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BROWN PE, 1914, IOWA AGR EXP STA RES, V18, P49
[3]
CALL LE, UNPUB
[4]
CLARKE FW, 1908, US GEOL SURVEY B, V330, P26
[5]
HALL AD, 1906, SOIL, P200
[6]
HALL AD, 1906, WIS AGR EXP STA RES, V14, P16
[7]
HART EB, 1911, WIS AGR EXP STA RES, V14
[8]
HART EB, 1915, J AGR RES, V5, P233
[9]
HILGARD EW, 1906, SOILS THEIR FORMATIO, P23
[10]
HOPKINS CG, 1910, SOIL FERTILITY PERMA, P75