Just done a search on winter wheat and came up with this article.
A thought just occurred to me that I could plant a crop of something in the container broad casting the seed thickly. Maybe turnips as they sprout and grow quickly to about 3 or 4 inches high. Just work the greens into the soil when transplanting time comes around.
I did read an article not to long ago where serious and knowledgeable container or yard gardeners manage their soil where they do not need to add fertilizers.
Green manure cover crops for Minnesota