Vertical Distribution of Magnesium in the Laterite Soils of South India
High levels of exchangeable potassium or ammonium can interfere with the uptake of magnesium by crops. This antagonism is a major concern in some tea growing soils with low magnesium. Soil samples were collected from tea fields and a nearby forest, at various depths. The pH of tea soil ranged between 4.60 and 4.88 and the values were lower when compared to the forest soils. This could be due to the continuous application of nitrogen containing fertilisers. Water soluble magnesium content was higher in forest area when compared to the tea fields which could be attributed to the application of magnesium in the form of carbonate once in four year. Total magnesium was estimated in both cultivated and forest soils and the amount of total magnesium was higher in forest area when compared to the cultivated area. The total magnesium content in forest area ranged between 370 and 1550 mg kg-1, while in tea soils it ranged between 228 and 968 mg kg-1. The available magnesium content ranged between 10 and 40 mg kg-1 in tea soil and 20 and 70 mg kg-1 in forest soil. Water soluble magnesium had negative correlation with pH of the tea soils. There existed a linear relationship between water soluble magnesium and electrical conductivity of cultivated soils. The magnesium status of tea soils is generally low when compared to the forest area due to continuous exploitation of this nutrient by tea plants. This study confirmed that there is need for soil application of magnesium fertilizer in tea fields to preserve the magnesium status.
South Indian tea soils are classified as latosol and it contains kaolinite
and gibbnite as predominant clay minerals with small amount of illite. In geological
origin, the soils are mainly derived from gneissic rocks containing lot of mica.
Tea is mono culture crop which had been planted in the forest soil and there
is feeling that the fertility of the old forest soils is being depleted by tea
cultivation. Soil magnesium is a moderately leachable nutrient. Compared to
calcium, greater amounts are often found in the subsoil than in the upper parts
of the soil profile, especially older, highly weathered soils. High levels of
exchangeable potassium or ammonium can interfere with magnesium uptake by crop
plants. This antagonism is a major concern in some tea growing areas with low
magnesium soils. About 150 to 300 kg of K2O is added every year to
overcome potassium deficiency and to obtain sustainable yield (Verma
and Palani, 1997; Venkatesan, 2006). The removal of
magnesium through leaf harvesting is 6 to 8 times lesser than that of potassium,
the higher potassium input strengthens the antagonism between magnesium and
potassium leading to magnesium deficiency which is observed in many tea field
in South India (Venkatesan, 2006). The correction is done
through foliar application of magnesium sulphate along with zinc sulphate, manganese
sulphate, boric acid and naphthalene acetic acid (Verma and
Palani, 1997). This study is expected to provide the basic information as
to what level the tea cultivation has influenced the magnesium nutrient present
in soils, when compared to the forest soils. The information available on magnesium
distribution is very much limited in tea, while there are many attempts under
various other crops (Mathan, 1991; Garg
et al., 2003; Cornfield and Pollard, 2006).
The data will be useful in understanding the magnesium fertiliser requirements.
The present study attempts have also been made to study the distribution and
interrelationship of different forms of soil magnesium influenced by tea cultivation
practices in comparison to the nearby forest soils.
MATERIALS AND METHODS
Study site and soil sampling: In South India tea is grown in Latosols
(Oxisols and Ultisols) in the humid regions of Western ghats, close to the west
coast of peninsular India, at latitudes varied between 8° and 13° N
elevation ranged between 600 and 2800 m above the mean sea level, the annual
rain fall ranged between 900 and 7500 mm. The soil profile was excavated at
three places in the tea fields at 500 m apart. The resulting trench was 2x2
m with depths of 2 m. Eight soil samples were taken by gently scratching each
trench wall at the following depth intervals like 0-25, 25-50, 50-75, 75-100,
100-125, 125-150, 150-175 and 175-200 cm. Samples were collected at each depth
at three places, pooled and mixed thoroughly by hand on a polythene sheet. At
the same time the soil profiles were excavated at three places inside the forest
situated nearer to the tea field. Soil samplings were done as in the case of
tea fields. The bulk quantity of the sample was reduced by quartering method
to make one composite sample. The samples were air dried and passed through
2 mm sieve. The experiment was conducted between 2007 and 2008 at UPASI Tea
Research Foundation, Valparai.
Physicochemical properties: Soil pH was measured in 1:2 soil water mixture
using Orion pH meter (Orion, 950) by Schofield and Taylor
(1955), electrical conductivity was measured using conductivity meter (Systronics,
304) by Mason and Obenshain (1939) method, organic matter
by Wakley and Black (1934) procedure, available phosphorus
by Bray and Kurtz (1945) method, potassium by Hanway
and Heidal (1952) Cation Exchange Capacity (CEC) and bulk density by Bhargava
and Raghupathi (2001).
Different pattern of magnesium: The remaining soil samples were analysed
for different forms of magnesium namely water soluble magnesium, 1 N ammonium
acetate extractable magnesium (i.e., available magnesium) and extractable by
1 N boiling nitric acid. Exchangeable magnesium was determined by subtracting
from water soluble magnesium (Jackson, 1977). Non exchangeable
magnesium was determined by subtracting 1 N HNO3 magnesium from available
magnesium (Wood and de Turk, 1940). Total magnesium
was determined by sodium carbonate fusion method (Jackson,
1977). Lattice magnesium was determined by subtracting total magnesium from
non-exchangeable magnesium (Jackson, 1977). The amount
of magnesium present in the soil solution was estimated by an Atomic Absorption
Spectrophotometer (GBC Avanta 908 AA).
Statistical analysis was carried out by the standard method (Gomez
and Gomez, 1984). The correlation coefficient was carried out using the
software SPSS 12.1 for windows.
Physico chemical properties of tea and forest soils: The pH of the tea
soils ranged between 4.60 and 4.88 and the values are lower when compared to
the forest soils (Table 1, 2). The electrical
conductivity of cultivated soil ranged between 0.02 and 0.05 dS m-1
which was greater than that of forest soils. Organic matter content was estimated
in both cultivated and forest soils. The results showed that the forest soils
are having slightly higher organic matter content when compared to the tea soils.
Organic matter content decreased with increase in soil depth. Further the cation
exchange capacity, an interrelated parameter to organic matter, showed a similar
trend like organic matter. The bacteria and fungai count of tea soils are lower
when compared to the forest soils. The microbial count generally decreased with
increase in soil depth. The bulk density of forest soil ranged between 1.03
and 1.20 g cm-1 and the values were higher when compared to the tea
soils. An irregular trend was observed in both the soils. The exchangeable potassium
content of soils under tea plantations was significantly higher than that of
forest soils up to 50 cm profile. The available phosphorus was estimated in
both cultivated and forest soils with respect to soil depth.
|| Physico-chemical characteristics of tea soils
|SEM±: Standard Error Mean; CD: Critical difference;
EC: Electrical conductivity; CEC: Cation exchange capacity; OM: Organic
matter; ND: non detectable
|| Physico-chemical characteristics of forest soils
|SEM±: Standard error mean; CD: Critical difference;
EC: Electrical conductivity; CEC: Cation exchange capacity; OM: Organic
matter; ND: non detectable
|| Vertical distribution of different forms of magnesium distribution
in tea and forest soils
|SEM±: Standard error mean; CD: Critical difference;
Mg: Magnesium; T: Tea soils; F: Forest soils
Results showed that the tea soils were having comparatively higher P content.
Available calcium content was higher in forest area when compared to tea fields.
It ranged between 57 and 81 mg kg-1 in tea soils and between 60 and
92 mg kg-1 in forest soils.
Different pattern of magnesium in tea and forest soils: Water soluble magnesium content was higher in forest area when compared to the tea fields. It ranged between 1.7 and 6.4 mg kg-1 in tea soils and between 3.8 and 9.0 mg kg-1 in forest soils (Table 3). It decreased with increase in soil depth. The available magnesium content of tea soils ranged between 10 and 40 mg kg-1 and in forest soils it ranged between 20 and 70 mg kg-1. It also decreased with increase in depth.
Both the soils were having comparable amount of non-exchangeable magnesium content. It varied between 90 and 352 mg kg-1 in tea soils (Table 3) and between 109 and 371 mg kg-1 in forest soils. The exchangeable magnesium content was lower in cultivated soils compared to the forest soils and varied between 8 and 34 mg kg-1 in tea soils and between 16 and 61 mg kg-1 in forest soils.
In the top layer of soil profile the total magnesium content in forest soils was almost two fold higher when compared to the tea soils (Table 3). The top layer (0 to 25 cm) and the next profile (25 to 50 cm) had similar quantum of total magnesium, but beyond that depth the values gradually decreased. Total magnesium content varied between 228 and 968 mg kg-1 in tea soils and between 370 and 1550 mg kg-1 in forest soils. Lattice magnesium was determined by subtracting the total and non-exchangeable magnesium. It was higher in forest soils when compared to the tea soils. It varied between 138 and 616 mg kg-1 in tea soils and between 261 and 1180 mg kg-1 in forest soils.
Different forms of magnesium and their relationship with physical properties:
Water soluble magnesium of cultivated soils showed positive and significant
correlation coefficients with EC, organic matter, cation exchange capacity,
exchangeable and non-exchangeable magnesium (Table 4), while
the same parameter observed in forest soils had positive correlation with different
forms of magnesium only and did not show any significant relationship with physical
properties of the soil. Water soluble magnesium has negatively correlated with
pH of the tea soils (Table 4), but in the case of forest soils
no significant correlation (Table 5). Exchangeable magnesium
content of cultivated soils showed positive correlation with available Mg, water
soluble Mg, non-exchangeable Mg, total Mg, EC and OM content and negative correlation
with soil pH. In the case of forest soils the available Mg has shown non-significant
correlation with pH. Available magnesium content of cultivated soils had positive
and significant relation with EC, CEC and OM. A negative correlation coefficient
was obtained for available magnesium and pH. But in the forest soils have shown
the positive correlation coefficients (Table 5). Lattice magnesium
showed positive and significant correlation with other forms of magnesium and
physical properties of cultivated soils and a negative correlation with soil
pH. But the forest soils have shown positive correlation with soil pH (Table
5). Lattice magnesium had a positive and significant correlation with other
forms of magnesium and poor correlation with pH and EC in the case of forest
soils. Similar kind of trend was observed in exchangeable Mg, non-exchangeable
Mg and total Mg (Table 4, 5).
|| Correlation coefficients among soil properties and different
forms of magnesium in tea soils
|*Significantly at 5% level; **Significantly at 1% level. EC:
Electrical conductivity; OM: Organic matter; CEC: Cation exchange capacity
|| Correlation coefficients among soil properties and different
forms of magnesium in forest soils
|*Significantly at 5% level; **Significantly at 1% level. OM:
Organic matter; CEC: Cation exchange capacity; EC: Electrical conductivity
Continuous applications of fertilisers have impacted two important physico
chemical properties of tea soils viz., pH and electrical conductivity. The decrease
in pH could be due to the application of acid forming nitrogenous fertilisers,
while the ions dissociated in soil moisture could have contributed for increase
in conductivity. The results are confirmative to the reports available at TRI
(Ranganathan and Natesan, 1985; Venkatesan
et al., 2003). The organic matter content of tea soil was slightly
lower than forest soils due to continuous removal of nutrients by harvesting.
On the other hand the forest systems accumulate organic matter at the top soils
by littering process, while translocating nutrients from deeper horizons through
deep root system (Hirekurabar et al., 2000; Verma
and Venkatesan, 2001). The removal in forest eco system is rather low. Due
to the same reason, at any given depth organic matter content of tea soil was
lower than the forest soils. Organic matter has been proved to be directly proportional
to the microbial population of soil (Klose and Tabatabai,
2000). Due to this reason, same kind of trend was observed in microbial
count also. Its decrease with increase in soil depth could be attributed to
the addition of leaf litter on the surface soils (Subba-Rao,
1995). The higher levels of potassium content observed in cultivated soils
could be because of application of potassium fertilisers to achieve commercial
levels of production (Venkatesan and Murugesan, 2006).
The exchangeable potassium content generally decreased with increase in soil
depth. This could be due to the addition of leaf litter on the surface soil
which releases liable K from organic residues and also due to upward translocation
by capillary rise of ground water (Hirekurabar et al.,
2000). In many depths the phosphorus content of tea soils was higher than
the forest soils, which could be due to addition of citric acid along with rock
phosphate (Venkatesan and Murugesan, 2004; Venkatesan,
As mentioned earlier dolomitic lime is the main source of magnesium, which is applied in tea fields as liming materials. It contains 12.6% of Mg in the form of MgCO3, which is not fully soluble in water. It normally takes a long time to become available form of Mg. This could be the main reason for obtaining low water soluble Mg in tea soils. Exchangeable Mg contents were lower in tea soils which indicates the fact of exhausted nature of cultivated soils due to continuous harvesting of tea shoots.
The same phenomenon could have affected the available Mg of tea soils which
was lower than forest soils. Since the soil available Mg was proved to be directly
proportional to the exchangeable and non-exchangeable form of Mg (Chu,
1985; Mathan, 1991; Berkowitz
and Wu, 1993; Cofie and Pleysier, 2004; Cornfield
and Pollard, 2006), the trend observed in available Mg was seen in other
two forms also. Since, tea soils are proved to have higher total potassium than
the forest soils (Jayaganesh and Venkatesan, 2006; Venkatesan
and Murugesan, 2006) and since they are antagonistic to each other, the
total magnesium content of forest soils was higher than tea soils. Similar kind
of observations were reported by Garg et al. (2003),
Komosa et al. (1999), Khan
et al. (1997) and Ashraf and Biddappa (1994)
in various soils. The decrease in all forms of magnesium distribution in both
the soil with increase in depth was probably due to variation in degree of leaching
of magnesium and fluvial characteristics of soils (Ashraf
and Biddappa, 1994; Mathan, 1991).
As mentioned elsewhere, 2000 to 3000 kg of dolomite lime was applied once in
pruning cycle to enhance the soil pH. Such a higher quantity could have increased
the conductivity behaviour in combination with fertiliser applications. Because
of this, different forms of Mg content positively correlated with Electrical
Conductivity (EC) of cultivated soils (Table 4). The insignificant
correlation coefficients revealed by forest soils could be due to lack of external
addition of Mg in any form. Similar trend was observed by Komosa
et al. (1999) and Khan et al. (1997).
Same kind of trend observed in non-exchangeable and total magnesium in both
the soils. This could be because of the fact that the distribution pattern of
total magnesium at different depths is mostly governed by parent materials and
physiographic characteristics of the soils.
Intensive applications of potassium can increase the leaching of Mg by displacing it from cation exchange sites so that the amount in the root zone is reduced. Calcium and potassium can also interfere with the root uptake of Mg by competition at the root surface even when available Mg is not reduced by application of these essential elements. Hence, the mining of magnesium might have been severe for several decades. Resultantly, the reservoir is appearing to be exhausted. In nutshell, different form of magnesium content in tea soils were lower when compared to the forest soils which shows that continuous cultivation of tea without magnesium fertiliser led to serious depletion of soil magnesium pool. Therefore the study advocates to enrich the soil Mg status.
The authors thank to Director for his constant encouragement and support during the course of study. The authors are thanking to Dr. N. Muraleedharan for appreciation and critical evaluation of the manuscript. We also acknowledge Mr. N. Palani for critically evaluating this manuscript. The financial assistance provided by the Department of Science and Technology (DST) is gratefully acknowledged.
Berkowitz, G.A. and W. Wu, 1993.
Magnesium, potassium flux and photosynthesis. Magnesium Res., 6: 257-265.PubMed | Direct Link |
Bhargava, B.S. and H.B. Raghupathi, 2001.
Analysis of Plant Materials for Macro and Micronutrients. In: Methods of Analysis of Soils, Plants, Waters and Fertilizers, Tandon, H.L.S. (Eds.). Fertiliser Association and Consultation Organization, New Delhi, pp: 49-82
Bray, R.H. and L.T. Kurtz, 1945.
Determination of total, organic and available forms of phosphorus in soils. Soil Sci., 59: 39-46.CrossRef | Direct Link |
Chu, C.H., 1985.
Relationship between exchangeable and total magnesium in Pennsylvania soils. Clays Minerals, 33: 340-344.Direct Link |
Cofie, O.O. and J. Pleysier, 2004.
Ion exchange involving potassium-calcium and magnesium-calcium in soil and organic matter fractions. Commun. Soil Sci. Plant Anal., 35: 2417-2431.CrossRef | Direct Link |
Cornfield, A.H. and A.G. Pollard, 2006.
The relative rates of release of potassium, calcium and magnesium from soils during electrodialysis. J. Sci. Food Agric., 3: 613-615.CrossRef | Direct Link |
Garg, V.K., P.K. Singh and K.K. Singh, 2003.
Distribution of nutrients in soils and plants of Sikkim. J. Indian Soc. Soil Sci., 51: 208-211.
Gomez, K.A. and A.A. Gomez, 1984.
Two Factor Experiments. John Wiley and Sons Inc., New York, pp: 84-129
Hanway, J.J. and H. Heidal, 1952.
Soil analysis methods used in Iowa state soil testing lab. Agronomy, 57: 1-31.
Hirekurabar, B.M., T. Satyanarayana, P.A. Sarangmath and H.M. Manjunathaiah, 2000.
Forms of potassium and their distribution in soils under cotton based cropping system in Karnataka. J. Indian Soc. Soil Sci., 48: 604-608.
Jackson, M.L., 1977.
Soil Chemical Analysis. Prentice Hall of India Private Limited, New Delhi
Jayaganesh, S. and S. Venkatesan, 2006.
Magnesium adsorption characteristics of South Indian tea soils. J. Plantn. Crops, 34: 286-289.Direct Link |
Khan, S.H., R.L. Bingham and P. Felker, 1997.
Micronutrient, magnesium and phosphorus effects on biomass and leaf tissue nutrient concentrations of field grown Leucaena leucocephala
. Agrofor. Syst., 37: 157-173.CrossRef | Direct Link |
Klose, S. and M.A. Tabatabai, 2000.
Urease activity of microbial biomass in soils as affected by cropping systems. Biol. Fert. Soils, 31: 191-199.CrossRef |
Komosa, A., E. Pacholak, A. Stafecka and W. Treder, 1999.
Changes in nutrient distribution in apple orchard soils as the effect of Fertigation and irrigation. II: Phosphorus, potassium and magnesium. J. Fruit Ornam. Plant Res., 7: 72-80.
Mason, D.D. and S.S. Obenshain, 1939.
A comparison of methods for the determination of soil reaction. Soil Sci. Soc. Am. J., 3: 129-137.CrossRef | Direct Link |
Mathan, K.K., 1991.
Magnesium distribution pattern in a soil toposequence in Doddabetta series of Nilgiris. J. Indian Soc. Soil Sci., 39: 368-370.Direct Link |
Ashraf, P.M. and C.C. Biddappa, 1994.
Adsorption characteristics of magnesium in four coconut growing soils in west coast. J. Ind. Soc. Soil Sci., 42: 236-239.
Ranganathan, V. and S. Natesan, 1985.
Potassium Nutrition in Tea. In: Potassium in Agriculture, Munson, R.D. (Ed.). ASA, CSSA, SSSA, Madison, WI USA., pp: 981-1022
Schofield, R.K. and A.W. Taylor, 1955.
The measurement of soil pH. Soil Sci. Soc. Am. J., 19: 164-167.CrossRef | Direct Link |
Subba-Rao, N.S., 1995.
Biofertilisers in Agriculture and Forestry. Oxford and IBH Publication Co., New Delhi, pp: 1-50
Venkatesan, S. and S. Murugesan, 2004.
Phosphorus fixing capacity of tea soils of Anamallais and Nilgiris. J. Plantation Crops., 32: 63-67.
Venkatesan, S. and S. Murugesan, 2006.
Influence of tea cultivation on soil characteristics with special reference to potassium. Int. J. Soil Sci., 1: 58-63.CrossRef | Direct Link |
Venkatesan, S., D.P. Verma and M.N.K. Ganapathy, 2003.
Targetted yield equation of nitrogen for clonal teas under south Indian conditions. J. Indian Soc. Soil Sci., 51: 178-183.Direct Link |
Venkatesan, S., 2006.
Notes and Amendments to the Recommendations on Manuring of Tea in South India. UPASI Tea Research Institute, Valparai, Tamil Nadu
Verma, D.P. and S. Venkatesan, 2001.
Organic manures in tea. The Planters Chronicle, 97: 111-119.
Verma, D.P. and N. Palani, 1997.
Manuring of tea in South India (Revised recommendations). In: Handbook of Tea Culture, UPASI Tea Research Institute, Valparai, Tamil Nadu, Section 11, pp: 33.
Wakley, A. and I.A. Black, 1934.
Determination of organic carbon in soils. Soil Sci., 37: 27-38.
Wood, L.K. and F.E. de Turk, 1940.
The absorption of potassium in soil in non-replaceable forms. Soil Sci. Soc. Am. Proc., 5: 152-161.