NURSERY PRACTICE FOR PRAIRIE-PLAINS PLANTING 55 



The amount to apply artificially will, of course, vary with the 

 precipitation during the season. In some years no supplemental water 

 may be needed, but in dry seasons it may be necessary to apply as 

 much as 10 to 20 inches of water by irrigation. Therefore, as an 

 insurance measure, a sufficient supply of irrigation water should be 

 available in drought years. The available flow for maximum effi- 

 ciency on an average-sized nursery of 30 acres should not be less than 

 300 gallons, per minute, and there may be required as much as 800 

 gallons per minute on larger nurseries up to 60 acres in area. 



In the prairie-plains region, however, quality of water available for 

 irrigation is as essential as quantity. No long-time studies of this 

 subject have as yet been completed, but from analyses of water used 

 in a number of nurseries in the plains and observations of its effect on 

 nursery stock, it appears the part of wisdom to select a site for decid- 

 uous stock where the total dissolved solids in whatever water must 

 be used for irrigation are not over 600 parts per million, and preferably 

 under 400 p. p. m., with the further stipulation that these be largely 

 in the form of the less toxic compounds. For conifer stock, the water 

 should have not over 200 p. p. m. of total dissolved solids, with even 

 more stringent limitations on the allowable percent of the toxic salts, 

 especially those of sodium and magnesium. 



These limitations are narrower than those for ordinary farm crops. 

 Irrigation experts state that a mineral content of 40 grains per gallon 

 of water (equivalent to 680 p. p. m. of dissolved solids) represents 

 the usual upper limit for irrigation of farm crops. If most of this 

 mineral content, however, is in the form of carbonates and sulfates 

 of lime, the total mineral content can be over 1,000 p. p. m. On the 

 other hand, if most of the dissolved material consists of sulfates, 

 chlorides, and carbonates of potash or sodium in concentrations as low 

 as 400 or 500 p. p. m., it can soon render a soil unfit for use. This is 

 especially true if an impervious substratum prevents proper under- 

 drainage, thus causing a build-up of alkali salts in the surface soil, 

 and also if the amount of irrigation water used must be large in com- 

 parison with rain water, as in dry years or in the western part of 

 the region. 



The toxicity of various alkalis and salts was studied by Breazeale 

 (5, pp. 238-256), in an interesting laboratory experiment in which 

 sprouted seedlings of wheat were placed in solutions of different con- 

 centrations. He found that there was a marked difference in the 

 toxic point of salt solutions, depending on the presence or absence of 

 calcium, which had an ameliorating effect and enabled the wheat 

 seedlings to withstand higher concentrations of the salts. For in- 

 stance, a pure solution of 4,000 p. p. m. of sodium chloride represents 

 the approximate limit of endurance of wheat seedlings, but if a small 

 amount of a soluble calcium salt is added, they can endure concentra- 

 tions of over 10,000 p. p. m. These values are, of course, absolute 

 maximum concentrations, and the weaker solutions of only one-half 

 to two-thirds of lethal dosages generally result in a marked slowing 

 down of growth rate. 



