We are now using a soil moisture sensor which has been made in house, I have done lots of experimenting with different materials.
You need a 30ml syringe which is used to make a mould which will be reused every time you need a new sensor, also we going to used the top of the syringe for the cap to secure wire connections and a plastic ring which goes at the bottom.and a plastic insert see fig 1a
You need to cut the syringe tube into three pieces, cut the syringe outer tube to the 20ml line, this piece of this tube is going to be used for the mould. You need to cut the top of the syringe at the 6ml line , this is going to make the cap which protects the connections and secures the connecting cable to the block from the remaining tube. Cut a  5ml piece of tube this makes the protection ring.
Bottom ring
The next stage is to make the electrodes. They are made from  (1) 1 ohms  resistor, you need to cut the resistor in Half
After cutting the resistor in Half  you need to make the loops, these are used to solder the cable to the block,simply make a small loop and twist them around the remaining resistor wire,
The next stage is to measure out 20g of plaster  the type I am using is used in the ceramic industry for making moulds, use whatever you can get hold of. Next we want a tiny amount of sodium bicarbonate add this to the plaster, mix the plaster and sodium bicarbonate together.

You should now have your electrodes ready,
Assemble the syringe with the 20ml tube and have the plunger at the bottom ,you need to support this in the upright position I used a small hobby vice.Next insert a plastic strip this needs to be tight to the wall of the syringe mould this makes a divisor between the two electrodes. Please note plastic strip stays permanently in the past block
Add 10g of distilled water or rainwater to the plaster it needs to be like a double cream consistency.   give the mixture a good mixing using a small stick or plastic strip  then put small amounts of the mixture into the mould. Tap the mixture down Into each side of the mould until the whole tube is full, then you need to put in the two electrodes in to the mixture, this must be done quickly as the plaster mixture will dry quickly.  The electrodes connecting loops are just above the plaster mixture, leave it to dry overnight.

Some info about the mixture: I wanted to try to replicate a sensor block that would resemble more like soil structure in the sense that if I used plaster on its own it would be one solid block and it would take a long  time to register the differences in  the moisture of the soil , by adding the sodium carbonate to the mixture it causes hundreds of little tiny air bubbles in the plaster making it easy to absorb the moisture. The idea of using resistors for the electrodes is to  make the whole block a resistor , the resistance will vary with the moisture content, I have placed 4 of these sensor blocks in a root area of my rhubarb which is approximately  4 inches depth of soil, from my observations when it starts to rain and the soil becomes more wet it is not long  before the sensor registers the  moisture , I don't know how long it will last as this is the first soil test so only time will tell.

Tip:  before  putting your mixture into the mould sparingly rub some liquid soap on the inside of the tube.
When the block is totally dry you can remove it, the best way to do this  is remove the plunger from the bottom of the mould put a small amount of water and put the plunger back in forcing the water up into the side of the mould, with practice they become very easy to remove and reuse the mould as many times as you like.

But remember, for each block made, you will need a new syringe to make the top and the  protection ring.
Assembly:  you need some cable this is to solder onto the 2 electrodes  , choose cable diameter to enable to go through the whole in the top of the cap you can make the hole bigger by drilling out some of the plastic to a larger size , feed the cable through the hole, it needs to be as tight as possible don't drill out too much plastic, strip the ends of the cable, thread some heat shrink tubing over the wires . Solder the wires onto the elecrodes then slide down the heat shrink tubing.This is going to be used to cover the solder connection and the loop from the electrode to give it some protection. If you need to if the cable is not tight enough in the cap , I use a cable tie to make a cable restrain, just fix a cable tie to the cable inside the  cap ut off any excess cable tie, push the cap on to the block it should be a very tight fit , then push on the protection ring to the bottom of the block. I am using a 1 wire hobby board soil moisture circuit board just follow the instructions and attach sensor to the relevant moisture circuit. Next stage is to calibrate weather display you need the moisture block to be bone dry, write-down the voltage information  from weather display then place the moisture block in water and soak overnight. Then take another reading from weather display of the saturation voltage reading . These measurements are displayed in the  weather display 1 wire reader,  go to where the soil moisture settings are and fill in the voltage for dry soil and the voltage for saturated soil slope adjuster is set to - 0.95.

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.

Welcome to Weather-Above
Plastic insert
Updated  05 Oct 2014
Synthetic breathable filter material is used on the outside of the block,This can be obtained from garden centres it is used in hanging baskets The synthetic material is white and is very easy for moisture to pass through it

this concept was designed in-house by Michael Parry-Thomas 2014
Plastic insert

30 mm x 35 mm
I am using the new hobby board moisture sensor module which  has 3 soil moisture sensors and a wet leaf sensor
Weather display Version 10.37S build 34   Works with the new hobby board soil moisture module, using a add-on utility in weather display software called 1 wire hobby board reader
ALL information and images on whether-above.com webpage is copyrighted 

When the soil moisture sensor block is finished and totally bone dry. Use a 3 mm drill bit to drill  out the vent holes.. This allows the sensor to be very fast in responding when watering takes place
I have had a set of these Sensors in the ground for over two years and there is still plenty of life left in them, I would recommend changing them every two years, which will give optimum results during this period
When the new hobby board soil moisture module is connected to weather display software. The readings for bone dry (199) totally saturated (25) connected to weather display software . The readings for bone dry (199)  totally saturated (25)

0-25 -------------   Saturated soil
25-55 ------------- Soil is adequately wet (except for coarse sands which are beginning to lose water)
55-85 ------------  Usual range for irrigation (except heavy clay soils)
85-125 -----------  Usual range for irrigation in heavy clay soils
125-200 ---------- Soil becoming dangerously dry for maximum production.