There a re several things here to consider, 1. Electrode materials must be made of the same materials and non reactive, Stainless steel is best as it 'generally' won't react with water or the chemicals used for fertilization. 2. The use of DC for sensing EC is not recommended as the conductive medium (solution or soil) WILL polarize or react to form a gas layer on the electrodes that will cause a small to medium drift in the EC measured. 3. EC is measured in Millisiemens, there is a wiki reference at Siemens (unit) - Wikipedia and 4. The 'common' home made electrodes are generally not so very usable if left in situ in the soil medium for those measurements I would recommend a Tensiometer or it's equivalent.
I designed EC meters in the late 90's to early 00's and we used a patented method for a portable EC meter that involved a frequency locked NE602, was stable after I finished. It had a datalogger and PH meter as options. The point I guess is, that what you want to do must be done in a clever manner as there are several 'Gotcha's' before you have designed a workable EC meter and that an accurate, repeatable and stable device is a non trivial task... Hope this helps and if you need more then message me.