Bastard trenching is carried out in the same way as digging except that the trench is made at least two feet wide and the subsoil thus exposed is broken up with a spading-fork before the topsoil from the next trench is turned over upon it. When digging, the soil should be broken up as finely as possible with the spade or fork, so as to leave no lumps, and all large stones should be thrown out. Digging or plowing should never be attempted when the ground is frozen or when it is so wet that the soil sticks to the tools used. After the soil has been turned over and broken up the rake should come into operation and the surface fined so as to fit it for seed-sowing. V FERTILIZERS The most important elements that it is necessary to apply to the soil in the form of fertilizer are nitrogen, phosphorus, and potash. _Nitrogen_ greatly stimulates the production of leaves, and an excess of it applied to crops grown for their fruit or roots is undesirable. It is contained in all the animal manures, associated with other fertilizing elements, and in a practically pure state in nitrate of soda. _Phosphorus_ is a valuable and necessary fertilizer which has a great influence on the production of fruits and seeds. It is found in greater or less quantities in animal manures, and in bone meal. Phosphatic rock that has been treated with acid is another important source of phosphorus. _Potash._ The rôle of potash in the soil is similar to that of phosphoric acid. It is considered to be a valuable fertilizer for all crops that are grown for their roots. It occurs in animal manures and in wood ashes. Muriate of potash and kainite contain potash in a more concentrated form, but are difficult to obtain at present. All of these three elements must be present in the soil for the production of healthy crops.