26 MASS. EXPERIMENT STATION BULLETIN 361 



suggests a gradual release of the soluble toxic salt into the drip. A 1 percent 

 solution of chemically pure zinc sulfate is very injurious to tomato foliage, and 

 Read and Orchard (35) found that a 0.3 percent solution is the maximum con- 

 centration tolerated by chrysanthemum plants without noticeable injury. 



In a final experiment six proprietary white paints selected to contain different 

 amounts of zinc oxide and varying otherwise in their composition were painted 

 on wooden panels. The same procedure of fumigation and collection of the 

 fumigated residue was followed, and 10 grams of dry residue were added to 40 c. c. 

 of distilled water. The extracts were tested for sulfides and sulfites and analyzed 

 for sulfates and zinc. Small samples of the extracts were atomized on tomato 

 foliage in the greenhouse. The degree of injury was recorded 3 days after the 

 treatment (Table 16). 



The analyses confirm the results of the previous experiment. Soluble sulfides 

 and sulfites were lacking or never more than a trace. The amount of zinc re- 

 covered from the extracts and the degree of injury to the test plants were in rather 

 definite proportion to the amount of zinc oxide pigment in the paint. The Lucas 

 paint, which contains 5 percent zinc oxide, yielded after fumigation the smallest 

 amount of sulfates and zinc and caused no injury. The DuPont, Lowe Bros., 

 and Kyanize paints, containing 27, 25, and 33 percent zinc oxide pigment re- 

 spectively, yielded the largest amounts of soluble sulfates and zinc salts and caused 

 the most injury to tomato foliage. 



The Lucas and particularly the white lead paint residues on the wooden sur- 

 faces were dry and firm upon the removal of the panels from the humid sulfured 

 chamber. The other paint residues were soft and greas)'. The problem of com- 

 pounding a paint for interior greenhouse use that will not be softened and shed 

 by the action of sulfuric acid from the burning of sulfur is beyond the scope of 

 this stud\', but its significance is to be noted as a matter of record. There is some 

 suggestion of the value of the carbonates of lead and calcium in the composition 

 of the paint as a deterrent to this condition. 



Judging from the results reported in Table 16, a small amount of zinc oxide 

 in the composition of paint, e. g., as much as 5 percent, may be tolerated. How- 

 ever, it would seem safer to exclude zinc oxide altogether from interior greenhouse 

 painting. Read and Orchard (35) stated that the burning of sulfur is not advised 

 except when the interior painting is free from zinc. The former precaution seems 

 preferable, since sooner or later the occasion may arise for burning sulfur between 

 crops as a sanitary pest control measure. Undoubtedly, the amount of plant 

 poison dissolved in the drip is increased slightly by the fact that all zinc pigments 

 in white paints contain a trace to small amounts of zinc sulfate (51). 



Reaction with Metal Surfaces and Trellis Wire 



Aside from the plant injury from metallic salts dissolved in the drip water after 

 fumigation with burning sulfur, the greenhouse interior metal construction is 

 corroded and the usefulness of the trellis wire is greatly reduced. On galvanized 

 metal surfaces the chemical salt formed during the fumigation period has the 

 properties of white vitriol (zinc sulfate). Sulfur dioxide from burning sulfur is 

 converted quickly into sulfurous and sulfuric acids as is suggested by the thick 

 deposit of the white crystalline salt on galvanized metal surfaces at the end of the 

 fumigating period. Kadow et al. (25) mention the formation of a greenish yellow 

 salt of possibly zinc thiosulfate and the probability of its oxidation to zinc sulfate. 



In the case of injury on tomatoes which came to the writer's attention in the 

 spring of 1937, all of the damage was confined to plants directly below rusted iron 

 pipes and span supports. In this instance an excessively large dosage of sulfur 

 was burned previous to the removal of the preceding planting of tomatoes. Such 



