Minor changes were made for the purpose of consistency in the book lists at the end, like adding period, etc. And the the following misprints have also been corrected:
"sufficiemt" corrected to "sufficient" (page 45)
Changed "—" to "-" in nitrogen-fixing (page 151)
Joined words "intel lectual" split over two lines (page 236)
Missing text "to" added after "incidental" (page 285)

Other than the corrections listed above, printer's inconsistencies in spelling, punctuation and hyphenation have been retained.