388 STIRRING THE SOIL, AND MANURING. 



having become choked up by the washing down of the soil. In this case, 

 the water, not escaping freely from the pot, produces all the evils of stagna- 

 tion already mentioned (821); the spongioles burst and the fibres rot, the 

 leaves become yellow and drop off, and the bark, being distended by moisture, 

 separates from the wood, the plant in the meantime being killed. Nothing 

 is more common than cases of this kind in the greenhouses and window- 

 gardens of amateurs ; and it is very frequent also in collections of plants in 

 pots, such as alpines, under the care of regular gardeners, whose workmen or 

 apprentices water them indiscriminately, with little or no regard to the state 

 of the plant or the soil hi the pot. The obvious manner of preventing this 

 evil is, whenever there is the slightest suspicion of overwatering, to turn the 

 plant out of the pot, examine the drainage, which will come out with the 

 ball, and take it off and replace it with fresh materials. It would be well 

 also, in the case of all plants that are likely to be overwatered, to use a 

 larger proportion of sand in the soil, and to put extra drainage in the bottom 

 of the pot, and also to introduce among the soil a considerable proportion of 

 fragments of freestone. 



829. Aquatic and marsh plants, being grown in water, or in soil saturated 

 with it, form exceptions to the treatment required for plants in general ; 

 nevertheless it has been observed of these that they always grow with most 

 vigour when the atmosphere is moist, whether produced in hot-houses by 

 watering over the top, or in the open air by rain. The cause, De Candolle 

 thinks, may be in part traced to the state of the electricity of the atmosphere 

 during rain ; and perhaps something also may be due to the temporary 

 cessation of excessive evaporation. 



830. Watering with liquid manure is necessarily confined to the soil, and 

 is most advantageous when given to plants in a growing state ; because, 

 though at other seasons a portion of it would still be absorbed by the roots, 

 yet the greater part would be washed into the subsoil. See xiii. 



831. To economise the water given to plants, more especially in the open 

 air, the surface is sometimes mulched with fibrous or littery matter, or even 

 with small stones or pebbles. Both materials retain moisture and heat ; 

 while stones or pebbles, by becoming soon dry, prevent surface-damp, and 

 reflect much heat during sunshine. The strawberry is sometimes mulched 

 with straw, and sometimes with tiles or slates, or pebbles, for the double 

 purpose of retaining moisture and keeping the ripening fruit clean ; and 

 the surface of the ground in the rose nurseries about Paris is sometimes 

 mulched with straw, to save watering, and prevent the rose-beetle from 

 depositing her eggs in the soil. 



XIII. Stirring the Soil, and Manuring. 



So much has already been said on these subjects that it is only necessary 

 here to advert to the chapters in pages 45 and 56, and to page 227. 



832. Stirring the soil is advantageous by the admission of air, rain, and 

 heat to the roots of plants, by promoting evaporation in moist soils, and by 

 retaining moisture in such as are dry. In the latter case the dry loose soil 

 on the surface acts as a mulching or non-conductor to the soil below ; and 

 in the former it acts by exposing a greater number of moistened particles to 

 the air than could be the case if these particles were consolidated. The 

 celebrated agriculturist Curwen found that an acre of pulverised soil eva- 



