14 HOW TO MAKE SCHOOL GARDENS 
plow should be followed in the same 
furrow with the subsoil plow. This 
breaks it up, makes it more porous, 
and allows the water to pass through. 
Small lots, too small to be economically 
plowed, should be spaded ; and, where the 
subsoil is impervious, it can be trenched. 
If the ground is in turf it should be well 
turned and then thoroughly harrowed. 
If the land was not in turf, or if the turf 
is decayed after plowing, it should be 
replowed, running the furrows in the 
opposite direction. If spaded, the sod 
should be turned deep. It can again be 
lightly spaded. The aim should be to 
give thorough tillage to obtain all possi- 
ble from the land, and then apply ferti- 
lizers to get more. Tillage sets at work 
the forces which unlock plant food. Fer- 
tilizers are more useful to the plant on 
well-tilled land. 
FARM MANURES 
The valuable plant food in farm manures 
is not so quickly available as in high-grade 
commercial fertilizers; but they have the 
