Greetings @chipmc, looping into your issue you are building in a wrong way your context, the context is a key-value dictionary, so your payload should be something like this:
{"SoilTemp":"19","Watering":{"value": "0", "context": {"key1":"Oops"}},"Moisture":"516"}' https://things.ubidots.com/api/v1.6/devices/{DEVICE_LABEL}
So please try with this command:
curl -X POST -H 'Content-Type: application/json;' -H 'X-Auth-Token: {PUT_HERE_YOUR_TOKEN} -d '{"SoilTemp":"19","Watering":{"value": "0", "context": {"key1":"Oops"}},"Moisture":"516"}' https://things.ubidots.com/api/v1.6/devices/{PUT_HERE_YOUR_DEVICE_LABEL}
Regards