412 The Woburn Pot- Culture Experiments , 1913. 
given were to the effect that thp presence of anything above 
*02 per cent, of the metal in a soil would exercise a toxic 
influence, but that in less quantity zinc might prove stimulating. 
In 1912 the salts experimented with were the phosphate, 
the nitrate and the carbonate, and in amounts containing 
respectively ’03 per cent., ’02 per cent, and ’01 per cent, of the 
metal zinc. In that year, however, the whole set of plants 
were attacked by a species of mildew, and this affected the 
results so much that it was felt desirable to repeat the work 
in 1913 on the same lines, and this was accordingly done. 
The soil used was that of Butt Furlong, one very fairly 
supplied with plant food; it contained lime 1*04 per cent., and 
phosphoric acid *24 per cent. 
The experiments were carried out in large earthenware 
pots, each holding 40 lb. of soil. The salts were mixed 
with the whole of the soil contained in each pot, and each 
experiment was in duplicate. 
Wheat was sown on November 27, 1912, twelve seeds in 
each pot, the number of plants being subsequently reduced 
to six. 
On the untreated sets the wheat all came up by Decem- 
ber 16 ; with zinc phosphate and zinc carbonate, even in the 
larger amounts, the prolongation of the incubation period was 
hardly marked, but where zinc nitrate had been applied there 
was a decided prolongation, more especially with the stronger 
applications. Thus, where '03 per cent, zinc had been used 
as nitrate, no plants appeared until December 30, and it was 
forty-three days before the whole twelve showed. With ’02 
per cent, zinc (also as nitrate) December 19 was the first date 
of appearance of the plant, and it took thirty-four days for 
all the twelve shoots to show. With ’01 per cent, only twenty- 
five days were required. 
During the earlier stages of growth the zinc phosphate and 
zinc carbonate plants were much alike in size, and did not differ 
greatly from the untreated ones, except as regards a marked 
increase in the tillering. With zinc nitrate much ranker 
growth and deeper colour of foliage were evident. Where 
03 per cent, of zinc had been used the plants were distinctly 
stunted.. In May, the phosphate and carbonate plants were 
much alike, but the nitrate ones began to show signs of mildew. 
The foliage also became very limp. 
In June it was only in the case of the heaviest application 
( 03 pei cent, zinc) that anything like a toxic effect was 
shown with carbonate or phosphate. With the nitrate, how- 
ever, more especially the heavier dressing, this toxic influence 
was very pronounced, and the tops of the ears were in all 
cases found to be practically 44 blind.” 
