QBARS - v11n4 Growth of the Azalea as Influenced by Ammonium and Nitrate Nitrogen

Growth of the Azalea as Influenced by Ammonium and Nitrate Nitrogen
By M. S. Colgrove, Jr., and A. N. Roberts
Oregon State College, Corvallis, Oregon
Technical Paper No. 953 Oregon Agricultural Experiment Station
Contribution of the Dept. of Horticulture

It has often been observed that a soil reaction satisfactory for the growth of many plants may result in poor growth and the development of chlorosis in the azalea. This condition is commonly attributed to a deficiency of available iron to the plant. However, it is questionable in many cases whether the chlorotic condition ascribed to a deficiency of iron is a direct result of a shortage of available iron at the root surface or is the indirect result of a lack of balance of other nutrient-elements rendering iron nom-functional in the plant tissue.

Recent research with the blueberry, an ericaceous plant, has shown that nitrate nitrogen may be harmful while ammonium nitrogen maybe essential. The fact that blueberry plants grown on nitrate nitrogen develop iron chlorosis symptoms, while those on ammonium nitrogen do not, has been interpreted by Cain (1) as indicating a close relationship between nitrogen and iron nutrition. He concluded that the superiority of ammonium nitrogen is not an effect on availability of iron in the growing medium, but on internal function, since plants receiving nitrate nitrogen and showing iron deficiency symptoms contain as much or more iron in their foliage as those showing no symptoms of deficiency.

Stuart (19) observed that azaleas brown in sand culture became chlorotic when calcium nitrate was used as a source of nitrogen, but the chlorotic condition could be corrected by adding ammonium sulfate to the solution. The chlorosis was attributed to the change in pH brought about by the residual ion associated with the nitrogen. Spencer and Shive (18) obtained similar results with Rhododendron ponticum . Leaf analyses of azaleas by Twigg and Link tell have shown that the requirements of these plants for phosphorous, potassium, calcium and magnesium are quite low compared to most horticultural plants.

With these concepts is mind, a study was undertaken to obtain a better understanding of the influence of ammonium and nitrate nitrogen on the growth of the azalea plant. A series of experiments was conducted in all attempt to show the conditions that favor one source of nitrogen over the other as evidenced by growth response and the development of chlorosis.

Since the completion of this study with azalea, Cain (2) has presented data which show a close relationship between blueberry leaf chlorosis, the accumulation of the basic ions calcium, magnesium and potassium and an increase in the pH of the leaf tissue. His results and interpretations are especially significant in the light of the results and interpretation, presented in this paper as they pertain to the azalea.

Materials and Methods

The Hexe variety of evergreen azalea was used in all experiment; reported here, although several other varieties were used in certain preliminary studies to determine variety response to the various treatments. The plants were grown in 1.3 liter glazed crocks containing Del Monte coarse sand.

The nutrient solutions were prepared from reagent grade chemicals. Hoagland and Arnon's (7) four-salt, nitrate solution and a similar solution containing ammonium sulfate as the sources of nitrogen were used for several experiments. The pH of these solutions ranged from 6.6 to 7.0. The solutions were applied by the slop culture method at the rate of 150 ml. per crock once of twice daily.

The method used to determine pH of the leaf tissue was essentially the same as that used by Kramer (12). A uniform sample of fresh leaves were weighed and then ground in a mortar. Distilled water was added to the crushed leaves at the rate of five times the leaf weight and ground until thoroughly mixed. The pH was determined on the resulting slurry. All pH determinations of plant tissue were made between 7:00 and 10:00 p.m. The pH determinations on solution leachate were made using the first 100 ml. collected from each crock after applying the solutions. All pH determinations were made with the glass electrode. Statistical analyses of pH were made by converting pH values to hydrogen ion concentrations and then converting hack to pH values.

Chlorosis and Nitrogen Deficiency Index

The chlorosis and nitrogen deficiency indexes are an average of the several plants in each treatment. These are arbitrary ratings from 1 to 5 with increasing severity of leaf deficiency symptoms. Plants designated 1 showed no symptoms and those designated 5 showed severe deficiency symptoms.

The chlorosis symptoms that appeared in these plants were similar to those often described as iron deficiency. The first symptoms appeared as an inter-veinal yellowing of the leaves, and eventually the whole leaf became an ivory color. Iron chlorosis was substantiated by applying a dilute ferric tartrate solution to the scratched surface of chlorotic leaves, which, in all cases, restored a normal green color along the injured areas. However, increasing the iron supply in the nutrient solution did not correct the chlorosis.

Results

The Effects of Ammonium, Nitrate and Hydrogen Ion Concentration. Several preliminary experiments showed that Hoagland's NO ₃ -N solution resulted in unsatisfactory growth and chlorosis, while plants supplied with NH ₄ -N solution produced excellent growth and were dark green in color. The following experiment was done in order to determine the influences of, and the interrelationships between, NH ₄ , NO ₃ and H ion concentration on growth and on the development of chlorotic symptoms in the azalea.

Rooted cuttings of the Hexe azalea were planted July B, 1954. The full strength NH ₄ -N and NO ₃ -N solutions (Table 1) were diluted to 1/4, 1/2 and 3/4 strength. Solutions of each of the three concentrations were adjusted to pH 3.5 and 6.5 with sulfuric acid. The solutions were applied once daily during the first month and then twice daily for the remainder of the summer. During the fall and winter the solutions were again applied once daily but at the rate of 200 ml. per crock.

Twelve plants were used in each of the twelve treatments. Five plants of each treatment were placed at random under a double layer of cheesecloth and the remaining seven plants were distributed at random in an un-shaded block. The final measurements of plant response were made between March 22 and 24, 1955.

The effects of NH ₄ -N and NO ₃ -N supplied at the two H ion concentrations on growth and leaf color are summarized in Table 2. These averages are a composite of three solution concentrations at two light intensities. It will be seen that the fresh weight of plants receiving NH ₄ -N was significantly greater than that of plants supplied NO ₃ -N at either solution pH. No significant differences in fresh weight were obtained between plants supplied NO ₃ -N at either of the two hydrogen ion concentrations. However, the data indicate that reducing the pH of the NH ₄ -N solution from 6.5 to 3.5 decreased slightly the fresh weight of the roots.

The average pH of the leachate shows the physiological acidity and alkalinity resulting from the two forms of nitrogen nutrition. Within a 24 hour period plants supplied NH ₄ -N reduced the pH of the solution in contact with the roots to approximately 3.0. Those supplied NO ₃ -N increased the pH toward 6.0.

Significant differences in the pH of the leaf tissue were obtained between plants supplied NH ₄ -N at pH 3.5 and those supplied NO ₃ -N at either 3.5 or 6.5. The pH of chlorotic leaf tissue was significantly greater than that of healthy leaf tissue, regardless of the form of nitrogen supplied.

The data show a relationship between pH of the plant tissue and fresh weight production in the aerial portions of the plant.

Table 3 shows the effects of solution concentration. The averages presented are a composite of the two pH levels at two light intensities. Each increase in NO ₄ -N solution concentration significantly reduced growth. Increasing the NO ₃ -N solution concentration had no effect on fresh weight. Fresh weight production was always significantly greater with NO ₄ -N than with NO ₃ -N solutions.

Plants supplied NH ₄ -N had a significantly lower leaf-tissue pH than those supplied NO ₃ -N. Increasing the NH ₄ -N concentration significantly reduced growth and increased slightly the pH of the leaf tissue. Plants supplied the highest NO ₃ -N solution concentration produced the least amount of growth and had the highest leaf tissue pH.
Solution concentration affected little the development of chlorotic symptoms, although the data show that an increase in the concentration of the NO ₃ -N solution increased slightly the severity of chlorosis. No symptoms of chlorosis developed in plants supplied NH ₄ -N, regardless of solution concentration.
During the first three months of this experiment, all plants supplied NO ₃ -N were chlorotic. However, by November, 1954, some of the plants receiving the NO ₃ -N solution adjusted to pH 3.5 began to grow and the chlorosis disappeared. By January, 1955, all plants supplied NO ₃ -N at that pH were green and showed evidence of new growth.
Day length or light intensity was apparently an important factor influencing chlorosis. The NO ₃ -N solution at pH 3.5 did not prevent the development of chlorosis during the summer months, but the same solution applied during the winter corrected chlorosis and brought about growth. Since shaded plants differed little from the un-shaded plants during the first two months of the experiment, the cheesecloth was removed in September, 1954. However, the first plants to show the growth and leaf color responses mentioned above were those that had been shaded during the preceding summer. The influence of the reduced light intensity could still be detected on January, 1955, since the plants that were previously shaded and supplied the NO ₃ -N solution at pH 3.5 were a darker green than the un-shaded ones.

The Effect of Varied Amounts of Ammonium and Nitrate Nitrogen. The object of this experiment was to determine the effect of various proportions of NH ₄ to NO ₃ ions in the nutrient solution on growth and the development of chlorosis. The development of iron chlorosis was followed to determine the proportion of NH ₄ -N to NO ₃ -N that was needed for preventing chlorosis under the conditions of these experiments. The influence of light on the expression of these growth responses was also considered.

Rooted cuttings of the variety Hexe were planted July 16, 1954. Quarter strength complete nutrient solutions containing a total of 52 ppm N with NH ₄ -N and NO ₃ -N at ratios of 4:0, 4:1, 2:2, 1:3 andO:4 were applied daily. As NH ₄ -N was reduced the SO ₂ and Cl content of the solution was correspondingly reduced. The five treatments were replicated 10 times as single plants in randomized blocks. Five of the blocks were shaded with cheesecloth. The un-shaded plants were removed from the pots March 27, 1955, and the pH and growth measurements were made.

The effects of decreasing the proportion of NH ₄ -N to NO ₃ -N in the nutrient solution are presented in Table 4 and in Figure 1. The data show that reducing NH ₄ -N and increasing the NO ₃ -N content of the solution decreased the fresh weight. Significant reductions in fresh weight occurred when the ratio of NH.-N to NO ₃ -N was reduced from 3:1 to 2:2. A further significant reduction in fresh weight occurred when the NH ₄ -N was reduced from 2:2 to 1:3. No significant fresh weight reductions resulted when the proportion of NH ₄ -N to NO ₃ -N was changed from 4:0 to 3:1, or from 1:3 to a complete NO ₃ -N solution.

The reduction in plant weight accompanying decreases in the proportion of NH ₄ -N in the nutrient solution was probably not related to the pH of the medium. As shown in Table 4, the pH of the leachate increased but slightly if (NH ₄ ) ₂ SO ₄ was used in the nutrient solutions.

There were no significant differences in pH of the leaf tissue among plants receiving nutrient solutions containing (NH ₄ ) ₂ SO ₄ .

The development of chlorosis followed a pattern similar to that observed in the previous experiment. Those plants supplied the 1:3 ratio solution and those supplied only NO ₃ -N developed typical chlorosis within a month and a half after the experiment was started.

Since both the shaded plants and the un-shaded ones responded similarly to the treatments, the cheesecloth was removed in September, 1954. The chlorotic plants supplied NH ₄ -N to NH ₄ -N at the 1:3 ratio started to produce new growth by January, 1955. As in the preceding experiment, plants that had been shaded were the first to renew growth and the leaves were a darker green than those on un-shaded plants of the same treatment. At the end of January, 1955, all plants were green and growing well with the exception of those plants supplied NO ₃ -N only.

The Effect of Chloride and Sulfate. The azaleas supplied NH ₄ -N in the previous experiments grew more than plants receiving NO ₃ -N. When anionic and cationic N were used in separate solutions, however, difficulty was encountered in balancing the ions so that each solution had identical compositions. The NH ₄ -N solutions in the previous experiments contained larger amounts of SO ₂ and Cl than the corresponding N solutions containing NO ₃ -N. Thus, it was questionable whether the superior growth of the azaleas was due entirely to the NH, ion or in part to the additional SO ₄ and/or CI ions. For this reason. NO ₃ -N solutions were prepared which contained either C1 (192 ppm). SO ₄ (216 ppm), or CL and SO ₄ ions. These amounts are equivalent to that found in the quarter strength NH ₄ -N solution. This was accomplished by using either Cl or SO ₄ salts and by the addition of HCl or H ₂ SO ₄ . A fourth treatment consisted of 1/4-Hoagland's NO ₃ -N solution. The solutions containing the additional anions had a lower pH than the standard NO ₃ -N solution.

Rooted cuttings of Hexe azaleas were planted August 12, 1951. Ten plants. arranged as a single randomized block, were used in each of the four treatments. Five plants in each treatment were supplied Fe in the form of ferric tartrate. The remaining five plants were given an equivalent amount of chelated Fe. This latter procedure was discontinued after two and one-half month, because no differences in growth or leaf color were observed, and ferric tartrate was then used in all solutions. Four plants from each treatment were harvested April 10, 1955. The results are shown in Table 5.

The plants supplied the NO ₃ -N solutions containing the additional anions and hydrogen produced a significantly greater amount of fresh weight than did those supplied the standard NO ₄ -N solution. The plants supplied additional SO, produced significantly greater fresh weight of tops and roots than those supplied additional Cl, Cl plus SO4 or the standard NO ₃ -N solutions. Plants receiving both C1 and SO, produced significantly less fresh weight than plants supplied additional Cl or SO, alone.

There appears to be a relationship between the pH of the leachate and the fresh weight of plants supplied the additional anions. The pH of the leachate increased as fresh weight of the plants increased. The data indicate that the pH of the leaf tissue was significantly lower when additional SO ₄ was added to the NO ₃ -N solution.

The addition of 1-42 ppm of Cl to the NO ₃ -N solution resulted in foliage of a lighter green color than when SO ₄ only was added to the NO ₃ -N solution. The appearance of the foliage suggested a lower rate of absorption or utilization of NO ₄ -N. The Cl plus SO ₄ solution resulted in a slight necrosis of the margins of the leaves.

Although chlorosis slid not develop in plants supplied the additional ions, light intensity, noted in the previous experiments, apparently influenced the rate of growth during the first two months of the experiment. Even though the pH of the leachate was similar to that of plants supplied NO ₄ -N, the plants supplied the additional anions in the NO ₃ -N solution were light green in color. However, in December, 1959, plants supplied additional SO ₄ began to grow and the foliage became dark green. By January, 1955, all plants receiving the additional ions were growing.

Effect of Concentration of Nitrogen. Total Salts. and Form of Nitrogen. This experiment was primarily designed to study the relationships between the total concentration of salts in the nutrient solution and the concentration and form of N used, since the results of the previous experiment indicated that large additions of Cl or SO ₄ to the NO ₃ -N solutions were not essential in preventing chlorosis. The foliage of azaleas receiving solutions lacking either anion was healthy, although better growth was obtained with large additions of SO ₄ . The increased H ion concentration associated with these additions was apparently the major factor responsible for the satisfactory growth and the absence of chlorosis, although the exact function of H in this respect was not understood.

Rooted Hexe cuttings were planted as described in the previous experiments and the nutrient solutions Table 6 were applied daily, beginning November 21, 1954. Treatments 1, 2, and 3 contained 40, 26, and 13 ppm of NH ₄ -N, respectively, in the form of (NH ₄ ) ₂ SO ₄ . Equal amounts of the other salts were used in these solutions. Nitrate nitrogen supplied as HNO ₃ was used at the same decreasing rates in treatments 4, 5, and 6, with the total salt content the same in all treatments. In treatments 7, 8, and 9 NO ₃ -N was constant at 90 ppm N, but with total salt content varied in the several solutions. These three treatments contained 305.0, 155.0, and 97.5 ppm total salts, respectively. No acidic or basic substances were used to control solution pH, but the pH of each solution and leachate was recorded periodically to determine more closely the effect of this ion on the development of iron chlorosis. Five plants, randomized in a single block on the greenhouse bench, were used in each treatment.

The various measurements of plant response to the several treatments summarized in Table 7 were made April 15. 1955. The fresh weights of the plant tops in treatments (40 ppm NH ₄ -N with 471.5 ppm total salts) and 2 (26 ppm NH ₄ -N with 436.0 ppm total salts) were significantly greater than for any of the other treatments. There were no significant differences in fresh weight between plants receiving 40 ppm NO ₃ N (Treatments 4 and 9) and those receiving 13 ppm NO ₄ -N (Treatment 3). The plants of treatment 4 (40 ppm NO ₃ -N with 97.5 ppm total salts) had a better leaf color (N-index, Table 7) than plants of treatment 3 (13 ppm NO ₄ -N with 391.0 ppm total salts), although treatment 3 produced the greatest fresh weight. This suggests that an increase in NO ₃ -N would have affected little the fresh weight of these plants.

Treatments 6 (13 ppm NO ₃ -N with 332.0 ppm total salts) and 7 (40 ppm NO ₃ -N with 305.0 ppm total salts) produced the least amount of growth. The fresh weight of roots showed about the same growth pattern as the tops for each treatment.

Although significant differences in growth response to treatments did not occur generally, certain trends were evident. Growth decreased progressively as NO ₃ -N was reduced from 40 to 13 ppm (Treatments 4, 5, and 6). Conversely, growth increased when the total salt content was reduced from 305.0 to 97.5 ppm with NO ₃ -N constant at 40 ppm. The increased H ion concentration in the NO ₃ -N solution (Treatment 4) was about as effective in promoting growth as the reduced salt content of treatment 9.

The source of N determined the pH of the leachate, as in the previous experiments. The pH of the leachates for the several NO ₃ -N treatments was not significantly different, with the exception of treatment 9, which had a lower pH.

The pH of the leaf tissue of plants supplied 40 and 26 ppm NH ₄ -N was significantly lower than that of plants supplied 40 ppm NO ₃ -N, with the exception of the plants in treatment 9 (97.5 ppm total salts). The pH of the leaf-tissue of plants supplied this solution was the lowest of all NO ₃ -N treatments in this experiment.

The development of chlorosis in the azalea appears to be closely associated with the pH of the leaf tissue. The chlorotic plants of treatment 7 had the highest pH of the leaf tissue.

The conditions that influenced the pH of the leaf tissue, and evidently the development of iron chlorosis as well, are apparent from the treatments and data presented in Table 6. The lowest leaf-tissue pH for all NO ₃ -N treatments was associated with the low total salts concentration of the NO ₃ -N solution of treatment 9. Low NO ₃ -N content of the nutrient solutions (Treatments 5 and 6) was also effective in lowering leaf tissue pH and preventing chlorosis as compared to treatment 7, even though treatment 6 had a higher solution pH. No significant differences in the pH of the leachates occurred between these treatments. A high concentration of H ions in the nutrient media, in addition to low salt or low NO ₃ -N, also lowered tissue pH and thereby reduced chlorosis. This is shown by treatment 4 which had the lowest pH of the leachate of all the NO ₃ -N treatments.

The Effect of Calcium and Potassium. In these studies, the 3/4-strength NO ₃ -N solution with 121.0 ppm total bases always produced chlorotic plants unless the H ion concentration was high, or the NO ₃ -N content low.

The following experiment was designed to compare the influence upon chlorosis development of a low total base solution (96 ppm total base), containing either a high Ca (75 ppm) and a low Ca (18 ppm) and a high K (75 ppm), each at pH 6 and at pH 3.5, with that of the 1/4 strength high total base solution (121 ppm total base). For additional comparison an NO ₄ -N solution (162 ppm total base) was employed. In all the NO ₃ -N solutions and in the NO ₄ -N solution a concentration of 37 ppm N was used. The differential pH of the low total base solutions was obtained, by varying the amount of HNO ₃ to NO ₃ salts used in the NO ₃ -N solutions. The solutions were applied daily starting March 1, 1955, until the final observations were made in July, 1955. The solutions used and the results are shown in Table 8 and Figure 2.

The data show that a relatively high Ca content (75 ppm) did not induce chlorosis, if the total base content was low (96 ppm). Solution pH influenced little these results. However, some of the plants supplied the high K solution (75 ppm) at pH 6.0 showed slight indications of chlorosis, although the chlorosis was not as severe and did not develop as rapidly as when the plants were supplied the standard NO ₃ -N solution. Although no analysis was made, chlorosis may have been due to rapid absorption and accumulation of K in the plant tissue, a characteristic plant response to a luxury supply of this ion.

In contrast to plants supplied the NO ₃ -N solutions, plants receiving NO ₄ -N produced the most growth and had the best leaf color, even though the total base content of the solution was high (162 ppm). Although the initial pH of the NO ₄ -N solution was high (9.5), the pH of the leachate was similar to that obtained from crocks supplied (NH ₄ ) ₂ SO ₄ solutions.

Discussion

Early in these studies it appeared that NO ₄ -N was superior to NO ₃ -N for the Hexe azaleas: however, it was not known whether NO ₃ -N had been supplied under conditions satisfactory for growth. The various adjustments in the NO ₃ -N solution that were made in an attempt to produce growth comparable to that obtained with NO ₄ -N have shown the importance of the relative concentrations of N, H and certain other ions.

Concentration of Nitrogen. The results indicate that the azalea requires less NO ₄ -N than NO ₃ -N for good growth and foliage color, although NO ₃ -N concentration was apparently not the limiting factor in these experiments. Nitrate supplied as high as 158 ppm N gave no significant increase in growth or improvement in leaf color over that of plants receiving 50 ppm of N as NO ₃ . Plants supplied 40 ppm NO ₃ -N had a better leaf color than plants receiving 13 ppm NO ₄ -N, but produced less total growth, indicating that factors other than NO ₃ -N concentration were limiting growth. Increasing the NO ₄ -N concentration in the solution to 105 ppm N resulted in a significant reduction in growth.

Hydrogen Ion Concentration. Of the several factors that influence the absorption and assimilation of N, the pH of the root medium is usually considered of great importance. In contrast to the results obtained with several plants (3, 4, 14, 20), good growth was obtained in these experiments with azaleas supplied NO ₄ -N solutions at pH 3.5. Since it was not possible to observe the effects of a high pH when NO ₄ -N was applied to the sand cultures, a limited series of aerated solution cultures was later prepared for this purpose. The 1/4 strength NO ₄ -N solution at pH 6.8 was used. The 10 liters of solution were renewed daily to prevent any change in pH. The azaleas grew normally and without chlorosis under these conditions for a period of two months, indicating that NO ₄ -N is absorbed and assimilated equally well by the azalea at a low or a high pH.

Plants supplied the standard, NO ₃ -N solution did not grow and developed severe chlorosis, unless the pH was very low. However, a low pH was not essential for normal growth if the NO ₃ -N content, or the total concentration of salts, was low. The function of the large amounts of H ions is apparently related here to the absorption of other elements. Hoagland and Broyer (8) found a substantially greater accumulation of cations and anions in barley roots at a high rather than at a low pH. Under conditions of high H ion concentration Ca was not absorbed and in some cases was lost from the plant. Prianishnikov (14) also found an antagonism between H and Ca.

The H ion is apparently similar to the NH, ion in its effect on absorption of other bases. That NO ₃ -N increases the absorption of the bases Ca, Mg and K over that absorbed from similar solutions containing NO ₄ -N has been demonstrated by Sideris and Young (16) with pineapple, and Holley, Pickett and Dulin (9) with cotton. Clark (3) found a lower organic acid content in tomatoes supplied NO ₄ -N, which indicates lower base absorption. Wadleigh and Shive (22) show that the NH ₄ ion has a greater influence in reducing the base content of corn plants than has the H ion. They state, however, that the high concentration of H ions on the root film may be partially responsible for the lower uptake of cations.

Concentration of other ions. Increasing the supply of SO ₄ and/or Cl in the NO ₃ -N solution resulted in an increase in growth over that obtained with the standard NO ₃ -N solution. The largest increase was obtained by increasing the SO ₄ concentration. However, reducing total salts (12 ppm SO ₄ ; Tables 7 and 8) also produced good growth. A wide range of SO ₄ and/or Cl concentration had little effect on growth when NH ₄ was used as the source of N.

Further evidence indicating that small amounts of SO ₄ are sufficient for good growth of the azalea was obtained in a series of deficiency studies. Solutions containing only trace amounts of SO ₄ , and with NH ₄ -NO ₃ as the source of N, produced good growth for one year. However, S deficiency has been reported to develop rapidly in certain species of the Ericaceae (11) . Since NO ₃ -N was the only source of nitrogen in the solution lacking SO ₄ , the symptoms reported were more likely those of iron chlorosis caused by the high pH and/or the relatively high base content of the medium.

Evidence showing that chlorosis and poor growth are results of a relatively high total base content of the NO ₃ -N solution was first obtained in the fourth experiment. Reducing total salts (Treatment 7, 8, and 9, Table 7) increased growth and prevented chlorosis. Thus, the total amount of bases as related to the pH of the root media appears to be closely linked to the incidence of iron chlorosis in the azalea.

Of the bases essential for plant growth, Ca has frequently been associated with the development of iron chlorosis. In these studies, the 1/4 standard NO ₃ -N solution with 50 ppm Ca and 121 ppm total bases always produced chlorotic plants unless the pH was low, NO ₃ -N content low, or the Mg and K content low. The chlorosis obtained when NO ₃ -N was supplied azaleas in these experiments has been shown to be associated with the total base content of the nutrient solution and not to any individual ration.

That the NH, ion acts in preventing chlorosis and promoting good growth of the azalea through its antagonistic action toward the bases Ca, Mg, and K is supported further by the low requirements of the azalea for the basic elements. Twigg and Link (21) established critical levels of 0.22, 0.17, and 0.80 per cent of dry foliage for Ca, Mg and K, respectively. They state that these are quite low compared to those of many cultivated plants.

A close relationship was found in these experiments between the concentration of NO ₃ -N, total bases, pH of the NO ₃ -N solution, and the pH of the leaf tissue of the azalea. The pH of the leaf tissue of plants supplied the standard NO ₃ -N solution was consistently higher than plants receiving NO ₄ -N. However, the pH of the tissue of plants supplied NO ₃ -N could be reduced by lowering the total base content of the solution, by lowering the NO ₃ -N content of the solution, or by lowering the pH of the solution. These solution adjustments that lowered the tissue pH also resulted in the best growth of plants supplied NO ₃ -N.

The inactivation of Fe in the plant tissue has been found to be closely related to the pH of the plant sap in other plants. Loehwing (13) found that limed soils increased the pH of the plant sap as much as 0.82 of a pH unit, as compared to that of unlimed soils. Ingalls and Shive (10), using nine different species of plants, found an increase in soluble Fe content as the H ion concentration of the sap increased. They showed that plants with a high pH of the sap had a low total but a higher soluble Fe content than plants with a higher pH of the plant sap. Rogers and Shive (15) found that Fe accumulations occurred in tissues having a high pH when these tissues were adjacent to tissues of low pH. Small (17) has reviewed these and other experiments concerning the H ion concentration of plants and concludes that the explanation of lime induced chlorosis has apparently been settled.

Environmental factors. The various adjustments of the NO ₃ -N solution in these experiments had little effect on plant growth under long days and/or high light intensity. Hibbard (G) found that full light intensity favored the absorption of Ca, and Fe in the plants studied, while short days decreased total salt uptake. The translocation of Fe in the aerial parts of the plant was more rapid under short days. Gericke (51 observed that wheat plants growing in solutions lacking Fe were more etiolated and smaller than shaded plants. Small (17) found that Rhododendron ponticum had a snore acid sap during the winter than during the summer. The influence of light in increasing the absorption of bases and thus increasing the pH of the plant sap could account in part at least for the results obtained in these experiments with azaleas supplied NO ₃ -N.

Summary and Conclusions

Several experiments were conducted in sand cultures to determine the conditions that promote good growth and foliage color of the azalea when applied either NO ₄ -N or NO ₃ -N NO ₃ -N. The relationship of light intensity to growth response was also considered.

NO ₄ -N supplied as sulfate or hydroxide, produced more plant growth and better foliage color and was required in smaller amounts than NO ₃ -N.

The standard NO ₃ -N solution used in these experiments resulted in poor growth and chlorosis of the azalea. (However, good growth and foliage color was attained when large additions of H and SO ₄ , ions were added to the solution or when the total base content was reduced to a low level. Large additions of Cl and H ions prevented the development of chlorosis, but did not improve N nutrition.

Reducing the NO ₃ -N concentration reduced plant growth. Chlorosis did not develop in these plant, although the pH and base content of these solution, always resulted in chlorosis at the higher NO ₃ -N levels.

Chlorosis was closely associated with the pH of the leaf tissue. Plant supplied NO ₄ -N had a lower tissue pH than those receiving NO ₃ -N. Chlorotic plants had the highest leaf tissue pH. The solution adjustments that promoted good growth with NO ₃ -N lowered tissue pH. Reducing the NO ₃ -N content of the nutrient solution also lowered the pH of the leaf tissue.

The NO ₃ -N solution adjustments that were made in an attempt to promote good growth with this form of nitrogen had little effect during periods of high light intensities and/or long days.

It is concluded from these data that the ammonium ion reduces the uptake of other cations and thereby reduces plant tissue pH. Nitrate apparently increases base absorption and Fe is inactivated in the plant tissue as a result of the higher tissue pH.

LITERATURE CITED

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7. Hoagland, D. R. and Arnon, D. I., 1950, The water-culture method for growing plants without soil, Calif. Agr. Exp. Sta., 347.

8. ………. and Broyer, T. C., 1940. Hydrogen-ion effects and the accumulation of salt by barley roots as influenced by metabolism, Amer. Jour. Bot. 27: 173-185.

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