The use of commas for an inserted clause

This novel is set in a Chinese village before World War One.
The protagonist lives with his wife, baby, and father, his wife was a maid with very rich family.
The province is suffering from a great famine.

Now Wang Lung's ox he had cared for as long as he could. He had given the beast a bit of straw and a handful of vines as long as these were gone. Then since there was no land to plough, since seed, if it were planted only dried in the earth, and since they had eaten all their seed, he turned the ox out to hunt for itself, sending the eldest boy to sit upon its back all day and hold the rope passed through its nostrils so that it would not be stolen.
[The Good Earth by Pearl S. Buck]
I was wondering why it should be ", if it were planted", not "if it were planted,"
Thank you in advance for your help.