The role of prosopis cineraria in an agropastoral system in western Rajasthan*

H.S. Mann and K.A. Shankarnarayan

Central Arid Zone Research Institute, Jodhpur

Influence on soil fertility conditions

Nodulation pattern

Soil moisture regime under p. Cineraria

Effect of P. Cineraria on the productivity of range grasses

Yields of leaf fodder and its chemical composition

Concluding remarks

Summary

References

Prosopis cineraria (Khejri) is a ubiquitous tree in the vast alluvial plains of Western Rajasthan and constitutes the principal component of the Desert Thom Forest (Champion, 1936). Although it has a wide ecological amplitude, its optimum density (150-200 per ha) and vigour is found in the districts of Sikar, Churu and Nagaur (300-400 mm rainfall). Its density decreases on both sides of this rainfall zone. Prosopis cineraria is found on soil types ranging from sandy loam to clay loam and flat plains to sand dunes.

The very fact that P. cineraria occurs and is encouraged by farmers to grow in all cultivated fields and village grazing grounds shows that its usefulness has been generally accepted by the farmers who have a strong conviction, based on generations of experience and observations, that vegetative growth under P. cineraria is better than in other areas under the same management conditions. The tree yields brushwood for fencing, top feed for cattle and timber for household and food from the fruits during famine years to tide over scarcity periods. For these reasons P. cineraria (Khejri) is known as Kalpa Vraksha and is not only protected every where but is also worshipped as a deity (Kailash Singh, 1960).

Influence on soil fertility conditions

In order to study the scientific reasons why crop growth under Khejri is better than the adjoining open areas, Singh and Lal (1969) investigated the profile characteristics, including levels of fertility, under both Prosopis cineraria and Acacia nilotica and compared them with those of open field conditions. They attributed the better crop growth under P. cineraria to higher organic matter status, total nitrogen, available O₂O₅, soluble calcium and lowest pH values (Table 1). The calcium carbonate content is sufficiently low in soils under P. cineraria as compared to open field soils. The soluble calcium (CaO) increased up to the fourth sampling depth in open fields. The combined values of CaCO₃ and CaO are maximum in P. cineraria. Again, although the O₂O₅ content generally decreased with increasing depth in both cases, the volume of the O₂O₅ content was greater under Prosopis than in the open. However only K₂O was higher in open field soils compared with P. cineraria.

Table 1. Mechanical and chemical composition of soils (expressed as percentage on oven dry basis).

Source: K.S. Singh and P. Lal 1969, Annals of Arid Zone, 8: pp 33–36.

With regard to mechanical composition of soils, it will be seen that the sand percentage decreases and silt + clay increases up to the depth of 120 cm in P. cineraria and at subsequent depths the trend is reversed, while in open fields there is a regular decrease in sand and increase in silt + clay up to 90 cm depth, below which the trends are not consistent.

These results suggest that the leaching is comparatively higher under P. cineraria trees which resulted in the transference of silt + clay to the lower depths in these profiles, whereas in open field conditions, the movement of silt + clay is restricted up to the depth of 90 cm.

The lower pH value of soils under Prosopis according to Singh and Lal (1969) was attributed to the presence of a higher content of organic matter, soluble calcium and comparatively lower content of CaCO₃.

The higher accumulation of organic matter under Prosopis was obviously due to the vigorous growth of vegetation which in turn left behind more organic matter in the form of dead roots and leaf litter. This also explains the higher content of total nitrogen and available P₂O₅ ,and K₂O under Prosopis.

Soil fertility studies under fourteen-year-old tree communities of P. cineraria, P. juliflora and bare sites, indicated (Tables 2 and 3) that there was an overall improvement in the soil organic matter, total nitrogen, total P₂O₅ and available macro and micro-nutrients under the P. cineraria community compared with P. juliflora on bare sites.

Table 2. Soil chemical characteristics under tree communities and bare fields.

Source: R.K. Aggarwal et al (1976) Indian Forester 102, No. 12, pp 863–872.

Table 3. Available macro and micro nutrient contents in soils under different tree plantations.

Source: R.K. Aggarwal et al (1976) Indian Forester 102, No. 12. pp. 836-872.

The data (Table 3) on DTPA extractable micro-nutrients for 0–15 cm and 15–30 cm depths indicated that available Zn, Mn, Cu and Fe contents vary from 0.08 to 2.28 ppm, 6.9 to 11.7 ppm, 0.26 to 1.28 ppm and 2.4 to 4.8 ppm respectively. Available Mn and Cu contents were highest in both the soil depths under P. cineraria while the Fe content was highest in the 0–15 cm soil depth and the Zn content in the 15–30 cm soil depth.

Aggarwal et al (1975) concluded that in general the growth of tree plantations and P. cineraria in particular, improved the available micro-nutrient status of the soil.

This view is further supported by studies conducted by Aggarwal et al (1976) on soil physico-chemical changes under twelve-year-old tree plantations in Western Rajasthan. Results (Figures 1, 2 and 3) show that organic matter, total nitrogen and P₂O₅ was highest in Prosopis cineraria at 0-15 cm depth compared with other tree species and bare sites. This is also reflected in the highest number of herbaceous plant species/m², mean plant density/m² and mean ground phytomass (Table 4).

Table 4. Plant density, number of species and above-ground phytomass

Source: R.K. Aggarwal et al (1976). Indian Forester, 102, No. 12, pp. 863-87.

Figure 1. Depth-wise distribution of soil organic matter under different tree plantations

Figure 2 . Depth-wise distribution of total soil N under different tree plantations.

Figure 3. Depth-wise distribution of total Soil P₂O₅ under different tree plantations.

Nodulation pattern

Basak and Goyal (1975) conducted in vitro nodulation studies of the native rhizobia on P. cineraria and P. juliflora present in the soils of the IARI Farm, Delhi. Their results showed that number of nodules/plant were higher on the lateral roots than on tap roots. The pattern and extent of nodulation was the same on both the tree species. It was further reported that the rhizobia from these legumes were akin to those of the cowpea group.

The microbial population in terms of bacteria and actinomycetes was higher in surface soils of P. cineraria in comparison to soils of bare sites.

Soil moisture regime under p. Cineraria

In view of its deep root system extending to more than 10 m, the fluctuations of the upper soil moisture have hardly any influence on the internal moisture balance of Prosopis cineraria and obviously the water balance of the upper soils is largely regulated by the evapotranspiration of the shallow-rooted herbaceous ground cover (Lahiri, 1965).

Comparative study on the soil moisture status in the 120 cm soil profile of different tree communities indicated that the moisture regime remained generally high under P. cineraria and Tecomella undulata compared with other species like P. juliflora, Albizzia lebbek and Acacia senegal. The soil moisture ranged from 27 to 50 mm during the rainless period from November to June and 115 to 140 mm during July to September under P. cineraria.

The interrelationship between soil moisture and growth of grass cover in a natural community of Prosopis cineraria trees at Jodhpur was measured under field conditions (Lahiri, 1965). Figure 4 shows the status of soil moisture up to the depth of 200 cm for the period May to October 1963. It may be observed that the fluctuations in soil moisture due to showers during this period were primarily restricted to about 50 cm depth from the surface. At these depths, soil moisture was below 1% from May to the middle of July. In general, moisture content varied from 1 to 2% between depths of 50 to 150 cm. Above 150 cm depth the moisture level was above 2%. Moisture content above 30% was observed only in small pockets around 175–200 cm depth with indications of higher soil moisture at greater depths.

Figure 4. Chrono-isoplets indicating the soil moisture status during the period between May to October, 1963.

The chrono-isoplets of soil moisture during the dry period from November 1963 to June 1964 (Figure 5) indicated that the moisture content, except during sporadic showers, hardly ever exceeded 2% up to the depth of 125 cm. These data on soil moisture regimes suggest that the plants with limited root growth can only grow during the short rainy season, the wilting point of this soil being about 2 to 2.5%.

Figure 5. Chrono-isoplets indicating the soil moisture status during the dry period extending from November, 1963 to June, 1964.

Effect of P. Cineraria on the productivity of range grasses

The experimental evidence relating to grass yields as affected by P. cineraria was reported by Shankar et al (1976). The dry matter yields of range grasses under Prosopis was found to be 2.3 t/ha as against 1.66, 1.32, 0.85 and 0.78 t/ha under Tecomella undulata, Albizzia lebbek, Prosopis juliflora and Acacia senegal respectively. A marked improvement in botanical composition and vigour of major grasses under Prosopis was observed. The sown Cenchrus ciliaris pastures showed gradual and significant differences in plant height, tussock diameter and forage yields as the distance from Prosopis trees increased.

Yields of leaf fodder and its chemical composition

As already mentioned, Prosopis cineraria is an important source of top feed during lean months from December to June. A moderate sized tree yields about 45 kg of dry leaf fodder locally known as 'loong'. Table 5 (Institute of Agriculture, Arond, Guyarat, 1960) indicated that the nutritional status of the leaves was most favourable in the winter season when the trees were generally lopped.

Table 5. Percentage nutrient contents in Prosopis cineraria in different seasons

Source: Progress Report of the Indian Council of Agricultural Research for Western Regional Animal Nutrition Centre (1960). Institute of Agriculture, Anand, Gujarat.

Concluding remarks

From the foregoing, it may be concluded that improvement of crop or plant growth under this tree can be attributed to the combined actions of a number of factors. Its favourable influence is clearly discernible in pearl-millet fields in sandy plains where Prosopis trees are found in abundance. However, quantified data on the yields of pearl millet under Prosopis are lacking and there is a need for obtaining systematic data in this regard. The effect of Prosopis on the forage yields of range grasses is, however, comprehensive enough. Soil moisture being a serious limitation, its subtle improvement below this tree may be considered as a direct cause for vigorous crop growth and yield increase. The stem flow enrichment of soil moisture and the non-exploitation of surface moisture by Prosopis which inherently has a profuse deep root system, encourages luxuriant growth of vegetation which obviously contributes towards organic matter build-up and fertility improvement below the tree over time as is amply borne out by soil analysis data. The shade effects coupled with microbial influence may perhaps hasten fertility build-up and minimise losses, particularly soil nitrogen. Apart from conferring such beneficial effects on soil moisture, and fertility build-up, the leaf fodder from this tree, which is very nutritious, is available for the sustenance of livestock at a period when no green grass is available for grazing. Although the important role of Prosopis cineraria in farm forestry systems is well recognized, there is a need for quantified data on the input/output from this system in order to draw up an overall balance sheet.

Summary

Prosopis cineraria is an ubiquitous tree in the sandy plains of Western Rajasthan. Investigations of the causes of better crop growth under this tree revealed that this is apparently due to higher organic matter status, total nitrogen, available P₂O₅ and soluble calcium and low pH values. The sand content decreases and silt + clay increases up to 120 cm depth. The micronutrient status generally increases. The moisture status under Prosopis remains generally high and this is perhaps attributable to the stem flow from these trees. While the fluctuation in soil moisture was restricted to 50 cm depth, it varied from 1 to 2% at 50 to 150 cm.

The dry matter yield of range grasses under Prosopis was highest, 2.3 t/ha compared with other trees. Apart from augmenting the fertility status and soil moisture which result in better growth of crops and vegetation, the leaves of Prosopis constitute a valuable nutritious fodder which becomes available at a period when no green grass is available for grazing.

References

Aggarwal, R.K. et al (1975).'Studies on soil physico-chemical and ecological changes under twelve year old five desert tree species of West Rajasthan'. Indian Forester, 102 (12) 863–872.

Basak, M.K. S.K. Goyal. (1975). 'Studies on tree legumes I. Nodulation pattern and characterisation of symboint'. Ann. Arid Zone, 14 (4): 367–370.

Champion, H.G. (1936). 'A preliminary survey of the forest types of India and Burma'. Indian For. Res. (N.S) Silv. 1–286. Forest Research Institute, Debra Dunn.

Institute of Agriculture, Anand, Gujarat. (1960). 'Progress Report of the Indian Council of Agricultural Research for Western Regional Animal Nutrition Centre'.

Lahiri, A.N. (1965). 'Some aspects of soil water relationship in the growth of plants in arid environment'. In: Growth and Development of Plants. Proceedings of the seminar held at Department of Botany, Punjab University, Chandigarh. New Delhi. Today & Tomorrow's Book Agency.

Shankar, V., N.K. Dadhich, S.K. Saxena. (1976). 'Effect of Khejri tree (Prosopis cineruria McBride) on the productivity of range grasses growing in its vicinity'. Forage Research 2: 91–96.

Sigh, Kailash, (1958). 'Farm forestry in Rajasthan'. Proceedings of Farm Forestry Symposium. ICAR, New Delhi.

Singh K.S. P. Lal. (1969). 'Effect of Khejri (Prosopis spicigera L) and Baool (Acacia arabica) trees on soil fertility and profile characteristics'. Annals of Arid Zone. Vol. 8, No. 1, pp. 33–36.