REVIEW OF LITERATURE
Intercropping of cereals and grain legumes is a neglected theme in agricultural science and practice in both conventional and organic farming systems (Dahlmann, and Von Fragstein2006). The fast rising population in many tropical countries is one of the reasons for enormous growing demand for food. The increasing urbanization due to world growing population has affected food production leading to irrevocable loss of arable land. Opening up new land for cultivation can enhance the decrease of agriculture. Farmers and researchers should be conscious that cost-benefit ratio bringing new land under cultivation is smaller than that of increasing production of already cultivated land, which may lead to increase in production per unit area.
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Intercropping tenders farmers the opportunity to engage nature’s principle of diversity on their farms. Spatial arrangements of plants, planting rates, and maturity dates must be considered when planning intercrops. Intercrops can be more fruitful than growing monocropping. Many different intercrop systems have been studied, including mixed intercropping, strip cropping, and customary intercropping provisions. Pest management benefits can also be realized from intercropping due to augmented diversity. Harvesting options for intercrops include hand harvest, machine harvest for on-farm feed, and animal harvest of the standing crop. Most grain-crop mixtures with similar ripening times cannot be machine-harvested to produce a marketable commodity since few buyers purchase mixed grains. Dispite its advantages intercropping is neglected due to complex nature of intercropping systems.
In intercropping systems an LER measures 1.0, it tells us that the amount of land required for crops grown together is the same as that for these grown in pure stand (i.e., neither loss nor loss due to intercropping over pure stands). LERs above 1.0 demonstrate an advantage to intercropping, while numbers below 1.0 diplay a disadvantage to intercropping. For example, an LER of 1.25 tells us that the yield produced in the total intercrop system would have required 25% more land if planted in pure stands. If the LER was 0.75, we know the intercrop yield was only 75% of that of the same amount of land that grew pure stands.
Pakistan is a subtropical country having sufficient resources with high intensity of sunlight required for plant growth. Therefore, possibility of intercropping of different crops on the same piece of land in a year needs to be explored for effective and efficient utilization of these natural resources. Intercropping is being looked as an efficient utilization of these natural resources and economical production system as it increases the production per unit area and time. Presently, interest in intercropping is increasing among the small growers because of their diversified needs and meagre farm returns from the monocropping system.
Planning of cropping system should be done yearly on entire catchment basis. The type of planning should lead to a proper balance between food, fiber and fodder crops. When the rainfall is between 500-700 mm with a distinct period of moisture surplus, intercropping system should be adopted for improved crop production. Even in higher rainfall areas (750- 1100 mm) intercropping facilitates growing either cereal-legume or legume-legume system of different maturity patterns. Intercropping minimize risk of crop failure in drylands. Mixed cropping (mixing seeds of two or more crops and broad casting the mixture) should be avoided as it hinders post-sowing operations. Choice of varieties with in the crops is very important to harness total intercropping advantage. Cereal-legume intercropping systems should be advocated to minimize fertilizer use,.? reduce pest and disease incidence,?? produce balance foods, ?provide protein rich legume fodder for cattle,? take full advantage of growing season.
Cereal-legume intercropping plays an important role in subsistence food production in both developed and developing countries, especially in situations of inadequate water resources (Tsubo et al., 2005). Intercropping cereals and grain legumes can be very potential for both organic and conservative farmers. The use of land equivalent ratio (LER) as a measure for calculating the cropping advantage of intercrops over sole crops is simple, ignoring weed inhibition, yield reliability, grain quality, and minimum advantageous yield are all relevant factors for farmers’ perspective (Prins and de Wit 2005).
Intercropped legumes secure most of their nitrogen from the atmosphere and not compete with maize nitrogen resources (Adu-Gyamfi et al. ,2007). Increased diversity of the physical structure of plants and increased leaf cover in an intercropping system facilitates to reduce weed infestations once crop are established (Beet1990). Having a variety of root system in the soil reduces water loss, enhances water uptake and reduce transpiration. The increased transpiration may make the microclimate cooler, which cools the soil and decrease evaporation (Innis 1997). In this way during times of water stress, intercropped plants utilize a larger percentage of available water from the field than monocropped plants. Creating windbreaks may also modify the microclimate. Rows of maize in a field with a short stature crop would reduce wind speed above the shorter crop and thus deceasechance of desiccation (Beet1990). Intercropped legumes fix most of their nitrogen from the atmosphere and not compete with maize for nitrogen resources (Adu-Gyamfi et al. ,2007; Vesterager et al.,2008).
Diversification of cropping systems, i.e. smaller fields and mixtures of crop species (intercropping) was much more in vouge Pre World War II. Intercropping, the simultaneously cultivation of more than one species in the same field, is a cropping method, which often result in a more efficient use of resources, cause more stable yields in problematic environments and a method to reduce problems with weeds, plant pathogens and nitrogen losses post grain legume harvest.In this context a greater introduction of longterm rotations, intercrops and grain legumes play an important role (Jensen 1997; Karlen1994). Intercropping of cereal and legume crops facilitates to maintain and improve soil fertility (Andrew, 1979).
Intercropping of legumes with cereals has been popular in tropics (Hauggaard-Nielsen et al.,, 2001; Tsubo et al.,,2005) and rain-fed tracts of the globe (Banik et al.,, 2000; Ghosh, 2004; Agegnehu et al.,, 2006; Dhima et al.,,2007) due to its benefits for soil conservation (Anil et al., 1998), weed control (Poggio, 2005; Banik et al.,,2006), lodging resistance (Anil et al.,, 1998), yield enhancemnent (Anil et al.,, 1998; Chen et al.,, 2004), hay curing, forage preservation over pure legumes, more crude protein percentage and protein yield (Qamar et al.,, 1999; Karadag and Buyukburc, 2004), and contols legume root parasite infections (Fenandez-Aparicio et al.,,2007).
Different seeding ratios or planting patterns for cereal-legume intercropping have been accomplished by many researchers (Tsubo et al.,, 2001; Karadag and Buyukburc, 2004; Banik et al.,, 2006; Dhima et al.,, 2007). Competition among mixtures is thought to be the major characteristic affecting yield as compared with monocropping of cereals. Species or cultivar selections, seeding ratios, and inter and intra specific competition among mixtures may influence the growth of the species grown in intercropping systems in rain-fed areas (Santalla et al.,, 2001; Karadag and Buyukburc, 2004; Carr et al.,, 2004; Agegnehu et al.,, 2006; Banik et al.,, 2006; Dhima et al.,, 2007).
Various competition indices such as land equivalent ratio (LER), relative crowding coefficient (RCC), competitive ratio (CR), actual yield loss (AYL), monetary advantage(MI) and intercropping advantage(IA) have been anticipated to portray competition within and economic advantages of intercropping systems (Banik et al.,, 2000; Ghosh, 2004; Agegnehu et al.,, 2006; Banik et al.,, 2006; Dhima et al.,, 2007). However, such indices have not been used for maize and common bean intercropping to determine the competition among species and also economic advantages of each intercropping system in the East Mediterranean region. Higher monetary returns were obtained compared to sole cropping when bush beans intercropped with sweet maize (Santalla et al.,, 2001). Higher seed yield and net income under planting pattern with changing mix-proportions may be explained in higher total productivity under intercropping with relatively less input investment (Banik et al.,, 2006).
Tsubo et al., (2005) formed a simulation model to find out the best planting methods for maize and bean intercrops in sub-arid South Africa. Based on 52 years of weather data, they compared the best planting time, optimal water saturation at planting, maize plant density, and bean plant density to receive the highest LER, energy value (EV), and monetary value (MV) from the intercropped field. For every combination of factors, a LER greater than 1.0 was found, indicating that intercropping of maize and beans increases total yield. The simulations show that initial soil water content has the greatest influence on intercropping productivity. Bean plant density had no influence on maize or bean yields, indicating that maize yield is not affected by bean intercropping, although bean yields were decreased in the intercropped system (Tsubo et al., 2005). High densities of maize maximized maize yield and calorie production, but high densities of beans maximized financial return. Decline of external inputs and increases of homegrown feed together with a more efficient nutrient use from leguminous symbiotic dinitrogen (N2) fixation (SNF) can result in a decrease of nitrogen and mineral losses. Maize-legume intercropping systems are able to lessen amount of nutrients taken from the soil in comparasion to a maize monocrop.
Organizing the complication of exchanges that are possible due to the physical constraints of diversity are present in the farm system is vital part of reducing the need for external inputs and moving toward sustainability (Herrera, 1974). Increasing diversity often allows better resources use efficiency in agro ecosystem because with higher diversity, there is larger microhabitat differentiation, allowing the components species and varieties of the system to grow in an environment ideally fitting to its unique requirements (Mazaheri and Oveysi, 2004; Willey and Reddy1981; and Yancey, 1994). A key and straight way of rising diversity of an agro ecosystem is intercropping system that allows interaction between the individuals of the different crops and varieties (Mazaheri, 2004; Willey, 1981 and Venkatswarlu1981).
Intercropping can add temporal diversity through the sequential planting of different crops during the same season (Yancey, 1994). Importance of multiple cropping is increasing world food supplies. An LER value of 1.0, indicating no difference in yield between the intercrop and the collection of monocultures (Mazaheri and Oveysi, 2004 and Kurata 1986). Any Value greater than 1.0 indicates a yield advantage for intercrop. A LER of 1.2 for example, indicates that the area planted to monocultures would need to be 20% greater than the area planted to intercrop for the two to produce the same combined yields (Laster and Furr,1972). Intercropping in cassava was beneficial in increasing the biological yield, tuber equivalent yield and land use efficiency. Cassava tuber equivalent yield, LER, ATER and AHER were higher in cassava + cowpea combinations.(Amanullah et al., 2006). Mixed culture (or intercropping) of legumes and cereals is an old practice in tropical agriculture that dates back to ancient civilization. The main objective of intercropping has been to maximum utilization of resources such as space, light and nutrients (Willey, 1990; Morris and Garrity, 1993; Li et al.,, 2003b), as well as to improve crop quality and quantity (Nel, 1975; Izaurralde et al.,, 1990; Mpairwe et al.,, 2002).
Other benefits include water quality control through least use of inorganic nitrogen fertilisers that pollute the environment (Crew and Peoples, 2004). The contemporary drift in global agriculture is to search for highly productive, sustainable and environmentally safe cropping systems (Crew and Peoples, 2004). This has resulted into renewed interest in cropping systems research (Vandermeer, 1989). When two crops are grown in association, interspecific competition or facilitation between plants may take place (Vandermeer, 1989; Zhang et al.,, 2003).Different studies have shown that mixtures of cereals and legumes produce higher grain yields than either crop grown unaccompanied (Mead and Willey, 1980; Horwith, 1984; Tariah and Wahua, 1985; Ofori and Stern, 1987a; Lawson and Kang, 1990; Watiki et al.,, 1993; Peter and Runge-Metzger, 1994; Skovgard and Pats, 1999; Rao and Mathuva, 2000; Olufemi et al.,, 2001; Mpairwe et al.,, 2002; Dapaah et al.,, 2003). In such crop mixtures, the yield increases were not only due to enhanced nitrogen nourishment of the cereal component, but also to other unexplored causes (Nel, 1975; Connolly et al.,, 2001).
Many of the unknown and less research processes occur in the rhizosphere of mixtures (Connolly et al.,, 2001; Zhang et al.,, 2003, 2004). The rhizosphere soil is the narrow zone of soil neighboring the roots where soil, micro-organisms and roots jointly play key roles in the soil ecosystem. Compared with the bulk soil, the rhizosphere has diverse biological, physical and chemical soil properties. It is rich in root exudates, and, therefore, play a major role in nutrient mobilisation and microbial activities (Dakora and Phillips, 2002; Dakora, 2003). So far however, little attention has been paid to rhizosphere effects on crops grown in mixtures (Connolly et al.,, 2001; Zhang et al.,, 2003; 2004), where interaction between different organisms is high. The major management practices employed in mixed cultures to attain good yield includes the enhancement of microclimatic conditions, improved utilisation and recycling of soil nutrients, improved soil quality, provision of favourable habitats for plants and stabilisation of soil, among others (Juma et al.,, 1997).
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Most of intercropping systems are intentionally made and manipulated to optimise the use of spatial, temporal, and physical resources both above-and belowground, by maximising positive interactions (facilitation) and minimizing negative ones (competition) among the components (Willey and Osiru, 1972; Willey, 1979; Mead and Willey, 1980; Horwith, 1985; Ofori and Stern, 1986, 1987a, b; Jose et al.,, 2000; Silwana and Lucas, 2002). An understanding of the biological and chemical processes and mechanisms involved in the distribution of resources in such systems is indispensable. The complex interactions in legume/cereal cropping systems such as those used by traditional farmers have received little research attention (Connolly et al.,, 2001; Zhang et al.,, 2004) because quantitative rhizosphere studies in the field involving complex mixtures are notoriously complex and cumbersome. These conditions are achieved by manipulating management practices such as planting patterns of the mixtures with the selection of appropriate cropping systems.
Interactions will occur in the growth process, especially when the component species are exploiting the resources above-and below-ground (Vandermer, 1989; Willey, 1990; Ong et al.,, 1996) from the same niche or at the same time. In crop mixtures, any species utilizing the same combination of resources will be in direct competition. However, based on differences in phenological characteristics of species in asocition, the interaction among them may lead to an increased capture of a limiting growth resource (Willey and Osiru, 1972; Willey, 1979; Mead and Willey, 1980; Horwith, 1985; Ofori and Stern, 1986, 1987a,b; Silwana and Lucas, 2002) and then amassing larger total yield than the collective production of those species if they were grown separately on an equivalent land area (Mead and Ndakidemi 2527 Willey, 1980; Horwith, 1984; Tariah and Wahua, 1985; Ofori and Stern, 1987a; Lawson and Kang, 1990; Watiki et al.,, 1993; Peter and Runge-Metzger, 1994; Myaka, 1995; Asafu-Agyei et al.,, 1997; Skovgard and Pats, 1999; Rao and Mathuva, 2000; Olufemi et al.,, 2001; Dapaah et al.,, 2003). Thus, mixed cropping systems between cereals and legumes may face a complex series of inter- and intra-specific interaction (Izaurralde et al.,, 1990; Giller and Cadisch, 1995; Evans et al.,, 2001; Li et al.,, 2003c) geared by modifications and utilisation of light, water, nutrients and enzymes.
Most annual crop mixtures such as those involving cereals and legumes are grown almost at the same time, and develop root systems that acquire the same soil zone for resources (Horwith, 1984; Chang and Shibles, 1985a,b; Reddy et al.,, 1994; Jensen et al.,, 2003). Under such circumstances, below-ground competition for resources such as nutrients is most likely to take place. For example, research has shown that activities in maize + cowpea intercropping take place between the top 30-45 cm of soil, and their intensity decreased with depth (Maurya and Lal, 1981; McIntyre et al.,, 1997). Because of these interactions, cowpea yields can be reduced significantly in relationto that of maize (Watiki et al.,, 1993).
In contrast to some negative effects on yield, root systems in mixtures may provide some of the major favorable effects on soil and plants. These include, amongst others, carbon enrichment through higher carbon return (Ridder et al.,, 1990; Vanlauwe et al.,, 1997), discharge of phenolics, phytosiderophores and carboxylic acids as root exudates by companion plants (Dakora and Phillips, 2002; Dakora, 2003). These compounds play a major role in the mineral nutrition of plants. For instance, some studies have displayed that, in P-deficient soils, pigeon pea roots utilize piscidic, malonic, and oxalic acids to solubilise Fe-, Ca- and Al-bound P (Ae et al.,, 1990). Once mobilised, P and Fe then become available for uptake by the pigeon pea plant as well as by plant species grown in association and micro flora in the cropping system.This is due to the fact that, thus far, research efforts on mixed cultures has centered on the intra- and inter-specific competition for light and water, and research reports on competition for nutrients in legumes and cereal mixtures (Connolly et al.,, 2001; Zhang et al.,, 2003, 2004). It is, therefore, imperative to discover how the rhizosphere systems of the associated plant species in mixtures interact under different legume-cereal cropping systems.
Rhizospheric pH changes in different management systems in legume/cereal mixtures
Many plants have the ability to alter the pH of their rhizosphere (Hoffland et al.,, 1989, 1992; Raven et al.,, 1990; Degenhardt et al.,, 1998; Muofhe and Dakora, 2000; Dakora and Phillips, 2002) and improve nutrient availability such as P, K, Ca, and Mg, which are otherwise fixed and not available to plants (Vandermeer, 1989; Hauggaard- Nielson and Jensen, 2005). For instance, legumes induce numerous reactions that modify the rhizosphere pH (Jarvis and Robson, 1983; McLay et al.,, 1997; Tang et al.,, 1998, 2001) and influence nutrient uptake (Brady, 1990; Vizzatto et al.,, 1999). For example, Dakora et al., (2000) have shown that due to pH changes in the rhizosphere, Cyclopia genistoides, a tea-producing legume native to South Africa, increased nutrient availability in its rhizosphere by 45 – 120% for P, 108 – 161% for K, 120 – 148% for Ca, 127 – 225% for Mg and 117 – 250% for boron (B) compared to bulk non-rhizosphere soil. Hence, legumes may take up higher amounts of base cations, and in the process of balancing internal charge, release H+ ions into the rhizosphere that results in soil acidification (Jarvis and Robson, 1983; McLay et al.,, 1997; Tang et al.,, 1998, 2001; Sas et al.,, 2001; Dakora and Phillips, 2002; Cheng et al.,, 2004).
Other legumes such as alfalfa, chickpea, lupines, and cowpea can release considerable amounts of organic anions and lower their rhizospere pH (Liptone et al.,, 1987; Dinkelaker et al.,, 1989, 1995; Braum and Helmke, 1995; Gilbert et al.,, 1999; Neumann et al.,, 1999; Rao et al.,, 2002; Li et al.,, 2004b), a condition favorable for the hydrolysis of organic P and hence improving P2O5 nutrition for plants and micro organism in the soil. In the same context, white lupine (Lupinus albus) exuded organic acids anions and protons that lowered rhizosphere pH and recovered substantial amount of P2O5 from the soil and made them more available to wheat than when it was grown in solitary cropping system (Horst and Waschkies, 1987; Kamh et al.,, 1999). Similarly, pigeon pea increased P2O5 uptake of the intercropped sorghum by exuding piscidic acid anions that chelated Fe3+ and subsequently released P2O5 from FePO4 (Ae et al.,, 1990). In a field trial, faba bean facilitated P2O5 uptake by maize (Zhang et al.,, 2001; Li et al.,, 1999, 2003b; Zhang and Li, 2003). In another comparative study, the ability of chickpea to mobilise organic P2O5 was shown to be greater than that of maize due to greater exudation of protons and organic acids by chickpea in relation to maize (Li et al.,, 2004a). Thus, in mixed cultures, plants such as cereals, which do not have strong rhizosphere acidification capacity can benefit directly from nutrients solubilised by legume root exudates. What is, however, not clearly known is the extent of rhizosphere pH changes in mixed cultures involving nodulated legumes and cereals and their influence on other biological and chemical processes in the soil.
N2 FIXATION IN LEGUMES AND THE ASSOCIATED
BENEFITS TO THE CEREAL COMPONENT
Biological nitrogen fixation by grain legume crops has received a lot of attention (Eaglesham et al.,, 1981; Giller et al.,, 1991; Izaurralde et al.,, 1992; Giller and Cadisch, 1995; Peoples et al.,, 2002) because it is a considerable N source in agricultural ecosystems (Heichel, 1987; Dakora and Keya, 1997). However, studies on N2 fixation in complex cereal-legume mixtures are few (Stern, 1993; Peoples et al.,, 2002). Intercropping usually includes a legume which fixes N2 that benefits the system, and a cereal component that depends heavily on nitrogen for higher yield (Ofori and Stern, 1986; Cochran and Schlentner, 1995). Controlled studies have shown a significant direct transfer of fixed-N to the associated non-legume species (Eaglesham et al.,, 1981; Giller et al.,, 1991; Frey and Schüepp, 1993; Stern, 1993; Elgersma et al.,, 2000; Høgh-Jensen and Schjoerring, 2000; Chu et al.,, 2004). There was evidence that the mineralisation of decomposing legume roots in the soil can boost N availability to the allied crop (Dubach and Russelle, 1994; Schroth et al.,, 1995; Evans et al.,, 2001). In mixed cultures, where row arrangements and the distance of the legume from the cereal are far, nitrogen transfer could decrease. Research has shown that competition between cereals and legumes for nitrogen may in turn kindle N2 fixation activity in the legumes (Fujita et al.,, 1990; Hardarson and Atkins, 2003). The cereal component effectively drains the soil of N, forcing the legume to fix more N2. Therefore it is important to manipulate and establish how the management practice in legume-cereal mixtures may influence N2 fixation and nutrition in cropping systems.
The microbial biomass is influenced by biological, chemical, and physical properties of the plant-soil system. Generally, soil and plant management practices may have greater impact on the level of soil microbial C (Gupta and Germida, 1988; Dick et al.,, 1994; Dick, 1997; Alvey et al.,, 2003). For instance, soil microbial C tend to show the highest values in cropland and grassland soils and the lowest in bare cultivated soils (Brookes et al.,, 1984; Gupta and Germida, 1988).Monoculture systems are expected to contain less amounts of microbial biomass and activities in comparison to those in mixed cultures (Moore et al.,, 2000). Studies have indicated that legumes accumulated larger amounts of soil microbial C in the soil than cereals (Walker et al.,, 2003). This is attributed to lower C : N ratio of legume than that of cereal (Uriyo et al.,, 1979; Brady, 1990). Microbial biomass activities could increase after the addition of an energy source. The stimulation of soil microbial biomass activity by organic amendments is elevated than that induced by organic fertilisers (Bolton et al.,, 1985; Goyal et al.,, 1993; Höflich et al.,, 2000). Soil organic matter content and soil microbial activities, vital for the nutrient turnover and long term productivity of soil, are enhanced by the balanced application of nutrient and/or organic matter/manure (Bolton et al.,, 1985; Guan, 1989; Goyal et al.,, 1993; Höflich et al.,, 2000; Kanchikerimath and Singh, 2001). Under conditions of adequate nutrient supply such as P2O5, the microbial biomass C will be increased due to improved plant growth and increased turnover of organic matter in the soil (Bolton et al.,, 1985). Whether the management practices in mixed cultures involving legumes and cereals may favour the stimulation of biological soil activity and, thus, result in a higher turnover of organic substrates in the soil that are utilized by micro-organisms is a good subject to be investigated. Although there is a lot of information that show the relationship between soil management and soil microbial activity, little is known about these effects under mixed cropping systems as practised by farmers in the tropical/ subtropical environments (Dick, 1984; Dick et al.,, 1988; Deng and Tabatabai, 1996). In this context, the measurement of their activities could provide useful information concerning soil health, and also serve as a good index of biological status in different crop production systems.
PHOSPHATASE ACTIVITY IN LEGUME/CEREAL MIXTURES
Plants have evolved many morphological and enzymatic adaptations to bear low phosphate availability. This includes transcription activity of acid phosphatases, which tends to increase under P2O5 starvation (Tarafdar and Jungk, 1987; Goldstein, 1992; Duff et al.,, 1994; del Pozo et al.,, 1999; Haran et al.,, 2000; Baldwin et al.,, 2001; Miller et al.,, 2001; Li et al.,, 2002). Phosphatase enzymes in the soil serve several important functions, and are good indicators of soil fertility (Dick and Tabatai, 1992; Eivazi and Tabatabai, 1997; Dick et al.,, 2000). Under conditions of P2O5 deficiency, acid phosphatase secreted from roots is greater than before (Nakas et al.,, 1987; Chrost, 1991;Hayes et al.,, 1999; Li et al.,, 1997). Gilbert et al., (1999) found that white lupin roots from P-deficient plants had significantly superior acid phosphatase activity in both the root extracts and the root exudates than comparable samples from P-sufficient plants. At various starvation levels, these enzymes release phosphate from both cellular (Bariola et al.,, 1994) and extra cellular (Duff et al.,, Ndakidemi 2529 1994) organic compounds. The transcripts and activity of phosphate transporters are increased to optimise uptake and remobilisation of phosphate in P-deficient plants (Muchhal et al.,, 1996; Daram et al.,, 1999; Kai et al.,, 2002; Karthikeyan et al.,, 2002; Mudge et al.,, 2002; Versaw and Harrison, 2002).
It is thought that these morphological and enzymatic responses to P starvation are coordinated by both general stress-related and P-specific signaling systems. The amount of acid phosphatase secreted by plants is genetically controlled, and differs with crop species and varieties (Izaguirre-Mayoral and Carballo, 2002) as well as crop management practices (Patra et al.,, 1990; Staddon et al.,, 1998; Wright and Reddy, 2001). Some studies have shown that the amount of enzymes secreted by legumes were 72 % higher than those from cereals (Yadav and Tarafdar, 2001). Li et al., (2004a) found that, chickpea roots were also able to secrete greater amounts of acid phosphatase than maize. The activity of acid phosphatases is expected to be higher in biologically managed systems because of higher quantity of organic C content found in those systems. In fact, the activity of acid and alkaline phosphatase was found to correlate with organic matter in various studies (Guan, 1989; Jordan and Kremer, 1994; Aon and Colaneri, 2001). It is, therefore, anticipated that management practices in mixed cultures that induce P stress in the rhizosphere, may also affect the secretion of these enzymes. To date, there have been few studies examining the influence of cropping system on the phosphatase activity in the rhizosphere of most legumes and cereals grown in Pakistan. Understanding the dynamics of enzyme activities in these systems is crucial for their assessment their interactions as in turn their activities may regulate nutrient uptake and plant growth in the ecosystem.
EFFECT OF ORGANIC, BIOLOGICAL ANDCHEIMCAL FERTLIZERS ON CROPS AND SOIL
Application of organic manures has various advantages such as increasing soil physical properties, water holding capacity, and organic carbon content apart from supplying good quality of nutrients. The addition of organic sources could increase the yield through improving soil productivity and higher fertilizer use efficiency (Santhi, and Selvakumari, 2000). High and sustained yield could be obtained with judicious and balanced fertilization combined with organic manures (Kang, B.T. and V. Balasubramanian, 1990). Protecting long-term soil fertility by maintaining soil organic matter levels to certain extent, sustaining soil biological activity and careful mechanical intervention, providing crop nutrient directly by using relatively insoluble nutrient sources which are made available to the plants by the action of soil micro-organisms, nitrogen self sufficiency through the biological nitrogen fixation (Hossain et al.,,2004) as well as effective recycling of organic materials including livestock wastes organic manuring (Safdar, 2002).Soil degradation which is brought about by loss of organic matter accompanying continuous cropping becomes aggravated when inorganic fertilizers are applied repeatedly. This is because crop response to applied fertilizer depends on soil organic matter (Agboola and Omueti, 1982).
Among differnret manues poultry manure is highly nutrient enriched organic manure since solid and liquid excreta are excreted simultaneously resulting in no urine loss. In fresh poultry excreta uric acid or urate is the most plentiful nitrogen compound (40-70 % of total N) while urea and ammonium are present in petite amounts (Krogdahl, and Dahlsgard. 1981). Cooperband et al., (2002) assessed phosphorus value of different- age poultry litter composts and raw poultry litter. Available soil P was the highest in plots amended with 15-month old compost, followed by raw poultry litter amended plots. Poultry manure is an excellent organic fertilizer, as it contains high nitrogen, phosphorus, potassium and other essential nutrients. In contrast to mineral fertilizer, it adds organic matter to soil which improves soil structures, nutrient retention, aeration, soil moisture holding capacity, and water infiltration (Deksissa et al.,, 2008). It was also indicated that poultry manure more readily supplies P to plants than other organic manure sources (Garg and Bahla, 2008). As the use of poultry manure becomes an integral part of sustainable agriculture, demand for poultry products increases and pasturelands as well as croplands become nutrient saturated, which has ultimately increased water quality and public health concerns. In addition to high N and P content, raw poultry manure has a potential source of pathogen or E .coli (Jamieson et al.,, 2002; Bustamante et al.,, 2007) and endocrine disruptors (Deksissa et al.,, 2007).
High and sustained crop yield can be obtained with judicious and balanced NPK fertilization combined with organic matter amendment (Kang and Balasubramanian, 1990).The benefits
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