SOIL OF THK WOODED SWAMI'S. 353 



may in the future be added hi the way of fertilizer's. It also gives a 

 stratum with sufficient capillary power to draw water from a consider- 

 able depth below, which would be of great value in times of drought. 

 The mean clay content of 15 per cent at 100 to 150 cm. is just mid- 

 way between that of the first and second 50 cm., and is not suffi- 

 cient in quantity to prevent a fairly free lateral movement of the 

 ground water at this depth. This is very important, since the success 

 of underground drainage depends largely upon the free lateral move- 

 ment of ground water at about the depth at which tile drains are 

 usually laid. The whole of the Dismal Swamp area will require thor- 

 ough underdrainage before it can be brought to a high state of culti- 

 vation, hence the importance of the texture of the soil at this depth. 

 The clay content of a soil, when coupled with its organic matter, is 

 the controlling factor in relation to its structure and water capacity. 



TEXTURE. 



The table on page 358 gives a complete mechanical analysis of eleven 

 samples of soils from the Dismal Swamp, one from West Norfolk, and 

 one from Illinois. As may be seen from the mean of the first nine 

 analyses, the soils of the swamp to a depth of L5 meters (5 feet) con- 

 tain, on an average, approximately 50 per cent of fine sand, the par- 

 ticles of which range from one-fourth to one-tenth of a millimeter in 

 diameter. The next largest separation is thatof (day, of which we have 

 already spoken. These analyses show that the soils would properly be 

 classed as sandy vegetable loam. The light texture of the upper 20 

 inches makes the soil easy of cultivation, and fields that have been 

 in crops annually for fifty years are still in the best of mechanical 

 condition. 



Only two samples were taken below 3 meters (10 feet) in depth, and 

 those show considerable difference in texture. No. 3928 was material 

 taken from the bottom of the canal by the dredge, and had been 

 exposed to the weather upon the canal banks for some time. It is 

 not at all improbable that the rains had carried away most of its silt 

 and clay, which may account for the small amounts of these separa- 

 tions shown in the analysis. Sample No. 3933, however, has a much 

 less amount of fine sand and much more of medium and coarse sand. 



Sample No. 302 is black prairie soil from Illinois, where under- 

 ground drainage is practiced on a large scale in preparing land for the 

 production of corn, grass, and wheat. A comparison of its texture 

 with that of the soils of the Dismal Swamp shows the latter to have 

 half as much clay as the former, and approximately one-fourth as 

 much fine silt and silt. For thorough drainage in the prairie soils the 

 lines of tile drains are seldom laid nearer than 150 feet from each 

 other, and, judging from the comparative texture of the two soils, suc- 

 cessful drainage could be accomplished in the swamp soils by having 



