Welcome to Weather-Above
Materials for the mould and electrodes

fig 2 - 3
For the mould and the top and bottom are made out of conduit plastic piping 25 mm diameter, you can get this in black and white.
1 Cut a length of conduit plastic piping to 60 mm in length, then you  need something to split the piping I used a dremel multi tool
with a cutting disc. This will make it easy to get the plaster casts out of the mould

fig 3a
2 Prepare the mould by putting a strip of insulating tape down the slit to stope any of the mixture leaking from the tube, you also need to blank off the bottom of the tube, you could use insulating tape or find a cap that will cover the end you want it so you can remove it. This will help when removing the solid block.

fig 6 - 4 - 5
3 You need to prepare electrodes take two resistors as illustrated in the image just where the bottom of the brown colour you need some strong wire cutters and cut the bottom part off , wear some safety glasses, then you need to make the loop which will be used to solder your cable to, please see image on how this is done. Please take care and leave a small tail pointing upwards this will help to keep the electrode in position and create a good anchor.

fig 3a -  5
4 You should now have your mould ready prepared, and your two electrodes.

fig 8 - 9
5 I have experimented with different mixtures of gypsum the most effective type I have found is the Brown Thistle plaster board finish.

You need to weigh out using accurate scales 50 g of gypsum with 20 g of distilled water makes it quickly fill the mould to the top,
tap the side of the mould to get as much of the air bubbles out as possible then you need to quickly place the electrodes into the mixture
leave enough of the electrical wire poking out so you can solder cable to it, I tend to use tweezers to position the electrodes and the mixture, they must go into the mixture when it wet,  you will need to work quickly and have everything to work with close at hand, leave the mould to one side and let the gypsum set hard.

fig 1
6 Next stage is to cut  2 pieces of conduit plastic pipe in 20 mm length this will form the top and the bottom.

7 When the gypsum is completely set hard remove the tape on the side and whatever you've used on the bottom and carefully
remove your solid gypsum block with electrodes from the mould be careful not to disturb electrodes.

fig 10 -11
8 Carefully push on the 20 mm ring on the bottom of the gypsum block you only need approximately 10 mm of the plastic ring over the block.

9 You are now going to fill the bottom end of the ring with hot melt glue, this will secure the ring and make a really good solid plug this will protect the gypsum block when being pushed into the ground.

10 When the hot melt glue has become solid and is cold the next stage is to work on the top of the electrodes you need to solder a length of 2 core cable, I've used red and black speaker wire. Carefully clean the electrode wire loop remove any gypsum residue then feed a small piece of cable through the loop and solder it repeat the process for the other electrode,

fig 12
11 Feed the cable through the remaining 20 mm ring then carefully push the ring onto the gypsum block as you did with the bottom ring.
The next stage is very important,  put a very small amount of hot melt glue around the electrodes wire and over any solder that is exposed
this will seal and stop any water from getting to the wire, take a small cable tie and wrap this round the cable ,clip it into place on the cable just above electrodes which you have covered in hot melt glue. This cable tie needs to be really tight on the cable as this is going to help to take any strain off the electrode wire and solder joint, you want the cable tie to be able to be covered with hot melt glue, continue using the hot melt glue until you fill the whole top ring, you should just have a two core wire sticking out the top ring, the cable tie should be buried within the hot melt glue please see images.

fig 13 - 13a
12 The next stage you need some absorbent type of material cloth this needs to be the thin synthetic dishcloth type material is normally blue-and-white and is very absorbent  or you could use some of the material that the using hanging baskets which is very thin whitish porous synthetic cloth material.  Cut a strip of this material and wrap it around the centre of the exposed gypsum, you can secure it in place by using two rubber O-rings. 

fig 14a
13 The last stage it's extremely important,  take a 4 mm drill bit and drill a hole through the hot melt glue in the centre of the bottom ring just until you see gypsum material on the drill bit end.

Use the same drill bit and drill a small hole in the side of the top ring please see images.

14 The last stage is to soak the soil moisture sensor block for 24-48 hours , connect it to your weather station when it's saturated it should read between zero and 10 cb  this may be slightly higher due to your cable length just number your soil moisture blocks and make a note of any sensors that are over 10 cb which you take off your readings, I made quite a few of these now and I'm very happy that they are within 3% of each other. The next stage is to dry out the soil moisture sensor and the reading should be approximately 199 cb when bone dry. Weather display software works really well with these moisture sensor blocks if your device can use a watermark soil moisture block then you can use this project please let me know on your results, and how long they are lasting in your soil types, I've had them in the soil for approximately two years and they are still working really well because they are so cheap to make I would recommend replacing them 18 months to 2 years depending on soil type. I made this project for hobby weather stations who wish to monitor soil moisture 

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fig 3a
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fig 13a
fig 14a
Soil moisture sensor block
1 ohms  resistor
The following is a Standard general guide for interpreting the readings.  {cbars}

   0-10 ------------- Saturated soil
   0-30 ------------- Soil is adequately wet (except for coarse sands which are beginning to lose water)
  30-60 ------------ Usual range for irrigation (except heavy clay soils)
  60-100 ----------- Usual range for irrigation in heavy clay soils
100-200 ---------- Soil becoming dangerously dry for maximum production.

Hobby board soil moisture module is connected to weather display software. The readings for bone dry (199) totally saturated (10)
First draft work in progress