Winter Irrigation. 171 
ing rate of evaporation, the character of tilth, etc., enter as fac- 
tors, and it becomes clear that he is fortunate enough who knows 
how much water to use on his own place. 
WHEN TO IRRIGATE, 
The outline of experience which has been given includes 
times for irrigation as well as amounts of water used, but when 
to irrigate is governed by local conditions and the needs of 
different fruits and can not be stated in general rules. There 
are, however, some principles involved which may be hinted at. 
Winter Irrigation.—On lands with sufficient depth of fairly 
retentive soil, the grower may artificially supplement a scanty 
rainfall by thoroughly soaking the land by winter irrigation, and 
then by careful summer cultivation he will be able to conserve 
enough water in the soil to carry deciduous fruit trees or vines 
through bearing and autumn bud formation without further 
water supply. But there are other situations in which no amount 
of winter irrigation nor rainfall will suffice for these ends. There 
are foot-hill orchard areas in which the winter rainfall is two or 
three times as great as in the valley situations where fruit is suc- 
cessfully grown without irrigation, and yet water must be applied 
in summer on those foot-hills or the fruit would be unmarketable 
and the trees in distress. The forty or more inches of rainfall 
falling on a shallow soil underlaid by a sloping bed-rock in some 
cases nearly sluices the cultivated soil from its foothold, and yet 
the oversaturation in winter avails nothing for summer growth, 
because most diligent cultivation can not retain moisture enough 
in shallow soil thus situated to sustain bearing trees in good 
crops of full-sized fruit. The same is true of valley soils under- 
laid by hard-pan. In such cases winter irrigation could add 
nothing but distress to the soil oversoaked by rainfall; and sum- 
mer irrigation, well-timed and adequate, is the secret of success 
in the orchard. The same conclusion must hold for soils under- 
laid by gravel or sand and thus too rapidly dried by leaching. 
But even this generalization must be accepted only for sit- 
uations endowed with conditions which justify it. There may 
be sloping hills with shallow soil where winter rainfall does not 
amount to saturation. Then winter irrigation to supply such 
saturation is desirable, and then, too, summer irrigation in 
proper amount and at proper intervals, will also be demanded. 
Among the foot-hills, also, there may be localities with depth of 
retentive soil in which water enough can be applied in winter 
to carry trees through the year. Thus we come again to the 
only safe generalization which can be made, and that is, that 
everywhere water must be adequate to the demands of the tree 
at the time it is needed, and whether it can best be applied in 
