4 BULLETIN" 479, U. S. DEPARTMENT OF AGRICULTURE. 
by capillarity. A subsoil of hardpan is to be avoided because it does 
not allow good drainage and the moisture retained may cause sour- 
ing of the soil and excessive freezing and heaving. 
AMOUNT OF LAND AVAILABLE. 
The future development of a nursery can seldom be forecasted. 
After the nursery is established the location, soil, or some other fea- 
ture may be found to make it especially suitable for the growing of 
certain species of trees; the stock may have to be retained in the 
nursery longer than was anticipated; wider spacing may prove 
advisable ; longer rotations of crops may become necessary ; or a por- 
tion of the 4 area may have to lie fallow longer than was expected. 
Any of these conditions will make extension of the nursery area 
desirable. A location should be chosen, therefore, where there is an 
additional contiguous area of good soil lying in such a position 
that the watering system can be readily extended to it. 
SLOPE AND EXPOSURE OF SITE. 
The nursery site should be as level as possible, because of the 
danger that the soil on a slope will wash during severe rains or arti- 
ficial watering. Of course, grades can be lessened by terracing. This 
is expensive, however, and on terraced areas plowing and harrowing 
are carried on with difficulty, and protection ditches are almost in- 
dispensable. Under no circumstances should the slope exceed 5 per 
cent, and even such a slope is excessive where irrigation is to be 
practiced. 
On slight slopes the exposure is of little import. There is but little 
difference in the temperature, frost danger, or length of growing 
season of the different aspects. On steeper slopes, north, northeast, 
and northwest aspects are more suitable for nursery purposes than 
south and west, because of the smaller range of temperature. Frost 
danger is greater on the south and west slopes than on the others, 
because the plants normally start growth sooner and continue it later. 
Alternate freezing and thawing, which causes heaving, is more de- 
cided on south and west slopes than on the other three. So-called 
frost holes, or depressions in which the air drainage is poor, and 
narrow valleys or canyons, should be avoided entirely because of 
frost danger. In general, a higher location should be given prefer- 
ence over a lower one. 
ACCESSIBILITY. 
The relative importance of accessibility to base of supplies and 
labor or to planting sites depends upon whether the nursery is large 
or small. If it is small and the amount of labor and supplies neces- 
