INTRODUCTION 7 



able to cohere. Many forms of organic matter have a great 

 capacity for absorbing water. This can be explained by 

 reference to some familiar instances. Ground linseed cake 

 will absorb about sixteen times its weight in water, peat 

 moss litter about ten times its weight of water, and gelatine 

 about twenty times its weight of water, whilst the material 

 known as agar, or dried seaweed, is capable of retaining 

 up to two hundred times its weight of water. The effect 

 of any manures of this class upon the water supply of the 

 soil is very pronounced. It will readily be seen that a 

 material which provides water for lasting out a droughty 

 period will confer a great advantage, and an equal advantage 

 will result from a material which will prevent surface washing 

 of the soil, by absorbing water during excessive rainfall. 

 It is quite impossible to find out, except by experiment on 

 the soil itself, what the value of any particular organic 

 manure may be as regards the water-holding capacity. On 

 very light soils the value will be due to retention of water, 

 and cohesion of the sandy particles. On heavy soils the 

 value will be due to the prevention of surface washing, by 

 absorption of excessive rain, opening up the soil to air, 

 and making the soil lighter for spade or plough to work. 



An important point in the consideration of the use of 

 fertilizers is the depth of penetration of the manures. 

 Nitrates will penetrate to practically any depth. Ammonia 

 compounds are entirely precipitated on the surface, and do 

 not usually go more than two or three inches deep. Amides, 

 such as urea and asparagine, penetrate perhaps to about 

 ten or twelve inches. Soluble albuminoids penetrate to a 

 depth midway between ammonia and amides. The insoluble 

 albuminoids filter out on the surface. Phosphates are precipi- 

 tated near the surface and rarely reach a depth of eight 

 inches. Super-phosphate will be found for the most part 

 at a depth of about four or five inches. Basic slag does not 

 readily penetrate more than about one inch. Potash pene- 

 trates a little further than ammonia. This, of course, 

 applies only to the immediate action. Secondary actions 

 of all these materials will alter their position. 



