Sách tham khảo

1895, Pennsylvania

1963

1964

1965, Bartholomew, W. V.

1965, Black, C.A.

1966, American Society of Agronomy

1967, Hamilton, Harry

1967

1957, Luthin, James N

1964, Army, T.J.

1964

1965, Black, C. A.

1965

1967

1967, Dinauer, Richard C.

1968, Smith, D. C

1961, Coffman, Franklin A.

1964

1965, Stelly Matthias

1965

1966, Bartelli, L.J.

1967, Drew, J. V.

1967, Whitney, Robert S

1968, Neuffer, M. G.