During the early period of American orange-growing in a commercial way the advice was given to keep up clean cultivation through the season and to rely mainly on commercial fertilizers. The natural outcome of this system has been the same as when practised with the deciduous fruits (125. Culture After Planting). With increased experience it was found that humus as well as fertilizers must be given to the soil. Growers now recognize the need of adding barn-yard manure, straw, or other refuse, or to grow leguminous cover-crops (126. Shading of Orchard Soils), to give the needed fresh humus and nitrogen to the soil.
The orange is a gross feeder that throws out wide-spreading roots with an unusual supply of root-hairs and •feeding-fibres. It must have water and a regular supply of vegetable humus to give the needed green color to the foliage and to carry a full crop of fruit to maturity. Experience has shown also that continued culture and the use of commercial fertilizers soon chance the mechanical condition of the soil, and that a given supply of water is sooner evaporated than on soils well supplied with vegetable matter.
In the fine orange orchards of Redlands, Riverside, Colton, and other orange centres of south California, many growers are now cultivating and watering in the usual way during the season of growth, but follow it with a cover-crop of the field-pea, cow-pea, or other legume to shade the ground during the period of fruit maturation and to add humus and nitrogen to the soil when plowed under the next spring. In the winter of 1897-98 the writer travelled many miles to observe the effect of this treatment on the color of the foliage and the perfection of the fruit. In all cases the orchards given the cover-crop treatment in connection with fifty pounds of potash and twenty-five pounds of phosphoric acid to the acre annually, were indicated afar off by the dark green of the foliage alone.
On the foot-hill slopes of the Salt River valley in Arizona the observed benefit of this system was still more apparent, as the cover-crop seemed to bring about a ripening of the wood prior to danger from frost that prevented much damage to leaves or points of growth when cultivated orchards were defoliated.