Deposition of Pesticides on the Soil Surface
Materials and methodsField experiments were carried out in three growing seasons from 1998 to 2001 on Research Centre Flakkebjerg near Slagelse. The soil type is classified as a sandy loam in all fields used for the experimental work.
The growth stages are given according to the BBCH scale (Meier, 1997). Figure 2 show the principal growth stages for the four crops included in the depostion studies. Danish translations are available for the general growth scale (Skovbo et al., 1995) and for some crops (Bromand et al., 1995; Schulz et al., 1995; Skovbo et al., 1995; Fertin et al., 2002).
Figure 2. BBCH growth scale for cereals, beet, potatoes and oilseed rape. The vertical projection of ground cover by the crop was measured analysing digital phtographies of plots covering 50-33 cm. The photographies was analysed with the GIPS image-processing program (Gade data/Image house A/S, Copenhagen). Additionally a non-destructive measurement of plant canopy reflectance in the red and the near-infrared spectrum was taken. The reflectance measurements were converted to vegetation indices that measure the photosynthetic size of plant canopies. A close correlation between plant biomass and vegetation index has been demonstrated (Jensen & Christensen, 1993). All measurements were taken on 4 replicates. The aim of this part of the project was to give estimates of plant cover at various growth stages for crops not included in the deposition studies. This part of the project was stopped after the first year due to the fact that a literature review on the same topic was published by Becker et al (1999). Data from this review fulfils the same purpose as intended by our study and the data is presented in the report. The second part of the project includes simultaneous measurements of soil deposits of spray and measurements of crop characteristics. Crop measurements included crop height, crop cover measured by photography, and crop growth stage based on the BBCH scale. Additional measurements included the vegetation index measurements in the first growing season. In the last growing season, canopy density was measured non-destructively by using a portable device (LAI-2000, LI_COR, Inc Lincoln, USA), measuring the diffuse light transmission through the canopy followed by a calculation of an approximate LAI (m2 m-2). LAI-2000 should only be used in diffuse light, and measurements were therefore carried out at dawn or dusk or under cloudy conditions. This part of the project was carried out during three growth seasons in winter wheat, spring barley, sugar beet and potatoes. The four crops were grown according to normal agricultural practice and
recommendations. This included a general weed control in all plots at an early growth stage. This means that the crop was kept weed free during the growing season and that weed plants did not contribute significantly to the plant cover measured at any application date. For details on cultivar, sowing date etc, see Table 3. The spray application followed by a conventional technique, which is used for, most applications in agricultural crops in Denmark. A self-propelled plot sprayer equipped with hydraulic flat fan nozzles with drop size characteristics normally recommended for the purpose was used. For details see Table 4. The sprayer used a driving speed of 6 km h-1 and the plots treated were 2.5 x 3 m
The spray solution used was water with addition of the tracer in the 1998-1999 season. In the two following seasons, 1999-2000 and 2000-2001 a non-ionic surfactant was added at a concentration of 0.1% to the spray solution of water and tracer. The tracer used was brillantsulfoflavin at a dose of 100 g ha-1. The product was delivered by Chroma-Gesellschaft with catalog number C.I. nr. 56205 1F 561. The surfactant used was a non-ionic linear alcohol polyethoxylate (Lissapol Bio, Zeneca, Denmark). The tracer was used at a dose of 100 g ha-1. Just prior to spray application, paper objects were placed in the plots in order to collect the spray. In winter wheat and spring barley the following technique was used throughout the season and the same technique was used in sugar beets and in potatoes after the time when these crop had reached a crop cover exceeding 50% of the soil surface. Four rectangular paper objects with a size of 1.8 x 12 cm were placed just above the crop and 3 x 4 objects at the same size was placed at the soil surface. The paper objects were placed on metal rods in order to obtain a horizontally oriented object, and in order to avoid contamination with soil on the objects placed at soil level.
Figure 3. Placement of objects to catch spray at the soil surface and above the crop in winter wheat. The objects placed above the crop was pooled to one sample and the objects placed at soil level was pooled into 3 samples each consisting of 4 pieces of paper. In the cereal crops, winter wheat and spring barley, a paper object could reach across from the middle of one row to the middle of the next row giving a very representative sample. In sugar beets and in potatoes, the paper objects at the soil surface were placed in such a way that each distance from the middle of the crop row and to the middle of the row between to rows were equally well represented in the samples. In sugar beets with a row distance of 50 cm, 4 papers with a length of 12 cm could be placed as a string from the middle of one row to the middle of the neighbour row. In potatoes with a row distance of 75 cm, the four paper objects were placed between to rows but with a distance of approximately 6-cm between each object. The treatments included each time 4 replicates. The deposit of spray on the soil surface was calculated from the measurements of tracer on the objects per area unit. The deposit is shown as a percentage of the applied per area unit and as a percentage of the spray measured per area unit just above the crop.
Figure 4. Placement of objects to catch spray at the soil surface and above the crop in potatoes when the plant cover exceeded approximately 50%. The limited plant density makes it difficult to place paper objects in a representative way at the early growth stages. Instead 6 plants in each plot were randomly selected and paper objects were placed on the soil surface below these plants. The object size used was 21.6, 100 or 200 cm2, depending on the size of the crop plant. When less than 50% of the paper was visible below the plant, the larger object size was used. Photography was taken of each individual plant in order to calculate the proportion of the paper which was covered by the crop and which part was visible from a vertical view. Photography was also taken in order to estimate the proportion of the soil with plant cover in the total plot. The filter papers from these six single plants were collected individually in order to calculate the collection efficiency of the individual plants. The deposit on these papers was related to the proportion of the paper covered by the crop. The deposit on the part of the plots with a crop cover was calculated from the mean value collected by the six measured plants. The deposit on the part of the plots not covered with crop was assumed to be equal to 100% of the applied spray. Figure 5. Placement of objects to catch spray at the soil surface and above the crop in sugar beets (picture) and potatoes until the plant cover exceeded approximately 50%. The deposit of spray liquid on the soil surface in the plots was then calculated the following way in a plot with 10% plant coverage and where the deposit of spray below the plant cover was 50%: 90% with no cover and 100% deposit + 10% with plant cover and 50% collection of the spray = 0.9*100 + 0.1*50 = 95% of the spray deposited on the soil surface. After spraying the filter papers were collected and stored in 100 ml amber glass bottles under dark conditions at 50C until the samples were analysed. Samples of the spray liquid were taken and stored the same way. In 2000 and 2001 four experiments two in winter wheat and two in spring barley were used to study the influence of pesticide formulation and application technique on the deposit of spray liquid on the soil surface below the two cereal crops. Four different formulations of a spray liquid were applied to winter wheat at growth stage 38 (BBCH) and to spring barley at growth stage 32-33 (BBCH) using either a fine atomising flat fan nozzle or a coarse atomising air induction nozzle. The formulations used were
The tracer brillantsulfoflavin was added to all formulations at a dose of 100 g ha-1 and was used to quantify soil deposits. Paper objects with a size, number and distribution as described above were used to catch the spray liquid above the crop and at soil level. The study included 4 replicates. Details on crop cover at application, characteristics of nozzles used etc are shown in Tables 5 and 6.
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