An e-publication by the World Agroforestry Centre
METEOROLOGY AND AGROFORESTRY
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An e-publication by the World Agroforestry Centre |
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METEOROLOGY AND AGROFORESTRY |
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section 6: agroforestry and animals Impacts of sheep grazing on soil properties and growth of rubber (Hevea brasiliensis) N.M. Majid, K. Awang and K. Jusoff
Faculty of Forestry, Universiti Pertanian Malaysia
Abstract The agrometeorologic importance of sheep grazing is receiving increased attention in rubber plantations in Malaysia. Measurements of soil physical and chemical properties, foliar and tree girth of rubber ( Hevea brasiliensis ) in a Rubber Industry Smallholders Development Authority (RISDA) mini-estate at Bukit Mahang, Ketereh in Kelantan showed 15 months of grazing did not cause soil compaction enough to restrict water movement through the soil profile especially during intense rainstorms. All measurements of soil physical properties, namely soil moisture content, particle density, bulk density, total pore space, macropores, micropores, saturated hydraulic conductivity, soil temperature, particle-size distribution, loss on ignition and resistance to root penetration indicated an aggradation at the soil surface (0-10cm depth) due to grazing. Except for K, soil nutrients — namely total N, P, Ca and Mg levels — and soil pH under analysis showed that N, P, Ca, Mg and Na increased with grazing. Potassium, however, decreased with grazing. Trees under grazing have higher girth increment than ungrazed area. The quantification and understanding of these parameters will allow proper agroforestry management practices to be adopted.
Integration of livestock under plantations is not a new form of husbandry. It is an old farming practice and has been carried out by many farmers in Asia and other parts of the world. What is perhaps new is the realization that the integration of livestock and crop can improve productivity per unit area of land. With the present rapid population growth, the allocation of land for agriculture and forestry does not appear to be able to meet the increasing demand for arable land. Thus, increasing productivity per unit area of land is very vital. In Malaysia, the integration of livestock under plantation crops such as rubber (Wan Mohamed and Abraham 1976; Wan Mohamed 1977; Lee, Ng and Goh 1978; and Rubber Research Institute Malaysia 1980), coconut (Selvadurai 1967), and oil palm (Chen et al. 1978) has been shown to be technically feasible and financially profitable. It has been found that such practice provides extra income to the farmers as well as providing effective biological control of weeds. With the introduction of sheep under rubber, the overall cost of weeding can be reduced by 15 to 25% compared to chemical control (Tajuddin 1984; and Rubber Research Institute Malaysia 1985). Apart from the biological and economic advantages, it is believed that the droppings of sheep can improve soil fertility. According to Tajuddin (1984), an adult sheep can produce every day about 186 g (dry matter basis) of manure containing 2.4% N, 0.5% P, 2.9% K, 1.8% Ca, 0.5% Mg, and 507 ppm Na. This amount of manure added to the soil will significantly increase the supply of nutrients for tree growth. A number of workers have also reported that grazing animals can cause changes in the chemical and physical properties of soil. According to Vitosh, Davis and Kaezek (1973), and Wallingford et al. (1975), soil chemical properties, particularly organic matter, soil nitrogen and exchangeable cations increase when manure is applied. Besides providing plant nutrients, manure also improves the physical properties of soil. These include increased aggregate stability of soils, water infiltration and water-holding capacity (Mathers and Stewart 1980; Hall, Hedrick and Keniston 1959; Hedrick and Keniston 1966). There is some evidence that the introduction of livestock in tree-crop areas may improve the yield of the crop.A study made by Fernandez (1970) showed that grazing cattle on pasture under coconuts improved yield from 5,780 to 110,180 nuts per ha over a four-year period. A similar finding was also reported by Nitis and Rika (1978) and Thomas (1978). Other studies also showed that poultry (Wan Mohamed and Kuan 1976; Wan Mohamed and Abraham 1976) and sheep grazing (Tajuddin 1984) under rubber plantation improves growth of rubber trees. Likely reasons for the increase in yield of tree crops were the reduction in weed competition and increased soil nutrient content originating from animal droppings. Although the benefits of animal manure and grazing on crop yields have been well-documented, animal-crop integration may also cause problems. Steinbrenner (1951) and Adams (1975) reported that livestock trampling drastically reduced soil infiltration rate, pore space, aeration, seepage, microfaunal activity as well as causing damage to the tree roots from poaching and decreased organic matter content. Gerald and Hawkins (1978) reviewed the hydrologic impact of grazing on infiltration. They reported that grazing effects on runoff, erosion, on-site water use, and consequent downstream impacts are especially obvious and of great concern. As these processes are affected by grazing, so are nutrient cycles, soil moisture patterns, erosion and sediment yields, downstream water quality, and productivity. Investigations by Leaf (1958) in southern Wisconsin showed that the weight and nutrient content of the litter were much lower in grazed stands and that soils in grazed stands were more acid and lower in organic matter, exchangeable K, Ca, and Mg. On the other hand, Lund, Doss and Lowry (1975) and Elliot and Stevenson (1977) reported that surface application of manure can pollute streams by contaminating runoff water. Heavy application of manure may produce forage with excessive nitrate levels or equivalent ratios of K/(Ca + Mg) that are detrimental to animal health. Excessive loading will often cause nitrate contamination of the ground water or excessive salt accumulation in the soil. There is little evidence in Malaysia that sheep grazing under rubber plantations is a problem. Moreover, little information has been reported concerning the impacts of grazing on soil characteristics and growth of rubber trees. Such information is important for better management of the agricultural land resource.. The objective of this paper is to examine the impacts of sheep grazing in a rubber plantation on the physical and chemical properties of the soil and the growth of the rubber trees.
Description of the study area The study was conducted in a RISDA estate at Bukit Mahang, Ketereh (latitude 5° 58'N, longitude 102° 16' E) in the state of Kelantan, Malaysia (Figure 1).
The area is characterized as having a tropical humid climate. Mean annual rainfall ranges from 3,048 to 3,302 mm. The wet season starts around November to January with the arrival of the north-east monsoon. The dry season lasts from February to August. Temperature is high and uniform. Mean annual temperature is 24 °C. The area has a mean annual relative humidity of about 96%. Soils of the study area have a loamy-sand texture. Detailed characteristics and historical background of the study area are given in Table 1.
Experimental design Two treatment plots (grazed and ungrazed) 50 m x 100 m in size were randomly established at the study site. Repeated observations were taken at several sites within the each plot.
Soil sampling was done in November 1986. Soil from 0 to 10 cm depth was randomly sampled from the areas between the rows of the rubber trees, using ELE soil augurs. These disturbed samples were used to determine chemical properties. For the determination of physical properties, 20 undisturbed soil cores were also randomly collected from each site at 0 to 10 cm depth with metal core rings 7.6 cm in diameter and 4 cm high. A depth of 10 cm was chosen because variabililty in soil properties was greater in the surface layer than in the lower zones (Gent, Ballard and Hassan 1983). Both ends of the cores were fitted with plastic covers to prevent loss of water by evaporation.
Soil temperature Soil temperature was measured at a depth of 5 cm with soil probe thermometers. Readings were taken three times a day for one week at each site and the average for the day was recorded.
A CL-700 Soil Test Inc. pocket penetrometer was used to measure resistance to root penetration on the grazed and ungrazed plot. The resistance to penetration test is commonly used to measure soil strength or root penetration resistance since soil engineering tests for shear strength, such as the triaxial and direct shear tests, have limited use due to the larger number of samples and tests required to obtain an adequate degree of precision in structured soils (Bradford 1980). Twenty replicated samples of each probe were taken in each of the treatment plots.
Tree girth at 0.5 cm height was used to measure growth on one hundred randomly-selected trees in each plot.
Leaf samples were collected as composite samples from three to five trees. For every selected tree, 20 leaves were randomly sampled. Twenty composite leaf samples were collected from each treatment plot.
Soil chemical properties The soil chemical properties determined were total N, P, K, Ca, Mg, Na and soil pH. Total N in the soils was determined by the micro-Kjeldahl method (Mackenzie and Wallace 1954); and available P by Deniege's method using a Spectronic 20 spectrophotometer. Exchangeable cations K, Ca, Mg, and Na were extracted with 50 ml ammonium acetate from a 10 g soil sample. Their concentrations were then determined with an atomic absorption spectrophotometer.
Soil physical properties determined were soil moisture content, particle density, bulk density, total pore space, macropores, micropores, loss on ignition and particle-size distribution. Moisture content was determined by the gravimetric method after oven-drying at 105 °C for 24 hours. Soil particle density was determined using a picnometer. Bulk density was calculated from oven-dry weight and measured volume of each core. Total pore space was calculated as: 100 x (1 - bulk density/particle density). Large-pore space (macropores) equalled the volume of water drawn from the saturated cores at tensions up to 60 cm of water. Small-pore space (micropores) was calculated as the difference between total porosity and large-pore space. Loss on ignition was determined using a high temperature kiln at 750 °C for two hours. Particle-size distribution of samples was determined by the pipette method.
The composite leaf samples were washed with distilled water and kept in paper envelopes before drying in a forced-draft oven at 65 °C for two days. The dry samples were then ground using a stainless Fritch pulverisette mill and then passed through a 1 mm sieve. Ground leaf samples of 0.25 g were subjected to rapid wet digestion (Thomas, Sheard and Moyer 1967) using concentrated sulphuric acid and hydrogen peroxide in the ratio 5:3 for the analysis of N, P, K, Ca, Mg and Na. These elements were determined by procedures similar to those used for the soil samples.
There is no replication in this experiment. The repeated observations within each plot do not constitute true replications. However they do indicate the level of repeatability of observations within the two plots. As much standard deviations are indicated in each table. However they should not be used to test for statistically sigificant differences due to the applied treatments. Table 2. Plot differences in physical properties of soil.
Soil physical properties Results are presented in Table 2. Total pore space, saturated hydraulic conductivity, resistance to penetration, and loss on ignition show large differences between the two plots, and these observations could be expected to be altered by grazing. The weather at Bukit Mahang, Ketereh, in November 1986 was wetter than normal. It was raining heavily when soil sampling was carried out. Thus, high soil moisture contents were recorded in both grazed and ungrazed plots. No difference in soil temperature was recorded between grazed and ungarzed plots. This may have been due to shading from the rubber trees. Particle-size distribution was relatively uniform among samples collected from the same depth. The percentage of clay was slightly higher in the grazed plot. This may indicate that gully erosion had taken place in the bare areas under rubber. Both grazed and ungrazed plots have a loamy-sand texture which may be associated with poor structure, loose consistency, excessive drainage and thus with low water-retention (Donahue 1977). Although particle-size distribution and particle density (or specific gravity) are usually unaffected by grazing per se as found in this study, they may be altered indirectly. Smith (1940) reported, for instance, that the proportion of clay in the surface soil increased as erosion progressed under intense grazing. Thus, similarity in texture and particle density between soils with and without sheep suggests that gully erosion was not accelerated sufficiently to affect particle size and density at the early stage of grazing. Due to the beating action of intense rains with large drop-size during soil sampling, both plots were subjected to compaction. In comparison with no grazing, grazing increased bulk density by 1% in the surface layer (Table 2). This increase was very small compared to those reported in other studies, especially those in North America. For example in South Dakota, surface soils averaged 1.22 g/cm in shelterbelts frequented by livestock, but only 1.01 g/cm where livestock was excluded (Read 1957). Bulk density of A horizon soils in the Allegheny River watershed, averaged 0.92 g/cm for the ungrazed woodlands, as compared to 0.52 g/cm (Trimble, Hale and Potter 1951). Animal concentration was unspecified in both instances. On Oklahoma range, the 10-to-15 cm layer averaged 1.72 g/cm in ungrazed exclosures (Rhoades et al. 1964); differences were even greater at the 30-to-60 cm depth. Heavy grazing consisted of stocking throughout the year at one animal per 5 ha. They found that 20 years of light grazing increased bulk density of a loamy fine sand to the 7 cm depth. Reduction in large pores of the surface soil layer on grazed land is readily understandable (due to animal treading) but reasons for responses of small-pore (micropore) space to grazing are less apparent. The increase in micropores under grazing, coupled with the sharp reduction in macropores, suggests a transformation of large pores into small pores. Van der Weert (1974); Dickerson (1976); Kamaruzaman and Muhamad (1986b); and Kamaruzaman, Muhamad and Desa (1986) reported that when forest soil is compacted following logging activities, total porosity is reduced at the expense of the large voids. Hence, the proportion of micropores increased because micropores were relatively less affected by compaction (Kamaruzaman Desa 1988; Kamaruzaman and Muhamad 1986a, 1986b). Saturated hydraulic conductivity (a measure of infiltration) was reduced by grazing (Table 2). During a one- hour period, 26.9 cm of water entered the soils on the grazed plot as compared to 54.2 cm on the ungrazed. In their review, Gifford and Hawkins (1978) showed considerable evidence of reduction in infiltration caused by animal trampling. A reduction in infiltration rate of grazed area may be due to the removal of vegetative cover crops from the soil and compaction upon grazing. Wilkinson and Ania (1976) reported a high rate of infiltration (25.4 cm/hr) into sandy soil (10% clay) under tropical forestfallow; and reduction of the rate of infliltration to 9.1 cm/hr after two consecutive years of maize cropping. Infiltration rate was reduced in this case because of the compaction effect by the maize roots. Grazed soil showed an increase in resistance to penetration (a measure of soil strength or bearing capacity) compared to ungrazed soil. The increase in soil strength that occurs as a result of grazing indicates that the soil is compacted by sheep trampling due to the large hoof pressure of sheep when walking with two to three hooves on the ground which could raise the hoof pressure to about 220 kPa. Another factor is that the hoof (estimated to have a pressure of 83 kPa) is not necessarily placed flat on the soil surface, again tending to increase the applied pressure. For comparison, the value obtained with a forestry tractor is 30 to 150 kPa (Sohne 1958). Table 3. Plot differences in soil chemical properties.1
Root growth may be inhibited as soil strength increases. There are many laboratory studies showing this effect; see for example, Gradwell (1968). Significant reduction in root growth could take place as penetration resistance increases by between 20% and 30% for the grazed plot as compared with the ungrazed plot. However, in this study, resistance to penetration increased by 26% but showed no pronounced effect on growth of rubber trees in the grazed plot, the reason being the abundance of sheep manure which provided fertilizer.
The mean values of the chemical analyses are shown in Table 3. On the average, total N and available P were higher in the grazed plot. The increase in total N was probably due to the increased output of organic matter from the sheep manure. In the case of available P, the increase could be attributed to the organic part of the manure retarding P fixation by mechanically separating soluble P from the mineral part of the soil. The higher level of the exchangeable cations (Ca, Mg and Na) in the grazed plot compared to the non-graze plot was most likeley due to the added manure and its sub- sequent decomposition. The decrease in K of the grazed soil may be due to the rduction in the water-soluble K which caused excessive salinity as a result of the addition of sheep manure. It is also likely that the highly mobile K is lost by leaching, especially in the loamy sand, since heavy rain showers were experienced during the taking of soil samples. Table 4. Plot differences in some foliar nutrients and girth difference.
Except for K, the other foliar minerals analysed showed an increase in the grazed compared to ungrazed plot (Table 4). The increase in P, Ca and Mg uptake is, however, small compared to N and Na under grazing. The variability of nutrient availability for uptake by the rubber trees under grazing may be due to the variation in the amount of sheep manure in the soil and leaching losses. Potassium is less available for tree uptake in the grazed area because of the lower amount of K as explained above. The tendency for higher nutrient uptake by the rubber trees in the grazed plot is evident from their larger girth increment (Table 4). Apart from the addition of organic matter deposited by the sheep, suppression of weeds by grazing led to lesser competition for nutrients and water.
The data suggest that 15 months of grazing slightly compacted soils under rubber as a result of trampling or treading. Nevertheless, sheep grazing improved soil fertility through the addition of nutrients to the soil. Grazing also reduced weed competition for soil and nutrients. This resulted in increased nutrient uptake by the rubber trees, thereby enhancing their growth. Future research should be directed towards the study of the relationships between latex production, stocking rate and intensity, change in pasture composition, and time of grazing with the presence of sheep under rubber plantations.
This study was supported by Universiti Pertanian Malaysia and Rubber Industry Smallholders Development Authority (RISDA). We wish to thank Nik Hassan Sulaiman, Director of RISDA, and his officers, for their assistance. The authors also express their appreciation to Nisanto Masrie and Muzammal Johan for their assistance in soil and foliar analyses.
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