I was under the impression from the GOV.UK website that NI contributions for the self employed did not need to be paid until profit was more that £6205 per year.
£500 income would easily be written off against annual depreciation on farm assets and/or other costs so there is no profit to declare.
Have I missed something else?
Actually I don't think you have and think I was wrong on the NI front - looks like things have changed since I was last self employed (versus limited company). Back then there was compulsory Class 2 NIC regardless of income/profit (used to be £2 a week or so) and Class 4 depending on level of profit. Looks like Class 2 has been cut back to the levels you mention above.
Like you say, small income easily covered by expenses incurred.
But most people making anything more than insignificant, occasional self employed income do need to register anyway.