The outside value for the amount of energy fixed in the soil is obtainable by combustion of the soil in a calorimeter, but much of this is not available to the soil organisms. The normal sedimentary soils of England still contain decomposition products of the débris of plants and animals originally deposited with them, but in the long course of ages much of the extractable energy has been utilised. The soil population is thus dependent on recently grown vegetation, and it is therefore largely confined to the layer, usually in this country about 6 inches thick, through which the recently dead vegetation is distributed. Below this level there may be sufficient air, water, temperature, etc., but there is insufficient source of energy for any large population. Unfortunately there is no ready means for distinguishing between the total and the actually available quantity of energy in the soil. But it is not difficult, by adopting the Rothamsted analytical method, to ascertain the approximate amount of energy that has been transformed in a given period. The Rothamsted plots are periodically analysed and a balance sheet is drawn up showing how much of each constituent has been added to and removed from the soil in the intervening period. For two of the Broadbalk plots the results are shown in Tables XV., XVI. The dunged plot receives 14 tons farmyard manure per annum, a quantity in excess of what would usually be given; the unmanured plot, on the other hand, has received no manure for many years and is abnormally poor. Normal soils lie somewhere between these limits, but tending rather to the value for the dunged than for the unmanured plot. It will be seen that each acre of the dunged land loses on an average 41,000 calories per day, while each acre of the unmanured land loses on an average 2700 calories per day. TABLE XV.--MATERIAL BALANCE SHEET: BROADBALK SOIL, ROTHAMSTED. (LB. PER ACRE PER ANNUM.) +-------------------------+-------------+---------------+ | | Farmyard | No Manure | | |Manure Added.| Added. | | +------+------+-------+-------+ | | C. | N. | C. | N. | +-------------------------+------+------+-------+-------+ |Added in farmyard manure| 3600 | 200 | nil | nil | |Added in stubble | 300 | 3 | 100 | 1 | +-------------------------+------+------+-------+-------+ | Total added | 3900 | 203 | 100 | 1 | |Taken from soil | nil | nil | 200 | nil | |Stored in soil | 200 | 30 | nil | nil | +-------------------------+------+------+-------+-------+ | Lost from soil | 3700 | 170 | 300 | nil[H]| | Per cent. | 95 | 84 | 100 | nil | +-------------------------+------+------+-------+-------+ Initial C : N ratio in farmyard manure, 18 : 1 Final C : N ratio in soil, 10 : 1.