Much as I hate to disagree with
3xQDM 
Some aircraft, such as taildraggers, were designed for grass runways & are MUCH easier to land on grass, than on a hard surface.
On the other hand, some modern tricycle aircraft, are much more suited to tarmac. In general, I have also found some high-winged A/c, C152/C17/C182, easier on grass than low-winged A/c.
You also need to consider what the undercarriage is made of. I have always found that PA28s with conventional oleos, will settle more easily on grass landings, than say the AA5s that have bonded (laminated?) glass fibre, main gear arms. This ensures that AA5s bounce nicely on any grass runway, that is not like a billiard table.
Unless both the pilot and the aircraft are very good at grass landings, it is prudent to add on a large safety margin, to the landing distance, & take-of run. eg I am happy to land an AA5 on 400m of tarmac, but not less than 600m of grass. But the Citabria (high-winged taildragger) that I used to fly, would both land & take-off in about 50m, so with a safety margin, I was happy to take it into a grass strip of 350m
Grim Reaper 14 . If you have not done any grass landings, you are being very wise getting some practice with an instructor. That is the only way to really understand the differneces to tarmac landings.
Although I do a fair number of grass landing every year, in assorted PA28s, AA5s & GA7s(twin), I will probably go to the Sywell Fly-in by car, as I do not like the sound of its grass runways. I will wait for the tarmac, next year
OK..... So I am a wimp! BUT... I am a live wimp