IOWA ACADEMY OF SCIENCE 
125 
A sample was also secured from plot 308. This had been in corn both years, 
was free from weeds and was in good tilth. 
None of these plots had received any manurial treatment. They were all 
check plots and differed only in crop and cultural treatments. 
Cocklehurs were supplied to two of these soils. They were freshly gathered 
burs, ground and added to the soil at the rate of 19 grams per pot, which is 
.5 per cent of the weight of the soil in the pot. The percentage of burs which 
might occur in the soil under field conditions is estimated at .2 per cent. The 
burs from one medium sized plant were found to weigh 70 grams. This plant 
occupied about two square feet of space, and at this rate the crop would be 
3,361 pounds per acre. When we take into consideration that the leaves and 
the stalks also fall on the soil, and that there are residues of previous crops, 
for the burs are very slow in decaying, it is not hard to believe that two tons 
is a fair estimate for a bur crop. A good crop of corn, including the stalks, 
is three or four tons per acre. Two tons incorporated in the top six inches of 
the soil is .2 per cent. Therefore the burs were supplied in the pots approxi- 
mately twice as heavily as they would be in the field. 
The 19 grams of burs contained 1.74 per cent of nitrogen, or .36 grams. They 
contained .4 per cent of phosphorus, or .076 grams. The potassium was not 
determined, but as in all the previous experiments it has not proved a contribut- 
ing factor in the growth of clover, the exact amount present was not material. 
The manure leachings had .2 grams of nitrogen and .0456 grams of phosphorus. 
Therefore the burs contained nearly twice the amount of plant food that the 
manure- did. 
A series of pots from these samples was planted October 19, 1906. Final 
weights were secured January 18, 1907, or ninety-one days after planting. The 
series of the different soils v/ith the treatments and the results are given in 
the following table. 
OCTOBER BUR SERIES. 
o 
ft 
d 
Soil plot 
Treatment 
Green 
weight 
No. 
plants 
wt. per 
plant g 
Relative 
weight 
Average 
200' 
113 
Check 
4.1 
10 
.41 
96 
201 
113 
Check __ _ 
4.4 
10 
.44 
108 
100 
202 
113 
Manure leachings 
7.9' 
10 
.79 
186 
203 
113 
Manure leachings 
9.5 
10 
.95 
223 
205 
■242 
113- 
Cocklehurs .'5% _ _ 
4.15 
7 
.593 
140 
243 
113 
Cocklehurs .5% 
4.8 
& 
.80 
188 
164 
226 
101 
Check 
6.2 
10 
.62 
146 
227 
101 
Check _ _ 
6.15 
10 
.615 
145 
145 
230 
208 
Check __ _ 
4.4 
10 
.44 
103 
231 
208 
Check 
6.0 
10 
,60 
141 
122 
234 
213 
Check 
3.0 • 
8 
.375 
90 
235 
213 
Check 
4.9 
10 
.49 
113 
102 
238 
213 
Cocklehurs .5% ___ 
5.35 
. 9 
.593 
140 
237 
213 
Cocklehurs .5% 
5.4 
10 
.54 
127 
33 
238 
308 
Check _ 
4.95 
10 
.495 
116 
239 
308 
Check 
5.1 
10 
.51 
120 
118 
The data given above contains considerable support to the theory that the 
burs grown the year previous had left a toxin in the soil, and that the burs 
when added to the soil also poisoned it. The soil from plots 113 and 213 which 
had run to burs in 1905 and grew practically no crop in 1906, were on a par 
for clover growing. Taking these as a standard, the soil from plot 101, which 
