162 GREEN MANURING 



condition, can soon be utilised by the crop and where the deficiency 

 of retentive power of the soil may not lead to much loss in drainage. 



On grass land, where top-dressing is the only practicable method 

 of applying manures, there are obvfous reasons for preferring short, 

 well-rotted manure. 



On arable land, farm-yard manure is usually ploughed in, some 

 time before the crop is sown, but on light soils, especially where the 

 subsoil is porous, it is often spread in the furrows immediately before 

 potatoes are planted. The. water-retentive power of the bulky organic 

 matter of the manure is then often of importance in dry seasons. 



The heat produced by the decay and fermentation of farm-yard 

 manure, of which advantage is taken in the construction of hot- beds 

 under frames, in the cultivation of certain vegetables, though doubtless 

 produced in the field, cannot be of much importance under ordinary 

 farming practice, since any rise of temperature due to this cause must 

 be extremely small. Nevertheless, the very heavy dressings often used 

 with potatoes, in some cases reaching thirty or forty tons per acre, may 

 owe some of their efficiency to this cause. 



Farm-yard manure, sea- weed, and other bulky organic manures re- 

 quire to be used in very large dressings. Ten, twenty, or even up to 

 thirty-five tons per acre are not unusual quantities, but obviously much 

 depends upon the crop to be grown. In market gardening, especially 

 according to the recently boomed French gardening system, immensely 

 greater quantities are employed. In this last-mentioned method, the 

 heat produced by fermentation becomes an important factor. 



Green Manuring. One of the great advantages of manuring with 

 farm-yard manure viz., the addition to the soil of a large quantity of 

 bulky organic matter, with a consequent improvement in its texture and 

 water-retaining power can, in a great measure, be obtained in another 

 way, by the practice of green manuring. This consists in sowing some 

 rapidly growing crop, which can often be done in the autumn after 

 the main crop has been harvested on the land, and, before it ripens, 

 ploughing it in. In this way, organic carbonaceous matter is added to 

 the soil, which will eventually form humus. The nitrates formed in the 

 soil during the growth of the crop are, to a great extent, absorbed by the 

 crop, and are prevented from being washed out in the drainage. 



The production of nitrates is usually greatest in autumn, and land 

 without a crop upon it suffers a considerable loss of nitrates at this 

 season. With the " catch crop " the nitrates, which would otherwise 

 be lost, are retained, and built up into complex organic compounds. 

 These can subsequently, when the remains of the crop decay, undergo 

 nitrification and again afford a steady supply of nitrates. 



The improvement in porosity, water-retaining power, and other 

 physical properties, is often very considerable; while, if the "catch 

 crop " grown be a leguminous one, the soil should not only be saved 

 from loss of nitrates, but should actually be enriched in nitrogen, 

 owing to the free nitrogen of the air being assimilated by the legume 

 by the aid of the Bacillus radicocola in its root nodules. 



