The interaction between K and Mg was found to be significant with the addition of Mg causing a significant increase in the N uptake at each level of K though the magnitude of increase diminished as levels of Mg increased. As the level of Mg application increased without K addition the N uptake was found to decrease. Addition of increased levels of K increased the uptake of phosphorus. The application of high rates of Mg decreases the favorable effect of K on P uptake and hence Mg is seen to have an antagonistic effect on P uptake. This reduction of P uptake in spite of increased P availability might be due to the interference of sulphate ions on the absorption of P by the plant. Increasing the rate of K application significantly increased the K uptake. As the level of Mg application increased the K uptake also showed a general increase, contrary to the reports that K and Mg are antagonistic. Thus Mg might have diminished only the solution concentration of K in soil and not the uptake. At higher levels of K, a significant increase in K uptake was noticed as Mg levels were increased. Thus a positive interaction existed between K and Mg on the uptake of K. This increase in K in the tissue with increasing Mg might be because of the decreased dry matter production associated with high Mg. Calcium uptake was significantly reduced with increasing K levels especially in the absence of Mg. Similar results were reported by Fageria who found an antagonistic effect of K application on Ca uptake.

When applied with high levels of Mg, though higher levels of K caused a reduction in the Ca uptake,hydroponic dutch buckets no significant decrease was generally obtained. Addition of increasing levels of Mg remarkably decreased the Ca uptake. The high Mg levels must have hindered the absorption of Ca by the plant due to the action of the Ca- Mg antagonistic effect. Such antagonism of Mg on the uptake of Ca was also reported by Kumar et al. . It is also seen that the antagonistic effect of increased levels of Mg occurred only in the absence of K. The decrease in tissue concentration and uptake of Ca with increasing concentration of Mg is presumably due to the replacement of Ca by Mg for the neutralization of negative charges within the vacuole and on the exchange sites in the apoplast of the plant cell . Though Mg uptake increases with its level of application, the presence of K especially at higher levels was found to decrease uptake by the crop. This may be because of an induced reduction in availability of Mg to the crop by the excess application of K, leading to less absorption of Mg by the plants. Thus, at higher Mg levels, by supplying additional excess levels of K, the antagonism of additional K is great enough to repress Mg absorption regardless of the Mg level. Thus at low soil K and sufficient exchangeable Mg levels, uptake of Mg is not hindered. The California strawberry industry produces about 85% of the strawberries grown in the United States, on 37,000 acres, with a value of $1.5 billion in 2008 . To control soil borne diseases and weeds, California strawberry fields have long been fumigated with methyl bromide plus chloropicrin . However, methyl bromide is being phased out as an ozone-depleting substance under the Montreal Protocol , an international treaty. Currently, some California strawberries can still be treated with methyl bromide under a critical-use exemption, subject to annual review by parties to the Montreal Protocol.

Alternative fumigants permitted for use in California strawberries are 1,3-dichloropropene , chloropicrin and, as of December 2010, methyl iodide. About 81% of California strawberries are grown in soils that were previously treated with chloropicrin , while 30% are also fumigated with 1,3-D and 43% with methyl bromide . Since soil treatments began in the 1960s, entire fields have been covered with polyethylene film to hold in the fumigant at concentrations needed to kill soil pests . More recently, a sizable portion of strawberry acreage has been treated with fumigants applied to beds via the drip irrigation system . The major alternatives to methyl bromide, 1,3-D and chloropicrin, are heavily regulated. The transition away from methyl bromide to alternatives has been complicated by regulations aimed at protecting workers and others from exposure to fumigants. 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 key strawberry production areas . The recent critical use nomination for strawberry indicates that “township caps currently limit the use of 1,3-D on 40% to 62% of total strawberry land” . In other words, methyl bromide use continues in California because restrictions on alternative fumigants leave few options. Among the reasons that fumigants are so heavily regulated in California is that they are classified as volatile organic compounds . Alternative fumigants such as 1,3-D are released into the air and, after reacting with nitrogen oxides, can convert to form ground-level ozone — a harmful air pollutant . Regulations have been developed to reduce the contribution of fumigants to ozone formation, which, for example, has seriously affected the use of fumigants in Ventura County, a key strawberry production area.impermeable film can greatly reduce fumigant emissions and enhance their distribution in soil, in comparison with conventional polyethylene films or uncovered soil .

VIF differs from traditional high-density polyethylene tarps in that it has additional gasimpermeable layers, such as nylon or polyaminides, between the polyethylene layers . Fumigant concentrations of 1,3-D and chloropicrin were higher under VIF than low-density polyethylene tarp, 1 to 4 days after drip fumigation . The improved retention of fumigants under VIF also provides more opportunity for them to degrade in the soil rather than be released into the atmosphere . A number of researchers have found that VIF as a tarp can reduce the amount of 1,3-D plus chloropicrin needed for effective soil disinfestations by 50% . Santos et al. found that reducing methyl bromide plus chloropicrin rates by one-half under VIF controlled nuts edge similarly to the full rate of 350 pounds per acre applied under standard films. A relatively new barrier, totally impermeable film , has been shown to apply easily and retain fumigant better than VIF . TIF is a five layer film with two thin ethylene vinyl alcohol layers embedded in three layers of standard polyethylene film . Our studies evaluated the compatibility of TIF and standard films with the two major fumigant application methods for strawberry, broadcast fumigation and chemigation. The primary objective was to compare fumigant retention under TIF and standard film. Secondary objectives were to measure the effects on strawberry fruit yield and weed control.We compared the retention of methyl bromide plus chloropicrin under TIF and standard films at a commercial farm near Salinas in 2007. The soil was a Chualar sandy loam. Methyl bromide 57% plus chloropicrin 43% and 1,3-D 61% plus chloropicrin 35% , both at 350 pounds per acre, were applied by a commercial applicator on Oct. 15, 2007. As the fumigant was applied,bato bucket it was immediately tarped by 13-foot-wide standard film  or 13-foot-wide TIF . The plots were 280 feet long and 33 feet wide to allow for three passes, each 11 feet wide, of the application tractor. The films were 13 feet wide overall with 1 foot on the leading edge used to anchor the film in the soil and 1 foot on the trailing edge used to glue to the leading edge of the previous pass, creating a 1-foot overlap. Hence, the applied film is like rows of overlapped roofing shingles. The proprietary glue used by the commercial applicator adhered to the TIF film and held it in place without incident. Each treatment was replicated two times and arranged in a randomized complete block design. Fumigant concentrations under the tarp were monitored with a MiniRae VOC meter at 3, 27, 51, 76, 97, 120 and 166 hours after application. The MiniRae VOC meter uses a photo ionization detector to measure the concentrations of volatile compounds such as fumigants. Fumigant samples were taken from airspace between the soil surface and the tarp at three random locations near the center of the plots. The film was cut and removed 192 hours after application. The field was then prepared for strawberry planting by the installation of 52-inch-wide raised beds with two drip irrigation lines per bed. ‘Albion’ strawberry was transplanted on Nov. 11, 2007. Strawberry fruit were harvested from two 40-plant sample stations per plot from April 18 to Sept. 1, 2008, and fruit were sorted into marketable and cull fruit by a trained crew. Weeds were sampled from two 125-square-foot sample areas on Feb. 15, April 28 and July 8, 2008.In 2008, we evaluated TIF at the USDA Agricultural Research Service farm on Spence Road near Salinas. The soil was a Chualar sandy loam. We injected 1,3-D 35% plus chloropicrin 60% plus an emulsifier 5%  through the drip irrigation system on Oct. 21, 2008, at 50, 100, 200, 300 and 400 pounds per acre, under both standard and TIF film . Briefly, the fumigants were injected in a closed system directly from nitrogen pressurized cylinders and metered into irrigation water with a flow meter . A static mixing device was installed at the point of injection to mix fumigants with irrigation water before distribution via the drip irrigation system.An emulsifiable formulation of methyl bromide 57% plus chloropicrin 43% was applied on Oct. 29, 2008, at 350 pounds per acre, also through the drip irrigation system. Each treatment was replicated four times, and the trial was arranged in a randomized complete block design.

Plot sizes were a single 52-inch-wide by 75-foot-long bed. Fumigant concentrations under the tarp were sampled at one location near the plot center with a MiniRae VOC meter as described above, at 3, 8, 24, 48, 72, 96, 144, 192, 240 and 336 hours after application. The MiniRae meter was calibrated with known concentrations of 1,3-D and chloropicrin prior to each sampling. The plastic films were left on the beds for the length of the strawberry season. Before transplanting strawberries, planting holes were punched in the bed, and ‘Albion’ strawberry was transplanted by hand into all plots on Nov. 24, 2008. Visual crop injury was estimated on Jan. 6, 2009, using a scale of 0 = safe to 10 = dead. On March 10, 2009, diameters were measured on 20 plants per plot. Fruit were harvested from 50 sample stations in each plot once or twice weekly as needed from March 30 until Oct. 30, 2009. Fruit were graded as described in the 2007 trial. Weed densities were measured in 2007 and 2008. In 2008, nylon bags containing yellow nuts edge tubers and weed seeds were buried in each plot before the fumigant application, at a depth of 6 inches. These species were evaluated because they represent a range of susceptibility to fumigants from difficult , to intermediate , to easy . Little mallow and chickweed are common in strawberry. Weed seeds were retrieved 2 weeks after the methyl bromide plus chloropicrin application, and their viability was determined. The yellow nuts edge was planted in potting soil and placed in an illuminated growth chamber at 85°F for 4 weeks. Weed seed viability was determined using tetrazolium assays. Weed density ratings were measured in 125-square-foot sample areas on the bed tops, on Feb. 15, April 28 and July 8, 2008 , and Dec. 11, 2008, and Feb. 3 and March 17, 2009 .The data was subjected to analysis of variance in SAS v. 9.1 , and Duncan’s multiple range test was used for mean separation for all data at the 5% significance level. Weed seed and yellow nuts edge tuber survival data were analyzed to evaluate the effects of fumigant rate, film, and the interaction between rate and film. Linear contrasts were used to compare weed seed survival under the TIF and standard films using SAS PROC GLM. To determine if there was any correlation between strawberry fruit yield and fumigant concentrations, the 2008 data was tested using the SAS PROC CORR routine.