Crop safety period of plastic film mulching means the appropriate choice of a photo-selective mulch film according to the transplant season, to the ecological conditions of the area under cultivation and, finally, to the type of crop. In the days before the transplant, the plastic mulching film should be basic whole, and it works to warm soil and to decrease soil moisture evaporation. Successively, these functions almost reduce.Soil moisture conditions can basically meet the needs of potato growth and temperature is the most important factor to be considered in intensive agricultural region of North China.This study will give a guide in mulching film management and the choice of appropriate degradable mulch in intensive agricultural region of North China in Qingdao by determining the safety period of plastic film mulching of the local crop. The effects of plastic film mulching on the growth of potato were studied, as well as the soil temperature in the 10cm soil layer and the yield, by removing the film at different times.In this experiment, conducted from March to June in 2016, six treatments were set up: planting potatoes without plastic film(T1); planting potatoes with plastic film and removing film after 30d (T2), 60d (T3), 75d (T4), 90d (T5), respectively, and planting potatoes with plastic film for the entire growth period (T6). The results indicated that the plastic film could promote the emergence of potatoes by 5 days, and the emergence rate was significantly higher than T1. It also showed that removing the plastic film after 60 days could shorten the growth period of potatoes and accelerate its growth. The effect of plastic film on soil temperature in the 10cm soil layer was significant between the sowing and seeding stage of the potatoes. During this period, the daily average temperature under plastic film was 1.1℃ higher than without it, and the accumulated temperature was 66.1℃ higher than without it, respectively. The disappearance time threshold of the film mulching effect was about 70 days after the plastic film. The results of the experiment showed that the soil accumulated temperature in the 10cm soil layer was highest at 60 days, which was 96.5℃·d higher than non-mulching in the whole growth period, indicating plastic mulching had an significant warming effect. Within the potato growth process, the warming effect of film mulching was weakened. Film mulching can significantly improve the temperature in the 10cm soil layer between 1 and 60 days after mulching. If removing the film after 60 days, there would be no significant difference among T3, T4, T5 and T6, which indicated that the effect of plastic film on soil temperature was weakened after 60 days. It further demonstrated that the yield under non-mulching had the lowest yield (40.3tha−1). The highest and the second highest yield were occurred when film were removed after 60 days (T3) and 75 days (T4), which were 13.4% and 10.4% higher than T1, respectively. Obviously, removing film after 60-75 days would produce the highest yield. Thus, the potato safety period of plastic film mulching is at 60-75 days in intensive agricultural region of Northern China, which should be the deadline for removing plastic film or degradable plastic film duration.
