We now have thousands of tiny seedlings rapidly growing in our garden. The field crop is rotated over three years. This is the first year there has been rapeseed planted. We have not experienced this infestation of seedlings in previous years when potatoes then wheat were grown. Could this be the rape seeding itself in our garden?
Unfortunately, your photo did not come through. I will not be able to identify the seedlings. It could, very well, be that it has self seeded. All plants in that family tend to self seed, readily.
Solarizing the soil may be necessary to get rid of them.
Here is an article that will help:
We have made cannels from the stream for irrigation,we are currently in the cold season
This is typical of phosphorus deficiency, but can be due to cold weather.
This article will help you to care for the plant: