I would say Grace renders the Law useless. Just like a physically challenged person needs a wheel chair but, if they are healed, the fact that they can walk makes their wheelchair useless. In the same way, Grace is a healing at the spiritual level and it makes the Law useless. It doesn't mean one doesn't abide by the Law any more; it simply means one does the will of God without needing to know the Law.
Very good point! I would point out, though, that fertilizers are bad for our health and for ecology so fallow is also a matter of love
More importantly, the two extra commandments Jesus gave us and you quoted (Matthew 22:36-40 / Mark 12:28-31) are the very reason why Jesus said he is the lord of the Sabbath (Matthew 12:8) and, if a sheep has fallen into a pit on the Sabbath, we should save it (Matthew 12:11). This means that
the "Law of Love" prevails over the Law as it is written. This is why I say Grace renders the Law useless. This is still true nowadays and this is why
Jesus warned us many Christians wouldn't enter the Kingdom of Heaven (Matthew 7:21-23), because they would be stuck in an intellectual understanding of the Law preventing them from doing God's will which, as you pointed out, comes down to Love, not as it can be understood but as it can spontaneously flow from the heart of someone who has been graced by God.