As a statistical measure of trade persistence, we can use a transformation of the standard chisquared test, Cramer’s C-statistic, suggested by Carolan et. al.. The C-statistic lies between zero and one, with one representing complete association between the beginning and the ending trade balance. From Tables 7, 8 and 9 we find the C-statistic is 0.66 for agriculture, 0.39 for other primary products, and 0.54 for manufactures. These results suggest there was the least change in the trade balances over the 1980-1996 time period for agriculture, because the C-statistic is relatively high. For manufacturing and other primary products the results suggest there was relatively more change in the trade balances over the time period studied, because the C-statistics are lower.Rather than just comparing the beginning and ending time periods, we can construct histograms for agriculture and manufacturing, based on the number of years each subgroup runs a surplus . Figure 3 shows histograms for agriculture, “other” primary products, and manufactures. Figure 3 classifies subgroups indicating how many years the subgroup was in surplus. This means, that a subgroup that was in surplus for each of the 18 years would be in the cell at the extreme right of the histogram. The histogram for agriculture displays the strongest evidence of bimodality, which indicates persistence in trade flows. These histograms are therefore consistent with the C-statistic results,cultivo de arandanos suggesting more persistence in the composition of agricultural trade, compared to the other two groups. Finally, the results of an additional test for association are reported in Table 6.

We regressed NB1994-96 on NB1980-82. The regression coefficients for all three groups are all statistically significant, but the coefficient for other primary products is smaller than for agriculture or manufacturing. Similarly, the R2 is relatively small for other primary products compared to either manufacturing or agriculture . These results support the conclusion that the trade patterns appear to change the most for “other” primary products, and the least for agriculture. The major finding of this paper is that China’s agricultural trade structure has not changed dramatically since 1980. China’s agricultural trade may loosely correspond to the basic principles of comparative advantage, but that in itself is not such a big achievement. Even under central planning, the obviousness of comparative advantage was such that in general terms, China’s agricultural trade corresponded to basic principles of comparative advantage. What is more striking is the modest and limited way that agricultural trade has expanded along comparative advantage lines, despite an increase in foreign trade overall, and the implicit evidence this provides of foregone opportunities for benefit from agricultural trade. There is little evidence of increased reliance on comparative advantage in China’s agriculture.California’s coastal districts, where 86% of the nation’s strawberries are produced on 38,600 acres, are the most productive strawberry-growing areas in the United States . To achieve this level of productivity, California strawberry producers need effective soil disinfestation, productive varieties and cultural practices such as polyethylene mulch and drip irrigation . Strawberries are very sensitive to soil pathogens, and growers with these highly productive systems have become dependent on preplant fumigation. Traditionally, they used methyl bromide plus chloropicrin as the basis for soil pest control.

Fumigation with these chemicals controls soilborne pathogens such as Verticillium dahliae, Phytophthora species, Pythium species, Rhizoctonia species, Fusarium oxysporum and Cylindrocarpon species, as well as nematodes, soil borne insects and weed seeds in the soil seed bank . In 1992 methyl bromide was classified as a Class I stratospheric ozone-depleting chemical. Since 2005, under the Montreal Protocol, the use of methyl bromide for fumigation in the United States has been permitted only through critical use exemption . The methyl bromide phase-out and other regulatory limitations make research on alternative pest control measures essential. Currently, some California strawberry fields can still be treated with methyl bromide under the critical use exemption, which is subject to annual review by the parties of the Montreal Protocol. However, methyl bromide costs have been increasing, and its use in strawberry production has been decreasing . Alternative fumigants being used are 1,3-dichloropropene and chloropicrin . In traditional fumigation of California strawberry fields, beginning in the 1960s and continuing until recently, growers applied methyl bromide plus chloropicrin to the total field area. This process was called flat fumigation; the entire field was covered with polyethylene film to hold the fumigant at the concentration needed to kill soil pests . In the last decade, a sizable portion of the strawberry acreage has been treated with fumigants applied to the strawberry bed by drip fumigation . The major alternatives to methyl bromide such as 1,3-D and chloropicrin are heavily regulated due to the potential for adverse health effects to workers and nearby populations, which has complicated the transition away from methyl bromide. In California, 1,3-D use per 36-square-mile township is limited to 90,250 pounds, called a township cap, which severely limits its availability in numerous key strawberry production areas .

A 2008 strawberry critical use nomination indicates that “township caps currently limit the use of 1,3-D on 40% to 62% of total strawberry land” . Chloropicrin is capped at a use rate of 125 pounds near sensitive sites such as day-care centers, and in some counties it cannot be used within one-quarter mile of such sites. Consequently, considerable methyl bromide use continues in California because restrictions on alternative fumigants leave few other options for much of the strawberry acreage. Currently about 68% of the California strawberry acreage is fumigated with alternatives to methyl bromide,macetas redondas de plastico primarily drip-applied mixtures of 1,3-D plus chloropicrin or chloropicrin emulsified formulation . Drip fumigation with these products costs less than broadcast shank fumigation with methyl bromide plus chloropicrin. However, there are limits to how much of the remaining 32% of the strawberry acreage can be converted from methyl bromide to alternative fumigants. Fumigants are difficult to apply evenly by chemigation on hilly fields where beds are not formed along contour lines. Also, all fumigant applications are restricted or not allowed within one-quarter mile of a sensitive site, such as a hospital, jail, school or day-care facility . The public has shown less and less tolerance toward agricultural fumigant use, and regulators have been forced to look for solutions that meet the demands of the public yet allow growers to farm. One strategy to reduce the potential for fumigant exposure from off-site movement of volatile fumigants is the use of barrier films, which trap the fumigant in the field.A gas-impermeable film can minimize fumigant emissions, increase fumigant retention over time and reduce the amount of fumigant needed for effective pest control . Compared to standard 1-mil polyethylene films or uncovered soil, virtually impermeable film can greatly reduce fumigant emissions and enhance retention of the fumigant in the upper soil layer . VIF differs from traditional single-layer high density polyethylene tarp because VIF has at least one gas-impermeable layer between polyethylene layers . Higher concentrations of 1,3-D and chloropicrin were measured under VIF than under standard film 1 to 4 days after drip fumigation . Improved retention of fumigants under VIF provides more opportunity for fumigants to degrade in the soil instead of being released into the atmosphere . Researchers have found that VIF can reduce the amount of 1,3-D plus chloropicrin needed for effective soil disinfestation by 50% . Santos et al. found that reducing methyl bromide plus chloropicrin rates by 50% under multilayer VIFcontrolled nuts edge similarly to full-rate methyl bromide plus chloropicrin applied under standard single-layer films. A relatively new barrier film, totally impermeable film, or TIF, has been shown to retain fumigant better than VIF . TIF is a five layer film with two ethylene vinyl alcohol layers embedded in three layers of standard polyethylene film. Fumigant-use regulations in Ventura County allow the application of twice as many pounds of chloropicrin per 48-hour period where TIF is used than under standard 1.25-mil film . Fennimore and Ajwa found that TIF was effective at trapping fumigants, improving weed control and boosting strawberry yields. By trapping the fumigant under TIF, higher fumigant concentrations kill a greater percentage of the weed seeds and soil pathogens, thus improving soil pest control and yields.We conducted field trials near Salinas at the USDA Agricultural Research Service Spence Farm and near Watsonville at the Monterey Bay Academy research facilities during the 2007–2008 season. Fumigants were applied at Monterey Bay Academy on Oct. 11, 2007, and at Spence Farm on Oct. 24, 2007. The fumigants tested were 1,3-D plus chloropicrin , methyl bromide plus chloropicrin , and chloropicrin .Each fumigant was applied under two types of film: 1.25-mil VIF and 1.25-mil standard polyethylene tarp. Approximately 4 weeks after fumigation, the bare root strawberry variety Albion was transplanted.

Beds were 54 inches wide, center to center, and two lengths: 30 feet long at Monterey Bay Academy and 100 feet long at Spence Farm. Due to differences in the land available at the two sites, final harvest plot size was 20 feet long at Monterey Bay Academy and 35 feet long at Spence Farm. Treatments were arranged in a split plot design, with film as the main plot and fumigant as the subplot, and replicated four times at each site. Conventional tillage practices were followed for strawberry production in each area. Fruit yield was evaluated once or twice weekly and sorted into marketable fruit and culls. Fruit yield data were analyzed using SAS version 9.3 . Data were analyzed for the effects of film on season-long fruit yields, and mean separation was performed using Fisher’s protected LSD. The emissions data were analyzed in EXCEL using a student’s t-test. The permeability of the two films to 1,3-D, chloropicrin, iodomethane and methyl bromide vapors was monitored using procedures described by Papiernik et al. . Film samples were taken before and after installation, and the average measurement of the flow rate of fumigant through the film determined. For each fumigant, the before and after coefficients varied less than 10%, which means that installation did not damage the impermeable layer . Across all fumigants, the coefficients varied between 2.7 and 16.9 cm h-1 for the 1.25-mil standard polyethylene tarp but less than 0.01 cm h-1 for VIF, a significant difference for all fumigants . The average mass transfer coefficient of VIF was less than 1% of the average coefficient of the standard tarp. The effect of film on fruit yields was not significant . The work with VIF suggested that it does indeed trap fumigants but does not necessarily improve fruit yields. Recent work with TIF indicated different results. Compared with 1-mil single-layer standard films, TIF resulted in higher fumigant concentrations under the film, higher strawberry fruit yields and better weed control . The work with VIF reported above used a three-layer film with only one impermeable layer; it was a first-generation barrier film. The TIF film, a second-generation film, tested in subsequent studies was a five-layer film with two impermeable layers. The extra impermeable layer in the TIF film may have resulted in greater tolerance to stretching, and thus fewer breaks in the film and better pest control.Presently registered alternative fumigants such as 1,3-D, chloropicrin, and 1,3-D plus chloropicrin combinations have been tested and are effective at controlling soil pests in strawberry . However, as described above, regulations limit the use of these products . Given the challenges to fumigant use in California, the options for growing strawberries without fumigants must be thoroughly explored. One such option is soilless production. Strawberry crops can be produced on clean soilless substrates. This production method is commonly used in Europe and does not require methyl bromide. In 2003, 2,815 acres of strawberries were produced using soilless culture in Belgium, the Netherlands, U.K., France and Italy . Soilless production of strawberry crops is also being evaluated in Florida . Soilless production traditionally used coir, peat or other soilless substrates enclosed in bags under plastic covers, that is, high tunnels . However, concerns about bag disposal have led to more-sustainable systems, including the raised bed trough system. The bed is made like a typical strawberry bed, with the exception that troughs are cut into it and lined with fabric designed to permit moisture penetration but not allow root penetration. The troughs are filled with clean planting material, steam-treated soil or soilless media; drip tape is installed, and the beds are tarped in the same way as conventional strawberry beds.