hi, i'm building a project that if the soil humidity is low the sensor will send me signal!!
if any one can help me for the right sensor!!! to use in my circuit!

 

Probes to go into houseplant pots are often simply two metal rods at a fixed spacing which are pushed 4-6 inches into the soil. Then the resistance is measured. It's more of a conductance (Siemens, inverse of resistance) than resistance, as the value is very high, and has a wide margin based on soil type.

Is this for houseplants, lawns, or crop irrigation?

 

it's for crop irrigation, is there more simple thing like i don't know a chip!

 

Google search is your friend

http://www.cheapvegetablegardener.com/2009/03/how-to-make-cheap-soil-moisture-sensor.html

hi, i'm building a project that if the soil humidity is low the sensor will send me signal!!
if any one can help me for the right sensor!!! to use in my circuit!

 

it's for crop irrigation, is there more simple thing like i don't know a chip!

You can buy increasingly accurate and reliable devices starting at ~$4 USD and moving up to several hundred dollars. The cheap ones all use the conductivity approach already mentioned, and you can build your own if you want. More costly meters use other means. The ultimate would be a laboratory measurement of water activity.

Since you are concerned about a large area, using multiple cheap sensors might be useful.