saw it on bottles of the famous Seasol™ fertilizer but now it’s used for practically all the brands.

The hose water flows over the end of a thin tube

If you want far too much detail for our humble gardening needs, Wikipedia has a pretty exhaustive explanation here.

A closeup of the pieces for the assembly.
A closeup of the pieces for the assembly.

You can get the venturi units from ebay for as low as AUD $5 with postage from China and you can get them to fit different sized irrigation pipes. I couldn’t get any from any of our local suppliers of irrigation equipment (at least, nothing remotely in the same price range).

The Venturi unit screws into standard female threaded to 19mm barbed adaptors that you can get at many hardware stores or garden centres. I got ours from our Gawler Mitre 10 store for a couple of bucks each. Make sure you wrap the threads in plenty of plumber’s tape as the threads are a bit loose.

It is fortunate for water sensitive gardeners that the pressure to achieve the effect is pretty low and well within the range of normal home irrigation. That means that we can add it to a dripper system and have the dilute fertilizer delivered right to the base of our plants .

How to install and use it

Connect a container via some clear aquarium hose, making sure that the aquarium hose reaches the bottom of the container. Connect the other end of the hose to the Venturi unit and away you go.

Turn on your tap slowly and watch the aquarium hose. When you see the fertiliser being drawn through it into the Venturi, you know you’ve got the right pressure. These units act as pressure limiters too. It doesn’t matter how much you crank up your garden water pressure, they’ll only let a certain proportion of the water through. This means that, unless your system is very small you can’t use the Venturi at the same time that you’re using another of our favourites – Low pressure sprinklers, which are listed on our Low Technology page too. I’ve made a bit of a manifold to allow switching between sprinklers and Venturi. I could probably just turn the ball valve on the sprinkler off but that’s not as much fun.

As long as there is water supplied, the Venturi won’t stop sucking. This means that when your fertilizer container is empty, it’ll be sucking air into your system. I’m still experimenting with ways to overcome this and will add them to this page as I work something out.

At the moment, the best option is to work out the time taken to empty your fertiliser container and either add a timer to the supply or get one of the servants to manually turn the tap off.

All of the parts. Those with white tape are threaded pieces.