Rainfall.—It is impossible to state any definite amount of rainfall which will or will not maintain the trees and enable them to bear regular crops of nuts, for so much depends not only on the variation in rainfall in different sections and in different years, as regards distribution throughout the year, but also on the time and intensity of the fall, the character of the weather following the rains and the ability of the soil to receive and retain the rain that falls. Ordinarily, however, with the above factors favorable, it is conceded that where the winter rainfall averages sixteen inches, almonds can generally be grown without supplementing the water supply by irrigation, if the orchardist exercises reasonable care to conserve the moisture for the use of the trees. Where the rainfall is inadequate some means of irrigation must be found to make up the deficit.
In some sections the annual rainfall varies greatly from year to year. Often it falls in such a way that a large proportion of it is lost in the surface run-off. In many places the soil is so leachy that it is incapable of holding sufficient water for the use of the trees throughout the summer, much of the winter rainfall being lost in the underground drainage. Under either of these conditions, 40 inches of rainfall might not be sufficient. Very often winter rains are followed by desiccating winds so that a considerable portion of the rain which falls is lost by evaporation before anything can be done to hold it.
Continued rainy, damp and cold weather at the time of blooming is apt to sour the pollen or actually wash it away and thus prevent the fertilization of the blossoms, without which a crop is impossible. Bees and other insects are the principal means of accomplishing the pollination of almonds and such weather prevents them from working.
SOIL
The almond is a deep-rooting tree and draws heavily upon the plant-nourishing elements of the soil. In ripening the large number of seeds which it is required to do, the tree must draw upon a considerable area of soil in order to supply the large amount of mineral matter that is needed to develop and mature the seeds. Analyses of almonds, as compared with other commonly grown fruits and nuts, made by Colby,[2] show that the almond leads in the total quantity of mineral matters withdrawn from the soil. Colby further states that “The stone fruits fall much below the almond in total ash (mineral matter) excepting the olive, the ash of which, however, is largely silica (nearly eight-tenths), an ingredient so plentifully distributed in all soils that it is of no pecuniary value.” [Table I] illustrates this statement.
These figures suggest the necessity of having a deep, rich, well-drained soil for best results. For this reason and because of the deep-rooting habit of the almond, the soil should be at least ten or twelve feet deep.
Hardpan.—Compacted substrata in the soil, whether they be hard clay layers or cemented layers of silicious, ferruginous or calcareous origin, are objectionable. They not only prevent the roots from foraging to a considerable depth as they normally tend to do, but they prevent proper drainage and aeration of the soil. If such layers are comparatively thin, that is, not more that two or three feet thick at the most, they may be shattered with dynamite so as to allow the moisture, air and roots to penetrate to the better soil below. Hardpan, therefore, should be avoided where it is too thick to be broken up or where it is not underlaid by desirable soil.
TABLE I
Soil Ingredients Extracted by the Almond as Compared with Other Orchard Trees, as Shown by Analysis of 1000 lbs. Each of the Crops in a Fresh Condition
| Fruit | Potash, lbs. | Lime, lbs. | Phosphoric Acid, lbs. | Total Ash, lbs. | Nitrogen, lbs. |
|---|---|---|---|---|---|
| Almond (hulled) | 5.49 | 1.72 | 4.33 | 15.00 | 16.40 |
| Almond (not hulled) | 9.95 | 1.04 | 2.04 | 17.29 | 17.01 |
| Walnut (hulled) | 1.50 | 1.81 | 2.78 | 7.50 | 10.20 |
| Walnut (not hulled) | 8.18 | 1.55 | 1.47 | 12.98 | 5.41 |
| Chestnut (hulled) | 3.72 | .71 | 1.89 | 8.20 | 8.00 |
| Chestnut (not hulled) | 3.67 | 1.20 | 1.58 | 9.52 | 6.40 |
| Prunes (green) | 2.66 | .13 | .53 | 4.03 | 1.48 |
| Apricots (green) | 2.83 | .18 | .71 | 5.16 | 2.29 |
| Olives | 8.85 | 2.32 | 1.18 | 94.63[*] | 5.85 |
* 80.7 pounds of which is silica.