Objective  Effects of ecologically friendly co-cultivation of crops and animals on the physiochemical properties and fertility of soil on rice paddy fields were analyzed.

  Methods  Principal component analysis (PCA) was used to study the effects on the soil of paddy fields in South China under 4 types of co-cultivation that combined rice-growing and duck-raising (RD), rice-growing and fish-aquaculture (RF), or rice-growing and shrimp-aquaculture (RS) in comparison to the conventional mono-crop practice on a rice paddy field (CK). Bulk density, porosity, acidification, and nutrient contents of the soil were determined.

  Result  All co-cultivations significantly reduced bulk density, improved porosity, alleviated acidification, and increased contents of nutrients and organic carbon in the soil. Of the 3 methods, RD rendered the greatest porosity, organic carbon, total nitrogen, total phosphorus, total potassium, alkali-hydrolyzable nitrogen, available phosphorus, and available potassium in the 0–10 cm surface paddy soil. The fertility scores on the co-cultivations as ranked by PCA were RD＞RF＞RS＞CK for the surface soil and RD＞RF＞CK＞RS for the 10–20 cm sub-surface soil. The primary factors contributing to the soil fertility included bulk density, organic carbon, porosity, humin carbon, total nitrogen, and alkali-hydrolyzable nitrogen, while the secondary factors, total potassium, pH, available potassium, total phosphorus, fulvic acid carbon, humic acid carbon, and available phosphorus.

  Conclusion  On a paddy field, cultivating rice with ducks (RD), or fish (RF) as the second choice, effectively improved the soil conditions and fertility. Nonetheless, under specific local considerations such maneuver could also be combined with other means to achieve an ecologically beneficial rice farming in South China.