Plant and Soil 38, 9-16 (1973) Ms. 1820

AMMONIUM FIXATION IN THE SUDAN GEZIRA SOILS

by M. BAKHEIT SAID

Gezira Agricultural Research Station, Wad Medani, Sudan

SUMMARY

Laboratory studies showed that the Gezira soils contain appreciable a- mounts of fixed ammonium. Values for the surface soils varied from 0.25-0.30 me/lOOg. The soils have the capacity to fix a large quantity of applied am- monium.

In the two soil profiles studied the amounts of fixed ammonium and the ammonium-fixing capacity increased with depth reaching a maximum in the grey layer at a depth of about 70 to 140 cm below which it decreased again. The fixed ammonium values varied from 0.28 to 0.40 me/lOOg and comprised from 5.0 to 5.9 per cent of the total ammonium-fixing capacity (oven-dry condition). When expressed as percentage of Kjeldahl nitrogen the fixed ammonium increased with depth, varying from 12.5 to 20.2 per cent. As ex- pected the amount of fixed ammonium was found to depend on the type and amount of the clay minerals present.

The current cropping and fertilization practices were found to have very little effect on the fixed ammonium content of the soil, but it is expected from the high-fixing capacity of these soils that the amount of fixed ammonium will increase with the increase in frequency and dose of NH4-N fertilizers application.

INTRODUCTION

The soils of the Sudan Gezira are classified as vertisols, being alkaline heavy clay soils with the clay mineral largely montmorillon- ite 7 8 10. They are inherently low in nitrogen and urea is commonly used to supply extra nitrogen for cotton and some other crops.

The clay minerals montmorillonite, illite, and vermiculite are known to be chiefly responsible for ammonium fixation in soils, whereas kaolinite does not fix any appreciable amount of ammo- nium 2. The fixation of ammonium by soils is an important factor in evaluating the efficiency of ammonium-containing nitrogen fertilizers and also nitrogen fertilizers which yield ammonium upon hydrolysis.

10 M. BAKHEIT SAID

No information is available on the ammonium fixation and am- monium-fixing capacity of the soils of the Gezira. The present in- vestigations were conducted (a) to determine the amount of fixed ammonium in a number of soils of varying properties, (b) to meas- ure the ammonium-fixing capacity of these soils, and (c) to study the effect of cropping on the level and distribution of fixed ammonium in soils.

MATERIALS AND METHODS

Ten surface soils, sampled from 030 cm zone, were used in this study. Soil No. 1 was collected from the continuous cotton observation plot (No. 4) started in 1918. Soils No. 2, 3, and 4 were collected from a long-term NPK experiment (Plot No. 38). The NPK experiment was initiated in 1941 and followed a three-course rotation, with nitrogen applied in the form of ammo- nium sulphate to the cotton phase only. Soil No. 2 was taken from the con- trol plots of this experiment, while soils No. 3 and 4 were taken from the plots receiving 67 kg and 134 kg N/ha respectively. These four soils fall within the general range of the characteristics of the Suleimi clay 1. Soils No. 5, 6, and 7 were sampled from tenant’s fields in the Gezira Scheme with generally the same cropping history. In all seven Gezira soils, the dominant clay mineral is montmorillonite. The recent alluvial soils (No. 8, 9) were collected from the west bank of the Blue Nile, near Wad Medani. According to B u u r s in k 4 the clay mineral composition of these soils on the whole is similar to that of the Gezira soils. The sandy soil (No. 10) was sampled from the Tonj sisal estate in southern Sudan and kaolinite is the principal clay mineral in this soil. The Tonj

TABLE 1

Some physical and chemical properties of the soils

No. Soil PH CEC, me/100 g

Clay, org. c,* Kjeldahl

% % N, pm

1. Plot No. 4 8.5 50.0 62.0 2. Plot No. 38, NO a.4 54.7 62.2 3. Plot No. 38. Nl a.5 55.8 62.0 4. Plot No. 38, N2 a.3 54.1 63.0 5. Hosh clay a.4 51.3 59.3 6. Loata clay 8.1 33.5 40.0 7. Loata clay a.7 36.3 43.0 8. Recent alluvium 7.6 33.7 24.0 9. Recent alluvium 7.6 6.5 4.5

10 Tonj sandy loam 6.8 5.8 15.4

* A recovery factor of 0.75 has been used to give these figures.

0.50 298 0.51 301 0.51 301 0.5 1 336 0.52 298 0.41 294 0.36 217 1.21 634 0.08 235 1.33 466

NHd-FIXATION IN SUDAN GEZIRA SOILS 11

soil together with the recent alluvial soils were included in this study for comparison. Some of the properties of the ten soils are given in Table 1.

Of the two soil profiles studied, one was located in the permanent fallow plot under natural vegetation (Plot No. 141), and the second was taken from a cropped plot (No. 132) adjacent to the permanent fallow plot. These profiles were typical of that described by Greene 9. The soil samples were dug from pit walls by horizon. These were: the auto-mulching surface layer followed by the dark brown layer, the grey layer and the yellowish brown layer.

All soil samples were air-dried, ground to pass a OS-mm sieve and analysed by the following methods: organic carbon by the Walkley - Black method, total nitrogen by the Kjeldahl method, clay content by the pipette method, pH by a glass electrode (soil/water ratio 1: 2.5) and cation exchange capacity (CEC) by the sodium acetate method 5. The CEC of the mineral fraction of the soil (Table 3) was determined after treating the soil with hypobromite to remove the organic matter.

The fixed ammonium was determined on a 2-g soil sample by the 1N HF: 1N HCl method described by Bremner 3. Exchangeable ammonium and or- ganic nitrogen compounds, which may yield ammonium under the conditions employed for release of fixed ammonium, were removed by heating the sample with 0.5 N KOH solution and by washing the residue three times with 1N KCl. In the ammonium-fixing capacity study a 2-g sample was NHd-saturat- ed by shaking the soil with 3 successive 20-ml aliquots of 1 N NH&l solution, centrifuging, and decanting between shakings. NH&l solution was added for the fourth time, the mixture shaken and allowed to stand for 24 hours. The soil was then washed free of the excess salt with 80 per cent ethanol until the washing gave a negative Nessler test. Total fixed ammonium was determined on moist, air-dry and oven-dry ( 100°C) samples by the 1 N HF: 1 N HCl method proposed by Bre mner 3. Exchangeable ammonium and organic nitrogen compounds were removed as described above.

RESULTS AND DISCUSSION

The results of the fixed ammonium and the ammonium-fixing capacity of the soils are given in Table 2. The difference between the total fixed ammonium and the initially fixed ammonium represented the amount of added ammonium fixed. The amounts of the fixed NHa-N varied from 0.08 me per 1OOg in the Tonj sandy loam to 0.30 me per 1OOg in the ammonium fertilizer-treated plots of the NPK experiment (Soil No. 4). These amounts are appreciable and agree with the results obtained by Moore and Ayeke 11 for some tropical soils. The soil (No. 1) from the continuous-cotton plot con- tained 0.26 me fixed NHa-N per 1OOg. As far as is known this plot has never received ammonium fertilization, and it may be presumed that this ammonium was fixed naturally. A comparison of the re-

12 M. BAKHEIT SAID

TABLE 2

Fixed ammonium and fixation of applied ammonium under moist, air-dry and oven-dry conditions in the different soils (me NH&-N/100 g)

No. Soil NH4-N fixed

NHI-fixation capacity under three conditions

Moist Air dry Oven dry

1. Plot No. 4 0.26 0.64 1.35 4.80 2. Plot No. 38, NO 0.27 0.62 1.41 4.85 3. Plot No. 38, N 1 0.28 0.62 1.39 4.98 4. Plot No. 38, N2 0.30 0.65 1.40 4.87 5. Hosh clay 0.27 0.8 1 1.55 5.54 6. Loata clay 0.27 0.53 0.87 3.59 7. Loata clay 0.25 0.55 0.90 3.50 8. Recent alluvium 0.29 0.97 1.27 3.18 9. Recent alluvium 0.12 0.23 0.25 0.63

10. Tonj sandy loam 0.08 0.07 0.09 0.15

sults for the three samples taken from the NPK experiment (soils No. 2, 3, 4) showed that ten applications of ammonium sulphate at the rate of 67 kg and 134 kg N per ha over a period of twenty eight years had increased the fixed NH4-N by 0.01 and 0.03 me per 1OOg respectively. The increase in fixed NHd-N corresponds to about 1 per cent of the total applied nitrogen. These results show that the current cropping and fertilization practices had very little effect on the fixed ammonium content of the soil. However, it is expected from the high-fixing capacities of these soils that the amount of fixed ammonium will increase with the increase in frequency and dose of ammonium-nitrogen fertilizers application.

Neither the quantity of fixed NH4 nor the NH4-fixation capacity was highly correlated with the clay content. As can be seen from Table 2, the Tonj sandy loam fixed less ammonium, in comparison to the Gezira soils, than would be expected from its clay content, but the two recent alluvial soils fixed more. In the case of the Tonj sandy loam this lower fixation is attributed to the fact that kaolinite is the dominant clay mineral in this soil. A possible explanation in the case of the recent alluvial soils is that the clay fraction of these soils contain an appreciable amount of a high-fixing clay mineral. The presence of such a mineral is indicated by the high CEC of these soils relative to their clay contents (Table 1). The observed differences between the three groups of soils (Gezira, recent allu-

NHd-FIXATION IN SUDAN GEZIRA SOILS 13

5

4

3

2

1

0

Fig. 1. NHJ-fixation capacities of the soils used in the experiment.

El GEZIRA SOILS

El z$:Z RECENT ALLUVIUM

TONJ SANDY LOAM

1 2 3 4 5 6 7 8 9

SOIL NUMBER

vium, Tonj) is more striking when the results for the total fixed NHa--N (moist condition) are expressed in terms of me per 1OOg of clay (Fig. 1). Correlation of the clay content with the amounts of fixed ammonium is expected to improve if both type and quantity of clay minerals were to be taken in consideration. The ammonium - fixing capacities of the soils show the usual characteristic of greater fixation with increasing degree of drying. The amounts of fixed ammonium in the Tonj sandy loam continued to be low under the three soil moisture ranges tested, as would be expected from its predominant kaolinite content.

The ammonium fixation values of soil samples down to 210 cm depth are presented in Table 3. In both profiles the amounts of

14 M. BAKHEIT SAID

TABLE 3

Clay contents, cation exchange capacities, fixed ammonium and ammonium-fixation capacities of the profile samples

Depth, cm

Clay content,

%

CEC of mineral Matter

me/ 1 OOg (%)*

NHI-fixa- Fixed NH4-N tion cap- acity **

me NHI-N/ me/lOOg (%I 1oog

***

Pernzanent-fallow plot profile

o-5 59.4 52.56 95.4 4.72 0.28 5.9 5-70 59.0 55.46 98.1 5.43 0.28 5.2

70-140 61.0 56.55 92.3 6.83 0.39 5.7 140-210 63.0 55.60 90.9 5.97 0.34 5.7

Cropped plot profile

o-5 60.6 52.56 96.0 4.95 0.28 5-80 61.7 57.64 99.4 5.80 0.29

80-140 62.9 58.00 91.9 7.03 0.40 140-210 63.7 54.74 95.6 6.15 0.35

* Percentage contribution of mineral matter to total CEC. ** Oven-dry condition.

*** Percent values represent proportion of total ammonium-fixing capacity

5.7 5.0 5.7 5.7

fixed ammonium and the ammonium-fixing capacity increased with depth, reached a maximum in the grey layer at a depth of about 70 to 140 cm, and then decreased again. The clay content showed a slight increase with depth in both profiles, but, as reported pre- viously, it was not well correlated with the ammonium fixation. However, the changes in fixation were closely related to changes in the CEC of the mineral fraction of the soil. These results indicate that there are differences in the types or relative proportions (or both) of the different clay minerals between horizons of these soil profiles. The results could be explained on the basis of the earlier finding of Fad1 7 who showed that most of the kaolinite was con- centrated in the coarse clay (2 - 0.2 pm), whereas more than 90 per cent of the fine clay (co.2 pm) consisted of montmorillonite, and that the fine clay increased with depth, reached a maximum at a depth of about 50 to 115 cm and then decreased again. His results also indicated that the depth at which maximum concentration of the fine clay took place varied with locality. As can be seen from the table slightly more ammonium was fixed by the cropped-plot profile

NH4--FIXATION IN SUDAN GEZIRA SOILS 15

samples than by the permanent-fallow plot profile samples. This might be attributed, at least in part, to the higher clay content of the cropped plot profile. The results confirm the finding pointed out above that the current cropping practices had slight effect on the fixed ammonium contents of these soils.

The proportion of the ammonium-fixing capacity (oven-dry condition) which was already satisfied with the fixed ammonium varied from 5.0 to 5.9 per cent. It is also noteworthy that the CEC distributions show that 91.9 to 99.4 per cent of total soil CEC resided on the inorganic matter and the rest on the organic matter. It should be noted, however, that the hypobromite treatment did not comple- tely remove the organic matter of the soils.

TABLE 4

Total organic carbon, Kjeldahl nitrogen, fixed ammonium and C/N ratios of the profiles samples.

Depth cm

Org. c.* Kjeldahl C/N Fixed NHd-N C/N % N, (uncor- (corrected)

PPm rected) mm (%I**

Permanewt-fallow plot profile

o-5 0.43 312 13.8 39 12.5 15.8 5-70 0.43 298 14.4 39 13.1 16.6

70-140 0.57 294 19.4 55 18.7 23.8 140-210 0.23 256 9.0 48 18.8 11.1

Cropped plot profile

o-5 0.46 312 14.7 39 12.5 16.8 5-80 0.43 266 16.2 41 15.4 19.1

80-140 0.60 294 20.4 56 19.0 25.2 140-210 0.27 242 11.2 49 20.2 14.0

* A recovery factor of 0.75 has been used to give these figures. ** Percent values represent proportion of Kjeldahl N.

The carbon and nitrogen contents of the two Gezira profiles are shown in Table 4. As reported by other workers 6 11 1s 13 the pro- portion of the soil nitrogen present as fixed ammonium increased with increasing depth, varying from 12.5 to 20.2 per cent of the total nitrogen. Since the permanent fallow plot has never received any fertilizer application, the ammonium in this soil must have been fixed naturally. Generally the permanent fallow plot contained more

14 NH4-FIXATION IN SUDAN GEZIRA SOILS

total nitrogen but less organic carbon than the cropped plot. As a result of this, the C/N ratios were lower in the permanent fallow plot profile than in the cropped plot profile. In both profiles the C/N ratio increased with depth reaching a maximum in the grey layer below which it narrowed again. As expected the C/N ratio widened when allowance was made for fixed ammonium. It will be noted that the C/N ratios are large because a recovery factor of 0.75, which is quite arbitrary for these soils, has been used to give the organic carbon figures in Table 4.

ACKNOWLEDGEMENTS

Grateful acknowledgement is made to the Director, Agricultural Research Corporation for permission to publish this paper and to Dr. F. Cope for going through the manuscript.

Received September 28, 197 1. Revised May 1972

REFERENCES

1 Ali, M. A., Private communication. 2 Allison, F. E. and Roller, E. M., Fixation and release of ammonium ions by clay

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(1959). 4 Buursink, J., Soils of Central Sudan. Ph. D. thesis, State University Utrecht (1971). 5 Chapman, H. D., Cation exchange capacity. In C. A. Black, (ed). Methods of soil

analysis, Part 2. Agronomy 9, 891-901 (1965). 6 Dhariwal, A. P. S. and Stevenson, F. J., Determination of fixed ammonium in

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the Kenana clay plains of the Sudan. J. Soil Sci. 22, 129-135 (1971). 8 Finck, A., Classification of Gezira clay soil. Soil Sci. 92, 263-267 (1961). 9 Greene, H., Soil profile in the eastern Gezira. J. Agr. Sci. 18, 518-530 (1928).

10 Jewitt, T. N., Gezira Soil. Bulletin No. 12, Ministry of Agriculture, Sudan (1955). 11 Moore, A. W. and Ayeke, C. A., HF-extractable ammonium nitrogen in four Niger-

ian soils. Soil Sci. 99, 335-338 (1965). 12 Rodrigues, G., Fixed ammonia in tropical soils. J. Soil Sci. 5, 264-274 (1954). 13 Stevenson, F. J., Carbon-nitrogen relationships in soil. Soil Sci. 88, 201-208 (1959).