30 University of Xew Hampshire fSta. Bull. MA- 



presses it: "Since at the extreme 96-97% of the samples lie within 1.5% 

 of the regression line, it may be assumed that the starch content of single 

 samples may be obtained with an error of ±1.5% by means of the deter- 

 mination of the specific gravity." 



While this review and data indicate that specific gravity is not infall- 

 ible as a means of characterizing the starch content, yet, it does indicate 

 that in a large majority of instances the starch content and specific grav- 

 ity may be used interchangeably. 



Tests on Quality of Potatoes by Specific Gravity 



After the preliminary tests on potatoes grown in 1937, and reported by 

 Haddock (14), larger samples were used in 1938 because it was found 

 that there was considerable variation even in the same sample of potatoes, 

 provided enough tubers were used. The data for 1938 reported by Blood 

 and Haddock (3) are presented. 



In Tables XX\T1I and XXIX tlie quality rating given is the average 

 specific gravity of the entire sample tested, with the first digits of the 

 specific gravity omitted. For example, if the average specific gravity of a 

 sample is 1.095, the rating is given as 95. In this way relative quality 

 ratings are noted, and denote the exact average specific gravity itself. 



It will be observed in Table XXVTII, Jackson farm potatoes, that in 

 the 1938 crop potatoes from the high phosphorus treatment have the high- 

 est rating. These are closely followed by the • plots receiving one-ton 

 equivalent of a high analysis fertilizer (12-24-22) made up from equal parts 

 of Ammo Phos "A", 11-48-0, and nitrate of potash, 13-0-44. It will also 

 be observed that the check plot potatoes (4-8-7) stand third in this list. 

 Potatoes from plots with high potash fertilizer, 4-8-14 rated 80.95 as 

 against 93.4 for the clu-ck treatment, while those which received extra 

 phosphorus and potash Ijoth, tested 83.9, slightly higher than the jiotatoes 

 treated with a high potassium fertilizer. 



Potatoes which were produced on plots treated with a 4-8-7 fertilizer 

 in which the ]:)hosphoric acid was derived from basic slag rated 87.4 as 

 compared with 93.4 for the check plots. Yields were not materially in- 

 creased under slag treatment, but the pH of the soil was increased sig- 

 nificantly and more scab developed here than a])peared to be warranted by 

 the pH change produced by this alkaline pho>])horus carrier. I'.oron 

 changed the appearance of the potatoes l)y imparting an external luster, but 

 did not have an appreciable effect on cooking cpiailty. 



Table XXIX gives similar results for Lane farm plots on potatoes 

 produced in 1938. These data support thf»se of Table XXVIII. since 

 tubers produced with high phosj)horus exhibit a higher quality rating, and 

 those with high potash have a lower rating than the check plots. Increas- 

 ing or (emitting the nitrogen ajipeared to have little effect on f|uality. but 

 omitting the phosphoric acid or potash, or omitting the fertilizer entirely 

 seemed to have a bemlicial effect on (|uality, although, of course, the yields 

 under these treatments were seriously reduced am! the potatoes were much 

 smaller than those from pkjts more favorably treated. 



'i'ablc XXX presents a summary of quality ratings of potatoes from 

 comparably fertilized plots from Jackson and l.ane farms for the 1938 

 season. 



