This paper studies the optical characteristics of organic matter

humus composition and form

and humic acid at different planting ages of alfalfa in soils on the Longdong Loess Plateau.Results show that as the year of planting alfalfa goes by

soil organic matter content is gradually increased and its quality is improved.However

the situations of both reclaimed land and abandoned land are opposite to that of alfalfa field.From soil surface to underlying layers

ratio of looseness to tightness in alfalfa field planted for 10～20 years is increased

which means that planting alfalfa mainly affects soil fertility in the soil layer of 20—40 cm.The analyses of color tone coefficient(logK) and relative chromaticity(RF) of humic acid indicate that when the time of continuous cropping is between 5 and 15 years

the quality of humic acid tends to be improved.If the time of planting alfalfa is too long

the quality of humus acid may be reduced

the structure of humic acid may become aging

and the quality of humic acid on abandoned land may be reduced as well.