The root terminated 9.1 M from the tree trunk and was 11.3 M long. At no place was the root more than 46 cm below the ground surface, and at the free end was only 15 cm below the surface. In the imperfectly drained east coastal soils of Florida, Ford reported that stabilizing the water table at a lower level increased the total rooting area and the newly developed roots survived without periodic destruction. Lowering the water table from 76-178 cm doubled the quantity of feeder roots in four years and increased the size of the tree. Cahoon, Harding, and Miller found that the higher the tree yields, the more feeder roots found in the irrigated row middles. In several high-producing orchards the amount of roots found between trees actually exceeded those found under the trees.Cahoon, Huberty, and Garber report on a differential furrow irrigation treatment applied to a Washington Navel orange orchard from 1934-1957 on sweet orange stock. The treatments were frequent versus infrequent. In 1957 root samples were taken to a depth of 122 cm. Trees irrigated on a frequent schedule produced fewer deep roots than the trees irrigated on an infrequent interval. The difference was more evident at the 61-92 cm levels. Samish in Israel reported essentially the same thing. Cahoon and Stolzy in California used a neutron moderation method to estimate root distribution as affected by irrigation and root stocks. They encountered troublesome problems with soil moisture variability,square plant pots soil profiles, etc. Ford found poor root growth in the leached zone of certain acid soils of the imperfectly drained Florida flat woods.
In laboratory tests poor root growth was not corrected by the application of adequate water and nutrients. In laboratory tests the Rough lemon produced the best feeder and lateral roots, even better than sour orange. Damage to the roots was more severe at low pH 5.0. Roots of Cleopatra were severely damaged at high and low pH 5.0-6.5. Ford also states that the relatively poor feeder root growth of trifoliate orange together with the root damage that occurred at pH 5.0 when flooded suggests this stock should be carefully evaluated. On the other hand, the satisfactory tolerance of Rangpur lime to flooding warrants further study. Ford says the citrus root system is capable of rapid and deep growth in sandy soils but will not grow into or exist long in a soil saturated with water. When the water table is within 60 cm of the surface, roots are confined to a shallow zone. Fluctuating water tables have a pronounced affect on the root system. He compared roots from trees in orchards with 1.8 M deep drain lines to an adjacent undrained orchard. In the undrained orchard the highest per cent of roots were at 0-50 cm, less at 25-50 cm, and almost none below 50 cm. In the drained orchard there was good rooting to 50 in. and some rooting even to 180 cm, but less as the distance from the chain line increased. Stabilizing the water table at a lower level increased the total rooting area and newly developed roots survived. Lowering the water table from 75 cm to 180 cm doubled the quantity of feeder roots in four years and increased the size of the trees. The feeder root concentration in the deep rooting zone 75-180 cm was greater than in the 0-25 cm level. In Israel, Cossman established a close correlation between the vigour of stocks on sandy soil and the osmotic pressure of their root cells. The slow-growing group represented by pummelo, grapefruit, and sour orange have remarkably low figures for their osmotic pressure.
The roots of this group are easily outclassed by the retentive forces of the soil particles whenever the wilting range is approached. In Texas, Adriance stresses the importance of the tap root system of sour orange, but points out that the major portion of the root system was between 46-61 cm. He emphasizes the importance of environment, natural habitat of species, aeration, water table, salt content and stratified soils. Adriance and Hampton examined the root systems of trees on sour orange grown on different soil types and subjected to different cultural practices. A poor-stunted tree grown on a very dense and compact soil had a spread of lateral roots 1.8-2.1 M. There were few roots 1.2-2.5 cm in diameter in the upper 21 cm of soil, a minimum of fibrous roots down to 61 cm, and no roots below that zone. A medium sized tree grown on a compacted soil and underlain with caliche at 125 cm showed roots were small but up to 2.5 cm in size and were well distributed although they did not penetrate deeply. A large tree grown on a good textured soil to a depth of 152 cm had roots down to 152 cm and below. Another tree in a tilled orchard which was disked to a depth of 10 in. had good roots around the tree, but there was little lateral spread beyond that distance. A tree under nontillage had roots with a lateral spread of 5.5-6.1 M. In California, Crider found that citrus roots were found to be distributed largely according to the character and the previous cultivation treatment of the soil. In the case of a 25-year-old tree there were practically no roots below 1.2 M due to a tenacious subsoil. A 30-year-old tree on a dry sandy soil showed good root development to a depth of 2.7 M. In a well-cultivated and fertilized orchard, with young trees 3-6 years old, 50 per cent of the roots were in the first 46 cm of soil. On the other hand older indifferently handled trees showed greater root accumulation in the 30-60 cm and 60-90 cm layers.
Young stresses the importance of soil texture, drainage, aeration, and moisture relationship to citrus root development. In Florida, Ford observed Hamlin and Valencia oranges on Cleopatra and Rough lemon at 15-21 years of age growing on a red sandy clay some 46 cm-4.8 M below the soil surface. Trees on the Cleopatra were 46-92 cm taller than the trees on Rough lemon where the roots penetrated into the clay. The height of the trees on Rough lemon decreased as the clay was closer to the surface. A restriction in root growth imposed by the clay did not consistently increase feeder root concentration above the clay. Root growth ceased when the clay percentage was above 28 per cent. Feeder root concentration of 15-year-old Hamlins and Valencia on Cleopatra growing in deep sandy soil was greater than Rough lemon, even though the trees were smaller than the same scions on Rough lemon. At Riverside, much of the area occupied by the citrus root stock trials initiated by Webber was underlain with impenetrable hardpan. At one location where the hardpan was approximately 1 M from the surface, some of the deep-tap-rooted trees such as sour orange had their tap roots growing down to the hardpan and then fusing together in a solid plate like a pedestal and then the roots diverged at a lateral angle. In the Azusa-Covina area of California where many of the soils are alluvial sandy loams, especially adjacent to washes which were subject to flooding and were underlain with sand and gravel substrata. In such soils where the alluvium was deep, the roots of sour orange penetrated to a depth of 2 M or more with few laterals. However,vertical farming equipment as the trees in the orchard approached the stream bed the sour orange roots penetrated only to a depth of 60 cm or less with no tap roots, but a well developed system of surface laterals . Fertilizers and nutrition also play a big part in citrus root development. In Florida’s deep sandy soils, Ford, Reuther, and Smith found nitrogen was the primary element influencing root development in two fertilizer experiments after six years of differential treatment. The high nitrogen plots had 37 per cent less feeder roots than the low nitrogen plots to a depth of 1.5 M. Neither potassium or magnesium had any appreciable effect on root development. In the second plot there were 38 per cent less feeder roots at 13-89 cm in the high nitrogen regime as compared to low nitrogen levels. They felt a direct salt concentration [“Check” appear here in typescript in the margin of the manuscript] was responsible for the effects. In California, Cahoon et al. examined the effects of various types of nitrogen fertilizer on root density and distribution as related to water infiltration in a long-term fertilizer experiment on a sandy loam soil. They found that various nitrogen treatments, particularly the long-term application of sodium nitrate, and ammonium sulfate reduced root concentrations in the first 10 cm of soil. In tropical Trinidad Gregory found the root systems of Marsh grapefruit on sour orange were more vigorous and extensive on manured trees than unmanured trees. The manured trees had several lateral roots which exceeded the average spread of the branches and extended 106 cm from the trunk on 3-year-old trees. Most were shorter, and feeder roots occurred 8-46 cm from the trunk. The unmanured trees had shorter roots and the main feeding roots were only 8-31 cm from the trunk.
In Florida’s deep sandy soils, Spencer found that phosphate applications markedly reduced the concentration of feeder roots, especially in the surface 30 cm of soil. Reductions in root growth were not noted in the deeper soil zones even at the highest phosphate rates. Similar observations were made by Smith and Ford . Smith and Specht suggested an increase of iron chlorosis in Florida was mainly caused by an accumulation of copper in the soil with consequent root damage. Since copper accumulates primarily in the top soil they suggested trees became chlorotic because of root damage in that area. Chelated iron applied to seedlings in soil solution did not overcome the stunted root system associated with high copper levels. Ford had shown that in Florida 70 per cent of the feeder root system of healthy trees growing in deep sandy soils are located below 25 cm. Ford found that feeder root damage in orange trees affected with severe iron deficiency was not confined to the topsoil. Feeder root damage like copper toxicity was found to a depth of 1.5 M in groves located near lakes and swamps. Soil pH in the 0-25 cm zone was below 1.5 M with the subsoil at pH 3.9-4.4. All the groves had a high concentration of copper in the topsoil. The application of FeEDTA chelate to chlorotic trees which showed extreme root damage to a depth of 1.5 M resulted in pronounced new growth of roots in the subsoil. The increase of root growth was proportionately greater with an increase in depth so that often there were more new roots in the 75-150 cm zone than the 25-75 cm zone. Where iron chelate resulted in new leaf and shoot growth there was a corresponding increase in feeder root growth which occurred mostly below the 25 cm depth. If feeder roots were found in the 0-25 cm zone under chlorotic trees, treatment resulted in an increase of the number of feeder roots on the laterals. If there were no lateral roots in the surface 25 cm, then no new feeder roots were present after treatment. Changes in soil pH greatly influenced the distribution of feeder roots throughout the entire root profile. Ford says that in general, root concentration is highest when nutrition elements are low but not deficient. At high levels of applications of fertilizers the concentration of roots is reduced for all major elements. The correlation did not apply to the micro-elements. A deficiency of iron severely reduced the root system and an excess of copper and manganese prevented growth of the feeder roots due to toxicity in certain soil horizons. He suggested that from the standpoint of the root system the lowest level of nutrients consistent with high yield and healthy trees was the best. The effect of excessive accumulations of micro nutrients like Cu, Zn, and Mn on mycorrhiza and in turn, on root development, has yet to be fully evaluated.