6 



BULLETIN 3 3, PORTO RICO EXPERIMENT STATION 



The addition of stable manure to the sandy soil at the rate of 1 

 ton per acre will supply the soil with organisms, and then cover 

 crops can be grown, although fertilizers may be necessary for the 

 first few crops. Ground limestone may also profitably be applied 

 to such soils at the rate of 2 to 5 tons per acre. A rock crusher 

 should be available for use on plantations where limestone is plenti- 

 ful because most of the soils in which citrus trees are planted are 

 greatly benefited by an application of lime. The poor sandy-clay 

 soils may be improved by an application of both ground limestone 

 and organic matter. Citrus trees should not be planted on areas 

 where the subsoil is high in iron and much cemented. 



The suitability of sandy soils containing small amounts of clay 

 and organic matter depends upon the permeability and water-holding 

 capacity of the subsoil, as well as upon the level of the water table 

 and its fluctuations. Provided that the water table does not reach 

 to within 10 to 12 feet of the surface at any time and the subsoil has 

 great water-holding capacity and is sufficiently pervious for root 

 formation, the citrus tree usually will not suffer much from lack of 

 moisture in most of the citrus districts in Porto Rico. In such soils 

 trees with a taproot are more likely to succeed than those without. 

 In fact, the presence or absence of a taproot is probably often the 

 cause of differences in tree growth in many such soils. Wherever 

 the level of the water table varies within wide limits the root system 

 will adjust itself to the highest level maintained for a considerable 

 length of time; consequently, the trees may suffer from drought 

 when the level falls. The cultivation practice suggesting itself un- 

 der such conditions consists in the growing of cover crops during 

 the rainy season and the maintenance of a dust mulch during the 

 dry season. 



In sandy soils the leaching of fertilizers may be a problem of 

 some importance. Usually, however, it is of minor importance, as 

 is illustrated by the following example : A soil which in situ weighs 

 40 kilograms per cubic foot and has a water-holding capacity of 15 

 per cent will retain moisture equal to 2.5 inches of rain per foot. 

 If such a soil is fertilized after having been saturated by a heavy 

 rain, the water-soluble salts will go into solution quickly, and if 2.5 

 to 3 inches of rain fall shortly after the soil is fertilized a great por- 

 tion of the soluble salts will be leached below the foot of soil. The 

 actual amount of salts so leached will of course depend upon the 

 amount of colloidal matter present in the soil. Another rainfall of 

 2.5 inches will wash an additional portion of soluble salts from the 

 topsoil and also some of the salts from the subsoil. When the root 

 system of the tree is deep and the subsoil contains considerable clay, 

 fertilizers are not so likely to be washed below reach of the roots 

 except during periods of unusually heavy rains. 



The differences in productivity of clay soils are difficult to describe 

 and difficult to determine by analytical means. The terms " heavy w 

 or " light," when applied to clays, do not refer to actual weight of the 

 soil but rather to soil plasticity, which is not governed by the con- 

 tent of clay or by any other constitutent, but rather by the state of 

 the entire mass of colloidal matter. A clay subsoil, for instance, is 

 usually termed "heavy" because the colloidal clay and iron com- 



