Okay, I'm back with results, if anyone is still interested Took "a while" cause been too busy gardening
It appears that the field I'm talking about is app. half clay and half sand. At least so it looks to me in the sample that I took as suggested above.
As summer goes on the situation just looks worse. Lots of cracks, stone hard ground.
It's a tough spot this field. I tried planting hazelnuts in on corner of it. Dug large holes for them and tried to make things nicer for the roots by adding horse manure and peat plus ash to the soil in the planting hole. I also built a wind break all around the hazelnuts as the field is very exposed and windy. But the hazelnuts did not seem to like it there and after a month of waiting I rescued them, ie. dug them out.
I've also sown a few rows of a robust bean species (Vicia faba hangdown, don't know what's it called in English), few rows of a certain pea species and a small area of "green manure" seed mixture. The Vicia faba hangdown is doing reasonably although looking quite thin in comparison with the huge ones I have in my garden. The green manure clover mixture and the peas are growing really slow despite regular watering. But there are no weed problems either as this spot is too tough for weeds too!
Now I'm wondering what to do with this land. The neighbour does not want it back on rent (I asked) so I won't have to worry about taking this field out of production
There are too main problems: the seriously compacted clay soil and the exposed location. The latter being maybe the more difficult to fix. It's too large an area to plant a living windbreak. There aren't many wind break trees that would do well there anyway. I tried Siberian pea but out of 10 only 1 is alive at the moment... I did leave the planting rather late so that might explain this. But anyway the area is 140 m x 50 m. I would need at least 190 meters of wind break (north side 140 m, east side 50 m) and it would probably be best to have a wind break also on the west side, another 50 m. It is VERY windy and the prevailing winds are from north and from west. The field is also on a slope. The eastern side tends to dry out as it's the highest point. To the north there are the neighbour's huge wheat fields. The western end is near a river which floods every spring so this end of the field is under water until May. Oh and the location is Southern Finland in case I haven't mentioned before.