io8 Commercial Gardening 



The cultivator therefore must always pay attention to the physical 

 or mechanical condition of his soil rather than to its chemical composition, 

 and he must regulate his operations accordingly. It may be stated as 

 a good general rule, that whenever a crop is ready to be placed on the 

 soil, it is a good plan to have it dug in advance whether it be spring, 

 summer, or autumn. 



7. PLANT FOODS IN THE SOIL AND AIR 



By means of experiment it has been proved that all green-leaved plants 

 at least require certain essential foods to enable them to perform their 

 functions properly. Some of these foods are absorbed from the air under 

 the influence of sunlight, and some are taken from the soil by the roots. 

 These essential foods are 



Nitrogen . Potash Iron 



Phosphorus Lime Soda 



Sulphur Magnesia Chlorine 



Silica. 



These thirteen foods are found in all plants in varying proportions. The 

 first four are gaseous and organic, and are driven from the plant by 

 burning. The other nine are found in the ash of plants, and constitute 

 the mineral or inorganic foods. When soluble in water, they are in a 

 condition to be absorbed from the soil under favourable conditions. 



Until about three hundred years ago it had been always thought that 

 the soil supplied all the foods of plants and made up the great bulk of the 

 tissues. Jean Baptiste van Helmont (b. 1577, d. 1644), a chemist of 

 Brussels, was the first to disprove the old theory that all plant foods came 

 from the soil alone. He planted a young Willow weighing 5 Ib. in a pot 

 containing 200 Ib. of soil. He watered the plant daily with rainwater, and 

 grew it for five years. He then weighed the plant and soil again and 

 found that the Willow had increased from 5 Ib. to 169 Ib., but the 200 Ib. 

 of soil had lost only about 2 oz. Van Helmont therefore concluded that 

 the extra 164 Ib. weight of the Willow came from the water alone. In 

 this he was wrong. He did not know that the invisible carbon in the 

 air had anything to do with the increased weight of his W T illow. Indeed 

 it was not until a Dutch scientist, Jan van Ingenhuisz, published his 

 researches in 1779 that it was discovered that the increase in weight was 

 due to the carbon that had been assimilated from the atmosphere by the 

 leaves during the daytime (see p. 44). It is evident, therefore, that only 

 a very small proportion of plant food is actually taken from the soil itself. 

 That little, however, is absolutely essential; and unless it is in a form 

 easily dissolved in water, so that it may be absorbed by the roots, it is 

 quite useless, and no growth can take place. 



The figures on p. 109, compiled chiefly from Dr. A. B. Griffith's works, 



