How to connect 70 analog sensor to a single Arduino (Mega or Uno) device?
There are fairly inexpensive chips called "analog multiplexers" which let you choose one input out of a range (8 or 16 is typical). You could take the reading, then switch to a different input, and take another reading.

I have a page about analog multiplexers.

Example of it in operation:



In this example you have 8 inputs, and by setting the A/B/C pins appropriately, one of the 8 is selected and appears at the O/I pin (output/input).

There is a 16-channel model as well. I see the 16-pin chip (74HC4067) available on eBay for $US 1.26 right now. You can get four of them (which would give you 80 inputs) for $4.88.

Also see the Wikipedia article on multiplexers.




  For this, I need to collect soil moisture value from 70 sections of the farm on a single Arduino. 


Yes, well how will these sections get their data into one place? That is probably the bigger problem.

You may need to use a lengthy cable using RS485 (balanced signals) or maybe cheap radio transceivers.

Trying to use a "single Arduino" on what might be quite a large farm is the issue. It's not the Arduino's fault. You have to have some way of sending the data from the further points to where it is to be collected.