THE REQUIREMENTS OF PLANTS 



3-5 



amount ; one was just moist, another moister, and a third still moister. 

 These were then each subdivided into three others, one receiving no 

 nitrogen compounds, another one dose, and the third two doses. Oats 

 were sown in all nine sets with results that are given in Table XIII. : 



TABLE XIII. INFLUENCE OF WATER SUPPLY ON THE EFFECTIVENESS OF MANURES. 

 VON SEELHORST AND TUCKER (257). 



DRY WEIGHT OF OAT CROP. 



K = i gram of K 2 O as K 2 CO 3 per pot ; P = i gram of P 2 O B as Ca(H 2 PO 4 ) 2 per pot 

 N = '5 gram of N as NaNO 3 per pot. 



When only little water is present the added -5 gram of nitrogen 

 is without effect, the supply in the soil being sufficient for the crop 

 needs : the water and not the nitrogen is the limiting factor. When 

 more water is added the plant can make more growth, and can there- 

 fore utilise more nitrogen : the added -5 gram now raises the crop by 

 10 grams. Again, however, the water supply sets a limit, and the 

 second -5 gram of nitrogen is without effect. When a liberal 

 supply of water is added the first -5 gram of nitrogen gives 20 

 grams of crop, double the previous increment ; but even this does not 

 represent the whole possibility, for the second '5 gram of nitrogen 

 gives a still further increase of 15-5 grams. 



The results of the phosphate series are somewhat different in detail, 



1 The moist soil contained 14-35 per cent, of water (41*6 per cent, of saturation), 

 the moister soil 15-41 per cent, at the beginning, increasing to 18-43 (51-7 per cent, of 

 saturation) as the experiment proceeded, and the wettest soil, 16-44 per cent, at the 

 beginning, increasing to 22-59 per cent. (63-7 per cent, of saturation). 



