THE WATER REQUIREMENTS OF FRUIT PLANTS 17 



same soil. The greater tolerance of the apricot, olive and peach for drought 

 probably is not due to a utilization of the soil water in contact with their 

 roots greater than that of other plants, or in other words, to the reduction 

 of the soil water content to a lower wilting coeiEficient. It is possibly 

 associated with a greater ability of their roots to exploit every bit of soil 

 within their range, or to a wider range that their roots may possess. 

 It is important that both factors be kept in mind, namely, the marked 

 uniformity in the wilting coefficient for different plants and the marked 

 difference in their ability to get along on a limited water supply, for 

 both are factors that may alter materially cultural methods, the choice 

 of stocks upon which the fruits are grown and planting plans. As a rule 

 it is not necessary to wait until wilting actually begins to determine when 

 the danger point is at hand. Most plants will show signs of distress 

 before the moisture supply of the soil reaches its wilting coefficient. 

 Many weeds or cover crop plants growing among the trees may wilt 

 noticeably before the trees give visible evidence of moisture deficiency. 

 Temporary v/ilting at the middle of the day is quite likely to be an indica- 

 tion that the water supply of the soil is approaching a critical point and 

 efforts should be made to deal with the situation promptly. 



Summary. — Water is an important plant constituent, composing 

 from 50 to 85 per cent, of most living tissue. It is the solvent for all 

 plant nutrients. The intake of from less than 30 to more than 1,000 parts 

 of water is required for each part of drj^ matter produced, the amount 

 varying with the species and with the conditions under which the plant 

 is grown, rWhen seepage, run-off and evaporation are included this 

 means that for the average deciduous fruit crop a precipitation of some- 

 thing like 30 inches is requiredJ Inability to secure the requisite amount 

 of water checks growth and reduces yield and often a relatively small 

 amount of additional available moisture at a critical period will make 

 possible material increases in the size of the crop. Planting distances 

 in the orchard should be determined largely by the available moisture 

 supply and the growing habits of the particular species or variety. The 

 minimum water requirement of the plant, in terms of units of water 

 per unit of dry matter, is correlated with thorough acclimatization and 

 optimum growing conditions. Favorable nutritive conditions in particu- 

 lar make for water economy. The final wilting coefficient is practically 

 the same for all plants in all soils, but it varies greatly for the same plant 

 with different soils, being low for soils of coarse texture and very high 

 for fine clays. In the field, temporary wilting generally occurs before 

 the wilting coefficient is reached, the evaporating power of the air being 

 an important determining factor. In practice it is therefore desirable\ 

 to maintain the soil moisture supply well above the wilting coefficient,'^ 

 Different species and varieties show considerable variation in their 

 ability to withstand drought. 



