Nitrogen deficiency

Here is a maple where you can see that the leaves are yellowing between the veins showing it is lacking nitrogen. In the grass around the maple you can see the purple vetch a nitrogen fixing legume is doing exceedingly well. These two facts may be related. It seems like the purple vetch is doing better this year than either of the previous two years. In the previous years birdfoot trefoil seemed to be the dominant legume creating large mats but now in the square where the trefoil has been uncut for the third year now, the trefoil is losing out to the vetch. On the westside where this is only the second year without mowing the treefoil is still going strong.

Here is a link discussing the use of trefoil as a pasture crop. One thing they note is that while it is a perennial it needs to self seed if it is to become permanently established. I think we might have a situation where the trefoil needs a more disturbed and open location to get established than the vetch and that when young can't compete in the long term with the faster growing grasses and the weeds.