The effect of biological nitrogen fixation on pearl millet production systems at Mannheim Research Station, Tsumeb, Namibia

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1 Journal of Scientific Research and Studies Vol. 4(2), pp , February, 2017 ISSN Copyright 2017 Author(s) retain the copyright of this article MRP Review The effect of biological nitrogen fixation on pearl millet production systems at Mannheim Research Station, Tsumeb, Namibia Alweendo T. E. Ministry of Agriculture Water and Forestry, P.M.B , Namibia. Tel , Fax Accepted 31 May, 2016 The two split blocks manure and no manure (cowpea with phosphorous, pearl millet nitrogen and phosphorous, intercropping with pearl millet with cow pea, pearl millet (micro plot), pearl millet in rotation with cowpea and phosphorous) and four cropping systems pearl mille/cowpea, intercropping and cow pea pearl millet in rotation was used. In both seasons, significant (P<0.05) differences in grain yield were observed. The yield estimated in 2010/11cropping season indicated that the treatment with pearl millet (micro plot) attained the highest yield (3.28 t/ha), followed by the pearl millet x cow pea (2.18 t/ha), pearl millet + nitrogen + phosphorous (1.87 t/ha), cowpea + phosphorous (1.80 t/ha), while pearl millet rotation with cowpea + phosphorous recorded the least yield of (1.70 t/ha). In the 2011/12 cropping season, the pearl millet x cow pea intercrop produced the highest grain yield (3.15 t/ha) followed by treatment pearl millet + nitrogen + phosphorous (3.03 t/ha), pearl millet rotation with cowpea + phosphorous (2.58 t/ha), pearl millet (micro plot (2.42 t/ha), cowpea + phosphorous which recorded the least yield of (0.79 t/ha). The results suggested that the inter-crops produced high grain yields that can be adopted by smallholder farmers in the area in order to increase crop productivity in similar cropping systems. However, practicing no tillage or minimum tillage encourage biological nitrogen fixation, and its longtime operation leads to yield increase as it improves most soil properties. Key words: Pearl millet, manure, cowpea, biological nitrogen fixation, Tsumeb. INTRODUCTION Relying less on inorganic nitrogen fertilizers and more on crop organic matter (residues) and biological nitrogen fixation (BNF) by legumes is an effective and sustainable way of managing cropping systems. A study was conducted at Mannheim research station in the northern central of Namibia on (sandy clay loam) for two consecutive cropping seasons starting from 2010/11and 2011/12 cropping season Pearl millet and cowpea are the main staple food crops for the communal areas in northern Namibia. In addition, grain legumes are good sources of food, fodder and cash for farmers. The legumes also fix atmospheric nitrogen into the soil through BNF thus reducing the need for applying inorganic fertilizers which are often beyond the reach of most communal farmers in the region. For instance, cowpea is one of the major legumes in the area which contributes to soil fertility through BNF. Nitrogen (N) is one of the most important mineral nutrients limiting pearl millet production in Namibian soils. The removal of N containing crop residues from the field during harvesting compounds the problem of N deficiency (Giller et al., 2009). Consequently, depletion of N leads to poor grain yields. The standard recommendation of N in the area is about 150 kg/ha of Nitrogen in the form of CAN which is recommended with 4 tons/ha of kraal manure. However, there are studies that have been conducted to quantify the BNF from cowpea in the area. A comparative microbiological investigation of soil was carried out also fewer than four different crop rotation systems in 1952 of wheat carrying plots and in 1953 of fallow, oats and wheat carrying plots of the same rotation systems. The experimental soil were gravely, fine sandy

2 32 J. Sci. Res. Stud. loam, deficient in nitrogen(n) phosphorus (P), potassium (K) and organic matter, with poor capacity for adsorbing metallic cation and according to the results of the microbiological analysis and crop yielding capacity of low fertility. During the winter of 1952, the rainfall was moderate and during few winter months in 1953, the rainfall was heavy with poor aeration of the experimental soils as a result. Soil temperatures for the two years did not differ much. In 1952, it was found that the bacterial population was highest in the plots carrying wheat preceded by Lucerne and lowest in the wheat - fallow plots with a tendency to increase during the winter. In the wheat plots following on lupins the bacterial population was between that of the two plots groups mentioned above. The same applied to 1953, but the population showed more or less a similar tendency. Both the number of microorganisms and their activities indicated distinct seasonal fluctuations (Louw et al., 1957). The influence of rotation system on the microbial activities, the application of rotation systems including s legume such as Lucerne or lupins stimulated the microbial activities in the soil; a funding confirming that of Lohnis (1926). Lohnis found that actinomycetes and other fungi indeed increased in soil containing legumes but not to the same extent as bacteria. It is clear from these studies that legumes alone will not solve the problem of sustaining crop rotation as a soil fertility in Semi arid areas due to the fact that both human and livestock population are already exacting immense pressure on the land which has to continue to support their sustenance. It has been discovered that combination of different crop residues will or may not give high yields than mono cropping where cotton/pearl millet will be grown alone in the first year at all sites. Four treatments that were tested in the trial plants receive only none fertilizer application in the first year. It was also focused to identify appropriate and suitable crops that are beneficial to soil fertility improvement and maintenance and the effects of legume crops and crop residues on cotton and fibre quality. Cow pea (Vigna unquiculata L. Walp) is the main legume food crop in the Sudano Sahelian zone of Mali (Alweendo and Horn, 2010). It is one of the main pulses contributing to the economy of nitrogen in cropping systems with low input through the biological fixation (Alweendo, 2001). This symbiotic nitrogen fixation can reduce the rate of depletion of cultivated soils where legume cereal rotation is practiced (Louw et al., 1957). If cowpea is nodulated by powerful strains of Bradyrhizobia, 90% of the required nitrogen for maximum yield can be derived from biological nitrogen fixation (Kanyanjua et al., 2007). According to N diaye et al. (2000), Ndakidem and Dakora (2007) and Bationo et al. (2006), the values of N contribution by various legumes, including Cowpea (V. unguilata L Walp) are important (50 to 300 kg N ha -1 per year. These values depend on the legume plants density of the cropping system and the legume genotype (Hilhorst and Muche, 2000). Van Kessel and Hartley (2000) showed that cowpea Bradyrhizobia nodulating population density depends very significantly on the cultural practices rather than rainfall. So far very little attention has been addressed to the agronomicrobiological aspects in cow pea research in Mali. In order to fill this gap, the present study was conducted from to 2009 at Cinzana Agronomic Research Station in Sudano Sahelian zone of Mali. The objective was to compare the potential of root nodulation and mycorrhizal infection of three cowpea varieties in three cropping systems and identify an efficient cropping system that can improve cowpea and millet yields. Poor soil fertility has been acknowledged as a major hindrance to high crop yield (Hihorst and Muche, 2000). Researchers have devised ways of alleviating this problem, some of which comprises application of organic and inorganic fertilizers. However, use of inorganic fertilizers by small holder farmers in sub Saharan Africa is inadequate (Bationo et al., 2006) due to high costs, unavailability and sometimes lack of knowledge on usage. Materials for organic fertilizers are also difficult to acquire as farmers prefer supplying stovers to livestock rather than leaving them in the field to decay and consequently release nutrients (Baijuka, 2004). These challenges have led to exploitation of other economical ways of supplying nutrients to the crops and one of these ways is biological nitrogen fixation. Biological nitrogen fixation (BNF) in legumes has for a longtime been a component of many farming systems throughout the world. However, this biological nitrogen fixation (BNF) process is primarily controlled by four principal factors effectiveness of rhizobia host plant symbiosis, ability of the host plant to accumulate N, amount of available soil N and environmental constraints (Van Kessel and Hartley, 2000). Therefore, the objectives of this study were to determine the (i) effect of BNF by cowpea on pearl millet using cowpea; (ii) determine the quantity of N taken up by pearl millet in four cropping systems and (iii) identify an efficient cropping system that can improve cowpea and pearl millet yield potential. MATERIALS AND METHODS Site description The Mannheim Research Station is located in the northern part of Tsumeb (Northern Central). During the duration of the study, the daily temperatures ranged between 25 and 38 C. The rainy season stretches from November to May as shown in Figure 1. The soils at the location are generally sandy clay loam with 54% sand, 12-14% silt and 32-34% clay. The texture is classified as

3 Alweendo 33 Figure 1. The distribution of rainfall at Mannheim Research Station over a three year cropping season starting in 2009/10. Table 1. Characteristics of the soil from Mannheim Research station (Tsumeb, Namibia) evaluated during the 2009/10 cropping season. Soil sample description Mannheim Research Station (Sample Reference Lab. No Soil depth in cm Top ( 0-30 cm) Top (0-30) Top (0-30cm) Fertility results Value Class Value Class Value Class Soil ph 7.87 Medium alkaline 7.97 Medium alkaline 8.00 Medium alkaline Total Nitrogen% 0.06 Low 0.07 Low 0.06 Low Organic carbon% 0.38 Low 0.37 Low 0.37 Low Carbon % 0.4 Low 0.4 Low 0.4 Low Sand % 54 Good 54 Good 54 Good Silt % 14 Good 12 Good 12 Good Clay % 32 Good 34 Good 34 Good Texture class SCL Sand clay loam SCL Sand clay loam SCL Sand clay loam Cation exchange capacity me% Phosphorus (ppm) 18 Adequate 22 Adequate 10 Adequate Potassium me% 0.17 Low 0.21 Low 0.15 Low Calcium me % 1.6 Low 1.8 Low 1.6 Low Magnesium me% 4.74 High 4.50 High 5.28 High Manganese me% 0.05 Low 0.02 Low 0.02 Low Copper (ppm) 0.6 Low 0.6 Low 0.5 Low Iron (ppm) 0.5 Low 0.7 Low 0.7 Low Zinc (ppm) 0.6 Low 0.5 Low 0.4 Low Sodium % 0.10 Adequate 0.10 Adequate 0.10 Adequate EC ms/cm 0.29 Low 0.35 Low 0.26 Low good soil for most of the agronomic activities. However, the soils are low in some macro and micro elements and organic matter (Table 1). sandy clay loam with 54% sand, 12-14% silt and 32-34% clay. The texture class is classified as good soil for most of the Agronomic crop growth, (Table1). Soil The experimental trial was conducted and evaluated in Soil samples analysis The first composite soil samples were collected at the

4 34 J. Sci. Res. Stud. 15N% (BNF) N15%(BNF) leaf root leaf root leaf root leaf root N%(BNF) % N % 15N excess Figure 2. Indicating Plant analytical data at Mannheim Research station taken 2010/11 cropping season analyzed at FAO/ International Atomic Energy Agency Agriculture and Biotechnology Laboratory, Soil Science Unit, Seibersdorf, Austria depths 0-30 cm before the establishment of experiment to determine the NPK, Soil Organic Matter/Carbon, ph, Cation Exchange Capacity (CEC), and Electrical Conductivity (EC). The second soil samples were taken from 0 30 cm and cm, 65 days after planting. The soil organic matter/carbon content was measured by the standard Walkey Black method and mineralization was done using concentrated sulphuric acid (H 2 SO 4 ) with dichromate of potassium (K2Cr 2 O 7 ), and solution was titrated with Iron sulphate (FeSO 4 ). The soil ph was determined by the electrometric method in solution with soil/water ration of 1/2.5. The Cation Exchange Capacity (CEC) and Exchangeable bases (Ca 2+, Mg +2, K +, Na + ) were determined by the method of extracting solution 0.5 m Ammonium Acetate. The total available soil phosphorous (ppm) was determined by the method of Olsen method in Sodium bicarbonate solution 0.5 m ph 8.5 (Extracting solution) dissolve 420 g NaHCO 3 in distilled water. The total nitrogen % was measured by Kjeldahl Oxidation method in solution of concentrated Sulphuric acid (H2SO 4 (95% sg. 1.84g/ml). Plant The plant materials analyzed were root and leaf s for pearl millet in the N15 (micro plot). The plant was measured using distilling unit (Kjeltech system) for the total Nitrogen determination; the distill and titration of N15 as the starting point and Emission Spectrometer (NOI 6PC) for the determination of % N15 excess, %N, δ 13C and % C (Figure 2). The plant materials were assessed at FAO/International Atomic Energy Agency at the Agriculture and Biotechnology Laboratory, Soil Science Unit, Seibersdorf, Austria as indicated in Figures 2, 3 and 4. Experimentation on Mannheim Research Station The experiment was conducted into two blocks with and without manure based on a complete randomized blocks design, with four replications. The treatments consisted of (cowpea with phosphorous, pearl millet nitrogen and phosphorous, intercropping with pearl millet with cowpea, pearl millet, pearl millet in rotation with cowpea and phosphorous) and two cropping systems pearl mille/cowpea, intercropping and cowpea with pearl millet rotation were used. The Pearl millet (Okashana 2 ) and Cowpea (Nakale) were used in the trial and spacing: 0.75 m between rows and 0.45 cm between plants (pearl millet) and x 0.30 cm cowpea, with estimate plants population of to plants per hectare. The three cropping systems consisted of pearl millet/cowpea, intercropping in alternate rows (1 row pearl millet/1 row cowpea); the pearl millet mono cropping and cow pea mono cropping, and cow pea/ pearl rotation with plot size 4 x 5 m = 20 m 2. The soil was prepared by ridging to the cultivator at 0.75 m spacing between 4 ridges of 5 m long and the seeds were planted manually on the ridges. The Urea 5% a.e@15kg N/ha= (300 kg/ha) of nitrogen had been applied at four weeks after planting in all Nitrogen treatments. RESULTS Soil analysis interpretation The soil texture is predominantly sandy clay loam in the first 0-30 cm, with 54% of sand, 14% silt and 32% clay.

5 Alweendo 35 15N%(BNF) Leaves 15N%(BNF) Leaves Leaves % 15N excess % N N% ( BNF) Figure 3. Indicating Plant with manure analytical data at Mannheim Research station taken 2011/12 cropping season analyzed at FAO/ International Atomic Energy Agency Agriculture and Biotechnology Laboratory, Soil Science Unit, Seibersdorf, Austria Figure 4. Indicating Plant analytical no manure data at Mannheim Research station taken 2011/12 cropping season analyzed at FAO/ International Atomic Energy Agency Agriculture and Biotechnology Laboratory, Soil Science Unit, Seibersdorf, Austria The upper horizon contains only 32% of clay content that is good to the development and the maintenance of good structure and it is chemically a very good soil. The soil ph is ( ) medium alkaline in the first 30 cm layer which is good for growing most of the agronomic crops especially the pearl millet. The soil is low in organic matter (OM %), this means during the land preparation, the application of 4 t/ha of well decomposed manure will be recommended. Adequate in available phosphorous (P < 10 18ppm) with high reserve of total phosphorous and very low in exchangeable bases (Ca 2+, Mg +2, K +, Na + ) and trace elements (micronutrients). Normally if pearl millet is to be planted, no fertilizer; a 150 kg/ha of CAN and 50 kg/ha of Muriate of Potash (MOP) or known as Potassium chloride(kcl) should be applied half at planting time and half when the crops are 30 cm high. Experimentation on Mannheim Research Station The rainfall distribution graph (Figure 5) indicated less

6 36 J. Sci. Res. Stud. Figure 5. In 2010/11 the monthly rainfall was below the 1 year average while that of 2011/12 was above. These distribution patterns certainly had some influence on crop growth habit and contributed to yield differences. Figure 6. The average grain yield at Mannheim research station during 2010/11 and 2011/12 cropping season. rain (433 mm) in 2010/11 compared to 2011/12 (495.1 mm) respectively. Cropping system effects on pearl millet yield performance Crop yields were determined and evaluated using Sigma stat 2.0 Statistical and Microsoft Office Excel 2010 packages. The result for the 2010/11cropping season indicated that treatment with pearl millet (micro plot) recorded the highest (3.28 t/ha) followed by treatment intercropping pearl millet with cowpea (2.18 t/ha), pearl millet + nitrogen + phosphorous (1.87 t/ha), cowpea + phosphorous (1.80 t/ha), while pearl millet rotation with cowpea + phosphorous recorded the least (1.70 t/ha; Figure 6). In 2011/12 cropping season, the manure treatment with pearl millet x cowpea intercrop produced the highest (3.15 t/ha) grain yield followed by treatment pearl millet + nitrogen + phosphorous (3.03 t/ha), pearl millet rotation with cowpea + phosphorous (2.58 t/ha), pearl millet (micro plot (2.42 t/ha) while cowpea + phosphorous produced the least (0.79 t/ha). In 2011/12 cropping season, the field with no manure and with pearl millet + cowpea produced the highest (3.28 t/ha) grain yield followed by pearl millet + nitrogen + phosphorous (2.18 t/ha), pearl millet rotation with cowpea + phosphorous (1.870 t/ha), pearl millet (micro plot) (1.80 t/ha), while, cowpea + phosphorous recorded the least (1.70 t/ha). Cropping system effects on cowpea and pearl millet plants density and yield The four way interaction between emergence, flowering date, plant density and maturity date on cowpea/pearl millet variety was highly significant (P< 0.001). Therefore the comparison for factor treatments had shown

7 Alweendo 37 Figure 7. Indicating the effects of cropping system with no manure interaction between emergence, flowering date, plant density versus maturity date of N15 NBF evaluation trial conducted at Mannheim research station during the 2010/11 cropping season. Figure 8. Indicating the effect of cropping system with manure interaction between emergence, flowering date, plant density, maturity date on N15 NBF evaluation trial conducted at Mannheim research station during 2011/12 cropping season. significant differences in some treatments (Figures 7, 8 and 9). DISCUSSION The results showed highly significant effect of cropping system on cowpea/ pearl millet on biological nitrogen fixation. 2010/11 cropping season The yield indicated that treatment with pearl millet (micro plot) recorded the highest (3.28 t/ha), followed by treatment intercropping pearl millet with cowpea (2.18 t/ha), pearl millet + nitrogen + phosphorous (1.87 t/ha), cowpea + phosphorous (1.80 t/ha), while pearl millet rotation with cowpea + phosphorous recorded the least (1.70 t/ha; Alweendo, 2010/11). The significant influence on cow pea and pearl millet yield performance showed significant differences (P<0.020) in the mean values among the treatment groups. 2011/12 cropping season The block with manure treatment and intercropped with pearl millet x cow pea produced the highest (3.15 t/ha) followed by treatment pearl millet + nitrogen + phosphorous (3.03 t/ha), pearl millet rotation with cowpea + phosphorous (2.58 t/ha), pearl millet (micro plot (2.42 t/ha), while cowpea + phosphorous recorded the least (0.79 t/ha; Alweendo, 2011/12). The significant influence on cowpea and pearl millet yield performance had shown that the difference in the mean values among the

8 38 J. Sci. Res. Stud. Figure 9. Indicating the effect of cropping system with no manure interaction between emergence, flowering date, plant density versus maturity date on N15 NBF evaluation trial conducted at Mannheim research station during 2011/12 cropping season. treatment groups are greater than be expected by chance, and there is a statistically significant difference (P= 0.012). 2011/12 cropping season The manure block indicated that treatment with pearl millet + cowpea recorded the highest (3.28 t/ha) followed by pearl millet + nitrogen + phosphorous (2.18 t/ha), pearl millet rotation with cowpea + phosphorous (1.870 t/ha), pearl millet (micro plot) (1.80 t/ha; Alweendo, 2011/12), while cowpea + phosphorous recorded the least (1.70 t/ha). The significant influence on cowpea and pearl millet yield performance had shown that the difference in the mean values among the treatment groups are greater than be expected by chance, and there is a statistically significant difference (P= 0.020). The differences between cropping systems effects on biological nitrogen can be explained by differences in plants density of the different systems. The results were consisted with those of the emergence, flowering and maturity dates; they did not show significant differences, due to difference in dates. However emergence, flowering and maturity dates had been affected by high or low rainfall and low availability of soil organic matter ranging from (OM %), this means during the land preparation, the application of 4 t/ha of well decomposed manure will be recommended. Hence soil organic matter increased the biological nitrogen fixation. The four month repartitions of rainfall of the rainfall of two years were significantly different, which indicated less rain (433 mm) in 2010/11 compared to 2011/12 (495.1 mm) respectively. The precipitation regime year factor also influenced grain yield and dry biomass yields even though the annual rainfalls were not equivalent respectively. The yield advantage in N15 on pearl millet was explained not only by the high plants density ( to plants per hectare), but also by the absence of competition with other crops. In 2010/11 treatment, pearl millet (micro plot) recorded the highest yield (3.28 t/ha) while in 2011/12 treatment block, manure treatment intercropping pearl millet with cowpea recorded the highest yield (3.15 t/ha). In the intercropping with pearl millet, cowpea and phosphorous, in this system photosynthesis reduction of cowpea occur because of the shade effects of the leaching pearl millet crop, resulting in low productivity of cowpea. This confirmed the results of 2011/12 of 3.15 t/ha. However in the intercropping with pearl millet cow pea, pearl millet takes all of its nitrogen needed from the soil reserves, therefore fewer quantity of nitrogen will be saved for ensuing crop (pearl millet). Conclusion The results of this study showed that biological nitrogen fixation contributed to the improvement in the grain yield of pearl millet. The tested N15 and cropping systems have a significant influence on yield of pearl millet and cowpea. Therefore the year factor (precipitation regime) also influenced grain yield and other yield components, although the annual rainfalls were equivalent (433 and mm, respectively). This was explained by the two months wet rain quantities, which were different during the two year pearl millet cropping years. It was also noted that due to the inception of rains, planting was done late in December to early January and yields were poor to moderate due to erratic or heavy rainfall distribution, which affected almost of the entire crop production system at Mannheim research station. The two way interaction between density x cropping yield cowpea/pearl millet variety was highly significant (P

9 Alweendo 39 < 0.001). Therefore the comparison for factor treatments had shown significant differences in some treatments. The soil texture was predominantly sandy clay loam in the first 0 30 cm, with 54% of sand, 14% silt and 32% clay. The upper horizon contains only 32% of clay content that is good for the development and the maintenance of good structure and it is chemically a very good soil. The soil ph is ( ) medium alkaline in the first 30 cm layer which is good for growing most of the agronomic crops especially the pearl millet. The soil is low in organic matter (OM %), this means during the land preparation, the application of 4 t/ha of well decomposed manure will be recommended. Adequate in available phosphorous (P < 10 18ppm) with high reserve of total phosphorous and very low in exchangeable bases (Ca 2+, Mg +2, K +, Na + ) and trace elements (micronutrients). Normally if pearl millet is to be planted, no fertilizer at 150 kg/ha of CAN and 50 kg/ha of Muriate of Potash (MOP) or known as Potassium chloride (KCl) should be applied half at planting time and half when the crops are 30 cm high. ACKNOWLEDGEMENT I wish to express sincere gratitude to all the staff who assisted with data collection and fellow scientists for their valuable inputs. REFERENCES Alweendo TE (2001). Proceeding of the Second National Annual Agriculture Research Reporting Conference hosted by The Directorate Agricultural Research and Training, Ministry of Agriculture Water and Forestry Private bag 13184Windhoek, from 8 10 th September, 1998, 6-10 th September 1999, th September 2000 and th September Alweendo TE (2010). The Effect of Biological Nitrogen Fixation on Pearl Millet Production Systems at Mannheim Research Station 2010/11, 2011/12. Baijuka FP (2004). Adapting to change in banana based farming systems of northern Tanzania, the potential role of herbaceous legumes. PhD thesis Wageningen, The Netherlands. Bationo A, Hartemink A, Lungu O, Naimi M, Okoth ANP, Sambling E, Thiombano L (2006). African Soils: Their productivity and profitability for use. Paper presented at the African Fertilizer summit, Abuja, - Nigeria June Giller KE, Witter E, Corbees M, Tittonel P (2009). Conservation Agriculture and Smallholder farming in Africa The heretics view, Field Crop Res. 114: Hilhorst T, Muche F (Eds) (2000). Nutrient on the move: Soil fertility dynamics in Africa Farming Systems. Rusesel Press Nottingham, p Kanyanjua SM, Muriithi JG, Gachene CKK, Saha HM (2000). Soil fertility management handbook for extension staff and farmers in Kenya. Lohnis F (1926). Nitrogen availability of green manures. Soil Sci. 22: Louw HA et al. (1957). Science Bulletin No. 378 Stellenbosch Elsenburg Scientic Series No. 54. Ndakidem PA, Dakora FD (2007). Yield components of nodulated cowpea (Vigna unguiculata) and maize (Zea mays) plants grown with exogenous phosphorous in different cropping systems. Aust. J. Exp. Agric. 47(5): N diaye MAF, Spencer MM, Gueye M (2000). Genetic variability in dinitrogen fixation between cowpea (Vigna unguiculata (L) Walp) cultivars determined using the nitrogen 15 isotope dilution technique. Biol. Fertil. Soils 32(4): Van Kessel C, Hartley C (2000). Agricultural management of grain legumes: has it led to increase in nitrogen fixation? Field Crops Res. 65: Alweendo TE, Horn LN (2010). Crop rotation as a soil fertility improvement strategy in cotton/maize and pearl millet production system in the Northern Crop Research Station Namibia. Procedures for the National Research symposium 2009/10.