A CLASSIFICATION OF HUMUS FORMS AND MICRO‐FABRICS OF TEMPERATE GRASSLANDS

Summary The humus form is recognized in a soil profile as the group of A horizons in which organic matter is concentrated, and it is thus the upper part only of the genetic humus type, the true measure of which is the complete organic profile extending into B and C horizons. Grassland humus forms are subdivided on a basis of field morphology into two groups: mull, in which the organic residues are largely incorporated as an intimate clay‐humus mixture, and mor, in which organic residues in varying stages of decomposition overlie the mineral soil with a sharp boundary but may show weak mechanical admixture of mineral grains. A mor‐like mull intergrade is recognized where the mull is very high in intimately incorporated organic matter. According to variation in horizon thickness, structure, and consistence, sixteen humus forms are recognized: strongly granular, weakly granular, massive or blocky, fine, matted, laminated, and lenticular mulls, strongly granular, weakly granular, and laminated mor‐like mulls, and granular, massive, matted, laminated, banded, and mullized mors. Where mineral grains are very abundant in mor a sandy phase is recognized. Each layer or horizon of a humus form may have one or more distinct micro‐structures or fabrics. Twelve fabrics have so far been recognized under grassland: weak mull humus, strong mull humus, mull‐like moder, mull‐like rendzina moder, leached mull‐like rendzina moder, silica moder, swollen moder, rendzina moder, leached rendzina moder, raw soil humus, sclerotial humus, and raw humus.