Effects of Plastic Film Mulching on Physical Characters of Soil and Yield and Yield Components of Sweet Potato

Chaofan ZHANG,Yanlan HUANG,Hong ZHOU,Ya ZHANG,Daowei ZHANG

Hunan Crop Research Institute,Changsha 410125,China

Soil temperature,moisture and texture have direct impact on plant root growth and metabolism,which play a controlling role in growth of plant shoots[1].Plastic film mulching can not only increase soil temperature and moisture content[2],but also improve soil water use efficiency[3].It is a feasible technology for improving crop production.In arid and semi-arid regions,plasticfilm mulching has important practical implications for agriculturalproduction.Currently,promotion of plastic film mulching is mainly concentrated in the cultivation of vegetables,economic crops and major field crops in North China[4].However,plastic film mulching is rarely applied in cultivation of sweet potato.In this study,comparative experiments were conducted among different types of plastic film mulching,and the effects of plastic film mulching on growth and development,yield and yieldcomponentsofsweetpotato were studied.The technical key points of plastic film mulching were cleared,providing theoretical basis for highyielding cultivation of sweet potato.

Materials and Methods

Test materials

The test materials are energyproducing starchy sweet potato cultivars,including Xushu 22(processing type)and Xiangshu 19 (fresh eating).They were all provided by the Changsha Comprehensive Test Station of National Sweet Potato Industrial Technology System.The used plastic films are agriculture-specific. The thickness of black and white plastic films is all 0.015 mm,and the width is all 90 cm.

The test was conducted in the test base of Hunan Crop Research Institute in 2012.The last-season crop was sweet potato.The test soil was red soil,and its basic physical and chemical propertieswere shown in Table 1[5].

Test design

Total three treatments were designed,including black plastic film mulching,white plastic film mulching and no mulching (CK).In the white plastic filming mulching and CK groups,the test soil was all sprayed with Jindu’er agent.The randomized block design was adopted.There were four replicates for each treatment.Among the four replicates,the sweet potato plants in the plot IV were used for storage root survey,while the sweet potato plants in the plot I,II and III were used for determination of yield and quality.The spacing between two adjacent rows was 83 cm.There were six rows in each plot,and the area of each plot was 25 m2.The planting density was 52 500 plants/hm2.The sweet potato seedlings were transplanted on May 15,2012.The growth of sweet potato plants encountered continuous rainy weather.The sustained high-intensity rainfall led to saturated moisture content in sweet potato fields.However,timely drainage ensured that there was no excessive water in sweet potato fields.The other management measures were same with those in general fields.

Measured indicators and methodsSampling methodThe representative sweet potato plants were sampled on day 30,60,90,120 and 150 after the transplanting ofsweetpotato seedlings.For each time sampling,total five sweet potato plants were collected from each plot.The tiller number,main stem length and leaf number,as well as fresh weights of shoot and root per plant,were measured.After the drying,the dry matter ratio and dry matter weight were calculated.

Measurement of soil temperatureThe soil temperature meters were arranged in soil layers at the depths of 0,10 and 20 cm.The soil temperature was recorded for 10 consecutive days since the transplanting of sweet potato seedlings.On each day,the soil temperatures were observed three times(8:00,12:00,16:00).

Determination of soil moisture content and bulk densityThe test soils at the depths of 0-10,10-20 and 20-30 cm were sampled on day 0,30,60,90,120 and 150 since the transplanting of sweet potato seedlings.The moisture contents and bulk densities in the soil samples were determined.The soil bulk density was determined using ring sampler,while the soil moisture content was determinedbydrying method.

Determination of dry matter weightThe fresh weight of shoots and roots was weighed,respectively.Subsequently,the shoots and roots of sweet potato plants were dried at 105℃for 1 h,and then at 60-80℃to constant weights.The dry matter weights in shoots and roots were obtained.

Determination of dry matter ratio in sweet potato storage rootsThe representative sweet potato storage roots were selected.The central parts of the selected sweet potato storage roots were sampled and shredded.A certain amount(100 g)of sweet potato shreds was spread in an alumimum box,and there were two replicates for each plot.The sweet potato shreds were dried at 40℃for 6 h,at 60-80℃for 6 h,and then at 100-105℃for 12 h.Subsequently,the dried sweet potato shreds were weighed.The dry matter ratio was calculated according to the following formula:

Dry matter ratio in sweet potato tuber=Dry weight of sweet potato shreds/Fresh weight of sweet potato shreds×100%.

T/R valueT/R value=Fresh weight of shoot/Fresh weight of storage root.

Increase rate of fresh weightIncrease rate of fresh weight=(Dry matter weight in sweet potato tuber N d after the transplanting-Dry weight in sweet potato storage roots in the early period)/Growth days.

Determination of yieldDuring the harvest,the survived plant number per plot and tuber number per plant were surveyed.The fresh weights of shoots and roots were weighed.The planting density(plants/hm2),tuber number per plant,shoot fresh weight(t/hm2),root fresh weight(t/hm2)and fresh weight per plant(g/plant)were expressed as the means of the three plots.

Results and Analysis

Effects of different types of plastic film mulching on soilphysical characters

Soil temperatureSoil temperature is an importantindicatorreflecting warming effect of plastic film.The three times of soil temperature measurement in a day was able to reflect the dynamics in insulation effect of plastic film betterly.Under conditions of plastic film mulching,the soil temperatures at different depths were all higher than those in CK.The shallower the depth was,the higher the temperature increment was.With the deepening of soil layer,the soil temperature was decreased gradually.The dynamics in soil temperature were basically same between the black and white plastic film treatments.However,the soil temperature in the white plastic film treatment group was increased faster compared with that in the black plastic film treatment group,which might be due to that white plastic film was transparent.At different times,the soil temperatures at different depths basically ranked as white plastic film treatment＞black plastic film treatment＞CK.At the depth of 0 cm,the soil temperature in the white plastic film treatment group was 0.6℃higher than that in the black plastic film treatment group,and was 1.6℃higher than that in the CK;at the depth of 10 cm,the soil temperature in the white plastic film treatment group was 0.5℃higher than that in the black plastic film treatment group,and was 1.3℃higher than that in the CK;at the depth of 20 cm,no significant differences were observed in soil temperature among the three groups.Among the three measurement times,the soil temperature was highest at 16:00(Table 2).

Soil moisture contentSoil moisture is an environmental factor for crop growth,but also a carrier of nutrients.Soil moisture plays an important role in crop growth and development.Plastic film mulching can fight against drought and waterlogging.When it rains,rainwater flows along the film surface and is concentrated in ditches and then discharged from fields,thus the soil moisture will not be oversaturated;when it does not rain,plastic film can block moisture transpiration,reducing the loss of soil moisture.

Continued(Table 2)

In the early and middle growth period of sweet potato,continuous rainy weather occurred in Changsha,and each time of sampling all encountered light to heavy rain,resulting in higher moisture content in the CK compared with those in the plastic film mulching treatments.As shown in Fig.1,soil moisture contents were relatively stable in the plastic film treatment groups,but differed significantly in the control group;there were no significant differences in soil moisture content between the black and white plastic film treatment groups.

Soil bulk densitySoil bulk density is an important indicator of the status of soil elasticity,and is a comprehensive reflection of soil texture,structure,porosity and other physical properties.The size of soil bulk density is essentialforpromoting enlargementof sweet potato tubers.If soil is too tight,air and moisture in soil are insufficient,soil nutrients are harder to be decomposed,and topsoil is easier to be compacted and cracked,which are all not conducive to growth and enlargement of sweet potato tubers.On the contrary,if soil is too loose,soil moisture and nutrients are harder to be retained,leading to easy loss of soil moisture and nutrients.

Soil bulk density is affected by mulching treatment,as well as by soil internal traits.In this study,at the depths of 0-10,10-20 and 20-30 cm,the soil bulk densities in the black and white plastic film treatment groups were all lower than those in the CK,but there were no significant differences between the black and white plastic film treatment groups.At the depth of 0-10 cm,the average soil bulk densities in the black and white plastic film treatment groups and the control group were 1.27,1.27 and 1.32,respectively;at the depth of 10-20 cm,the average soil bulk densities were 1.35,1.31 and 1.46,respectively;at the depth of 20-30 cm,the average soil bulk densities were 1.39,1.41 and 1.50,respectively.It suggests that black and white plastic film mulching allcan improve soilphysicaland chemical properties and reduce soil bulk density,but there is no significant difference between black and white plastic mulching treatments.

Table 3 Effects of different types of plastic film mulching on T/R value (fresh weight)of sweet potato

Effects of different types of plastic film mulching on growth and development and yield of sweet potato

T/R valueThe results showed that the T/R values of Xushu 22 and Xiangshu 19 in the black and white plastic film treatment groups were all decreased significantly compared with those in the CK(Table 3).This decrement was more obvious in the early and middle growth stage of sweet potato plants. It indicates that mulching can rapidly reduce T/R value in the early and middle growth stage,which is conducive to early formation and enlargement of sweet potato tubers.In terms of effect on T/R value,black plastic film mulching was better than white plastic film mulching,which was consistent with the study result about sweet potato yield.

Increase rate of fresh weight of sweet potato storage rootsIn the early growth period (0-50 d after the transplanting),no significant differences were observed in increase rate of fresh weight of sweet potato storage root between cultivars or black and white plastic film treatment groups.During the growth interval of 50-90 d,the increase rates of tuber fresh weight of Xushu 22 and Xiangshu 19 were all increased rapidly,and they all reached the peaks on day 90.After then,the increase rates were decreased gradually.Since the 50th d,the increase rates of fresh weight of Xushu 22 storage root and Xiangshu storage root 19 in the black and white plastic film treatment groups were all increased significantly compared with those in the CK;but in the late growth period,there were no significant differences in increase rate of fresh weight among the three groups.

Yield and yield componentsof sweet potato

The yield analysis showed that black and white plastic film mulching significantly increased sweet potato yield.Compared with that in the CK,the yield of Xushu 22 in the black plastic film treatment group was increased by 21%,and in the white plastic film treatment group was increased by 16.7%.Compared with that in the CK,the yield of Xiangshu 19 in the black and white plastic film treatment groups were increased by 15.3%and 13.0%,respectively.It indicated that the increment of yield of Xushu 22 was larger,and Xushu 22 was more sensitive to mulching.Under condition of black plastic film mulching,the yield of the two sweet potato cultivars was increased by 17.7%in average;under condition of white plastic film mulching,the yield of the two sweet potato cultivars was increased by 14.8%in average.Blackand white plastic film mulching showed similar effects on drying rate of sweet potato.Mulching was conducive to improvement of drying rate of sweet potato storage roots,but there was difference between cultivars.The drying rates of Xushu 22 inthe white and black plastic film treatment groups were 2.9%and 1.3%higher than that in the CK,while the drying rates of Xiangshu 19 in the white and black plastic film treatment groups were 0.6%and 1.8%higher than that in the CK(Table 4).

Table 4 Effects of different types of plastic film mulching on yield of sweet potato

Table 5 Effects of different types of plastic film mulching on yield components of sweet potato

From the view of yield components,the proportion of large and medium storage roots in the black plastic film treatment group was higher than that in the white plastic film treatment group,and the proportion of large and medium tubers in the white plastic film treatment group was higher than that in the CK.It indicated that mulching could improve the proportion of large and medium storage roots,but there was difference between cultivars.The difference in proportion of large and medium tubers of Xushu 22 was only 0.2%between the black and white plastic film treatment groups,but the proportions of large and medium tubers of Xiangshu 19 in the black and white plastic film treatment groups were 8.2%and 2.5%higher than that in the CK (Table 5).It suggested that the proportion of large and medium tubers of Xiangshu 19 was more sensitive to mulching,which was consistent with the study result about yield.

Conclusions and Discussion

During the early growth period of sweet potato plants,the rainfall in Changsha region was higher than those in previous years,resulting in higher moisture content in the CK compared with those in the black and white plastic film treatment groups,but small difference in soil moisture content between the black and white plastic film treatment groups.Soul bulk density is an important indicator reflecting soil elasticity.With the deepening of soil layer,soil bulk density was increased continuously.Mulching also showed great impact on soil bulk density.In the black plastic film treatment group,the soil bulk densities at depths of 0-10,10-20 and 20-30 cm were 12%,9.9%and 8.6%lower than those in the CK;in the white plastic film treatment group,the soil bulk densities at depths of 0-10,10-20 and 20-30 cm were 7.1%,2.8%and 2.6%lower than those in the CK.

Mulching significantly increased the soil temperature.At the depth of 0 cm,the soil temperature in the white plastic film treatment group was 0.6℃higher than that in the black film treatment group,and was 1.6℃higher than that in the CK;at the depth of 10 cm,the soil temperature in the white film treatment group was 0.5℃higher than that in the black film treatment group,and was 1.3℃higher than that in the CK;at the depth of 20 cm,no significant differences were observed in soil temperature among the three groups.It suggests that with the deepening of soil layer,the change in soil temperature becomes smaller,and the effect of external factors on soil temperature also becomes smaller.In conclusion,with the deepening of soil layer,the insulation effectoffilm mulching is weakened,and the best insulation effect of film mulching(soil temperature being increased by 1.6℃)is shown at the depth of 0 cm undercondition ofwhite plastic film mulching.However,Luoet al.[6]reported that mulching could increase soil temperature by 3-4℃.The difference in insulation effect of film mulching may be caused by the difference in cultivation season.

Film mulching could significantly reduce T/R value of sweet potato,which was more obvious in the early and middle growth period.In the early and middle growth period,the T/R value was reduced,indicating rapid accumulation of carbohydrates,which was conducive to formation and enlargementofsweetpotato tubers.From the perspective reducing T/R value,black plastic film mulching was better than white plastic film mulching.There were no significant differences in increase rate of tuber fresh weight between the black and white film treatment groups in the early or late growth period.However,in the middle growth period(50-90 d),the increase rate of tuber fresh weight differed significantly between the two mulching treatments.Particularly,on day 90,the increase rates of Xushu 22 in the black and white film treatment groups were increased by 24.5%and 36.7%compared with that in the CK,while the increase rates of Xiangshu 19 in the black and white film treatment groups were increased by 19.7%and 31.8%compared with that in the CK.

Film mulching significantly improved the yield of sweet potato.Black plastic film mulching increased the yield of Xushu 22 and Xiangshu 19 by 17.7%in average,while white plastic film mulching increased the yield of Xushu 22 and Xiangshu 19 by 14.8%in average,indicating that yield-improving effect of black film mulching was better.Under condition of black film mulching,the yields of Xushu 22 and Xiangshu 19 were increased by 21%and 15.3%,while under condition of white film mulching,their yields were increased by 16.7%and 13.0%,indicating that the yield increment of Xushu 22 was larger.In terms of yield components,the tuber number per plant in both black and white film treatment groups was decreased,but the proportion of large and medium storage roots was increased.Between the two mulching treatment groups,the decrement of tuber number per plant and increment of single tuber weight were all larger in the black film treatment group.It suggested that sweet potato compensated for decreased tuber number per plant by increasing single tuber weight,thus higher yield of sweet potato was obtained.

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