350 
SOILS AND AGRICULTURE OF NOREOLK. 
(c) It must contain bacteria capable of dealing with 
organic matter and converting the nitrogen to nitrates. 
(d) The texture must be good, that is, the soil must con- 
tain both small and large particles (sand and clay) in 
such proportion that it is sufficiently porous, while at 
the same time the clay, from which most of the mineral 
plant food is obtained, should form a fair proportion 
of the total mass of soil. 
We may regard the practice of agriculture as the systematic 
cultivation of certain plants under the most favourable con- 
ditions that can be obtained for them, for the sake of the 
stored products of plant activity and anabolism. 
A seed when placed in the ground quickly germinates and 
begins to grow. Growth is rendered possible by the fact that 
the embryo or seed plant is surrounded by a mass of food 
stuff, stored by the mother plant, which enables it to pro- 
duce leaves and rootlets and so obtain further supplies of 
food on its own account. Plant food is of two kinds, organic 
and inorganic. 
(A) Inorganic, including such elements as calcum, mag- 
nesum, potassium, iron, aluminium, nitrogen, phosphorus 
and silicon. 
All these elements can only be taken up in solution as 
salts, the nitrogen almost invariably in the form of nitrates. 
They are dissolved out in small quantities by the water sur- 
rounding each particle of soil, solution being assisted by a 
slight acidity of the water due to the presence of weak 
organic acids resulting from the decay of humus. 
This solution is taken up by the root hairs and passed into 
the vascular bundles and so conveyed all over the plants. 
Carbon in the form of carbon dioxide is taken in by the 
leaves, which, under the influence of sunlight, build up an 
organic body — Starch — from water and gases taken in. 
