HARM DONE BY WEEDS 7 



broad flat leaves, under which grass does not grow, the 

 removal of the weed leaving a bare patch. Under ideal 

 conditions the whole of the cultivated area should be 

 occupied, even if not covered, by the planted crop. 



(b) Weeds rob cultivated Crops of (i) Food; (2) Light, 

 Air, and Heat; (3) Moisture. When we see a cornfield 

 crowded with weeds we may be quite sure that these 

 not only take up much space, but that they also rob the 

 cultivated crop of food, light, and moisture. 



(i) A large crop of weeds not only takes up much 

 carbonic acid gas from the atmosphere, but needs a 

 considerable quantity of mineral food, which can only 

 be obtained from the soil and from the manures applied 

 for the sown crop. That weeds absorb soluble in- 

 gredients from the soil in considerable quantity is clearly 

 shown by analysis. Some analyses made at Konigs- 

 berg, and lately reported by Professor Stutzer and L. 

 Seidler, 1 show that the amounts of nitrogen, phosphoric 

 acid, potash, and lime which are removed are deserving 

 of serious consideration. A number of weeds without 

 their roots were collected from oat fields, the soil of 

 which was fairly heavy and poor in humus. In the 

 case of the Wild Radish or White Charlock the plants 

 had already formed many seed-pods, but the other 

 weeds were in full bloom. The table on page 8 

 shows the* percentage of ingredients in the dry matter. 

 These figures indicate in a general way the amount 

 of the chief plant foods required by weeds. The nitro- 

 gen in the Persicaria nearly equalled 20 per cent, and 

 that in the Sow Thistle nearly 15 per cent of albumi- 

 noids in the dry matter. Phosphoric acid was chiefly 

 taken up by Spurrey and Persicaria ; potash by the 

 Sow Thistle and Spurrey ; and lime by Persicaria, 

 Yarrow, and Cornflower. 



1 Fiihling's Landwirtschaftliche Zeitung, June 15, 1908, p. 429. 



