Although farmyard manure possesses the great virtues mentioned it must be used with care and intelligence. In some places, where large and cheap supplies are available, the soil is saturated with manure. The greater the quantity of manure incorporated with a soil the greater the necessity for plenty of fresh air to bring about decomposition, and ultimately humus. Now, if a soil has not been deeply dug or trenched, and it happens to be of a heavy nature, it is possible that the rains will not pass away readily. Then the manure begins to get sour, fresh air with its oxygen is driven out, carbonic acid gas develops too freely, and the beneficial bacteria are suffocated or annihilated by their enemies which come into being owing to the lack of fresh air. To avoid these troubles the soil should be well and deeply dug, and whenever extra large quantities of manure are used the soil should afterwards be dressed with lime or chalk, basic slag, or nitrolim, to keep it in a sweet condition.