PROFILE JUNAID NAWAB
COLUMN FOR THE RECORD
ENVIRONMENT A SOIL CANCER "SALINITY" AND PLANT GROWTH
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A SOIL CANCER "SALINITY" AND PLANT GROWTH

 

The serious threat to maintaining adequate food supply

 

By S M Alam
Jan 13 - 19, 2003
 
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Wide spread soil salinity and ever increasing human population are the serious threat to maintaining adequate food supply. It has been opined that in future this phenomenon will require cultivation of salt tolerance species and efforts to check further spread of salinity. Studies were directed towards identifying suitable species/cultivars of leguminous/non-legume crops of importance rhizobia associated with N-fixation, trees, shrubs, which could withstand moderate salinity levels. Repeated plantings of various species in saline lands, failed to achieve a satisfactory growth of plant communities. The situation became worse when such fields remain inundated for several months after the monsoon rains. Presence of salinity had more adverse effect on N-fixation by leguminous plants than on their vegetative growth. A substantial amount of nitrogen was anyway fixed even under these hostile conditions. It was concluded that limited success may be achieved in reclaiming saline soils with EC less than 6, but a shallow water table less than 1.5m deep may prove a major hurdle in achieving this goal and may make the task of maintaining productivity even more difficult. Plant requires salts in the root medium for growth. When the concentration of these salts reach such levels that it is harmful for growth salinity is said to have developed.

Saline soils normally contain neutral soluble salts comprising of chlorides and sulphates of sodium, calcium and magnesium. Most of these soils occur in arid/semi-arid zones, but they generally possess good physical condition and permeability due to flocculating effect of neutral salts. Sodic/saline sodic soils on the other hand, have high pH and exchangeable sodium. They have greatly impaired physical and chemical condition due to de-flocculating effects of sodium on the soil, i.e. surface crusting, compaction of sub-soil, reduced filtration/hydrantic conductivity. These soils are worldwide occurrence (Table 1).

Pakistan is no exception where, according to an estimate (Table 2), about half of the irrigated area is affected by salinity/sodicity/waterlogging. Yet another report shows it to be about 80 per cent of the total arable lands. The economic loss is assessed to be in the region of about 1.44 billion rupees annually. The cultivation of salt tolerant species and amelioration of salt-affected lands are the two logical approaches. The threshold for salt tolerance, especially in our common field crops is generally low and there comes a stage when economic returns are no more feasible. This makes utilization and amelioration of such lands imperative.

TABLE 1.
WORLDWIDE OCCURRENCE OF SALINE/SODIC SOILS (MILLION HECTARES).

 

Total area

Affected

% of total

North America

2137.80

17.72

0.83

South America

1753.47

129.16

7.37

Africa

2963.63

80.54

2.72

Australia

788.66

357.33

45.30

Europe

472.96

50.80

10.74

South Asia

678.02

84.83

12.50

North and Central Asia

1103.01

211.69

19.19

South East Asia

897.62

19.98

2.23

Total

10795.17

952.05

8.82

 

 

 


 

TABLE 2.
STATISTICS OF PAKISTAN (MILLION HECTARES)

TOTAL LAND AREA OF PAKISTAN = 80 MILLION HECTARES (MHA).

Province

 

Cultivated Irrigated

Saline/sodic/ water logged

(Total)

(Total)

(Report 1)

(Report 2)

Punjab

11.04

9.19

2.88

3.28

Sindh

5.45

3.15

3.13

3.06

NWFP

1.62

0.65

0.60

0.61

Balochistan

1.18

0.48

- -

Total

19.29

13.47

6.61

6.95

 

 

Whereas the saline soils can be improved by simple leaching, with sodic soils having pH above 8.5 the process is more difficult. Here the exchangeable sodium is replaced with calcium before leaching. Leaching can, however, be affected where water table is not shallow and soil is easily permeable. If feasible, lowering of such water tables by laying tile drains or sinking tube wells for pumping out the water will greatly help. These are costly approaches, hence beyond the reach of every farmer, and can not cover the entire affected area. Even under a most efficient system of management, it may also not be possible to get rid the soil completely of salts, which may, anyway, rebuild in the absence of necessary precautions. This requires measures to check the capillary rise of salts. Providing a vegetative cover on these lands, reduces evaporation from the soil surface, which is the main cause of this upward movement of salts. Such lands should hence not be left fallow for any extended period.

Identification of appropriate gene pools is a prerequisite for gainful utilization of saline soils. Our efforts on screening for salt tolerance have identified many plant species/cultivars, which show minimum depression in growth under saline conditions. Some salient findings of these efforts are given as. The studies on rice, wheat and soybean were conducted in large cemented tanks filled with river sand, which could be irrigated from separate reservoirs containing nutrient solutions of desired salinity. A reasonable check of pH and EC could hence, be maintained. Screening of trees/shrubs was done under field conditions, while other species were tested in pots. Nitrogen fixation in legumes is achieved through symbiotic association with rhizobia. A wide range of variation is known to exist in salt tolerance of different strains. If more tolerant strains of rhizobia, suited for particular conditions, are isolated, or their salt tolerance limit could somehow be enhanced, these better adapted strains may subsequently be used to induce tolerance in otherwise sensitive hosts.

Screening of other crops of importance identified a number of tolerant and sensitive cultivars of cotton (Gossypium hirsutum, G. arboreum); mustard (Brassica campestris, Brassica juncea); maize (Zea mays), sorghum (Sorghum vulgare); millets (P. typhoides) and some other grasses. Acacia ampliceps, Atriplex lentiformis, Atriplex amnicola have shown best tolerance, followed by Acacia stenophylla, Acacia machonocheane, Acacia nilotica, Eucalypfus microtheca, Casuarina glauca, Conocarpus lancifolius, while the growth of Acacia auriculiformis, A. salicina, A. victoriae, Eucalyptus occidentalis, E. camaldulensis, Cassia sturtii, Azadirachta indica has not been satisfactory. Planting on ridges, application of N, P-fertilizers and mulching with wheat straw resulted in better performance in the field.

The reclamation of saline/sodic soils could be achieved through physical, chemical and biological measures. These soils may be improved physically by land leveling and sub-surface loosening, mulching with sand or straw and lowering of water table through drainage. The chemical approach makes use of gypsum to replace sodium, particularly in sodic soils, followed by leaching. It may be even more beneficial for the land owners to ameliorate these lands during gainful utilization hence the need to go for biological measures. It is generally observed that under our conditions, the farmers keep cultivating a land till it gives them economic returns and, if salinity creeps in, they move to a better area. These abandoned lands deteriorate further with time because removing vegetative cover of any land, let alone a saline one, causes further degradation. The micro-environment of vegetation is helpful in checking spread of the menace. Besides, the roots open the soil and improve permeability and the organic matter of the decomposing roots and the litter fall enrich the soil. Survey of literature shows that periodic cultivation of alfalfa, clover, sesbania, berseem, lucerne, deep incorporation of these (i.e. green manuring) and cattle manure has helped in improving saline lands. Gypsum may be necessary for amelioration of sodic soils. The treatments generally resulted in decrease in EC, soil bulk density, clay dispersion, increase in permeability (pore space) and soil water content which ultimately increased yields from such soils.

 

 

The vegetation was strongly influenced by the salinity of the soil. The intensity of the species present at low salt levels, gradually decreased and other species became dominant with increasing substrate salinity. These latter species were almost absent at lower salt levels suggesting that they required certain salts for their optimum growth. It was observed that the texture is predominantly clay and the clay content increases as we go down the profile. The water table stands at a depth of nearly 2 meters. As generally common in such areas, the salt content is highly variable with a range between 2 and 18 mS. The salt contents reduce with depth. The pH is around neutrality and never exceeds 8.00. Na and Ca contents are quite high and K is low. All these ions are also higher in upper horizon than the lower. The low salt plots generally have uniformly distributed salts upto the depth of 60 cm, whereas in soils with higher salt, the concentration is more on the surface than deeper down.

It may also be worth mentioning here that those soils difficult to reclaim, which had shallow and highly saline (20000 to 60000 ppm) underground water. These conditions are very similar to our field site. It has also been observed that even at low salt levels drilling the seed like normal agriculture may often not result in good germination. It is advisable in such situations, to broadcast the seed in standing water. This dilutes the salts and relieves the stress at germination and early seedling stages. It may, however, later create problems in cultural operations. For better survival and growth, trees and shrubs should be planted on raised beds at nearly one third of the way below the peak. Mulching with straw and adequate fertilizer application may help seedling establishment and survival.

The availability of nitrogen is generally an important limitation on yields in these situations. Experiment, using N15 labeled fertilizer has shown that a substantial amount of nitrogen was fixed by the legumes. This could hence supplement the existing N-pool and help in improving the fertility. Alternatively, a 20-25 per cent higher dose of nitrogen (preferably in the form of ammonium sulphate) over that recommended for normal soils, should be applied. At high pH values of sodic soils, availability of calcium is often reduced. This necessitates amendment with gypsum, but phospho-gypsum and iron pyrite are also used. Addition of farmyard manure enhances the efficacy of gypsum. In some cases, sulphuric acid has been applied to lower the pH. This of course requires extra precaution due to corrosive action of the acid. Where shallow water table is not a problem, rice cultivation helps in improving these soils. Closer spacing higher plant population and use of older seedlings, i.e. either weekold instead of six week at transplanting, yield better under such conditions. With low delta crops, light but frequent irrigation is recommended. Large scale adoption of above practices requires concerted efforts by the relevant agencies. Lowering of water table through tile drains and tube wells are costly. Even if they are installed, disposal of underground water of very bad quality is another problem.

The cost of the ameliorates may not be a major constraint. Gypsum is already being supplied at subsidized rates. Its proper use has to be taught to the farmers, as it may not be required everywhere and where needed, the application should be according to the requirement of the soil. This involves technical advice on not only the quantity and right manner of application but also on correct choice of crop, cropping sequence, cultural practices and other necessary precautions. Biological amelioration should be encouraged as it is the most cost-effective method. There is however, a need to identify species suited for specific sites and establish demonstration plots. This should be a joint venture involving all the relevant agencies, i.e. agricultural departments (research and extension), foresters, universities and other research institutes. Keeping in view the vast expanse of affected lands in the country, the task seems enormous but so also are the gains.