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Relationships of Particle and Pore Sizes to the Growth of Sunflowers

(Helianthus annuus, L.)¹

ABSTRACT

Sunflowers (Helianthus annuus, L.) were grown in 20 compacted soil separates, ranging in diameters from 4,760 to 2.31µ, which contained pores of diameters between 529 to 2.23µ. Two moisture levels, 20-cm. water tension and moisture content at "flex point," were imposed on the 20 separates. Nutrient solution was applied to the separates.

Noticeably different growth rates of sunflowers were measured at the two moisture levels. Maximum rate of stem elongation at 20-cm. water tension was nearly three-fold greater than was that at "flex point" moisture percentage. Maximum growth of sunflowers at 20-cm. water tension was obtained from separates between 52.3 and 210µ in diameter or pores with mean diameters of 17.7 to 43.5µ. Optimum growth of sunflowers at "flex point" moisture was on separates 13.1 to 9.25µ in diameter whose pores were about 4µ in diameter. Aeration appeared to limit the rate of stem elongation when air occupied less than 4% of the pore space.

Maximum growth rate of sunflowers at both moisture levels apparently was determined by the influence of pore size upon aeration and area of root-solution contact. The greater the area of root-solution contact, as determined by pore size and moisture tension, the more favorable was the growth of roots and shoots, provided aeration was not limiting.

¹ Contribution from the Department of Agronomy, Nebraska Agr. Exp. Sta., Lincoln, Nebr. Published with approval of the Director as Paper No. 786, Journal Series, Nebraska Agr. Exp. Sta. Presented before Div. I, Soil Science Society of America, Nov. 14, 1956, at Cincinnati, Ohio.

² Soil Scientist, Soil and Water Conservation Research Division, A.R.S., U.S.D.A., Research Center, Mesa Experimental Farm, Univ. of Arizona, Mesa, Ariz., and Professor of Agronomy, Univ. of Nebraska, respectively.

Received for publication October 13, 1956. Accepted for publication March 7, 1958.