Page 30 



BETTER FRUIT 



May, ipiQ 



Possible Cause of "Sour Sap" 



I'.oiitiiuird from pnf^o 11. 



the active temperature in the orchards 

 are available. Experiment Station rec- 

 ords, however, show that the lowest 

 temperatures for April at Corvallis oc- 

 curred when the trees were in lull 

 bloom, and were as follows: April 2, 

 27°; April 3, 21% and April 4, 26'. At 

 Hermiston and Stanfield in Eastern 

 Oregon, while the temperature fell to 

 25°, 17° and 18°, respectively, o-n April 

 1, 2 and ,3, and during the weeks end- 

 ing April 20 and 27 the minimum tem- 

 peratures were 26° and 25°, respec- 

 tively, and the crop was ruined, no, 

 appreciable injury was done to the 

 trees. Likewise in the vicinity of Med- 

 ford the temperature fell to 21° in cer- 

 tain orcliards, and while the crop was 

 more or less reduced no apparent in- 

 jury was done to trees, either young or 

 old. It is my belief that temperature in 

 the orchard sections of the humid 

 Northwest rarely, if ever, falls low 

 enough in April or May to cause serious 

 injury to the trees, although it may 

 rarely cause the loss or partial loss of 

 the crop. 



^^^lat, then, is the cause of "spring 



injury 



I do not know, but during 



recent months I have developed a 

 theory which I believe accounts for it 

 and offers hope of very largely pre- 

 venting it. In brief it is this: 



Spring injury is due to acute nitro- 

 gen starvation during the most active 

 vegetative periods in the life of the tree. 



The first objection to this theory no 

 doubt will be that the most serious 

 spring injury often occurs in orchards 

 which have shown none of the charac- 

 teristics of nitrogen starvation, and 

 that the most vigorous trees are often 

 most seriously injured; the second will 

 be that it often occurs in orchards 

 growing upon soils which are known 

 to contain a fair amount of nitrogen — 

 or at least are not known to be mark- 

 edly deficient in nitrogen. 



The answer to the first objection is 

 that if acute nitrogen starvation is the 

 cause of spring injury we should expect 

 that the most vigorous trees at their 

 most active vegetative period would 

 suffer most since their needs are 

 greater. To the second objection the 

 answer is that though nitrogen may be 

 present in the soil in normal amounts 

 it may not be available for the use of 

 the tree during the critical period of 

 its most active vegetative development. 



Plants can obtain the necessary nitro- 

 gen only when it is in the soil in the 

 form of nitrates. But nitrates are 

 leadily soluble; and at the lime of 

 spring injury — at the time the trees are 

 most in need of an adequate supply of 

 nitrates — the soils of the humid por- 

 tions of the Pacific Northwest have 

 been subjected to a drenching and 

 leaching process for a period of approx- 

 imately six months. Is it not possible — 

 even probable — that the nitrates, even 

 though they may have been present 

 during the preceding summer and fall 

 in normal amounts, have been so de- 

 pleted that the trees may suffer from 

 acute nitrogen starvation during a few 

 brief but critical weeks? Soil analyses 



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