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If memory serves soybeans are less than 1 ft tall , 1/8 -1/16 " tall. Simplelest would be to use plaster or foam to make the ground , paint it , then use some ground foam for the plants. There are more complex things you could do , but it is N scale.
It depends on what month of the growing season you are talking about. Soybeans around here can get thigh high. But they dont look that tall. Also they are a darker green than corn.
In N scale it will be hard to discern whether or not the fields are soybeans. The fields up the street from me can get 24" - 32" tall, and they are always planted in clearly defined rows about 3' apart. These pictured are not mature yet.
I would use a piece of corrugated cardboard with one side removed, paint it an earth color and then glue coarse ground foam to the ridges.