Change conditioning in real time
if (moisture &lt;= 350) //if moisture is less than reading 350 {



That is wrong, because you have the brace { after the comment. You need it before the comment, eg.

 if (moisture &lt;= 350) { //if moisture is less than reading 350


Or preferably in my opinion:

if (moisture &lt;= 350) //if moisture is less than reading 350
  {





  I'm just asking if it's possible to change the if (moisture &lt;= 350)


Yes, of course. That's what variables are for. You could read a potentiometer, and the value from that could replace 350 in the code. You could use a simple LCD or LED display to show what the current reading is.




  delay(600000) //water for ten minutes


Your problem here is that the sketch will be totally unresponsive (it would not react to your changing the moisture level) during the time you are watering. You might want to read How to do multiple things at once which shows how you can keep doing other things, while waiting for time to elapse.