The geographical distribution of soil black pigment

1. Soils having a relatively high organic matter content do not necessarily have the highest pigment content, nor are they always the darkest in color. 2. The second 6 inches of all samples studied were lower than the top 6 inches in relative pigment content. 3. The relative humus color of the soil extract is proportional to the relative pigment content. 4. In regions of approximately equal rainfall, a sigmoid is obtained when organic matter content or relative humus content is plotted against mean annual temperature. 5. Equal differences in precipitation influence the relative pigment, organic matter, and humus contents of the soil more in eastern Nebraska than in western Nebraska. 6. Generally speaking, it can be said that for every fall of 18 degrees F (10 degrees C) in mean annual temperature along the two isohyetal lines studied the average soil organic matter and humus contents are approximately doubled, the relative pigment content of the soil is increased two to six times, and the relative humus color is increased two to three times. 7. Equal differences in mean annual temperature have a greater effect on the relative pigment, organic matter, and humus contents of the soil in the area of greater precipitation. 8. With increasing precipitation along an isothermal line the average soil organic matter, relative humus content, and relative pigment content increase, the graphical nature of these relationships being a curve; but the relative humus color increases linearly. 9. With increasing temperature, along the isohyets, a linear decrease of relative pigment content and relative humus color was noted. 10. The relative pigment content and relative humus color of soil or of sand cultures were not appreciably increased by the addition of organic materials and subsequent decomposition over a period of 62 weeks. 11. Of all soil groups studied the relative humus content was highest in the podsol sample, followed in order by the gray-brown forest soils, the red and yellow soils, the chernozem samples, and finally the laterite. 12. The relative pigment content and humus color, excluding the surface few inches of the podsol, were greatest for the chernozem soils followed by the gray-brown forest soils, the laterite, and red and yellow soils. 13. The laterite and red and yellow samples were fairly high in humus content but low in relative pigment content.