however, when supplemental water can increase 

 yield or avert a crop failure. Supplemental irriga- 

 tion for high-value crops will undoubtedly increase 

 in humid areas in spite of the fact that much capi- 

 tal is tied up in irrigation equipment during years 

 in which little or no use is made of it. 



The range of temperatures in the humid region 

 in which supplemental irrigation is an appreciable 

 factor is almost as great as that mentioned for the 

 arid and semiarid areas. It ranges from the spe- 

 cialty crop production in the short growing season 

 of upstate New York and Michigan to the continu- 

 ous growing season of southern Florida. But in 

 the whole of this area, the most unpredictable fac- 

 tor in crop production is the need for additional 

 water for optimum crop production. 



Soils 



The soils of the humid region contrast with 

 those of the West primarily in being lower in avail- 

 able nutrients. They are also generally more acid 

 and may have problems with exchangeable alu- 

 minum. The texture of soils is similar to that 

 found in the West and ranges from sands to clays. 

 Also, some are too permeable while others take 

 water very slowly. 



Soils of the humid region generally have clay 

 minerals of lower exchange capacity than soils of 

 the arid and semiarid regions and hence lower 

 buffer capacity. They are more easily saturated 

 with anions and cations. This is an important con- 

 sideration if irrigation with brackish water is nec- 

 essary to supplement natural rainfall. Organic mat- 

 ter content ranges from practically none on some 

 of the Florida sands to the high amounts found in 

 some irrigated mucks and peats. 



One of the most important characteristics of 

 many of the soils of the humid Southeast is the un- 

 favorable root environment of the deeper horizons 

 containing exchangeable aluminum and having a 

 strong acid reaction. In fact, the lack of root pene- 

 tration of these horizons by most farm crops is the 

 primary reason for the need for supplemental irri- 

 gation during short droughts. If soil and water 

 management practices would permit roots to pene- 

 trate another foot or two, many irrigations would 

 not be needed. Sometimes normally deep-rooted 

 crops such as alfalfa will wilt or stop growing when 

 there is plenty of available water at a depth as 

 shallow as 2 feet. 



Though there are some relatively level irrigated 

 areas in the humid region, as a whole the land- 

 scape is more uneven than the irrigated areas of 

 the arid and semiarid regions. Because of this, and 



because of the occasional nature of supplemental 

 irrigation in the humid area, sprinkler systems are 

 used almost exclusively. The nature of the land- 

 scape limits the naturally available supplies of 

 water that can be used for supplemental irrigation. 



Specific Difference Between Humid and 

 Arid Regions 



The effect of a specific water quality deterrent 

 on plant growth is governed by related factors. 

 Basic principles involved are almost universally 

 applicable, but the ultimate effect must take into 

 consideration these associated variables. It has 

 been previously stated that the effect of any given 

 water quality deterrent on plant growth is greatly 

 affected by the sensitivity of that plant to the de- 

 terrent, soil characteristics, and the climatic en- 

 vironment. The amount of irrigation water used 

 and soil drainability are also contributing factors. 

 For this reason, water quality criteria for supple- 

 mental irrigation in humid areas may differ from 

 those indicated for arid and semiarid areas where 

 the water requirements of the growing plant are 

 met almost entirely by irrigation. 



Plant sensitivity to a given deterrent is a fixed 

 characteristic of a given species. When irrigation 

 water containing a deterrent is used, its effect on 

 plant growth may vary, however, with the stage of 

 growth at which the water is applied. In arid areas, 

 plants may be subjected to the influence of irriga- 

 tion water quality continuously- from germination 

 to harvest. Where water is used for supplemental 

 irrigation only, the effect on plants will depend not 

 only upon the growth stage at which applied, but 

 to the length of time that the deterrent remains in 

 the root zone (95). Leaching effects of interven- 

 ing rainfall must be taken into consideration. 



Quality of water applied by sprinkler irrigation 

 will affect both foliar absorption and absorption of 

 the constituents found in that water. Although 

 some sprinkler irrigation is found in arid and semi- 

 arid regions, it is the dominant type used for sup- 

 plemental irrigation in humid regions. It is, there- 

 fore, of primary concern in the latter. 



Climatic differences between humid and arid 

 regions also influence criteria for use of irrigation 

 water. The amount of rainfall determines in part 

 the degree to which a given constituent will accu- 

 mulate in the soil. Other factors associated with 

 salt accumulation in the soil are those climatic con- 

 ditions relating to evapotranspiration. In humid 

 areas, evapotranspiration is generally less than in 

 arid regions and plants are not as readily subjected 

 to water stress. The importance of climatic condi- 



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