Evaluating soil fertility improvement effects of chemical fertilizer combined with organic fertilizers in a red paddy soil using the soil fertility index

Objectives Correct evaluation of soil fertility is the base of choosing efficient management method for improving soil fertility. This study investigated the soil fertility via total dataset (IFI_TDS) and minimum dataset (IFI_MDS).

Methods A three-year (2015–2017) field experiment was conducted in a paddy soil derived from red earth in middle and lower reaches of the Yangtze River. Seven treatments in the field included: no fertilizer (CK), sole chemical NPK fertilizer (CF) and treatments that replaced half NPK input levels by cattle manure (+CM), pig manure (+PM), chicken manure (+ChM), milk vetch manure (+GM) or straw incorporation (+SR). After harvesting of late rice in 2017, top soil samples were collected for the determination of physical, chemical and biological property indicators. The indicators closely related to rice yield were screened, and the main limiting factors in soil fertility quality were investigated by radar plotting. The soil integrated fertility index (IFI) under each organic fertilizer treatment was assessed.

Results The soil aggregates, pH and OM (organic matter) belonged to weakly variable fertility factors (CV < 10%), while other soil properties belonged to moderate variable fertility factors (10% < CV < 100%). Among the moderately factors, available Zn, available P and acid phosphatase activity were relatively sensitive to fertilization. According to the Radar plot of membership, cation exchange capacity (CEC), readily available K and total P were the major constraints for soil fertility. The soil fertility index based on total data set (IFI_TDS) and that based on minimum data set (IFI_MDS) showed similar evaluation result on the soil fertility of fertilizer treatments. Both the IFI_TDS and IFI_MDS in +PM and +ChM treatments were significantly higher than those in +CM and +GM treatments. The IFI_TDS of +SR treatment was significantly higher than those of +CM and +GM treatments, but IFI_MDS did not differ among the three replacement treatments. Both the values of IFI_TDS and IFI_MDS were significantly related with rice yield.

Conclusions Both the IFI_TDS based on total dataset and IFI_MDS based on minimum dataset can objectively reflect soil fertility. According to the IFI_TDS and IFI_MDS of soils under each treatment, chemical NPK fertilizer combined with pig manure has the best soil fertility improvement effect, followed by the treatment with chicken manure and straw incorporation, while treatments with cattle manure and milk vetch manure have the weakest effects.