Specific Kieselguhr Increased Hop Yield Over Several Years

The yields of farm crops are strongly affected by rainfall. Cultivation methods that can alleviate rainfall shortage have been long awaited to ensure stable and sustainable crop production. Porous materials, such as kieselguhr and perlite, were applied during hop cultivation, and their contribution to yield and beer quality was analyzed. An increase of over 20% in yield was confirmed following the application of burnt coarse kieselguhr. This increase was sustained for at least 3 years from the time of burnt coarse kieselguhr application in the first year. Water content around the hop root was maintained at a high level after burnt coarse kieselguhr application. Therefore, it appeared that burnt coarse kieselguhr absorbs water in its porous space during rainfall and releases water to the plants thereafter, including during drought conditions. However, the quality of the hops was not changed following burnt coarse kieselguhr application. Burnt coarse kieselguhr application around the plant root is expected to alleviate the effect of climate change and contribute to the stable and sustainable production of various crops that need a constant water supply.


Introduction
Hop (Humulus lupulus L.) is a perennial dioecious climbing plant.Flowers (hop cones) that form on the female plants are ingredients used in beer production.In beer brewing, hops add a crispy bitterness without any lasting flavor, and produce a fresh hoppy aroma and fine foaming beer (Schönberger & Kostelecky, 2011;Steenackers, Cooman, & Vos, 2015).Its important position in beer brewing was established during the Medieval Ages in Europe, not only to enhance the flavor of the beer, but also for quality stabilization of the antibacterial activity of humulones.Compounds that affect flavor, such as humulones, which contribute to bitterness, and terpenes, which contribute to the hoppy aroma, are localized in the resin-like lupulin in the hop cone (Patzak, Krofta, Henychova, & Nesvadba, 2015;Wang et al., 2008).Moreover, xanthohumol, which contains a prenyl group and is a notable functional compound providing health benefits, is also localized in lupulin (Nagel et al., 2008).The bract of the hop contains a variety of phenols, which contribute to the fullness of the beer (Inui, Okumura, Matsui, Hosoya, & Kumazawa, 2017).
Hop is cultivated in cold districts, where the average temperature in mid-summer is under 25 °C.The major hop growing countries are the United States, Germany, Czech Republic, China, Australia, and New Zealand (Meier, 2017).Over 100 cultivars that have specific qualities are grown in the countries mentioned above (Kishimoto, Kobayashi, Yako, Iida, & Wanikawa, 2008;Kovacevic & Kac, 2002).Hop grows to a height of 7-9 m attached to a dedicated hop trellis (Matsui, Inui, Ishimaru, Hida, & Oka, 2012).Hop sprouts in early spring and grows to its maximum height in only 3 months; thereafter, it blooms, and harvesting is completed upon maturity, 1-2 months after blooming (Matsui, Inui, Oka, & Fukui, 2016).The time of harvest is judged based on the maturity condition of the flavor-related compounds for determining beer quality (Cocuzza, Lutz, & Muller-Auffermann, 2013;Kavalier, Ma, Figueroa, Kincaid, & Matthews, 2014;Keukeleire et al., 2003;Sharp, Townsend, Qian, & Shellhammer, 2014).These plants require considerably more water than other crops do, and the yield depends to a large extent on the amount of water received from rainfall or irrigation systems.Therefore, the yield of a hop garden that contains an irrigation system is stable.Most hop gardens in the United States install irrigation systems because rainfall is low during the hop cultivation period in the hop growing areas of the United States.
The Saaz variety, which is cultivated in the Saaz region of the Czech Republic, is famous for its pleasant hoppy aroma and mild bitterness, and it is referred to as a "Fine aroma hop" in breweries worldwide.In the past 11 years, yields in 4 years (2006, 2007, 2012, and 2015) were lower than the standard yield, and requisitions during these 4 years for this hop variety from breweries could not be satisfied.Neither the growers nor their cultivation methods differed across years in the districts cultivating the Saaz variety (approximately 4,000 ha); therefore, it was speculated that the climate conditions affect yield instability.According to the growers, the lower the rainfall is, the lower the yield.Long-term climate modelling analysis of the Saaz region has shown that rainfall will decrease gradually in the future (Hanel et al., 2018;Mozny et al., 2009), and hop yield is related to rainfall during the cultivation period (Mozny et al., 2009).Irrigation systems are installed in only 30% of the hop gardens because of the lack of a water source in the Saaz hop growing districts.It is thus possible that periods of lower yields might occur more often in the future.Importantly, effective methods to mitigate the effects of rainfall shortage must be developed in the Saaz hop growing regions.
In other crops, such as corn and strawberry, it has been determined that adding kieselguhr to the soil increases yield and improves quality (Escobar, Espejo, & Rodríguez, 2014).The mechanism underlying this effect might be that kieselguhr holds water and nutrients in its porous space and constantly releases them.This mechanism might be effective for the stabilization of hop yield during periods of rainfall shortage.In the present study, we placed different porous materials around hop roots to identify measures to obtain stable yields during periods of rainfall shortage.Kieselguhr and perlite were the porous materials tested.Kieselguhr is the fossil of diatoms (Bacillariophyceae).Diatoms absorb silica during their lifetime; after their death, all the components, except silica, are removed over millions of years of fossilization.Kieselguhr is mainly used for filtering liquids such as beer, juice, and oil.In addition, it is used as a component of materials for the construction of the walls of houses because it absorbs moisture under high humidity conditions.Perlite is produced from volcanic ash at high temperature and high pressure and also has a porous structure.Although these porous materials have been used previously for soil improvement (Aksakal, Angin, & Oztas, 2012) in other crops, they have not been used for hops to date.Kieselguhr is classified into burnt and unburnt types, and its particle size is variable.The water absorption ability of kieselguhr might be affected by these factors (Nascimento et al., 2018).The type of kieselguhr most suitable for providing water to crops has not yet been determined.In the present study, we assessed two categorizations of kieselguhr-burnt vs. unburnt and fine vs. coarse for their suitability for the stabilization of hop yield over a period of 3 years beginning 2015.

Average Yield of Saaz Cultivar and Climate Data in Saaz-cultivating Districts
The average yield of hop (dried hop cone) of the Saaz cultivar grown in the Saaz region, Czech Republic, originated from Bohemia Hop Co. Ltd. (Zatec, Czech Republic).Temperature and precipitation data were obtained from the climate observation systems administrated by the Hop Research Institute Co. Ltd. (Zatec, Czech Republic).

Porous Materials
Three types of kieselguhr and one type of perlite were used in the present study (Table 1).All the materials were obtained from the Showa Chemical Industry Co. Ltd. (Tokyo, Japan).

Hop G
The hop g The plantin

Porous
Porous ma L of soil a Perlite and roots was r

Figure
Figure 1.A Abbreviations

Figure
Figure 5. S blue dots)

Table 1 .
Porous materials used in hop gardens since 2015

Table 2 .
Li org Garden garden used fo ng density was s Material App aterial applicat around the hop d soil were pla removed and 2 Application of ist of tests perf est No.