Soybean Seedling Growth Responses to Light Reflected From Different Colored Soil Surfaces

Spectral balance (quality) of light influences many aspects of plant growth and development; and spectral balance of reflected light in the seedling establishment zone is affected by the color of soil surfaces, plant residues, and mulches. Soybean [Glycine max (L.) Merr.] seedlings were grown over white, red, and black surfaces in greenhouse studies. Painted insulation panels, cloth‐covered insulation panels, and different colored soils over insulation panels were used to test the effects of reflected light while minimizing root zone temperature differences. Reflected light from 400 to 800 nm was measured at 5‐nm intervals and expressed as the percentage of direct sunlight at each measured wavelength. Photosynthetically‐active light (400–700 nm) reflected from the white and black surfaces was about 40 and 5%, respectively, of that in direct sunlight. Blue light (400–500 nm) was reflected at about 40% from white and 5% from either black or red panels. Red surfaces reflected similarly to the black from 400 to 570 nm and similarly to the white from 620 to 800 nm. Soybean seedlings were grown in 1‐L containers of vermiculite mounted below the insulation panels, inoculated with Bradyrhizobium japonicum, and supplied with N‐free nutrient solution. Plants grown over white surfaces developed the shortest stems, the most lateral roots and nodules, and the smallest shoot:root dry matter ratios. The amount of blue light as well as the far‐red/red ratio in reflected light contributed to these morphogenic effects. It was concluded that the surface color of soil can substantially alter the growth and nodulation of soybean seedlings even though root zone temperatures under variou surface colors are similar.