292 QUEENSLAND AGRICULTURAL JOURNAL. [1 Sxer., 1901. 
certain crops might still be grown on it. But a soil may be said to be quite 
exhausted, as regards any particular crop, whenever the cost of cultivation 
comes to more than the crop is worth. Suppose it cost a farmer £3 to produce 
an acre of maize, and that the crop on that acre in a good season only ira: 
him in £2 19s. 6d., then you would say that the land was exhausted for that 
particular crop, although it might still produce a good crop of cotton or cow- 
peas. But the farmer wants to grow maize on the land, so he must do one of 
two things: He must either manure it properly, or he must let it lie ‘“‘fallow” 
for a year. This means that the land is either left untouched, or it is ploughed 
frequently to destroy weeds, and, by ploughing them under, turn them into 
green manure, which gradually produces humus. The old Roman farmers used 
to leave a field fallow for a year after taking off a crop. But where the land 
is artificially drained there is no need for this “bare fallow” as it is called. 
On good clean soils or light soils bare fallowing is not necessary. On such 
soils, what are called fallow crops are taken and ploughed under for green 
manure; but in dry climates, on heavy, clay soils, on weedy land or sour soils, 
when the exposure to air and rain will sweeten and improve the soil, destroy 
insect life and clear off weeds, bare fallowing is of great advantage. Thus 
fallowing thoroughly cleans the soil and exposes it to the beneficial effect of rain 
and air, after which rest it may once more be put under cultivation. 
Every plant taken from a field helps to exhaust the soil, but all plants do 
not exhaust it equally. Indeed, some plants, such as cow peas, beans; and many 
leguminous plants which draw “nitrogen” (a very valuable plant-food) from 
the air, restore this to the soil, but wheat, oats, barley, maize, &c., draw it from 
the soil. 
You have now some idea of the necessity both for manuring and for giving 
rest to an exhausted soil. As I told you before, such a soil will, however, grow 
certain green crops, and the poorer the land the greater will be the necessity 
for a green crop to be ploughed under as manure. Many of these crops will 
renovate the soil and put it in condition to grow a crop of maize or wheat. 
Cow peas, velvet beans, rape, and even thistles and other sueculent weeds will 
supply to the soil the materials from which ‘‘ nitrates” (you will be told what 
nitrates are by and by) are produced. They also bring plant-food to the 
surface—that is, the soluble plant-food which is drawn up through the soil b 
the roots. Much of this is consumed by the plants, but far more than endure 
is drawn up, and thus remains for the benefit of the next crop sown. 
Tf you have ever seen an exhausted lucerne field ploughed up and planted 
with potatoes, you would at once see how the lucerne fertilises the soil when 
the potatoes are harvested. Clover has the same effect, and many old farmers 
say that there is no need to manure a field which has carried clover or lucerne, 
because, although clover removes more plant-food, such as potash, lime, phos- 
phorie acid, &c., from the soil than almost any other crop, and three times as much 
nitrogen as a crop of wheat, yet a great quantity of nitrogenous matter is left 
on the surface in the shape of dropped leaves, and still more in the roots. When 
lucerne is grown for seed, the leaves fall thickly on the ground, and thus pro- 
vide a quantity of plant-food. Now, we will just run over this lesson, and see 
what we can derive from it. First, you learned that certain crops will grow on 
alight soil, whilst others require solid food, such as is contained in a rich soil. 
Then you were shown that, however rich a soil may be, if it is constantly 
cropped without anything being returned to it, it will eventually refuse to 
produce a crop worth as much as the cost of growing it. How the rerrinrry 
of the soil is kept up, why the land requires rest, and what is meant by 
ExHAuSTION of the soil, you now understand. Fallowing 1 have only 
mentioned to prepare you for a harder lesson hereafter on what is called the 
rotation of crops, but you can now at least understand what is the object of, 
and what is gained by fallowing. Lastly, I have pointed out to you that certain 
plants restore plant-food to the soil whilst others withdraw it. Now, let us 
propose the usual questions. 
