CITE US CULTURE IK" PORTO RICO 



7 



pounds present are compacted. When brought to the surface such 

 subsoil may become hard on drying and plastic on wetting or it may 

 become crumbly. Crumb structure does not usually develop in 

 such soils until after they have become thoroughly mixed with 2 per 

 cent or more of humus. It may erroneously be inferred from this 

 that the plowing under of 15 tons of dry grass per acre will result in 

 the formation of a 6-inch layer of topsoil. Theoretically this is cor- 

 rect, but in reality only a small portion of the organic matter re- 

 mains in the soil. In fermenting, the humus becomes thoroughly 

 incorporated with the soil, and a crumb structure results. Several 

 annual applications of dry grass each of 15 tons would be required 

 to produce the desired effect and at the same time leave a deposit 

 of 2 per cent of humus in the soil. 



A 6-inch layer of soil is not sufficient for root formation, especially 

 in groves where the topsoil is disturbed periodically by cultivation. 

 Usually seedling trees the roots of which are not disturbed by cul- 

 tivation are shallow rooted where the soil layer is thin and the sub- 

 soil hard. Hundreds of such trees when blown over by the hurri- 

 cane of September, 1928, were found to have a very shallow root 

 system. In cultivated groves having a shallow surface soil the 

 subsoil must of necessity be pervious to the tree roots, or growth 

 will be very slow. A great many of the groves were found to con- 

 tain small and large areas having a shallow surface soil when the 

 trees were planted. Holes for planting were dug in the subsoil and 

 sometimes dynamite was used to break up the surrounding soil area, 

 but the tree growth usually was very unsatisfactory nevertheless. 



Before planting is done it is often difficult to determine whether 

 the subsoil is permeable enough for the roots to penetrate, but after 

 the trees are planted and have attained the age of about 10 years 

 the suitability of a soil can readily be judged by the root develop- 

 ment. Unfortunately an impervious subsoil can not readily be 

 improved, and usually it is more profitable to discontinue cultivating 

 a grove on such soils than to attempt to improve them. Improve- 

 ment, when practicable, may be had by the practice of deep 

 subsoiling. 



The water table in clay soils is not usually high enough to inter- 

 fere with root development, but often drainage is slow, and the soil 

 remains saturated for prolonged periods. An excess of water will 

 cause the roots to decay because the area occupied by them is then 

 poorly aerated. In the deep, well-drained clay soils of the Manati 

 Valley the trees do not suffer from an oversupply of water even 

 when the rains are heavy and prolonged. These soils, because of 

 their crumb structure, are in some respects similar to sandy soils. 

 The water drains quickly from them and evaporates from the surface 

 to such an extent as to make drought a problem unless the dust- 

 mulch method of cultivation is practiced. The crumb structure in 

 these soils is readily destroyed by cultivating when the moisture 

 content is too high. This is true of all soils, but some are much more 

 difficult to handle than others. One type of soil in the Cidra Valley 

 retains its crumb structure much better than do the soils of the 

 Manati Valley, and these again better than most of the soils in the 

 Bayamon section. 



