What is going on?
I compared both pictures of the field (left out the one with the tractor, as it suffers from the same problem as the other over-exposed picture, IMHO) in After effects. The image above is a composition of all that I did: First, the composition of both your original images that I made in AE (white canvas added only here), then both vectorscope readouts (each picture separately) and then, the waveform parade of both (separated by the black border in the composition, which is shown as thick line at 0).
What we can see:
- The left picture is much brighter,
- The left picture is clipping a lot in the red channel, while the right one is not clipping as much,
- The greenish colors have a wider range of tones (stretched closer to yellow) in the left picture.
A bit of trying around showed me that with a gamma correction of
1 / 2.2 = 0.4545, the left picture gets quite close to the right picture (or, if wanted, a gamma correction of 2.2 gets the right one close to the left one):
Why is that?
Looks like overexposure and its effects on JPEG to me. If you saved the images as RAW, you could try to lower the exposure. It is hard to give tips or guess why this happened without knowing how you accomplished the shots: If you exposed manually, then you simply got it wrong a bit. If it was an auto-exposure, then your camera got it wrong somehow. The white balance seems to be slightly off (too blue), but that is not the main problem here.
A wild guess: You were shooting in M mode. You set your exposure settings when it was a bit cloudy - the first shot got properly exposed. Then, the sun came out a bit more, but you did not notice and therefore, your shots got slightly overexposed.
Setting things (roughly) right in GIMP:
Beware: This is the first picture I ever edited with GIMP - and I'm bad at editing non-RAWs, anyway. So what you see here is merely what you can accomplish as a newbie - and in 5 minutes time, without any talent. Also, please understand that over-exposed parts of a JPEG cannot be recovered (easily) - you can easily see that the flower's petals are blown out.
What I did: