17 



amounts of nutrient removal, there were no appreciable changes on soil 

 organic carbon, total N and available P and K (114). On double cropping 

 of paddy rice in Taiwan for 48 years, average rice yields were similar 

 among fertility treatments with the same amount of N added (lOO) . The 

 effects on chemical properties were also similar with those observed 

 by Nair et al. (114), and suggested an equilibrium level without major 

 differences among treatments. 



Continuous cropping does not always result in stable or increased 

 yields. Yield levels of succeeding crops depend on resultant soil 

 fertility which is influenced by changes in soil chemistry (143). 

 Double or triple cropping seq_uences involving sweet potato, taro 

 ( Colocasia esculenta L. Schott) , sorghum, and cowpea conducted in New 

 Guinea on volcanic alluvial soils showed progressive yield decreases 

 with time (15). These decreases were related to decreases in soil 

 fertility parameters. Crops were not fertilized but when cropping !-ra,s 

 alternated with legumes or green manure, the fertility decline was 

 delayed. Intensive cropping of this natiore required fertilization to 

 sustain long-term sequential cropping (116) . In a 2-year continuously 

 cropped rotation of early and late crops including com, cotton, bean, 

 sweet potato, peanut, finger millet (Sleusine corocana L.), and sorghum, 

 yields of all crops declined steadily during the first two cycles (153) » 

 but application of N, P, K, and farm yard manure increased yields. 

 After a few years of continuous cropping, K deficiency limited yields, 

 especially of sweet potato. 



