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Abstract [Objectives] This study was conducted to investigate the control effects of silicon fertilizer on growth of kohlrabi （Brassica oleracea L. var. caulorapa DC.） and Plutella xylostella. [Methods] Yougui and Guishen were selected and sprayed onto kohlrabi for 1， 2 and 3 times， forming difference treatments. [Results] Spraying the two kinds of silicon fertilizers for different times promoted the growth of leaf length， leaf width and leaf thickness of kohlrabi， improved chlorophyll contents in kohlrabi leaves and corm yield， and adversely affected P. xylostella. Furthermore， higher treatment times led to a more remarkably effect and higher harm to P. xylostella. [Conclusions] In production， spraying silicon fertilizer onto maize for 2-3 times could realize the purposes of improving kohlrabi yield and alleviating damage by P. xylostella.
Key words Silicon fertilizer； Kohlrabi； Growth； Plutella xylostella
Silicon is acknowledged as the fourth plant nutrients following nitrogen， phosphorous and potassium in the international soil science circles. In agricultural production in China， nitrogen， phosphorous and potassium fertilizers were applied in large quantities， and in order to ensure healthy growth of agricultural crops and improve yield， certain quantity of silicon fertilizer also should be applied[1]. Experimental results show silicon fertilizer has a remarkable effect of improving yield of agricultural crops， and has been applied to crops including rice， wheat， soybean， peanut， sugarcane and fruit trees currently[2-3]. Firstly， silicon fertilizer could alleviate the occurrence of pests and diseases on vegetables and crops and reduce the consumption of agrochemicals[4]. Secondly， silicon fertilizer could inhibit pathogenic microbes in soil， and could effective prevent the survival and propagation of various pathogenic microbes including mould[5]. Thirdly， after vegetables and crops absorb silicon， silicified cells are formed in them， the cell walls of the surfaces of stems and leaves are thickened， forming a hard protective layer， which is difficult to be bitten by insects and infected by pathogenic microbes[6-7].
In China， kohlrabi has a very long planting history and is popular[8]. As a kind of vegetable， it has certain health-caring and anti-cancer effects[9]. Supplementation of available silicon by spraying effective silicon onto leaf surface is an effective way to improve crop yield and quality[10]. In this study， the effects of silicon fertilizer on kohlrabi （Brassica oleracea L. var. caulorapa DC.） and Plutella xylostella were studied， so as to serve practical production. Materials and Methods
Experimental materials
Silicon fertilizers
Yougui was produced by Nangong Huayang Chemical Plant， and Guishen was produced by Cheng Pu Fertilizer Industry.
Tested kohlrabi
Kohlrabi was produced by Tianjin Shennong Seed Co.， Ltd.
Tested pests
P. xylostella was collected from the greenhouse of Agricultural College， Yanbian University. The pests were fed with cabbage leaves in laboratory. The pests were cultured at 23-27 ℃ under 16 （light） L ： 8 （dark） D and a relative humidity of 50%-60%.
Experimental instruments
Electrodyne （3JWB-16A）； plant chlorophyll meter （SY-S02）； leaf thickness measuring instrument （LS-3）； electronic scale （JM-A2003）.
Experimental methods
Sowing of kohlrabi
On June 5， the field in greenhouse of the Agricultural College was turned over and ridged， and each hole was sown with 3 seeds according to a row placing of 0.6 m and a plant spacing of 0.4 m. On July 5， only one healthy plant was reserved in one hole.
Silicon fertilizer treatments
Each plot included 3 rows， each of which included 5 plants， and there were 15 kohlrabi plants in one plot. From July 20， silicon fertilizer treatment was performed every 10 d. Yougui and Guishen were diluted by 750 times， and sprayed onto the leaves with the electrodyne. In order to achieve well adsorption of the silicon fertilizers onto kohlrabi leaves， 2 000 times dilution of Tween 80 was added into the electrodyne. The experiment was designed with seven treatments， i.e.， control （CK）， Yougui 1 （spraying Yougui once， similarly hereinafter）， Yougui 2 and Yougui 3， and Guishen 1， Guishen 2 and Guishen 3.
Investigation of kohlrabi growth
Measurement of leaf length and leaf width of kohlrabi
On August 25， upper leaves on kohlrabi were measured for leaf length and leaf width， and the values were recorded.
Measurement of chlorophyll content and leaf thickness of kohlrabi
Also， on August 25， the chlorophyll content and thickness of kohlrabi leaves were measured with the chlorophyll meter and the measuring instrument of leaf thickness， respectively， and the values were recorded. During the determination of chlorophyll， the measurement was performed at the same time period. During the measurement of leaf thickness， the leaf position was kept the same when keeping away from leaf veins.
Test of P. xylostella
On August 26， upper leaves were collected from the 7 treatments （CK， Yougui 1， 2 and 3 and Guishen 1， 2 and 3）， respectively. Leaf segments （2 cm×5 cm） were cut and placed into nursing bottles. Then， 10 second instar larvae were added into each nursing bottle. Each group included three nursing bottles as three replicates. The larvae were fed with upper leaves of kohlrabi every day， and the death number of P. xylostella was investigated on the third day and recorded. On August 28， lower leaves damaged by P. xylostella were collected from the 7 treatments， and placed and photographed according to silicon fertilizer and treatment times. Investigation of yield per plant of kohlrabi
On August 30， kohlrabi was pulled out， and the leaves and roots were removed. The weight of single corm of each treatment was measured with the electronic balance.
Analysis software
The data were analyzed with DPS v7.05， and multiple comparisons were performed by Duncan’s new multiple range method.
Results and Analysis
Effects of the two kinds of silicon fertilizers on growth of kohlrabi
It could be seen from Table 1 that among the different treatments， Guishen 3 exhibited a leaf length of 30.7 cm， which was significantly longer than that of CK （23.5 cm）， while the leaf lengths of other treatments were longer than that of CK， but the differences were not significant.
It could been seen from the leaf width data in Table 1 that among the different treatments， the length widths of Yougui 3， Guishen 3， Guishen 2 and Yougui 2 were， respectively， 19.2， 17.7， 17.5 and 16.6 cm， which were significantly longer than that of the CK （12.40 cm）， while the leaf widths of Yougui 1 and Guishen 1 were longer than that of the CK， but the differences were not significant.
It could be seen from the leaf thickness column in Table 1 that among the different treatments， the leaves in Yougui 3， Yougui 2， Guishen 3， Yougui 1， Guishen 2 and Guisheng 1 were thicker than those in the CK， but the differences between any two of the treatments were not significant.
It could be seen from Table 1 that with the spraying times increasing， the leaf length， leaf width and leaf thickness of kohlrabi also increased. Overall， Yougui had a better effect than Guishen.
Effects of the two kinds of silicon fertilizers on chlorophyll content and yield of kohlrabi and P. xylostella
It could be seen from the chlorophyll column in Table 2 that among the different treatments， Yougui 3， Guishen 3， Yougui 2， Guishen 2， Guishen 1 and Yougui 1 had the chlorophyll contents higher than CK， but the differences were not significant.
It could be seen from the yield column in Table 2 that among the different treatments， Yougui 3， Yougui 2， Guishen 3 and Guishen 2 had the yields per plant， respectively， of 873.5， 800.2， 682.8 and 644.8 g， which were significant higher than the CK （363.1 g）， while the yields per plant of Yougui 1 and Guishen 1 were higher than that of the CK， but the differences were not significant.
The death rate data of P. xylostella in Table 2 showed that among the different treatments， Yougui 3， Guishen 3， Yougui 2 and Guishen 2 had the death rates of P. xylostella， respectively， of 31.0%， 28.0%， 24.0% and 20.0%， which were significantly higher than the CK （9%）， and though the death rates of P. xylostella in Yougui 1 and Guishen 1 were higher than the CK， the differences were not significant. It could be seen from Table 2 that with the spraying times increasing， the chlorophyll content and yield of kohlrabi and the death rate of P. xylostella also increased. Overall， Yougui had a better effect than Guishen.
Effects of the two kinds of silicon fertilizers on damage by P. xylostella
It could be seen from Fig. 1 and Fig. 2 that the treatments with the two kinds of silicon fertilizers greatly affected the damage by P. xylostella， and with the applying times increasing， the damage on the leaves of kohlrabi got lighter.
Discussion and Conclusions
The treatments with the two kinds of silicon fertilizers could promote the growth of leaf length， leaf width and leaf thickness of kohlrabi， and different treatments differed in the promoting effect. Among them， only Guishen 3 exhibited the leaf length significantly longer than the CK； the leaf widths of Yougui 3， Guishen 3， Guishen 2 and Yougui 2 were significantly wider than the CK； and the thicknesses of all the treatments were not significantly different from the CK.
The treatments with the two kinds of silicon fertilizers could improve chlorophyll content in kohlrabi， as well as kohlrabi yield and the death rate of P. xylostella， but there were also differences between treatments. Though the chlorophyll contents of all the treatments were higher that of the CK， the differences were not significant. For the yield of kohlrabi， the yields of Yougui 3， Yougui 2， Guishen 3 and Guishen 2 were significantly higher than that of the CK， and Yougui 3， Guishen 3， Yougui 2 and Guishen 2 exhibited the death rates of P. xylostella significantly higher than the CK， indicating that the Yougui and Guishen treatments were beneficial to the increase of kohlrabi yield and the improvement of death rate of P. xylostella.
To sum up， spraying 2-3 times of any of the two kinds of silicon fertilizers could promote growth of kohlrabi， improve yield of kohlrabi， as well as alleviating the damage by P. xylostella and increasing its death rate. Therefore， silicon fertilizer could be used rationally in the production of kohlrabi.
References
[1] FENG YQ. Siliceous fertilizer to become a new fertilizer product in expansion of agriculture in China[J]. Chemical Fertilizer Industry， 2000， 27（4）： 9-13.
[2] ZHOU CX， ZHANG JY， LI BX. Current status and developing prospect of silicate fertilizer[J]. Chemical Industry and Engineering， 2006， 27（6）： 48-53.
[3] LIU JH， CHEN Y. Application and developing prospect of silicon fertilizer[J]. Henan Chemical Industry， 2000（1）： 7. [4] SONG LY. Effect of silicon fertilizer the production of pollution-free vegetable[J]. Modern Agricultural Science and Technology， 2013（5）： 43.
[5] SEEBOLD KW， DATNOFF LE， CORREA-VICTORIA FJ， et al. Effect of silicon rate and host resistance on blast， scald， and yield of upland rice[J]. Plant Disease， 2007（84）： 871-876.
[6] MATYCHENKOV VV， AMMOSOVA YM. Effect of amorphous silica on some properties of a sod-podzolic soil[J]. Eurasian soil Science， 1996， 28（10）： 87-99.
[7] MATYCHENKOV VV， CALVERT DV， SNYDER GH. Prospective of silicon fertilizer for citrus in Florida[C]. Proceedings-Soil and Crop Science Society of Florida， 2000（59）： 137-141.
[8 ] YANG ES， ZHANG DC. Story of vegetables[M]. Beijing： Electronic Industry Press， 2015.
[9] WANG C. Nutrition and health function of cabbage vegetable[J]. Food Research and Development， 2002， 23（5）： 66-67.
[10] WANG YL， LI CH， WANG J， et al. Application and prospect of slow/controlled release fertilizers in maize production[J]. Chinese Agricultural Science Bulletin， 2009， 25（24）： 254-257.
Key words Silicon fertilizer； Kohlrabi； Growth； Plutella xylostella
Silicon is acknowledged as the fourth plant nutrients following nitrogen， phosphorous and potassium in the international soil science circles. In agricultural production in China， nitrogen， phosphorous and potassium fertilizers were applied in large quantities， and in order to ensure healthy growth of agricultural crops and improve yield， certain quantity of silicon fertilizer also should be applied[1]. Experimental results show silicon fertilizer has a remarkable effect of improving yield of agricultural crops， and has been applied to crops including rice， wheat， soybean， peanut， sugarcane and fruit trees currently[2-3]. Firstly， silicon fertilizer could alleviate the occurrence of pests and diseases on vegetables and crops and reduce the consumption of agrochemicals[4]. Secondly， silicon fertilizer could inhibit pathogenic microbes in soil， and could effective prevent the survival and propagation of various pathogenic microbes including mould[5]. Thirdly， after vegetables and crops absorb silicon， silicified cells are formed in them， the cell walls of the surfaces of stems and leaves are thickened， forming a hard protective layer， which is difficult to be bitten by insects and infected by pathogenic microbes[6-7].
In China， kohlrabi has a very long planting history and is popular[8]. As a kind of vegetable， it has certain health-caring and anti-cancer effects[9]. Supplementation of available silicon by spraying effective silicon onto leaf surface is an effective way to improve crop yield and quality[10]. In this study， the effects of silicon fertilizer on kohlrabi （Brassica oleracea L. var. caulorapa DC.） and Plutella xylostella were studied， so as to serve practical production. Materials and Methods
Experimental materials
Silicon fertilizers
Yougui was produced by Nangong Huayang Chemical Plant， and Guishen was produced by Cheng Pu Fertilizer Industry.
Tested kohlrabi
Kohlrabi was produced by Tianjin Shennong Seed Co.， Ltd.
Tested pests
P. xylostella was collected from the greenhouse of Agricultural College， Yanbian University. The pests were fed with cabbage leaves in laboratory. The pests were cultured at 23-27 ℃ under 16 （light） L ： 8 （dark） D and a relative humidity of 50%-60%.
Experimental instruments
Electrodyne （3JWB-16A）； plant chlorophyll meter （SY-S02）； leaf thickness measuring instrument （LS-3）； electronic scale （JM-A2003）.
Experimental methods
Sowing of kohlrabi
On June 5， the field in greenhouse of the Agricultural College was turned over and ridged， and each hole was sown with 3 seeds according to a row placing of 0.6 m and a plant spacing of 0.4 m. On July 5， only one healthy plant was reserved in one hole.
Silicon fertilizer treatments
Each plot included 3 rows， each of which included 5 plants， and there were 15 kohlrabi plants in one plot. From July 20， silicon fertilizer treatment was performed every 10 d. Yougui and Guishen were diluted by 750 times， and sprayed onto the leaves with the electrodyne. In order to achieve well adsorption of the silicon fertilizers onto kohlrabi leaves， 2 000 times dilution of Tween 80 was added into the electrodyne. The experiment was designed with seven treatments， i.e.， control （CK）， Yougui 1 （spraying Yougui once， similarly hereinafter）， Yougui 2 and Yougui 3， and Guishen 1， Guishen 2 and Guishen 3.
Investigation of kohlrabi growth
Measurement of leaf length and leaf width of kohlrabi
On August 25， upper leaves on kohlrabi were measured for leaf length and leaf width， and the values were recorded.
Measurement of chlorophyll content and leaf thickness of kohlrabi
Also， on August 25， the chlorophyll content and thickness of kohlrabi leaves were measured with the chlorophyll meter and the measuring instrument of leaf thickness， respectively， and the values were recorded. During the determination of chlorophyll， the measurement was performed at the same time period. During the measurement of leaf thickness， the leaf position was kept the same when keeping away from leaf veins.
Test of P. xylostella
On August 26， upper leaves were collected from the 7 treatments （CK， Yougui 1， 2 and 3 and Guishen 1， 2 and 3）， respectively. Leaf segments （2 cm×5 cm） were cut and placed into nursing bottles. Then， 10 second instar larvae were added into each nursing bottle. Each group included three nursing bottles as three replicates. The larvae were fed with upper leaves of kohlrabi every day， and the death number of P. xylostella was investigated on the third day and recorded. On August 28， lower leaves damaged by P. xylostella were collected from the 7 treatments， and placed and photographed according to silicon fertilizer and treatment times. Investigation of yield per plant of kohlrabi
On August 30， kohlrabi was pulled out， and the leaves and roots were removed. The weight of single corm of each treatment was measured with the electronic balance.
Analysis software
The data were analyzed with DPS v7.05， and multiple comparisons were performed by Duncan’s new multiple range method.
Results and Analysis
Effects of the two kinds of silicon fertilizers on growth of kohlrabi
It could be seen from Table 1 that among the different treatments， Guishen 3 exhibited a leaf length of 30.7 cm， which was significantly longer than that of CK （23.5 cm）， while the leaf lengths of other treatments were longer than that of CK， but the differences were not significant.
It could been seen from the leaf width data in Table 1 that among the different treatments， the length widths of Yougui 3， Guishen 3， Guishen 2 and Yougui 2 were， respectively， 19.2， 17.7， 17.5 and 16.6 cm， which were significantly longer than that of the CK （12.40 cm）， while the leaf widths of Yougui 1 and Guishen 1 were longer than that of the CK， but the differences were not significant.
It could be seen from the leaf thickness column in Table 1 that among the different treatments， the leaves in Yougui 3， Yougui 2， Guishen 3， Yougui 1， Guishen 2 and Guisheng 1 were thicker than those in the CK， but the differences between any two of the treatments were not significant.
It could be seen from Table 1 that with the spraying times increasing， the leaf length， leaf width and leaf thickness of kohlrabi also increased. Overall， Yougui had a better effect than Guishen.
Effects of the two kinds of silicon fertilizers on chlorophyll content and yield of kohlrabi and P. xylostella
It could be seen from the chlorophyll column in Table 2 that among the different treatments， Yougui 3， Guishen 3， Yougui 2， Guishen 2， Guishen 1 and Yougui 1 had the chlorophyll contents higher than CK， but the differences were not significant.
It could be seen from the yield column in Table 2 that among the different treatments， Yougui 3， Yougui 2， Guishen 3 and Guishen 2 had the yields per plant， respectively， of 873.5， 800.2， 682.8 and 644.8 g， which were significant higher than the CK （363.1 g）， while the yields per plant of Yougui 1 and Guishen 1 were higher than that of the CK， but the differences were not significant.
The death rate data of P. xylostella in Table 2 showed that among the different treatments， Yougui 3， Guishen 3， Yougui 2 and Guishen 2 had the death rates of P. xylostella， respectively， of 31.0%， 28.0%， 24.0% and 20.0%， which were significantly higher than the CK （9%）， and though the death rates of P. xylostella in Yougui 1 and Guishen 1 were higher than the CK， the differences were not significant. It could be seen from Table 2 that with the spraying times increasing， the chlorophyll content and yield of kohlrabi and the death rate of P. xylostella also increased. Overall， Yougui had a better effect than Guishen.
Effects of the two kinds of silicon fertilizers on damage by P. xylostella
It could be seen from Fig. 1 and Fig. 2 that the treatments with the two kinds of silicon fertilizers greatly affected the damage by P. xylostella， and with the applying times increasing， the damage on the leaves of kohlrabi got lighter.
Discussion and Conclusions
The treatments with the two kinds of silicon fertilizers could promote the growth of leaf length， leaf width and leaf thickness of kohlrabi， and different treatments differed in the promoting effect. Among them， only Guishen 3 exhibited the leaf length significantly longer than the CK； the leaf widths of Yougui 3， Guishen 3， Guishen 2 and Yougui 2 were significantly wider than the CK； and the thicknesses of all the treatments were not significantly different from the CK.
The treatments with the two kinds of silicon fertilizers could improve chlorophyll content in kohlrabi， as well as kohlrabi yield and the death rate of P. xylostella， but there were also differences between treatments. Though the chlorophyll contents of all the treatments were higher that of the CK， the differences were not significant. For the yield of kohlrabi， the yields of Yougui 3， Yougui 2， Guishen 3 and Guishen 2 were significantly higher than that of the CK， and Yougui 3， Guishen 3， Yougui 2 and Guishen 2 exhibited the death rates of P. xylostella significantly higher than the CK， indicating that the Yougui and Guishen treatments were beneficial to the increase of kohlrabi yield and the improvement of death rate of P. xylostella.
To sum up， spraying 2-3 times of any of the two kinds of silicon fertilizers could promote growth of kohlrabi， improve yield of kohlrabi， as well as alleviating the damage by P. xylostella and increasing its death rate. Therefore， silicon fertilizer could be used rationally in the production of kohlrabi.
References
[1] FENG YQ. Siliceous fertilizer to become a new fertilizer product in expansion of agriculture in China[J]. Chemical Fertilizer Industry， 2000， 27（4）： 9-13.
[2] ZHOU CX， ZHANG JY， LI BX. Current status and developing prospect of silicate fertilizer[J]. Chemical Industry and Engineering， 2006， 27（6）： 48-53.
[3] LIU JH， CHEN Y. Application and developing prospect of silicon fertilizer[J]. Henan Chemical Industry， 2000（1）： 7. [4] SONG LY. Effect of silicon fertilizer the production of pollution-free vegetable[J]. Modern Agricultural Science and Technology， 2013（5）： 43.
[5] SEEBOLD KW， DATNOFF LE， CORREA-VICTORIA FJ， et al. Effect of silicon rate and host resistance on blast， scald， and yield of upland rice[J]. Plant Disease， 2007（84）： 871-876.
[6] MATYCHENKOV VV， AMMOSOVA YM. Effect of amorphous silica on some properties of a sod-podzolic soil[J]. Eurasian soil Science， 1996， 28（10）： 87-99.
[7] MATYCHENKOV VV， CALVERT DV， SNYDER GH. Prospective of silicon fertilizer for citrus in Florida[C]. Proceedings-Soil and Crop Science Society of Florida， 2000（59）： 137-141.
[8 ] YANG ES， ZHANG DC. Story of vegetables[M]. Beijing： Electronic Industry Press， 2015.
[9] WANG C. Nutrition and health function of cabbage vegetable[J]. Food Research and Development， 2002， 23（5）： 66-67.
[10] WANG YL， LI CH， WANG J， et al. Application and prospect of slow/controlled release fertilizers in maize production[J]. Chinese Agricultural Science Bulletin， 2009， 25（24）： 254-257.