The unique character of Rough lemon under these conditions is the relatively high concentration of feeder roots at the 76-152 cm depths. On the poorly drained soils of the east coast of Florida, the results would be different. Results would also be different on the cooler and heavier soils of California. Frequent concentration in this zone may be greater than the total amount of roots in the upper 25 cm of soil. With sour orange the feeder roots have been found 3-4 M deep at nine years of age, which compares favorably with Rough lemon. Mature trees on sour orange usually have fewer feeder roots below 76 M than Rough lemon and differ from Rough lemon in having more feeder roots concentrated in the 0-25 cm zone and less in the 76-152 cm zone. Sweet orange differs from Rough lemon and sour orange in that the sweet orange has the greatest concentration of feeder roots of any stock used commercially.The high concentration of roots in rather limited zones may account for why growers in sandy soils claim sweet orange must be irrigated more frequently than Rough lemon. Ford also reports that 30-year-old trees on grapefruit roots had a shallow root development.He also says that Cleopatra is deep rooted. Temple oranges on Cleopatra at nine years of age had roots to 5.2 M. In regard to Webber’s comments regarding citrus root systems, the author is taking the liberty of interspersing his own comments since Webber did not live to the termination and removal of many of his root stock trials in 1960. At the time of the conclusion of these experiments, the root system of each tree was carefully analyzed by Kirkpatrick and Bitters , in which the roots of every tree and combination were counted, sized, and measured. Also, during the tristeza era in Southern California, when thousands of trees were pushed and pulled, it provided an opportunity to observe numerous root systems under diverse soil and environmental conditions. At the time of Webber’s demise ,best vertical garden system sour orange was still the world’s most popular root stock and the most widely used root stock in California. Webber points out that the sour orange has commonly been considered to be best adapted to growth on low, moist, and fairly heavy soils.
It first became established in Florida on such soils. On the very light sandy soils of the so-called “ridge” section of central Florida, it has been a failure. In California, especially with lemons , it was equally successful on light sandy soils such as in the Upland area. In root stock experiments it gave better results on a fairly light sandy soil at Riverside than a much heavier loam soil at Fillmore. His explanation to these different reactions was the difference in climatic and soil conditions as they reacted on the deep rooted sour orange. In California, where alkali in sub-soils and high water table is sometimes a problem, such as in the Imperial Valley, a deep rooted tree like sour orange may be seriously injured. Aeration may be a problem in high water tables and root asphyxiation may occur, or damage from high salt content. As the lower roots die back, they may be attacked by soil organisms like Phytophthora and cause further damage. A shallower-rooted tree would sustain little or no damage. The sour orange develops one or more tap roots which can grow to considerable depths, but lateral root development is more limited than in Rough lemon, sweet orange, or grapefruit. The tap root of sour orange cut back at transplanting from the nursery commonly branches and forms a small group of tap roots. Halma found one mature Eureka lemon grove in which the average number of main roots per tree was fifteen for 64 trees, of which 65 per cent were typical tap roots, whereas sweet orange was devoid of tap roots. Deep tap root penetration on deep soils renders the sour orange, to some degree, resistant to the effects of drought. Hume states that both nursery and grove trees on sour orange suffer much less in periods of protracted drought under identical conditions than Rough lemon, but Evans states in Dade County, Florida, orchard trees on grapefruit and sour orange may be actually dying of drought when adjacent trees on Rough lemon are satisfactory. Hume’s statement would not apply, perhaps, to results on the deep, sandy “ridge” soils of Florida which were planted mostly after 1926.
Webber further states the sweet orange does not develop a well differential tap root and is usually moderately shallow rooted. It does, however, have an abundant system of lateral roots. The author’s observations in California would agree. Mills states the sweet orange is a surface growing stock which has few or no penetrating roots, which does not agree with the observations of Ford . Webber makes no comment on mandarin root systems. However, when the old root stock plantings at Riverside were pulled, the author found the most extensive and massive root systems of all stocks observed with Washington navel, Valencia, grapefruit, and Eureka and Lisbon lemons to be on Cleopatra mandarin. It penetrated nearly as deeply as sour orange, had extensive laterals, but moderate fibrous roots on the sandy loam soil at Riverside which is underlain by a caliche hard pan at 1 M or more in depth. Dancy, Clementine, and Oneco were similar in structure to Cleopatra, but to a slightly lesser extent. Even the tractor driver removing the trees commented that the trees on Cleo were the most difficult to remove. Webber also made no statement regarding the pummelos. With all scion varieties at Riverside the root system of the pummelos was similar to that of sour orange, with deep penetrating multiple tap roots, a moderate lateral root system extending somewhat obliquely as in sour orange, and a scarcity of fibrous roots. On Rough lemon root stock, he states it develops a very wide spreading abundant system of lateral roots and commonly exhibits no marked tap root. The trees in these experiments did have abundant spreading laterals, profuse fibrous roots, but no tap roots. The root systems were not nearly as extensive as sweet orange, Cleopatra mandarin and several other stocks, and may account for the smaller tree size on the Rough lemon stocks. Webber also made no comment regarding lime root stocks. In the root-stock experiments at Riverside, the root system of sweet lime was similar to, but inferior to, Rough lemon. There were no tap roots, the laterals did not extend as far, but there were abundant fibrous roots. In other California experiments, the West Indian Lime had a root system similar to that described by El Azzouni and Wali , no tap root, extensive surface laterals with limited extension, and profuse fibrous roots.
The Rangpur lime had a somewhat similar root system but did not duplicate the mandarin root stocks in character. The citron also had no tap root, no extensive surface laterals and only fair amounts of fibrous roots. Many of these trees were “leaners”, further indicating that they had a weak root system. Relative to trifoliate orange, Webber remarks that it develops a well-branched root system with very abundant fibrous roots, but that the roots did not extend laterally as far as Rough lemon or sweet orange. The author’s observations on many trees indicate the complete absence of deep laterals,growing strawberries very shallow penetration, a very sharp angle of diversion from the trunk area and a moderate amount of fibrous roots. Most of the laterals were flattened in proximity to the trunk area with no surface laterals and many fibrous roots. No references were made to any of the citranges, although Morton, Savage, Rush, Coleman and Cunningham were in the plantings and Troyer and Carrizo were in later plantings. Most of the citranges have a poorly developed tap root system which only penetrates to a depth of about 1 M. They have, however, many laterals which emerge at an oblique angle and descend downward and may penetrate deeper than the tap roots. Fibrous roots are less extensive than the sweet orange parent, and in the surface layers less than the trifoliate orange parent. The root systems of Troyer, Carrizo, Savage and Morton were more extensive than the others observed and agrees well with Savage, Cooper, and Piper . No reference was made to Sampson tangelo. As expected from its parentage, its root system was extensive, but not as extensive as either mandarin or grapefruit. The tap root system was not well developed but the surface and subsurface laterals were prolific, and fibrous roots were moderate. Yuzu was very deep rooted with good surface laterals, but lacking in fibrous roots. The author would agree with Webber’s description of Calamondin, which has very large penetrating tap roots, at least several extending straight down to 150-180 cm or more. There were few laterals, only near the surface, and a scarcity of fibrous roots. Of 25 root stocks Webber observed, he felt Calamondin exhibited the most marked tap root system, followed by C. webberii, and sour orange next. The author’s observations would agree with this except, the shaddocks would also have to be included.
The root systems of C. macrophylla and C. pennivesiculata are very similar to the lemon-lime group, that is no pronounced tap roots. Extensive trees on Severinia buxifolia were the smallest of any observed and it was surprising that the roots could support andanchor the tree. There was no tap root per se, but extensive surface laterals, most of which were less that 5 cm in diameter and did not extend more than 150-180 cm from the tree trunk, and very few fibrous roots. The root systems of cuttings varied tremendously with the scion cultivar. Those of navel were the poorest, consisting of only 3-5 very large surface laterals with little penetration and few fibrous roots. Those of grapefruit cuttings were intermediate. The Valencia cuttings, however, were very similar to sweet orange root stocks except they didn’t penetrate as deeply. There were lots of surface laterals and lower tier laterals and extensive fibrous root development. Huberty states that the dominant factors in determining root distribution of plants appeared to be irrigation and soil types and sub-soils. However, in an irrigation experiment at the Citrus Experiment Station which provided for irrigation on a two, four, and six-weeks’ schedule, no noticeable difference was apparent in the amount, or the pattern, of the roots as affected by the various irrigation treatments. A marked difference was shown by the type of root stock. Smith, Kinnison, and Carns in Arizona report the effect of variable frequency of irrigation treatments on the root development of young Marsh grapefruit trees on sour orange root stock in the light sandy soils of the Yuma Mesa. In these experiments, irrigation intervals of one, two, three, four, five, and six to nine weeks were followed during the summer irrigation season for three years. The weekly irrigation schedule followed on Plot I kept the surface soil moist and at a lower range of temperature than in the other plots, and permitted an extensive root development the first year in the top 15 cm of soil. The root development in the top 15 cm of Plot II, irrigated every two weeks, was quite pronounced, but in the remaining plots it was appreciably less. This related condition prevailed for several years and then indicated a tendency toward relatively shallow root development irrespective of soil-moisture conditions. The effect of increasing the interval between irrigations seemed to limit the total root structure rather than to force development into the lower soil depths where favorable moisture conditions existed. Huberty also points out that part of the root system of a 25-year-old Washington Navel orange tree on sweet orange root was exposed by careful digging. This tree was planted on a contour row in a sandy loam soil exceeding 150 cm in depth. The longest root found was growing along the tree row in soil which was not cultivated and to which irrigation water was not directly applied.