Frost Protection - UTL Repository

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] F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S [ more than sand and clay soils when wet. However, the thermal conductivity is quite low regardless of the soil water content. Consequently, the diffusivity is low and crops on organic soils are considerably more prone to frost damage. When selecting a site in a region prone to frost, avoid planting on organic (peat) soils. Note that the diffusivity is highest for the sand and clay soils at 20 percent volumetric water content (Figure 6.9). This implies that heat is transferred and stored more efficiently when the soil is moist but not saturated. Therefore, if the soil is wetted to improve heat storage before a frost event, it should be wetted a day or two early to allow for drainage of gravitational water from the surface layer. There is little change in soil temperature below 0.3 m on a daily basis (Figure 6.8), so there is no advantage from irrigating to greater depths. As a practical recommendation, one should attempt to maintain the upper 0.3 m of soil at near field capacity, so allow 1-2 days of drainage before a frost event. Although capturing and storing more heat in the soil is beneficial for some crops (e.g. citrus) it can be problematic for deciduous trees and vines. Deciduous crops grown on these soils tend to bloom earlier in the spring, when there is a greater probability of subzero temperatures. Once planted the soil type in an orchard or vineyard cannot be changed, but planting varieties with greater chill requirements will delay bloom and may reduce the chance of frost damage to orchards planted on dark, heavy soils. After planting, the soil should be managed to maintain the thermal conductivity and heat capacity as high as possible, which will maintain the highest possible minimum soil surface temperature. A simple method to determine the best soil water management to use for frost protection is to measure the minimum soil surface temperature when the soil is exposed to different management. You will need several minimum-registering thermometers to conduct the experiment. On each day for 5–7 days, wet a different 1.0 m 2 or larger plot of soil down to about 30 cm depth. Then, for several days and nights with relatively clear skies, monitor the observed surface minimum temperatures using minimum thermometers laid horizontally on the soil surface of each plot. Whichever of the plots has the highest observed minimum temperature has the best soil water content for that soil. Note the number of days after soil wetting that gave the best result. Then, if a frost is predicted, wet the soil that many days in advance to achieve the best protection. 124
PASSIVE PROTECTION METHODS FIGURE 6.9 Typical thermal properties for sandy, clay and peat (organic) soils (based on Monteith and Unsworth, 1990) BULK DENSITY (Kg m -3 ) 2500 2000 1500 1000 500 0 5.0 sand clay peat 0 20 40 0 20 40 0 40 80 VOLUMETRIC WATER CONTENT (%) sand clay peat HEAT CAPACITY (x10 6 Jm -3 K -1 ) 4.0 3.0 2.0 1.0 0.0 0 20 40 0 20 40 0 40 80 VOLUMETRIC WATER CONTENT (%) THERMAL CONDUCTIVITY (Wm -1 K -1 ) 2.5 2.0 1.5 1.0 0.5 0.0 sand clay peat 0 20 40 0 20 40 0 40 80 VOLUMETRIC WATER CONTENT (%) THERMAL DIFFUSIVITY (x10 -7 m 2 S -1 ) 10.0 8.0 6.0 4.0 2.0 0.0 sand clay peat 0 20 40 0 20 40 0 40 80 VOLUMETRIC WATER CONTENT (%) 125
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- F A O E N A N D V I MO N I T O R
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FOREWORD Agrometeorology deals with
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ACKNOWLEDGEMENTS We want to thank D
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LIST OF PRINCIPAL SYMBOLS Roman Alp
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T n °C Observed minimum temperatur
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CHAPTER 1 INTRODUCTION OVERVIEW Whe
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INTRODUCTION Freeze and frost defin
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INTRODUCTION FIGURE 1.2 Mean air an
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INTRODUCTION FIGURE 1.4 Air (T a )
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INTRODUCTION TABLE 1.2 Categories a
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INTRODUCTION example, because bloom
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INTRODUCTION south in Florida in re
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INTRODUCTION assistance might be in
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CHAPTER 2 RECOMMENDED METHODS OF FR
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RECOMMENDED METHODS OF FROST PROTEC
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CHAPTER 4 FROST DAMAGE: PHYSIOLOGY
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FROST DAMAGE: PHYSIOLOGY AND CRITIC
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FROST DAMAGE: PHYSIOLOGY AND CRITIC
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CHAPTER 8 APPROPRIATE TECHNOLOGIES
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APPROPRIATE TECHNOLOGIES frost prot
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APPROPRIATE TECHNOLOGIES fashion. O
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APPROPRIATE TECHNOLOGIES CROP COUNT
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APPROPRIATE TECHNOLOGIES CROP COUNT
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APPROPRIATE TECHNOLOGIES other crop
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APPENDIX 2 CONSTANTS Specific heat
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- FAO ENVIRONMENT AND NATURAL RESOU
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