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Behaviour of some pesticides in a nutrient-film and in a rock-wool system
Scientia Horticulturae, 25 (1985) 1--9 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
1
B E H A V I O U R O F SOME P E S T I C I D E S I N A N U T R I E N T - F I L M A N D I N A ROCK-WOOL SYSTEM
S.J.H. CRUM 1, W.TH. RUNIA~, M. LEISTRA1 and J.H. SMELT 1 1Institute for Pesticide Research, Marijkeweg 22, 6709 PG Wageningen (The Netherlands) Glasshouse Crops Research and Experiment Station, P.O. Box 8, 2670 AA Naaldwijk (The Netherlands) (Accepted for publication 17 August 1984) ABSTRACT Crum, S.J.H., :Runia, W.Th., Leistra, M. and Smelt, J.H., 1985. Behaviour of some pesticides in a nutrient-film and in a rock-wool system. Scientia Hortic., 25: 1--9. Adsorption of ethoprophos, tetrachlorvinphos and etridiazole onto rock-wool was found to be much weaker than adsorption onto soils. The rates of transformation in water and in water plus rock-wool at 20°C were low. The concentration of etridiazole was measured in a nutrient-film system with recirculating nutrient solution. The decrease was rapid in the first few hours but was more gradual afterwards, to low values at 8 days after application. Etridiazole penetrated poorly into the lower part of a rockwool system when the solution was trickled near the stem-base of the tomato plants. Keywords: adsorption; nutrient-film; pesticides; rock-wool; transformation.
INTRODUCTION T h e g r o w t h o f h o r t i c u l t u r a l c r o p s o n r o c k - w o o l has b e e n e x t e n d i n g fairly r a p i d l y in r e c e n t years. I n T h e N e t h e r l a n d s , the surface area o f g r e e n h o u s e s in w h i c h this s u b s t r a t e is used n o w a m o u n t s t o a b o u t 1 0 0 0 ha. T o m a t o e s , c u c u m b e r s , paprikas (sweet peppers) a n d aubergines (egg-plants) are t h e m a i n c r o p s g r o w n o n r o c k - w o o l . T h e g r o w t h o f c r o p s in nutrient-films is also promising. I n these n e w g r o w i n g systems, t h e r o o t s o f t h e c r o p m a y suffer f r o m fungal diseases o r f r o m a t t a c k b y n e m a t o d e s and a r t h r o p o d s . So it m a y be n e c e s s a r y t o p r o t e c t t h e c r o p s w i t h pesticides. V e r y little i n f o r m a t i o n is available o n t h e b e h a v i o u r o f pesticides in these g r o w i n g systems. T h e a d s o r p t i o n o n r o c k - w o o l and o n plastic materials (tubes, sheets) s h o u l d be k n o w n t o m a k e a first e s t i m a t e o f t h e a p p l i c a t i o n rate n e e d e d as c o m p a r e d w i t h t h o s e applied t o soil. T o o high a rate c a n lead t o t o x i c effects o n plants a n d t o high residues in t h e p r o d u c t s . T h e a d s o r p t i o n o n r o c k - w o o l d e t e r m i n e s t h e e x t e n t to w h i c h t h e t r a n s p o r t o f t h e pesticides is r e t a r d e d w i t h r e s p e c t t o t h e water. High c o n c e n t r a t i o n s
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© 1985 Elsevier Science Publishers B.V.
2 will seriously enhance the e x t e n t to which soil and water courses are polluted b y surpluses o f nut r i ent solution containing pesticide. The rate o f decrease of t he active ingredient in the systems determines h o w o f t e n the ~Compounds should be applied. When t h e decrease is slow, m u ch pesticide can be saved and the discharge into the environment can be reduced. The decrease is the result of various processes and their relative i m p o r t a n c e should be know n to allow estimation o f the rate for conditions t h a t have n o t been studied. Two o r g an o p hos pha t e pesticides, e t h o p r o p h o s [O-ethyl S,S-dipropyl p h o s p h o r o d i t h i o a t e ] : and tetrachlorvinphos [O,O-dimethyl 2-chloro-1(2,4,5-trichlorophenyl)vinyl phosphate Z isomer], were selected for this study. Their adsorption on soils has recently been measured, so t hat the adsorption on rock-wool can be c om par e d with those data. The adsorption o f e t h o p r o p h o s on soils was m o d e r a t e to fairly strong (Leistra, 1979) and t h a t o f tetrachlorvinphos was strong (Leistra et al., 1984). Etridiazole [5-ethoxy-3-(trichloromethyl)-l,2,4-thiadiazole] was selected because it was being tested for use as a fungicide in nutrient-film and rock-wool systems. Th e solubility of etridiazole in water is variously r e p o r t e d to be 50 mg 1-1 at 25°C (Worthing, 1979) and less than 200 mg 1-1 (Spencer, 1982). Values stated for the vapour pressure o f this c o m p o u n d are: 0.013 Pa at r o o m t e m p e r a t u r e (Worthing, 1979), and 13 Pa at 72°C (Spencer, 1982). The mo b ility o f etridiazole measured by thin-layer c h r o m a t o g r a p h y on soil was very low (Helling et al., 1974). Rock-wool is made by melting a m i x t u r e of 60% diabase, 20% limestone and 20% coke at a t e m p e r a t u r e of about 1600°C. The melt is sprayed into fibres 5 p m thick and these are pressed into slabs with a bulk density o f only 70 kg m -3 . Substances (including phenol resin) are added during cooling to give the p r o d u c t firmness and to allow water retention. T he volume fraction o f pores is 0.96 m 3 m -3, and with decreasing water pressure around --103 Pa, a lot o f water is already lost (Verwer and Welleman, 1980). METHODS o n t o r o c k - w o o l . - - The pesticides were dissolved in water with CaC12 at a c o n c e n t r a t i o n of 0.007 mol U 1. The initial concentrations o f the pesticides at two or t hr e e levels were in the range 2--17 mg 1-1, and t h e y were measured exactly. A volume o f 40 ml of these solutions was added to 1.5 g of rock-wool (in little pieces, bulk volume a b o u t 20 ml) in centrifuge tubes (90 ml). T he tubes were closed with ground-glass stoppers and r o t a t e d for 24 h (20°C) at a rotational f r e q u e n c y of 10 min -1. After this, th e tubes with c o n t e n t were centrifuged for 3 min at 2000 min -1 (450 g). Samples o f t h e equilibrium solutions were ext ract ed with hexane and th e concentrations were t h e n measured b y gas chrom at ography. The adsorption was again measured after the tubes with t he rock-wool and remaining solution had been stored for a b o u t 1 week. A fraction of
Adsorption
the solution and the remaining fraction plus rock-wool were e x t r a c t e d and analysed separately. T he r e were no clear differences bet w een t he adsorption coefficients obtained before and after storage, so the values can be presented together. R a t e o f t r a n s f o r m a t i o n in w a t e r and in w a t e r p l u s rock-wool. -- The cont ent s
o f t h e centrifuge tubes used in t he adsorption study were stored for 7 days ( e t h o p r o p h o s and tetrachlorvinphos) or for 6 days (etridiazole) at 20°C in the dark. Af ter this period, the amounts of t he pesticides remaining were analysed and c o m p a r e d with t he initial amounts to see how m u c h of the pesticides was transformed. Solutions o f t h e pesticides in water (with CaC12 at 0.007 mol 1-1) were stored unde r the same conditions and the e x t e n t of t r a n s f o r m a t i o n was c o m p a r e d with t h a t in water plus rock-wool. The rate o f tr ans f or m at i on o f etridiazole in a n u t r i e n t solution was also measured. This solution w a s taken f r om a circulation t ank of a n u t r i e n t film system in a greenhouse t ha t had been in operat i on for a few months. Etridiazole was added bot h as wettable p o w d e r and as emulsifiable concentrate at a calculated initial c o n c e n t r a t i o n of 100 mg 1-1, and was intensively mixed for 2 h. One flask f or e a c h t r e a t m e n t was placed in the dark at 20°C and a n o t h e r in the l a b o r a t o r y in daylight. Concentrations in the solution were measured at intervals up t o 28 days after a p p l i c a t i o n . in n u t r i e n t - f i l m systems. -- T h e behaviour of etridiazole in systems with a circulating nutrient-film was studied in a heated greenhouse. In a preliminary experiment, the rate of dissolution of etridiazole in water f r o m the wettable p o w d e r (35%) was slow. Even after shaking intensively fo r 30 min, only 3/4 o f the final c o n c e n t r a t i o n was reached, so this formulation was n o t usable here. T he emulsifiable c o n c e n t r a t e (etridiazole 700 g 1-1) was added to t he nutrient solution (50 1) in circulation tanks and intensively mixed f o r 5 m i n , which proved adequate. Etridiazole at an initial c o n c e n t r a t i o n of over 25 mg 1-! was added to two tanks for film systems (F1 and F2), and an initial c o n c e n t r a t i o n o f over 50 mg 1-1 to t w o o t h e r tanks (F3 and F4). T he nut r i e nt solution in a tank was p u m p e d through tubes t o five gutter segments (2.5 m long, 0.25 m w i d e ) l i n e d with p oly(ethene) sheet. After flowing t hr ough a segment (slope 1.5%), t h e solution f l o w e d back t o the t a n k t hr ough a tube. The flow t h r o u g h the segments was c o n t i n u o u s and t h e rate was 1 1 min -1. Each gutter segment contained five well-developed t o m a t o plants at t he end o f their p r o d u c t i o n period. T he e x p e r i m e n t was in t he second half o f N o v e m b e r 1983. The transpiration by t he crop was estimated f r o m solar radiation data to be 7 m m in 8 days (Ph. Hamaker, personal c o m m u n i c a t i o n , 1984), The t e m p e r a t u r e of the water in t h e tanks a n d g u t t e r s was a b o u t 21 ° C. The solution in the tanks and at t he outl et of t h e gutter segments were sampled at i n t e r v a l s over 8 days to measure the etridiazole concent rat i on. Etridiazole
Before adding the c o m p o u n d , the nutrient solutions were sampled and t h e y did n o t contain interfering substances. Only after the sampling on Day 4 were the tanks replenished with fresh nutrient solution (30 l) w i t h o u t etridiazole. Etridiazole in r o c k - w o o l systems° -- The behaviour of etridiazole in rock-
wool systems was studied in t he same period in the greenhouse, Emulsifiable c o n c e n t r a t e of etridiazole was added to the tanks R1 and R2 of two rockwool systems and intensively m i xe d with the 250 1 o f nutrient solution. The initial c o n c e n t r a t i o n of etridiazole was almost 25 mg 1-1. In two ot her rock-wool systems, no fungicide was added to the tanks, but 500 ml of etridiazole solution (over 25 mg 1-1, applied as the same formulation) was p o u r e d close to the stem base of each t o m a t o plant. The nut ri ent solution was p u m p e d th r ough tubes to tricklers close to t he stem base of t he plants (one trickler per plant). Twice a day, 330 ml of nutrient solution was applied to each plant by trickling for 7.5 min. Two plants germinated in rock-wool blocks (7.5 cm 3) had been placed on each rock-wool mat, which was 1 m long, 0.15 m wide and 0.075 m deep. In each rock-wool system, 24 plants were grown. The mats were wrapped r o u n d with p o l y ( e t h e n e ) sheets and the surplus nut r i e nt solution leaked away to the soil t hrough slits. On one of the sides, there was a slit bet w e e n each pair of plants at a height of a b o u t 1 cm above t he b o t t o m of the mats. At th e sampling times in a period of 8 days, about 10 ml of nut ri ent solution was withdrawn with a syringe f r o m t he b o t t o m part of t he mats at 20 places per system and these samples were combined. The n u t r i e n t solutions with etridiazole in t h e tanks were sampled a few times. Gas c h r o m a t o g r a p h y . - - T h e concentrations of e t h o p r o p h o s were measured with a P y e Unicam gas c h r o m a t o g r a p h ( t yp e GCV) equipped with a flamespectrometric d e t e c t o r (phosphorus filter). The glass col um n (length 0.9 m, inner diameter 2 mm) was filled with 3% silicone OV-225 on Chrom osorb WHP (0.15--0.18 mm). T he flow rate o f t he carrier gas N2 was 55 ml min -1 and th at o f th e d e t e c t o r gases H: and air was 30 ml min -1. T he t e m p e r a t u r e in t h e injector part was 210°C, in t he col um n oven 190°C and in the d e t e c t o r part 220 ° C. The r e t e n t i o n time of e t h o p r o p h o s was 1.1 min. Tetrachlorvinphos was measured with t he same apparatus, but the temperatures were higher; injection part 230°C, col um n oven 250°C, and d e t e c t o r part 230°C. T he r e t e n t i o n time of this c o m p o u n d was 1.2 min. The co n cen tr a t i ons o f etridiazole in t he extracts were measured with t h e same gas ch rom at ogr a ph, but now the flame-spectrometric d e t e c t o r was provided with a s ul phur filter. T he glass col um n (length 0.9 m, inner diameter 2 mm) was filled with 4% silicone SE-30 + 6% silicone SP 2401 on S u p e l c o p o r t (0.13--0.15 mm). T he flow rates o f t he gases were t he same as before. The t e m p e r a t u r e in t he injector part was 170°C, in t he col um n oven 170 ° , and in the d e t e c t o r part 200°C. The r e t e n t i o n time o f etridiazole was 1.4 min.
T h e i n j e c t i o n o f t h e s o l u t i o n s t o b e m e a s u r e d was i n t e r c h a n g e d w i t h t h e injection of standard solutions with known concentration, T h e p a r t i t i o n ratio o f e t r i d i a z o l e b e t w e e e n h e x a n e a n d w a t e r was m e a s u r e d to c h e c k t h e s u i t a b i l i t y o f h e x a n e as e x t r a c t i o n solvent. T h e r a t i o was f o u n d t o b e higher t h a n 125, w h i c h s h o w s t h a t h e x a n e is suitable. T h e t e n d e n c y o f e t r i d i a z o l e t o volatilize was c h e c k e d b y e v a p o r a t i n g h e x a n e f r o m a s o l u t i o n o f k n o w n c o n c e n t r a t i o n w i t h an air-flow. T h e r e s u l t s c o n f i r m e d t h e d a t a , i n d i c a t i n g t h a t e t r i d i a z o l e is a s o m e w h a t volatile c o m p o u n d . RESULTS A d s o r p t i o n o n t o rock-wool. -- T h e a d s o r p t i o n o f e t h o p r o p h o s o n r o c k - w o o l was so w e a k t h a t it c o u l d n o t easily be m e a s u r e d . T h e a d s o r p t i o n o f t e t r a c h l o r v i n p h o s c o u l d be m e a s u r e d , a n d t h e average value o f t h e a d s o r p t i o n c o e f f i c i e n t Kr/1 a m o u n t e d t o 2.0 d m 3 kg -1 (n = 6, s = 0.4 d m 3 kg-1). T h e r e was no clear d i f f e r e n c e in t h e a d s o r p t i o n o f e t r i d i a z o l e o n r o c k w o o l at t h e t h r e e c o n c e n t r a t i o n s , a n d storage f o r 6 d a y s h a d n o clear e f f e c t . T h e overall value o f K r / l was 1.8 d m 3 kg -~ (n = 24, s = 0.9 d m 3 kg-1). T h e v a r i a t i o n in t h e m e a s u r e d values was increased b y t h e l o w b u l k d e n s i t y o f r o c k - w o o l . Because t h e c o n t e n t s o f t h e t u b e s s h o u l d m i x t h o r o u g h l y , t h e ratio b e t w e e n m a s s o f r o c k - w o o l a n d v o l u m e o f s o l u t i o n c o u l d n o t b e m u c h increased. R a t e o f t r a n s f o r m a t i o n in w a t e r plus rock-wool. - - T h e f r a c t i o n o f t h e initial e t h o p r o p h o s r e m a i n i n g in t h e t u b e s c o n t a i n i n g w a t e r plus r o c k - w o o l a f t e r 7 d a y s o f s t o r a g e was 95% (n = 4, s = 6%) o f t h e initial a m o u n t . O n l y 3% o f t h e initial a m o u n t was m e a s u r e d to b e t r a n s f o r m e d in w a t e r w i t h o u t rock-wool. T h e f r a c t i o n t r a n s f o r m e d o f t e t r a c h l o r v i n p h o s in w a t e r plus r o c k - w o o l was s o m e w h a t higher. O f t h e initial a m o u n t , 83% (n = 4, s = 6%) r e m a i n e d a f t e r 7 days. Again t h e e x t e n t o f t r a n s f o r m a t i o n in w a t e r w i t h o u t r o c k - w o o l was l o w e r , a n d a m o u n t e d t o 8%. T h e f r a c t i o n o f etridiazole f o u n d a f t e r 6 d a y s o f storage in w a t e r plus r o c k - w o o l was 92% (n = 6, s = 3%) o f t h e initial a m o u n t , w h i c h m e a n s t h a t t r a n s f o r m a t i o n is slow. In w a t e r w i t h o u t r o c k - w o o l , h a r d l y a n y t r a n s f o r m a t i o n o c c u r r e d : 99% r e m a i n e d (n = 4, s = 2%). T h e r a t e o f t r a n s f o r m a t i o n o f e t r i d i a z o l e in a n u t r i e n t s o l u t i o n at 2 0 ° C in t h e d a r k was o n l y slow; a f t e r 28 d a y s , 8 0 - - 9 0 % o f t h e initial a m o u n t r e m a i n e d . T r a n s f o r m a t i o n u n d e r l a b o r a t o r y c o n d i t i o n s in t h e light w a s slightly m o r e rapid; a f t e r 28 d a y s , a l m o s t 80% r e m a i n e d . T h u s , t h e p e s t i c i d e was n o t p h o t o s e n s i t i v e u n d e r t h e s e c o n d i t i o n s . E t r i d i a z o l e in n u t r i e n t - f i l m systems. -- T h e results o f t h e m e a s u r e m e n t s o f e t r i d i a z o l e in t h e c i r c u l a t i o n t a n k s o f t h e n u t r i e n t - f i l m s y s t e m s are given in T a b l e I. T h e values are averages f o r t w o s y s t e m s w i t h e q u a l s t a r t i n g c o n c e n -
t r a t i o n , a n d t h e m e a n d i f f e r e n c e b e t w e e n t h e t w o values was 7% o f t h e i r average, w i t h a m a x i m u m o f 20%. T h e d e c r e a s e i n c o n c e n t r a t i o n was r a p i d f o r t h e first f e w h o u r s a f t e r a p p l i c a t i o n . Firstly, d i l u t i o n o c c u r r e d w i t h t h e w a t e r in t h e g u t t e r - s e g m e n t s (15 1 p e r s y s t e m ) s h o r t l y a f t e r a p p l i c a t i o n . S e c o n d l y , t h e fungicide was a d s o r b e d o n t o t h e plastic m a t e r i a l s [poly( v i n y l c h l o r i d e ) t a n k s a n d t u b e s , p o l y ( e t h e n e ) sheets] a n d o n t o t h e p l a n t r o o t s . A f t e r t h e first f e w hours, t h e decrease b e c a m e m o r e gradual until o n l y l o w c o n c e n t r a t i o n s w e r e left a f t e r 8 days. TABLEI Concentrations (mg 1-1) of etridiazole measured in solution in the circulation tanks of nutrient-film systems Time (days) 0 Circulation tanks F1 and F2 F3 and F4
29.0 54.5
0.10 11.2 26.4
0.25 8.8 19.6
1 4.7 10.9
2
4
8
4.0 8.4
2.2 4.3
0.8 1.2
I n t h e s y s t e m s w i t h t h e l o w e r initial c o n c e n t r a t i o n , t h e c o n c e n t r a t i o n s o f e t r i d i a z o l e at t h e o u t f l o w o f t h e s e g m e n t s w e r e 14.5 m g 1-1 a f t e r 15 m i n a n d 5.6 m g 1-1 a f t e r 1 d a y . I n t h e s y s t e m s w i t h t h e higher initial c o n c e n t r a t i o n s , r e s p e c t i v e values w e r e 29.1 m g 1-1 a f t e r 45 m i n a n d 12.1 m g 1-1 a f t e r 1 d a y . T h e s e c o n c e n t r a t i o n s w e r e close t o t h o s e m e a s u r e d f o r t h e c i r c u l a t i o n t a n k s ( T a b l e I), so t h a t t h e r e d i s t r i b u t i o n o f t h e c o m p o u n d t h r o u g h o u t t h e s y s t e m s w a s rapid. in rock-wool systems. - - T h e c o n c e n t r a t i o n s o f e t r i d i a z o l e in t h e t a n k s R 1 a n d R 2 o f t w o r o c k - w o o l s y s t e m s w e r e 2 2 . 5 m g 1-' i m m e d i a t e l y a f t e r m i x i n g , 2 0 . 8 m g 1-1 a f t e r 1 d a y a n d 18.1 m g 1-1 a f t e r 4 d a y s . T h e c o n c e n t r a t i o n s in t h e s o l u t i o n p o u r e d close t o t h e s t e m - b a s e s was 25.3 m g
Etridiazole
TABLE II Concentrations (mg 1-1) of etridiazole measured in nutrient solution in the bottom part of rock-wool mats Time (days) 0.10 Mode of application To tanks R1 and R2 To stem base
<0.1 <0.1
0.25 <0.1 <0.1
1
2
<0.1 0.1
<0.1 0.4
4
8
0.3 0.2
1.5 0.2
1-1. These c o n c e n t r a t i o n s were as e x p e c t e d . T h e c o n c e n t r a t i o n s m e a s u r e d in t h e n u t r i e n t s o l u t i o n sampled f r o m t h e l o w e r p a r t o f t h e r o c k - w o o l mats are given in T a b l e II. In t h e first few days, t h e c o n c e n t r a t i o n s o f etridiazole were b e l o w or close t o t h e d e t e c t i o n limit. In t h e w h o l e measuring p e r i o d , t h e c o n c e n t r a t i o n s r e m a i n e d very low. Thus, w i t h n o n e o f t h e m e t h o d s o f a p p l i c a t i o n did t h e etridiazole solutions p e n e t r a t e t h o r o u g h l y into t h e mats. GENERAL DISCUSSION T h e a d s o r p t i o n o f t h e o r g a n o p h o s p h o r u s pesticides o n t o r o c k - w o o l can be c o m p a r e d w i t h t h e i r a d s o r p t i o n o n soils. T h e a d s o r p t i o n o f e t h o p r o p h o s o n t o soils is m o d e r a t e t o fairly strong, w i t h a d s o r p t i o n c o e f f i c i e n t s Ks/1 = 0.8 d m 3 kg -1 f o r a sandy loam soil w i t h 1.7% organic m a t t e r , Ks/1 = 1.9 d m 3 kg -1 f o r a l o a m soil with 2.6% organic m a t t e r , and Ks/1 = 4.4 d m 3 kg -1 for a h u m i c sand soil with 7.0% organic m a t t e r (Leistra, 1 9 7 9 ) . T h e adsorpt i o n o f t e t r a c h l o r v i n p h o s o n t o softs was f o u n d to be strong, with Ks/l = 20 d m 3 kg -1 f o r a h u m i c sand soil w i t h 3.1% organic m a t t e r , Ks/1 = 20 d m 3 kg -1 f o r a s a n d y loam soil w i t h 3.6% organic m a t t e r , and Ks/l = 115 d m 3 kg -1 f o r a l o a m soil with 9.7% organic m a t t e r (Leistra et al., 1 9 8 4 ) . T h e a d s o r p t i o n o f e t h o p r o p h o s o n t o r o c k - w o o l ( t o o w e a k to be m e a s u r e d ) and t h a t o f t e t r a c h l o r v i n p h o s o n t o r o c k - w o o l (Kr]1 = 2.0 d m 3 kg -1) were t h u s m u c h w e a k e r t h a n t h e a d s o r p t i o n o f t h e same c o m p o u n d s o n t o soils. T h e r e t a r d a t i o n o f t h e m o v e m e n t t h r o u g h a r o c k - w o o l m a t o f etridiazole can be e s t i m a t e d with respect t o water. S u p p o s e t h a t t h e w a t e r flows t h r o u g h a v o l u m e f r a c t i o n o f p o r e s ~ = 0.80 d m 3 d m -3, and t h a t t h e d r y b u l k density o f t h e m a t is 0.07 kg d m -3. T h e a d s o r p t i o n c o e f f i c i e n t was Kr/l= 1.8 d m 3 kg -1, so t h e c a p a c i t y f a c t o r b e c o m e s C a p = ~ +Pb Kr/1 = 0 - 9 3 dm3 d m -3. This implies t h a t 86% o f etridiazole is in t h e w a t e r phase, while o n l y 14% is a d s o r b e d . T h e rate o f t r a n s p o r t o f this c o m p o u n d t h r o u g h t h e mat will t h u s be a b o u t 0.86 t i m e s t h e rate o f w a t e r flow, w h i c h m e a n s t h a t r e t a r d a t i o n is o n l y low. In practice, t h e a d s o r p t i o n o f a c o m p o u n d like etridiazole in r o c k - w o o l systems will b e substantially greater t h a n w o u l d f o l l o w f r o m t h e measurem e n t s f o r " c l e a n " r o c k - w o o l alone. In view o f t h e r a t h e r low w a t e r solubility and t h e r a t h e r high h e x a n e / w a t e r p a r t i t i o n ratio, a t e n d e n c y t o fairly strong a d s o r p t i o n o n t o organic materials m a y be e x p e c t e d . T h e c o m p o u n d was m i x e d w i t h n u t r i e n t s o l u t i o n in plastic tanks and t h e s o l u t i o n was p u m p e d t h r o u g h plastic t u b e s and trickierS. I n t h e r o c k - w o o l mats, t h e r e was a lot o f r o o t material and t h e m a t s were w r a p p e d in plastic sheets. T h u s retardat i o n o f t h e t r a n s p o r t in p r a c t i c e will o f t e n be greater t h a n w o u l d f o l l o w f r o m calculations f o r " c l e a n " rocl~-wool. T h e rate o f t r a n s f o r m a t i o ~ o f e t h o p r o p h o s , t e t r a c h l o r v i n p h o s and etridiazole i n c u b a t e d in w a t e r and in w a t e r plus r o c k - w o o l was fairly slow. T h e r e is no clear i n d i c a t i o n t h a t t h e r o c k - w o o l c a t a l y s e d t h e t r a n s f o r m a t i o n o f these c o m p o u n d s . H o w e v e r , in practical r o c k - w o o l and n u t r i e n t - f i l m systems,
a s u b s t a n t i a l m i c r o b i a l a c t i v i t y c a n build up, w h i c h m a y be e x p e c t e d t o s p e e d u p t h e r a t e o f t r a n s f o r m a t i o n . T h e v a p o u r pressures r e p o r t e d in t h e l i t e r a t u r e a n d e x p e r i e n c e in t h e l a b o r a t o r y i n d i c a t e t h a t v o l a t i l i z a t i o n is a n o t h e r i m p o r t a n t p r o c e s s in t h e decrease o f etridiazole in n u t r i e n t - f i l m s y s t e m s . T h e r e is no i n d i c a t i o n t h a t p h o t o s e n s i t i v i t y is i m p o r t a n t f o r etridiazole in s u c h s y s t e m s . The extent to which the nutrient solution penetrated into the rock-wool m a t s can be e s t i m a t e d . T h e r a t e o f d r i p p i n g n e a r e a c h p l a n t was 6 6 0 c m 3 d a y -1, and t h e r a t e o f t r a n s p i r a t i o n p e r p l a n t w o u l d be 4 4 0 c m 3 d a y -1. A surplus o f 2 2 0 c m 3 passing t h r o u g h a f l o w p a t h w i t h a cross-sectional a r e a o f 6 0 c m 2 (15 c m wide, 4 c m high) f l o w s a d i s t a n c e o f 3.7 c m . So t h e nutrient solution could only gradually penetrate the lower part of the rockw o o l m a t s . B e c a u s e o f t h e fairly strong a d s o r p t i o n o f etridiazole o n t o plastics a n d p l a n t r o o t s , t h e p e n e t r a t i o n o f this fungicide i n t o t h e m a t s w o u l d h a v e b e e n e v e n slower t h a n t h a t o f t h e n u t r i e n t s o l u t i o n . A f t e r p o u r i n g 500 c m 3 etridiazole s o l u t i o n o n t o t h e m a t s a n d d r i p p i n g at 6 6 0 c m 3 d a y -1, o n e w o u l d e x p e c t fairly high c o n c e n t r a t i o n s in t h e l o w e r p a r t o f t h e m a t s w i t h i n o n l y a f e w days. T h e l o w c o n c e n t r a t i o n s m e a s u r e d i n d i c a t e t h a t e t r i d i a z o l e p r e s u m a b l y m o v e d in a l a y e r o f w a t e r o n t o p o f a s t a g n a n t w a t e r l a y e r in t h e b o t t o m p a r t o f t h e m a t s . T h e s a m e t y p e o f p r o b l e m w a s experienced by van Noordwijk (1978). He found that a "new" nutrient s o l u t i o n s p r e a d o n t o p o f t h e " o l d " s o l u t i o n in t h e b o t t o m p a r t o f a r o c k w o o l m a t a n d t h a t m i x i n g o f t h e s e s o l u t i o n s was v e r y slow. P e n e t r a t i o n c o u l d b e i m p r o v e d b y s u p p l y i n g t h e s o l u t i o n at a higher r a t e a n d b y m a k i n g t h e slits in t h e w r a p p i n g - s h e e t s at t h e b o t t o m . H o w e v e r , e n h a n c e d f l o w r a t e s w o u l d increase t h e l e a k a g e o f n u t r i e n t s o l u t i o n w i t h p e s t i c i d e t h r o u g h t h e slits into t h e soil, w h i c h c o u l d lead to c o n t a m i n a t i o n o f g r o u n d w a t e r a n d w a t e r courses (via t h e drain t u b e s ) . P r e f e r a b l y , f u r t h e r d e v e l o p m e n t s s h o u l d go in t h e d i r e c t i o n o f r e c i r c u l a t i o n s y s t e m s . H o w e v e r , p r o b l e m s w i t h t h e q u i c k s p r e a d o f diseases t h r o u g h t h e s e s y s t e m s a n d w i t h t h e salt a c c u m u l a t i o n in t h e n u t r i e n t s o l u t i o n will h a v e t o be o v e r c o m e .
REFERENCES
Helling, C.S., Dennison, D.G. and Kaufman, D.D., 1974. Fungicide movement in soils. Phytopathology, 64: 1091--1100. Leistra, M., 1979. Computing the movement of ethoprophos in soil after application in spring. Soil Sci., 128: 303--311. Leistra, M., Tuinstra, L.G.M.Th., van der Burg, A.M.M. and Crum, S.J.H., 1984. Contribution of leaching of diazinon, parathion, tetrachlorvinphos and triazophos from glasshouse softs to their concentrations in water courses. Chemosphere, 13 : 403--413. van Noordwijk, M., 1978. Zoutophoping en beworteling bij de teelt van tomaten op steenwol (Distribution of salts and root development in the culture of tomatoes on rock-wool). Report No. 3-78, Institute for Soil Fertility, Haren, The Netherlands, 21 PP.
Spencer, E.Y., 1982. Guide to the chemicals used in crop protection. Publication 1093, 7th edn., Research Branch, Agriculture Canada, 595 pp. Verwer, F.L. and Welleman, J.J.C., 1980. The possibilities of Grodan rock-wool in horticulture. Proc. Fifth Int. Congr. Soilless Culture, Wageningen, pp. 263--278. Worthing, C.R., 1979. Pesticide manual, 6th edn., British Crop Protection Council, Cradley, Malvern, Worcestershire, England, 655 pp.
1
B E H A V I O U R O F SOME P E S T I C I D E S I N A N U T R I E N T - F I L M A N D I N A ROCK-WOOL SYSTEM
S.J.H. CRUM 1, W.TH. RUNIA~, M. LEISTRA1 and J.H. SMELT 1 1Institute for Pesticide Research, Marijkeweg 22, 6709 PG Wageningen (The Netherlands) Glasshouse Crops Research and Experiment Station, P.O. Box 8, 2670 AA Naaldwijk (The Netherlands) (Accepted for publication 17 August 1984) ABSTRACT Crum, S.J.H., :Runia, W.Th., Leistra, M. and Smelt, J.H., 1985. Behaviour of some pesticides in a nutrient-film and in a rock-wool system. Scientia Hortic., 25: 1--9. Adsorption of ethoprophos, tetrachlorvinphos and etridiazole onto rock-wool was found to be much weaker than adsorption onto soils. The rates of transformation in water and in water plus rock-wool at 20°C were low. The concentration of etridiazole was measured in a nutrient-film system with recirculating nutrient solution. The decrease was rapid in the first few hours but was more gradual afterwards, to low values at 8 days after application. Etridiazole penetrated poorly into the lower part of a rockwool system when the solution was trickled near the stem-base of the tomato plants. Keywords: adsorption; nutrient-film; pesticides; rock-wool; transformation.
INTRODUCTION T h e g r o w t h o f h o r t i c u l t u r a l c r o p s o n r o c k - w o o l has b e e n e x t e n d i n g fairly r a p i d l y in r e c e n t years. I n T h e N e t h e r l a n d s , the surface area o f g r e e n h o u s e s in w h i c h this s u b s t r a t e is used n o w a m o u n t s t o a b o u t 1 0 0 0 ha. T o m a t o e s , c u c u m b e r s , paprikas (sweet peppers) a n d aubergines (egg-plants) are t h e m a i n c r o p s g r o w n o n r o c k - w o o l . T h e g r o w t h o f c r o p s in nutrient-films is also promising. I n these n e w g r o w i n g systems, t h e r o o t s o f t h e c r o p m a y suffer f r o m fungal diseases o r f r o m a t t a c k b y n e m a t o d e s and a r t h r o p o d s . So it m a y be n e c e s s a r y t o p r o t e c t t h e c r o p s w i t h pesticides. V e r y little i n f o r m a t i o n is available o n t h e b e h a v i o u r o f pesticides in these g r o w i n g systems. T h e a d s o r p t i o n o n r o c k - w o o l and o n plastic materials (tubes, sheets) s h o u l d be k n o w n t o m a k e a first e s t i m a t e o f t h e a p p l i c a t i o n rate n e e d e d as c o m p a r e d w i t h t h o s e applied t o soil. T o o high a rate c a n lead t o t o x i c effects o n plants a n d t o high residues in t h e p r o d u c t s . T h e a d s o r p t i o n o n r o c k - w o o l d e t e r m i n e s t h e e x t e n t to w h i c h t h e t r a n s p o r t o f t h e pesticides is r e t a r d e d w i t h r e s p e c t t o t h e water. High c o n c e n t r a t i o n s
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2 will seriously enhance the e x t e n t to which soil and water courses are polluted b y surpluses o f nut r i ent solution containing pesticide. The rate o f decrease of t he active ingredient in the systems determines h o w o f t e n the ~Compounds should be applied. When t h e decrease is slow, m u ch pesticide can be saved and the discharge into the environment can be reduced. The decrease is the result of various processes and their relative i m p o r t a n c e should be know n to allow estimation o f the rate for conditions t h a t have n o t been studied. Two o r g an o p hos pha t e pesticides, e t h o p r o p h o s [O-ethyl S,S-dipropyl p h o s p h o r o d i t h i o a t e ] : and tetrachlorvinphos [O,O-dimethyl 2-chloro-1(2,4,5-trichlorophenyl)vinyl phosphate Z isomer], were selected for this study. Their adsorption on soils has recently been measured, so t hat the adsorption on rock-wool can be c om par e d with those data. The adsorption o f e t h o p r o p h o s on soils was m o d e r a t e to fairly strong (Leistra, 1979) and t h a t o f tetrachlorvinphos was strong (Leistra et al., 1984). Etridiazole [5-ethoxy-3-(trichloromethyl)-l,2,4-thiadiazole] was selected because it was being tested for use as a fungicide in nutrient-film and rock-wool systems. Th e solubility of etridiazole in water is variously r e p o r t e d to be 50 mg 1-1 at 25°C (Worthing, 1979) and less than 200 mg 1-1 (Spencer, 1982). Values stated for the vapour pressure o f this c o m p o u n d are: 0.013 Pa at r o o m t e m p e r a t u r e (Worthing, 1979), and 13 Pa at 72°C (Spencer, 1982). The mo b ility o f etridiazole measured by thin-layer c h r o m a t o g r a p h y on soil was very low (Helling et al., 1974). Rock-wool is made by melting a m i x t u r e of 60% diabase, 20% limestone and 20% coke at a t e m p e r a t u r e of about 1600°C. The melt is sprayed into fibres 5 p m thick and these are pressed into slabs with a bulk density o f only 70 kg m -3 . Substances (including phenol resin) are added during cooling to give the p r o d u c t firmness and to allow water retention. T he volume fraction o f pores is 0.96 m 3 m -3, and with decreasing water pressure around --103 Pa, a lot o f water is already lost (Verwer and Welleman, 1980). METHODS o n t o r o c k - w o o l . - - The pesticides were dissolved in water with CaC12 at a c o n c e n t r a t i o n of 0.007 mol U 1. The initial concentrations o f the pesticides at two or t hr e e levels were in the range 2--17 mg 1-1, and t h e y were measured exactly. A volume o f 40 ml of these solutions was added to 1.5 g of rock-wool (in little pieces, bulk volume a b o u t 20 ml) in centrifuge tubes (90 ml). T he tubes were closed with ground-glass stoppers and r o t a t e d for 24 h (20°C) at a rotational f r e q u e n c y of 10 min -1. After this, th e tubes with c o n t e n t were centrifuged for 3 min at 2000 min -1 (450 g). Samples o f t h e equilibrium solutions were ext ract ed with hexane and th e concentrations were t h e n measured b y gas chrom at ography. The adsorption was again measured after the tubes with t he rock-wool and remaining solution had been stored for a b o u t 1 week. A fraction of
Adsorption
the solution and the remaining fraction plus rock-wool were e x t r a c t e d and analysed separately. T he r e were no clear differences bet w een t he adsorption coefficients obtained before and after storage, so the values can be presented together. R a t e o f t r a n s f o r m a t i o n in w a t e r and in w a t e r p l u s rock-wool. -- The cont ent s
o f t h e centrifuge tubes used in t he adsorption study were stored for 7 days ( e t h o p r o p h o s and tetrachlorvinphos) or for 6 days (etridiazole) at 20°C in the dark. Af ter this period, the amounts of t he pesticides remaining were analysed and c o m p a r e d with t he initial amounts to see how m u c h of the pesticides was transformed. Solutions o f t h e pesticides in water (with CaC12 at 0.007 mol 1-1) were stored unde r the same conditions and the e x t e n t of t r a n s f o r m a t i o n was c o m p a r e d with t h a t in water plus rock-wool. The rate o f tr ans f or m at i on o f etridiazole in a n u t r i e n t solution was also measured. This solution w a s taken f r om a circulation t ank of a n u t r i e n t film system in a greenhouse t ha t had been in operat i on for a few months. Etridiazole was added bot h as wettable p o w d e r and as emulsifiable concentrate at a calculated initial c o n c e n t r a t i o n of 100 mg 1-1, and was intensively mixed for 2 h. One flask f or e a c h t r e a t m e n t was placed in the dark at 20°C and a n o t h e r in the l a b o r a t o r y in daylight. Concentrations in the solution were measured at intervals up t o 28 days after a p p l i c a t i o n . in n u t r i e n t - f i l m systems. -- T h e behaviour of etridiazole in systems with a circulating nutrient-film was studied in a heated greenhouse. In a preliminary experiment, the rate of dissolution of etridiazole in water f r o m the wettable p o w d e r (35%) was slow. Even after shaking intensively fo r 30 min, only 3/4 o f the final c o n c e n t r a t i o n was reached, so this formulation was n o t usable here. T he emulsifiable c o n c e n t r a t e (etridiazole 700 g 1-1) was added to t he nutrient solution (50 1) in circulation tanks and intensively mixed f o r 5 m i n , which proved adequate. Etridiazole at an initial c o n c e n t r a t i o n of over 25 mg 1-! was added to two tanks for film systems (F1 and F2), and an initial c o n c e n t r a t i o n o f over 50 mg 1-1 to t w o o t h e r tanks (F3 and F4). T he nut r i e nt solution in a tank was p u m p e d through tubes t o five gutter segments (2.5 m long, 0.25 m w i d e ) l i n e d with p oly(ethene) sheet. After flowing t hr ough a segment (slope 1.5%), t h e solution f l o w e d back t o the t a n k t hr ough a tube. The flow t h r o u g h the segments was c o n t i n u o u s and t h e rate was 1 1 min -1. Each gutter segment contained five well-developed t o m a t o plants at t he end o f their p r o d u c t i o n period. T he e x p e r i m e n t was in t he second half o f N o v e m b e r 1983. The transpiration by t he crop was estimated f r o m solar radiation data to be 7 m m in 8 days (Ph. Hamaker, personal c o m m u n i c a t i o n , 1984), The t e m p e r a t u r e of the water in t h e tanks a n d g u t t e r s was a b o u t 21 ° C. The solution in the tanks and at t he outl et of t h e gutter segments were sampled at i n t e r v a l s over 8 days to measure the etridiazole concent rat i on. Etridiazole
Before adding the c o m p o u n d , the nutrient solutions were sampled and t h e y did n o t contain interfering substances. Only after the sampling on Day 4 were the tanks replenished with fresh nutrient solution (30 l) w i t h o u t etridiazole. Etridiazole in r o c k - w o o l systems° -- The behaviour of etridiazole in rock-
wool systems was studied in t he same period in the greenhouse, Emulsifiable c o n c e n t r a t e of etridiazole was added to the tanks R1 and R2 of two rockwool systems and intensively m i xe d with the 250 1 o f nutrient solution. The initial c o n c e n t r a t i o n of etridiazole was almost 25 mg 1-1. In two ot her rock-wool systems, no fungicide was added to the tanks, but 500 ml of etridiazole solution (over 25 mg 1-1, applied as the same formulation) was p o u r e d close to the stem base of each t o m a t o plant. The nut ri ent solution was p u m p e d th r ough tubes to tricklers close to t he stem base of t he plants (one trickler per plant). Twice a day, 330 ml of nutrient solution was applied to each plant by trickling for 7.5 min. Two plants germinated in rock-wool blocks (7.5 cm 3) had been placed on each rock-wool mat, which was 1 m long, 0.15 m wide and 0.075 m deep. In each rock-wool system, 24 plants were grown. The mats were wrapped r o u n d with p o l y ( e t h e n e ) sheets and the surplus nut r i e nt solution leaked away to the soil t hrough slits. On one of the sides, there was a slit bet w e e n each pair of plants at a height of a b o u t 1 cm above t he b o t t o m of the mats. At th e sampling times in a period of 8 days, about 10 ml of nut ri ent solution was withdrawn with a syringe f r o m t he b o t t o m part of t he mats at 20 places per system and these samples were combined. The n u t r i e n t solutions with etridiazole in t h e tanks were sampled a few times. Gas c h r o m a t o g r a p h y . - - T h e concentrations of e t h o p r o p h o s were measured with a P y e Unicam gas c h r o m a t o g r a p h ( t yp e GCV) equipped with a flamespectrometric d e t e c t o r (phosphorus filter). The glass col um n (length 0.9 m, inner diameter 2 mm) was filled with 3% silicone OV-225 on Chrom osorb WHP (0.15--0.18 mm). T he flow rate o f t he carrier gas N2 was 55 ml min -1 and th at o f th e d e t e c t o r gases H: and air was 30 ml min -1. T he t e m p e r a t u r e in t h e injector part was 210°C, in t he col um n oven 190°C and in the d e t e c t o r part 220 ° C. The r e t e n t i o n time of e t h o p r o p h o s was 1.1 min. Tetrachlorvinphos was measured with t he same apparatus, but the temperatures were higher; injection part 230°C, col um n oven 250°C, and d e t e c t o r part 230°C. T he r e t e n t i o n time of this c o m p o u n d was 1.2 min. The co n cen tr a t i ons o f etridiazole in t he extracts were measured with t h e same gas ch rom at ogr a ph, but now the flame-spectrometric d e t e c t o r was provided with a s ul phur filter. T he glass col um n (length 0.9 m, inner diameter 2 mm) was filled with 4% silicone SE-30 + 6% silicone SP 2401 on S u p e l c o p o r t (0.13--0.15 mm). T he flow rates o f t he gases were t he same as before. The t e m p e r a t u r e in t he injector part was 170°C, in t he col um n oven 170 ° , and in the d e t e c t o r part 200°C. The r e t e n t i o n time o f etridiazole was 1.4 min.
T h e i n j e c t i o n o f t h e s o l u t i o n s t o b e m e a s u r e d was i n t e r c h a n g e d w i t h t h e injection of standard solutions with known concentration, T h e p a r t i t i o n ratio o f e t r i d i a z o l e b e t w e e e n h e x a n e a n d w a t e r was m e a s u r e d to c h e c k t h e s u i t a b i l i t y o f h e x a n e as e x t r a c t i o n solvent. T h e r a t i o was f o u n d t o b e higher t h a n 125, w h i c h s h o w s t h a t h e x a n e is suitable. T h e t e n d e n c y o f e t r i d i a z o l e t o volatilize was c h e c k e d b y e v a p o r a t i n g h e x a n e f r o m a s o l u t i o n o f k n o w n c o n c e n t r a t i o n w i t h an air-flow. T h e r e s u l t s c o n f i r m e d t h e d a t a , i n d i c a t i n g t h a t e t r i d i a z o l e is a s o m e w h a t volatile c o m p o u n d . RESULTS A d s o r p t i o n o n t o rock-wool. -- T h e a d s o r p t i o n o f e t h o p r o p h o s o n r o c k - w o o l was so w e a k t h a t it c o u l d n o t easily be m e a s u r e d . T h e a d s o r p t i o n o f t e t r a c h l o r v i n p h o s c o u l d be m e a s u r e d , a n d t h e average value o f t h e a d s o r p t i o n c o e f f i c i e n t Kr/1 a m o u n t e d t o 2.0 d m 3 kg -1 (n = 6, s = 0.4 d m 3 kg-1). T h e r e was no clear d i f f e r e n c e in t h e a d s o r p t i o n o f e t r i d i a z o l e o n r o c k w o o l at t h e t h r e e c o n c e n t r a t i o n s , a n d storage f o r 6 d a y s h a d n o clear e f f e c t . T h e overall value o f K r / l was 1.8 d m 3 kg -~ (n = 24, s = 0.9 d m 3 kg-1). T h e v a r i a t i o n in t h e m e a s u r e d values was increased b y t h e l o w b u l k d e n s i t y o f r o c k - w o o l . Because t h e c o n t e n t s o f t h e t u b e s s h o u l d m i x t h o r o u g h l y , t h e ratio b e t w e e n m a s s o f r o c k - w o o l a n d v o l u m e o f s o l u t i o n c o u l d n o t b e m u c h increased. R a t e o f t r a n s f o r m a t i o n in w a t e r plus rock-wool. - - T h e f r a c t i o n o f t h e initial e t h o p r o p h o s r e m a i n i n g in t h e t u b e s c o n t a i n i n g w a t e r plus r o c k - w o o l a f t e r 7 d a y s o f s t o r a g e was 95% (n = 4, s = 6%) o f t h e initial a m o u n t . O n l y 3% o f t h e initial a m o u n t was m e a s u r e d to b e t r a n s f o r m e d in w a t e r w i t h o u t rock-wool. T h e f r a c t i o n t r a n s f o r m e d o f t e t r a c h l o r v i n p h o s in w a t e r plus r o c k - w o o l was s o m e w h a t higher. O f t h e initial a m o u n t , 83% (n = 4, s = 6%) r e m a i n e d a f t e r 7 days. Again t h e e x t e n t o f t r a n s f o r m a t i o n in w a t e r w i t h o u t r o c k - w o o l was l o w e r , a n d a m o u n t e d t o 8%. T h e f r a c t i o n o f etridiazole f o u n d a f t e r 6 d a y s o f storage in w a t e r plus r o c k - w o o l was 92% (n = 6, s = 3%) o f t h e initial a m o u n t , w h i c h m e a n s t h a t t r a n s f o r m a t i o n is slow. In w a t e r w i t h o u t r o c k - w o o l , h a r d l y a n y t r a n s f o r m a t i o n o c c u r r e d : 99% r e m a i n e d (n = 4, s = 2%). T h e r a t e o f t r a n s f o r m a t i o n o f e t r i d i a z o l e in a n u t r i e n t s o l u t i o n at 2 0 ° C in t h e d a r k was o n l y slow; a f t e r 28 d a y s , 8 0 - - 9 0 % o f t h e initial a m o u n t r e m a i n e d . T r a n s f o r m a t i o n u n d e r l a b o r a t o r y c o n d i t i o n s in t h e light w a s slightly m o r e rapid; a f t e r 28 d a y s , a l m o s t 80% r e m a i n e d . T h u s , t h e p e s t i c i d e was n o t p h o t o s e n s i t i v e u n d e r t h e s e c o n d i t i o n s . E t r i d i a z o l e in n u t r i e n t - f i l m systems. -- T h e results o f t h e m e a s u r e m e n t s o f e t r i d i a z o l e in t h e c i r c u l a t i o n t a n k s o f t h e n u t r i e n t - f i l m s y s t e m s are given in T a b l e I. T h e values are averages f o r t w o s y s t e m s w i t h e q u a l s t a r t i n g c o n c e n -
t r a t i o n , a n d t h e m e a n d i f f e r e n c e b e t w e e n t h e t w o values was 7% o f t h e i r average, w i t h a m a x i m u m o f 20%. T h e d e c r e a s e i n c o n c e n t r a t i o n was r a p i d f o r t h e first f e w h o u r s a f t e r a p p l i c a t i o n . Firstly, d i l u t i o n o c c u r r e d w i t h t h e w a t e r in t h e g u t t e r - s e g m e n t s (15 1 p e r s y s t e m ) s h o r t l y a f t e r a p p l i c a t i o n . S e c o n d l y , t h e fungicide was a d s o r b e d o n t o t h e plastic m a t e r i a l s [poly( v i n y l c h l o r i d e ) t a n k s a n d t u b e s , p o l y ( e t h e n e ) sheets] a n d o n t o t h e p l a n t r o o t s . A f t e r t h e first f e w hours, t h e decrease b e c a m e m o r e gradual until o n l y l o w c o n c e n t r a t i o n s w e r e left a f t e r 8 days. TABLEI Concentrations (mg 1-1) of etridiazole measured in solution in the circulation tanks of nutrient-film systems Time (days) 0 Circulation tanks F1 and F2 F3 and F4
29.0 54.5
0.10 11.2 26.4
0.25 8.8 19.6
1 4.7 10.9
2
4
8
4.0 8.4
2.2 4.3
0.8 1.2
I n t h e s y s t e m s w i t h t h e l o w e r initial c o n c e n t r a t i o n , t h e c o n c e n t r a t i o n s o f e t r i d i a z o l e at t h e o u t f l o w o f t h e s e g m e n t s w e r e 14.5 m g 1-1 a f t e r 15 m i n a n d 5.6 m g 1-1 a f t e r 1 d a y . I n t h e s y s t e m s w i t h t h e higher initial c o n c e n t r a t i o n s , r e s p e c t i v e values w e r e 29.1 m g 1-1 a f t e r 45 m i n a n d 12.1 m g 1-1 a f t e r 1 d a y . T h e s e c o n c e n t r a t i o n s w e r e close t o t h o s e m e a s u r e d f o r t h e c i r c u l a t i o n t a n k s ( T a b l e I), so t h a t t h e r e d i s t r i b u t i o n o f t h e c o m p o u n d t h r o u g h o u t t h e s y s t e m s w a s rapid. in rock-wool systems. - - T h e c o n c e n t r a t i o n s o f e t r i d i a z o l e in t h e t a n k s R 1 a n d R 2 o f t w o r o c k - w o o l s y s t e m s w e r e 2 2 . 5 m g 1-' i m m e d i a t e l y a f t e r m i x i n g , 2 0 . 8 m g 1-1 a f t e r 1 d a y a n d 18.1 m g 1-1 a f t e r 4 d a y s . T h e c o n c e n t r a t i o n s in t h e s o l u t i o n p o u r e d close t o t h e s t e m - b a s e s was 25.3 m g
Etridiazole
TABLE II Concentrations (mg 1-1) of etridiazole measured in nutrient solution in the bottom part of rock-wool mats Time (days) 0.10 Mode of application To tanks R1 and R2 To stem base
<0.1 <0.1
0.25 <0.1 <0.1
1
2
<0.1 0.1
<0.1 0.4
4
8
0.3 0.2
1.5 0.2
1-1. These c o n c e n t r a t i o n s were as e x p e c t e d . T h e c o n c e n t r a t i o n s m e a s u r e d in t h e n u t r i e n t s o l u t i o n sampled f r o m t h e l o w e r p a r t o f t h e r o c k - w o o l mats are given in T a b l e II. In t h e first few days, t h e c o n c e n t r a t i o n s o f etridiazole were b e l o w or close t o t h e d e t e c t i o n limit. In t h e w h o l e measuring p e r i o d , t h e c o n c e n t r a t i o n s r e m a i n e d very low. Thus, w i t h n o n e o f t h e m e t h o d s o f a p p l i c a t i o n did t h e etridiazole solutions p e n e t r a t e t h o r o u g h l y into t h e mats. GENERAL DISCUSSION T h e a d s o r p t i o n o f t h e o r g a n o p h o s p h o r u s pesticides o n t o r o c k - w o o l can be c o m p a r e d w i t h t h e i r a d s o r p t i o n o n soils. T h e a d s o r p t i o n o f e t h o p r o p h o s o n t o soils is m o d e r a t e t o fairly strong, w i t h a d s o r p t i o n c o e f f i c i e n t s Ks/1 = 0.8 d m 3 kg -1 f o r a sandy loam soil w i t h 1.7% organic m a t t e r , Ks/1 = 1.9 d m 3 kg -1 f o r a l o a m soil with 2.6% organic m a t t e r , and Ks/1 = 4.4 d m 3 kg -1 for a h u m i c sand soil with 7.0% organic m a t t e r (Leistra, 1 9 7 9 ) . T h e adsorpt i o n o f t e t r a c h l o r v i n p h o s o n t o softs was f o u n d to be strong, with Ks/l = 20 d m 3 kg -1 f o r a h u m i c sand soil w i t h 3.1% organic m a t t e r , Ks/1 = 20 d m 3 kg -1 f o r a s a n d y loam soil w i t h 3.6% organic m a t t e r , and Ks/l = 115 d m 3 kg -1 f o r a l o a m soil with 9.7% organic m a t t e r (Leistra et al., 1 9 8 4 ) . T h e a d s o r p t i o n o f e t h o p r o p h o s o n t o r o c k - w o o l ( t o o w e a k to be m e a s u r e d ) and t h a t o f t e t r a c h l o r v i n p h o s o n t o r o c k - w o o l (Kr]1 = 2.0 d m 3 kg -1) were t h u s m u c h w e a k e r t h a n t h e a d s o r p t i o n o f t h e same c o m p o u n d s o n t o soils. T h e r e t a r d a t i o n o f t h e m o v e m e n t t h r o u g h a r o c k - w o o l m a t o f etridiazole can be e s t i m a t e d with respect t o water. S u p p o s e t h a t t h e w a t e r flows t h r o u g h a v o l u m e f r a c t i o n o f p o r e s ~ = 0.80 d m 3 d m -3, and t h a t t h e d r y b u l k density o f t h e m a t is 0.07 kg d m -3. T h e a d s o r p t i o n c o e f f i c i e n t was Kr/l= 1.8 d m 3 kg -1, so t h e c a p a c i t y f a c t o r b e c o m e s C a p = ~ +Pb Kr/1 = 0 - 9 3 dm3 d m -3. This implies t h a t 86% o f etridiazole is in t h e w a t e r phase, while o n l y 14% is a d s o r b e d . T h e rate o f t r a n s p o r t o f this c o m p o u n d t h r o u g h t h e mat will t h u s be a b o u t 0.86 t i m e s t h e rate o f w a t e r flow, w h i c h m e a n s t h a t r e t a r d a t i o n is o n l y low. In practice, t h e a d s o r p t i o n o f a c o m p o u n d like etridiazole in r o c k - w o o l systems will b e substantially greater t h a n w o u l d f o l l o w f r o m t h e measurem e n t s f o r " c l e a n " r o c k - w o o l alone. In view o f t h e r a t h e r low w a t e r solubility and t h e r a t h e r high h e x a n e / w a t e r p a r t i t i o n ratio, a t e n d e n c y t o fairly strong a d s o r p t i o n o n t o organic materials m a y be e x p e c t e d . T h e c o m p o u n d was m i x e d w i t h n u t r i e n t s o l u t i o n in plastic tanks and t h e s o l u t i o n was p u m p e d t h r o u g h plastic t u b e s and trickierS. I n t h e r o c k - w o o l mats, t h e r e was a lot o f r o o t material and t h e m a t s were w r a p p e d in plastic sheets. T h u s retardat i o n o f t h e t r a n s p o r t in p r a c t i c e will o f t e n be greater t h a n w o u l d f o l l o w f r o m calculations f o r " c l e a n " rocl~-wool. T h e rate o f t r a n s f o r m a t i o ~ o f e t h o p r o p h o s , t e t r a c h l o r v i n p h o s and etridiazole i n c u b a t e d in w a t e r and in w a t e r plus r o c k - w o o l was fairly slow. T h e r e is no clear i n d i c a t i o n t h a t t h e r o c k - w o o l c a t a l y s e d t h e t r a n s f o r m a t i o n o f these c o m p o u n d s . H o w e v e r , in practical r o c k - w o o l and n u t r i e n t - f i l m systems,
a s u b s t a n t i a l m i c r o b i a l a c t i v i t y c a n build up, w h i c h m a y be e x p e c t e d t o s p e e d u p t h e r a t e o f t r a n s f o r m a t i o n . T h e v a p o u r pressures r e p o r t e d in t h e l i t e r a t u r e a n d e x p e r i e n c e in t h e l a b o r a t o r y i n d i c a t e t h a t v o l a t i l i z a t i o n is a n o t h e r i m p o r t a n t p r o c e s s in t h e decrease o f etridiazole in n u t r i e n t - f i l m s y s t e m s . T h e r e is no i n d i c a t i o n t h a t p h o t o s e n s i t i v i t y is i m p o r t a n t f o r etridiazole in s u c h s y s t e m s . The extent to which the nutrient solution penetrated into the rock-wool m a t s can be e s t i m a t e d . T h e r a t e o f d r i p p i n g n e a r e a c h p l a n t was 6 6 0 c m 3 d a y -1, and t h e r a t e o f t r a n s p i r a t i o n p e r p l a n t w o u l d be 4 4 0 c m 3 d a y -1. A surplus o f 2 2 0 c m 3 passing t h r o u g h a f l o w p a t h w i t h a cross-sectional a r e a o f 6 0 c m 2 (15 c m wide, 4 c m high) f l o w s a d i s t a n c e o f 3.7 c m . So t h e nutrient solution could only gradually penetrate the lower part of the rockw o o l m a t s . B e c a u s e o f t h e fairly strong a d s o r p t i o n o f etridiazole o n t o plastics a n d p l a n t r o o t s , t h e p e n e t r a t i o n o f this fungicide i n t o t h e m a t s w o u l d h a v e b e e n e v e n slower t h a n t h a t o f t h e n u t r i e n t s o l u t i o n . A f t e r p o u r i n g 500 c m 3 etridiazole s o l u t i o n o n t o t h e m a t s a n d d r i p p i n g at 6 6 0 c m 3 d a y -1, o n e w o u l d e x p e c t fairly high c o n c e n t r a t i o n s in t h e l o w e r p a r t o f t h e m a t s w i t h i n o n l y a f e w days. T h e l o w c o n c e n t r a t i o n s m e a s u r e d i n d i c a t e t h a t e t r i d i a z o l e p r e s u m a b l y m o v e d in a l a y e r o f w a t e r o n t o p o f a s t a g n a n t w a t e r l a y e r in t h e b o t t o m p a r t o f t h e m a t s . T h e s a m e t y p e o f p r o b l e m w a s experienced by van Noordwijk (1978). He found that a "new" nutrient s o l u t i o n s p r e a d o n t o p o f t h e " o l d " s o l u t i o n in t h e b o t t o m p a r t o f a r o c k w o o l m a t a n d t h a t m i x i n g o f t h e s e s o l u t i o n s was v e r y slow. P e n e t r a t i o n c o u l d b e i m p r o v e d b y s u p p l y i n g t h e s o l u t i o n at a higher r a t e a n d b y m a k i n g t h e slits in t h e w r a p p i n g - s h e e t s at t h e b o t t o m . H o w e v e r , e n h a n c e d f l o w r a t e s w o u l d increase t h e l e a k a g e o f n u t r i e n t s o l u t i o n w i t h p e s t i c i d e t h r o u g h t h e slits into t h e soil, w h i c h c o u l d lead to c o n t a m i n a t i o n o f g r o u n d w a t e r a n d w a t e r courses (via t h e drain t u b e s ) . P r e f e r a b l y , f u r t h e r d e v e l o p m e n t s s h o u l d go in t h e d i r e c t i o n o f r e c i r c u l a t i o n s y s t e m s . H o w e v e r , p r o b l e m s w i t h t h e q u i c k s p r e a d o f diseases t h r o u g h t h e s e s y s t e m s a n d w i t h t h e salt a c c u m u l a t i o n in t h e n u t r i e n t s o l u t i o n will h a v e t o be o v e r c o m e .
REFERENCES
Helling, C.S., Dennison, D.G. and Kaufman, D.D., 1974. Fungicide movement in soils. Phytopathology, 64: 1091--1100. Leistra, M., 1979. Computing the movement of ethoprophos in soil after application in spring. Soil Sci., 128: 303--311. Leistra, M., Tuinstra, L.G.M.Th., van der Burg, A.M.M. and Crum, S.J.H., 1984. Contribution of leaching of diazinon, parathion, tetrachlorvinphos and triazophos from glasshouse softs to their concentrations in water courses. Chemosphere, 13 : 403--413. van Noordwijk, M., 1978. Zoutophoping en beworteling bij de teelt van tomaten op steenwol (Distribution of salts and root development in the culture of tomatoes on rock-wool). Report No. 3-78, Institute for Soil Fertility, Haren, The Netherlands, 21 PP.
Spencer, E.Y., 1982. Guide to the chemicals used in crop protection. Publication 1093, 7th edn., Research Branch, Agriculture Canada, 595 pp. Verwer, F.L. and Welleman, J.J.C., 1980. The possibilities of Grodan rock-wool in horticulture. Proc. Fifth Int. Congr. Soilless Culture, Wageningen, pp. 263--278. Worthing, C.R., 1979. Pesticide manual, 6th edn., British Crop Protection Council, Cradley, Malvern, Worcestershire, England, 655 pp.