Ich sollte = I should/ought to do it
Ich soll = It is expected of me to do it

Ich soll = I am supposed to
Ich sollte = I ought to (I should do)

Slight difference, but a difference nevertheless...

There are different possibilities.

"ich soll" is equivalent to "someone told me to do something and probably I will do"

ICH SOLL ESSEN.

"ich sollte" is comparable to "I should", but it's not clear if I will do

EIGENTLICH SOLLTE ICH ESSEN, ABER ICH HABE KEINEN HUNGER.

"ich sollte" can also be the past tense, but still it's not clear if the person has actually done or not

GESTERN SOLLTE ICH EINEN BRIEF SCHREIBEN...