NATIONAL COUNCIL OF HORTICULTURE 15 



films are too thin and therefore too strongly held to be removed. 

 From the standpoint of plant function, we have the paradoxical condi- 

 tion of a sandy soil, containing perhaps one per cent of water, being 

 effectively more moist than a puddled clay soil containing 20 to 30 per 

 cent or than a peaty soil containing perhaps 40 to 50 per cent. 



But when the finest clay soils are put in a highly granular condi- 

 tion, with the kernels having the order of coarseness of the sandy 

 soils, these compound grains may themselves become invested with 

 water films which are thick and therefore available to crops. By such 

 a change of structure, therefore, the clay soils not only retain their 

 enormous surfaces, carrying water in which plant food may develop 

 and accumulate, but by the bunching of the fine particles there has 

 been superadded to the already enormous surface an additional large 

 area which now is able to retain much water in available form and so 

 advantageously placed that the plant food from the moisture within 

 the soil kernel can diffuse out into the available film and thus also 

 become available to the crop. Tilth, or the physical condition of the 

 soil, then, must be of very great importance in determining the pro- 

 ductive capacity of fields, first of all because it limits the availability 

 of the soil moisture and through this, at the same time, the availability 

 of plant food itself. Without the coarse grained texture and openness 

 of structure there must be imperfect drainage, inadequate soil ventila- 

 tion and a lack of freedom for movement and of room for the proper 

 development of either the roots of crops or the multitudes of soil 

 organisms whose activity is so indispensable to the maintenance of soil 

 fertility. The full significance of this openness of structure may be 

 better appreciated when it is stated that exact measurement has shown 

 that when soils of the coarse sandy, loamy and finest clay types arc 

 reduced to their single grain condition the rates of air and water 

 movement through them become as 900 to 36 to 1; the flow being 900 

 times as rapid through the coarse sandy soil as through the finest clay 

 type. Put in another way, if 2.5 hours are required to remove an 

 excess of rainfall from the coarse sandy soil, then four months would 

 be insufficient to effect the same result in a field of the finest clay 

 type when in the condition of its single grain structure ; while some 

 four days would be required for the loamy soil, and this is longer than 

 the average interval between rains in humid climates. More than this, 

 in the properly open soils there is but 2.5 hours between rainfalls dur- 

 ing which diffusion can carry the soluble plant food into the water 

 draining away, while in the other condition this loss by drainage is 

 continuous. 



If a high productive capacity of fields is to be secured and main- 

 tained then in some manner must all soils be given an openness of 

 structure approaching that possessed by our coarse sandy types. The 

 factor of paramount importance in securing prime tilth, or the best 

 possible structure, is an abundance of organic matter deeply and thor- 

 oughly incorporated in the soil; and with this must always be asso- 

 ciated ample underdrainage which fortunately generally exists where 

 structure is right. For ordinary field conditions this incorporation of 



