THE SOIL 193 
The Ghirriya, the northern part of the Nile delta, was in 
Roman times the Garden of Egypt, and produced the rice 
that fed the slave populations in Rome and in the Serbian 
mines ; when the administration of Egypt was undertaken 
by Britain in 1883 the Ghirriya was a barren waste owing to 
the accumulation of sodium carbonate in the soil. This 
salt was removed by successive washings by the Nile floods, 
and the ground was restored to its former fertility. Some 
of the low land in Mesopotamia is faced with a greater difh- 
culty, as the slope to the Persian Gulf is too slight to remove 
the irrigation water and its removal by evaporation leaves a 
residue of sodium carbonate. 
Nitrogen, the most important organic constituent of soil, 
occurs mainly as a constituent of humus, which gives most 
soils their brown or black colour. Humus is due to bacteria, 
which prevents the organic matter being decomposed into 
carbon dioxide; as that change takes place most quickly 
with an excess of air humus is less abundant in light porous 
soils than in clay. The amount of organic matter may be 
as much as 20 per cent. in virgin soils, while it may be almost 
absent from sandy soils in the tropics where the humus 
is decomposed in the dry season. 
Knowledge of the chemical composition of the soil enables 
agriculture in some cases to be managed with the precision 
of an industrial operation, as the material removed in each 
crop can be replaced in fertilizers. In general farming, 
however, this knowledge proved less useful than was ex- 
pected. An average English wheat crop withdraws from 
each acre about 50 Ib. of nitrogen, 20 Ib. of phosphoric acid, 
and 30 lb. of potash. Ordinary soil has enough nitrogen 
to supply this amount for 50 crops, enough phosphorus for 
120 crops, and potash for 70 crops. Only, however, a small 
fraction of these constituents can be withdrawn by the 
plants ; and then the soil requires rest until more has become 
available as plant food. Efforts to distinguish between the 
total and available amounts have been made by treating 
soil with a weak organic acid; but this test does not fully 
overcome the difficulty, as the clods are not penetrated by 
the soil waters, whereas during an analysis the solvent is 
brought into contact with all the particles. 
Soir TEXTURE AND WATER CaraciTry—Mechanical analysis 
13