The long-term use of mineral fertilizers has decreased the soil fertility in papaya (Carica papaya
L.) orchards in South China. In situ earthworm breeding is a new sustainable practice for improving soil fertility. A field experiment was conducted to compare the effects of four treatments consisting of the control (C), chemical fertilizer (F), compost (O), and in situ earthworm breeding (E) on soil physico-chemical properties and soil enzyme activity in a papaya orchard. The results showed that soil chemical properties, such as pH, soil organic matter (SOM), total nitrogen (TN), available nitrogen (AN), and total phosphorus (TP) were significantly improved with the E treatment but declined with the F treatment. On 31 October 2008, the SOM and TN with the O and E treatments were increased by 26.3% and 15.1%, respectively, and by 32.5% and 20.6% compared with the F treatment. Furthermore, the O and E treatments significantly increased the activity of soil urease and sucrase. Over the whole growing season, soil urease activity was 34.4%~40.4% and 51.1%~58.7% higher with the O and E treatments, respectively, than that with the C treatment. Additionally, the activity of soil sucrase with the E treatment was always the greatest of the four treatments, whereas the F treatment decreased soil catalase activity. On 11 June 2008 and 3 July 2008, the activity of soil catalase with the F treatment was decreased by 19.4% and 32.0% compared with C. Soil bulk density with the four treatments was in the order of O ≤ E < F < C. The O- and E-treated soil bulk density was significantly lower than that of the F-treated soil. Soil porosity was in the order of C < F < E < O. Soil porosity with the O and E treatments was 6.0% and 4.7% higher, respectively, than that with the F treatment. Meanwhile, the chemical fertilizer applications significantly influenced the mean weight diameter (MWD) of the aggregate and proportion of different size aggregate fractions. The E treatment significantly increased the MWD, but the F treatment decreased it. The MWD with the E treatment was 14.5% higher than that with the F treatment. The proportion of >2 mm size aggregates in the O and E treatments was vastly improved. In conclusion, in situ earthworm breeding in orchards performed better than traditional compost and chemical fertilizer in improving soil aggregation, chemical properties, and enzyme activity. This is a new, organic fertilizer application for improving soil structure, chemical properties, and soil enzymes due to the activities of the earthworms and the production of vermicomposting.
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