Fruit cracking in citrus is caused by an uneven or irregular water supply. When
citrus fruit takes in water faster than the skin can stretch, the skin cracks. A few cracked fruit are normal, however, if a high percentage of fruit crack, you have an irrigation management problem.
In Santa Maria citrus require about twenty inches of water per year. This amount includes rainfall. This assumes that the rains are evenly spread. During the winter (Dec, Jan, Feb) the tree requires from an eighth to one quarter inch of water per week. From March to June the requirements increase to a quarter to half an inch per week. During the warmest time of year – July to Sept – an inch of water per week should be supplied. In the fall a quarter to half an inch per week is again required. These are approximate numbers, and you need to alter them based on soil type. Sandy soils may need a bit more, while clay soils will require less. The important thing is consistency, i.e., a regular water supply.
A second factor in fruit cracking may be tree nutrition. Your orange should receive some fertilizer each year. Use an “all purpose” or “citrus fertilizer.” Both are available at garden stores. A mature citrus tree should receive between one and two pounds of elemental nitrogen per year. This amount should be applied in several applications, i.e., spring, summer and fall. For example, if you buy a fertilizer that contains ten percent nitrogen, you need to apply ten pounds of this product to the tree per year, i.e., about four pounds in the spring, and three pounds each application in the summer and fall. Apply the fertilizer evenly under the tree from the trunk out to the drip line of the tree.