Analyzing nutrient elements cycle of agricultural ecosystem is one of the principal bases for people to understand its functions

carry out artificial regulation to raise productivity and keep durative of the system.Through located observation and investigation of 3 villages

in the middle-south Loess Plateau

Feimahe and Nangou of Baota District in Yan'an municipality as well as Xipo of Chunhua county

the writers analyzed plant nutrient cycles and system's nutrient cycle.Results show:(1)Absorptions of N and K by plants are more than that of P while storage of the three elements in soil manifests K>P>N.Absorption of N

P and K varies from plant to plant

and generally speaking absorption of the elements by crops is more than that by fruit trees.N

P and K return quantities change from plant to plant

and general law is that fruit tree is more propitious to the return of soil N compared to crops.Referring to the turnover period of N

P and K

fruit tree shows that N turns over fastest while crops wheat

millet and corn manifest that K turns over quickest except rape acquits that P does fastest.(2)Cycling coefficients of nutrients N

P and K in the crop-fruit ecosystem are Feimahe village>Nangou village>Xipo village

and turnover periods are Nangou