For decades, scientists have observed that crops grow better and yield more, often significantly more, when the soil they are grown in is fumigated with a gaseous biocide prior to planting. The term, “increased growth response” or IGR came into use early to describe the phenomenon without explaining it. Chloropicrin, also known as tear gas, was one of the first fumigants used commercially, which was for control of nematodes and other soilborne pathogens of pineapple in Hawaii starting shortly after WWII. Combinations of chlorpicrin and methyl bromide came into commercial use for strawberries in California in the 1950s and 60s following the work of California plant pathologists Steve Wilhelm and Albert Paulus, with often spectacular yield increases due mainly to control of root diseases—with weed control as a bonus.
Along with the term IGR came three explanations for the phenomenon: 1) it is nothing more than a response to the nitrogen, phosphorus, and other nutrients released from the microbial biomass killed by the fumigant; 2) it is due to plant growth promotion by microorganisms as they recolonize the fumigated soil; and 3) it is due to elimination of root pathogens, showing what the crop should look like with a healthy root system.
As a student of Steve Wilhelm at UC Berkeley, I decided early in my career with the USDA Agricultural Research Service responsible for a research program on root diseases of wheat to use fumigation as a research tool in my field plots. Soil fumigation would never be a commercial reality for wheat like it was and remains today for high-value fruit and vegetable crops, but it has proved to be an invaluable research tool in showing what wheat with healthy roots should look like.
I started with the 1-pound cans of methyl bromide introduced under a plastic tarp sealed around the edges with soil, but soon went to the 50-pound cylinders as the number of field trials grew. Small plots such as in the picture above were used to provide “pathogen-free” checks in my field trials seeded with our plot equipment and as replicated trials with and without fumigation in grower’s fields seeded by the grower. Starting in 1974, and continuing for the next five years, WSU plant pathologist William Haglund at the WSU experiment station at Mount Vernon hauled his fumigation equipment to eastern Washington where we carried out experiments designed to test different fumigants with and without plastic tarp, determine the full yield potential for wheat in different rotations, and sort out the role of root disease versus the flush of nutrients from the killed microbial biomass. All along, my goal was to find rotations and economically affordable treatments that would produce the same or nearly the same increased growth and yield response but without using fumigation.
Without exception, the growth and yield of wheat, both winter and spring wheat, was significantly better in fumigated compared with nonfumigated plots. In the photograph below, for example, the yield of winter wheat in the nonfumigated plot in the foreground averaged a respectable 90 bushels/A (6.2 mt/ha) for the four replicate nonfumigated plots but that in the background averaged a spectacular 120 bushels/A (8.2 mt/ha), with the same available water, nitrogen application, growing season, etc. The IGR is these experiments was obvious as taller plants, as shown in the photo below, and more tillers with heads (spikes), often up to 20-25% more tillers with head, thereby accounting for more grain.
Of the many experiments conducted in grower’s fields during the 1970s and early 1980s, five experiments were conducted in fields where winter wheat was grown every other year (2-year rotations), alternated with spring peas of spring lentils, and five experiments were conducted in fields where winter wheat was grown every third year (3-year rotations), with spring barley as the third crop planted between the winter wheat and the spring legume. While not designed to test the interaction between soil fumigation and 2-year versus 3-year rotation, it was significant that the average yield increase in response for fumigation ranged between 13 and 36% where winter wheat was grown in 2-year rotations and between 3 and 12% where winter wheat was grown in 3-year rotations.
One of the best fumigation treatments was a mixture of 1,3 dichlorpropene (trade name Telone) mixed with chloropicrin at 17% by volume (Trade name Telone C17) shanked in at 25 gal/A (240 liters/ha) and roller-sealed without a plastic tarp (see photo above). Telone by itself is a good nematicide whereas Telone C17 is both a nematicide and fungicide. At two locations, we were able to show that both Telone and Telone C17 produced a significant flush of ammonium nitrogen, possibly because of killing the micro-fauna abundant in soil. Also in at these two locations and in response to both fumigants, the ammonium remained as ammonium well into the growing season, no doubt because both fumigants knocked out the nitrifying bacteria responsible for converting ammonium to nitrate. However, only Telone C17 produced the increased growth response. Soil analysis showed further that Telone C17 but not Telone by itself virtually eliminated Pythium spores from the soil as measured by dilution plate counts using a selective medium. To my knowledge, these are the first results that experimentally separate the IGR from the flush of nitrogen when soils are fumigated. Read Abstract here.
It was also revealing that winter wheat headed earlier when grown in plots fumigated with Telone C17 but not in response to Telone by itself. This is apparent in the photo below where winter wheat on the right and fully headed was grown in soil fumigated with Telone C17 whereas plot on the left was fumigated with Telone alone, which was no different than a nonfumigated check. One of the most consistent symptoms of Pythium and Rhizoctonia root rots of wheat is to delay maturity. These two root diseases, by eliminating roots and rootlets, also limit ability of the crop to explore the soil for immobile nutrients such as phosphorus. The earlier heading is likely a response of the crop to better uptake of phosphorus made possible by the availability of roots to take up phosphorus. (See also Root Diseases of Wheat and Barley: What do they look like and what do they do to the crop?)
We also analyzed for ammonium and nitrate nitrogen in the top 4 feet (120 cm) of soil profiles in fumigated and nonfumigated plots immediately following harvest. Again, the differences were clear; considerable nitrogen as nitrate was left unused in the profile of nonfumigated plots but was close to zero available at each incremental depth down to and including the 4th foot in fumigated plots. Thus, similar to the likelihood that the earlier heading is due to greater uptake of phosphorus, the taller plants and greater tillering that translates into more heads and hence higher grain yield is likely due to greater uptake of nitrogen, not because the fumigant releases more nitrogen, but because the crop with healthy roots is better able to use and makes a greater demand on the available nitrogen.