Full Length Research Paper
ABSTRACT
Low use efficiencies of inorganic fertilizers coupled with their rising costs has diverted attention of farmers towards organic sources. A study was conducted in Yatta sub-county between October 2012 to February 2013 short rains and April-August 2013 long rainy seasons to evaluate how tillage, cropping and organic inputs influenced soil nutrient status. A randomized complete block design with a split-split plot arrangement replicated three times was used. The main plots were tillage practices (TP): Split-plots comprised the cropping systems (CS) while split-split plots were organic inputs, plus the control. The test crops were sorghum and sweet potatoes (Impomea batata) with Dolichos (Dolichos lablab) and chickpea (Cicer arietinum L.) added either as intercrops or in rotation. Soil was randomly sampled at 0 to 30 cm depth at the onset of the experiment and at maturity of test crop for NPK and % organic carbon (OC) analysis. Significant (P≤0.05) high level of K (1.91 Cmol/+kg), available P (51.45 ppm), total N (0.19%) and OC (2.19%), in combined TR, intercrop sorghum/chickpea with application of Minjingu rock phosphate (MRP)+ farmyard manure (FYM) during SRS of 2012 compared to the other treatment combinations was observed. Comparing different organic inputs, tillage practices and cropping systems combined tied ridges (TR), intercrop of sorghum/chickpea and MRP+FYM and FYM increased the soil nutrients status. Therefore, soil organic inputs such as MPR and FYM are viable alternatives to inorganic fertilizers for improving the soil nutrient status. The study therefore recommends incorporation of the organic inputs in combination with TR, as well as intercropping with legumes in their cropping systems to improve soil health and resilience.
Key words: Cropping systems, tillage practices, organic inputs, semi-arid, soil nutrients.
INTRODUCTION
Low soil fertility and moisture deficits are major constraints to crop production in the semi-arid areas of Kenya. Many interrelated factors, both natural and managerial, lead to soil fertility decline either through leaching, erosion, and crop harvesting (Donovan and Casey, 1998). The low soil fertility is majorly contributed by agriculture intensification particularly in developing countries (Rezig et al., 2012) due to the ever-increasing food demand for the rising population. Unless the nutrients are replenished using organic or mineral fertilizers, partially returned through crop residues or rebuilt more comprehensively through traditional fallow systems that allow restoration of nutrients and reconstruction of soil organic matter, soil nutrient levels will decline continuously. Therefore, the use of species different from the main crop such as legumes contributes to the nutrient balance, which may consequently increase soil fertility level over time. Leguminous species are known for their capacity to fix atmospheric di-nitrogen in association with rhizobia bacteria and hence narrow the C: N ratio, resulting in faster residue decomposition (Aita and Giacomini, 2003) with consequent release of accumulated N, P and K to the soil (Borkert et al., 2003). Legume green manures are also efficient at mobilizing P from the soil (Knight and Shirtliffe, 2005) pool through de-composition and release in a labile form that enhances P nutrition of succeeding crops (Cavigelli and Thien, 2003).
Farmers in the Eastern part of Kenya use farmyard manure (FYM) as a cheaper alternative source of plant nutrients as opposed to the more expensive inorganic fertilizers (Gichangi et al., 2007). Farmyard manure acts as an alternative source of fertility enhancement as they release nutrients more slowly and steadily over a period of time and also improve the soil fertility status by activating the soil microbial biomass (Belay et al., 2001; Karuku and Mochoge, 2016). Consequently, inputs from organic sources such as FYM, play a pivotal role in the productivity of many farming systems by providing nutrients through decomposition and substrate for the synthesis of soil organic matter (SOM). SOM has been shown to improve crop growth and yield by supplying nutrients or by modifying soil physical properties (Rees et al., 2000; Karuku et al., 2012, 2014) and environment.
Furthermore, SOM acts as a bonding and dispersing agent by increasing inter-particle hydrophobicity and cohesion within aggregates (Mullins, 2000; Abiven et al., 2009). It is well known that manures are sources of all-necessary macro- and micro-nutrients in available forms, thereby improving the physical, chemical and biological properties of the soil (El-Magd et al., 2005; Zhang, 2005). Due to the slow process of decomposition, manures are usually applied at higher rates, relative to that of inorganic fertilizers, to meet crop nutrient requirements and the excess have positive residual effects on the growth and yield of succeeding crops (Makinde and Ayoola, 2008). Application of manures to soil similarly provide other potential benefits such as improved fertility and structure, increased soil organic matter buildup and improved water holding capacity (Phan et al., 2002; Blay et al., 2002). In addition, tillage practices such as Tied ridges and Furrows and Ridges may allow rainwater retention on open furrows for longer duration as the water infiltrates into the soil or soil management techniques that favor prolonged rainwater infiltration and retention, thus raising the overall soil moisture availability and hence improved crop production in arid and semi-arid lands (ASALs) (Itabari et al., 2003). This study evaluated effects of tillage practices, cropping systems and organic inputs on NPK and organic carbon in Yatta sub County.
METHODOLOGY
The study was carried out in Yatta Sub County, Kenya, coordinates: 1.4667°S and 37.8333°E at 944 m asl. The sub-county falls under agro-ecological zones IV, classified as semi-arid lands (Jaetzold and Schmidt, 2006). Yatta Sub County comprises a suite of soils that includes Acrisols and Luvisols in association with Ferrallisols (WRB, 2015). In most places, the topsoil is loamy sand to sandy loam, sandy clay to clay with low nutrient availability (Kibunja et al., 2010).
The mean annual temperature vary from 18 to 24°C; also, the area experiences bimodal rainfall with long rains in early April to May (about 400 mm) and short rains commencing from early October to December (500 mm). Most farmers in the Sub County practice small-scale mixed farming with crops grown ranging from maize (Zea mays)), beans (Phaseolus vulgaris), pigeon pea (Cajanus cajan), green grams (Vigna radiata), sorghum (Sorghum bicolor), and cowpea (Vigna unguiculata) (Macharia, 2004).
Initial soil analysis indicated that the soils at the site were acidic sandy clay, low in fertility, with low amounts of total nitrogen (TN), organic carbon (OC) and available P (Table 1). This was attributed to farmers’ reliance on one continuous cropping system without application of organic inputs. Continuous cropping of land with little or no nutrient returns lead to their depletion, hence decline in soil fertility (Smalling et al., 1997).
Farm yard manure used in the study was slightly alkaline at a pH of 8.2, OC of 18.6%, TN at 2.1% and P and K contents of 0.4% and 2.7%, respectively. The Ca content was 3.4% (Table 2).
The treatments were tillage practices (Oxen plough, tied ridges, and furrows and ridges), cropping systems (mono cropping, intercropping, and crop rotation), organic inputs (farmyard manure, rock phosphate, and combined farmyard manure and rock phosphate) and control. The experiment was in a randomized complete block design with split-split plot arrangement. The main plots (150 m × 60 m) were tillage practices (Oxen plough, tied ridges and furrows, and ridges). Split plots (10 m × 4 m) were cropping systems (mono cropping, intercropping, and crop rotation) and split-split plots (2.5 m × 1 m) were organic inputs (farmyard manure, rock phosphate and combined FYM and rock phosphate). A control (no organic inputs applied) was also included as a split-split plot. The test crops were sweet potatoes (Ipomea batatas lam) and sorghum (sorghum bicolor L.) with Dolichos (Dolichos lablab) and chickpea (Cicer arietinum L.) either as intercrops or in rotation.
Field practices
Land was prepared manually using oxen plough in late September and planted in October short rain of 2012 and April long rain season of 2013. Manure was broadcasted at a rate of 5 tha-1, Minjingu rock phosphate (MRP) at 498 kgha-1 equivalent to 60 kgP ha-1, and thoroughly mixed with the soil before the vines and seeds were placed in the holes. Sweet potatoes (wabolinge variety) were propagated through 30 cm long cuttings at a spacing of 90 cm between rows and 30 cm within rows. Weeding was done 5 weeks after planting and harvesting after 6 months when the leaves turned yellow and dry. The harvesting was done using a sharp hoe to remove all tubers. Sorghum (serendo variety) was sown at spacing of 75 cm by 30 cm while Dolichos and chickpea were planted at a spacing of 30 cm within the sorghum and sweet potato rows. Weeding was done after 5 weeks of planting and harvesting after three months when it had reached physiological maturity.
Soil samples were collected in a transect (in a zigzag manner from one edge of the field) for initial soil analysis. Soil samples were later taken at maturity of sweet potato and sorghum as main crops, at three samples per treatment which were then composited into a single sample and mixed thoroughly. The sample was air-dried by spreading it out in a clean, warm, dry area for two days before being analyzed for N by micro-Kjeldahl method as described by Bremner (1996); P by double acid method; K by flame photometry and organic carbon determined following Walkley and Black (1934) as described by Nelson and Sommers (1996). Soil pH-H2O and pH-CaCl2 was determined with a pH meter in a 1:2.5 ratio extract. Electrical conductivity (ECe) was measured on a 1:2.5 ratio extract with an EC meter.
Statistical analysis
Data was subjected to general analysis of variance using GenStat statistical software (Payne et al., 2005) version 18. Means were separated using least significant difference at a probability level of 5%.
RESULTS AND DISCUSSION
Potassium (K) content was significantly (P≤0.05) affected by the organic inputs as increased level were recorded with application of MRP + FYM in all tillage practices and cropping systems, compared to the other organic inputs MRP, FYM and their controls. Increased K content was observed under combined oxen plough (OP), sorghum mono cropping with application of MRP+FYM (3.37 Cmol+/kg) and intercropping of sweet potato/dolichos (3.08 Cmol+/kg) as compared to other tillage practices, combined furrows and ridges, intercropping of sorghum/dolichos with application of MRP+FYM (2.01 Cmol+/kg) and intercropping of sweet potato/dolichos (2.14 Cmol+/kg) and tied ridges along with intercropping of sorghum/chickpea (1.91 Cmol+/kg) and intercropping of sweet potato /chickpea (2.06 Cmol+/kg).
Increased K content under MRP+FYM application was attributed to the fact that when farmyard manure and Minjingu rock phosphate are mixed, it enhances release of other nutrients such as K through increased microbial activity in the soil. Same applies when FYM was applied. Low K content under Tied ridges (1.91 Cmol+/kg); Furrows and ridges (2.11 Cmol+/kg) compared to OP (2.95 Cmol+/kg) could be attributed to increased soil moisture content leading to loss of the nutrients down the profile due to leaching of K in the upper profile as compared to OP. Under different cropping systems, increased K content was observed under intercrop and crop rotation of both chickpea and dolichos in all tillage practices. This was attributable to the effects of exudates such as H+ and other organic acids released by the legumes in the rhizosphere along with works on the organic materials applied, thus mineralizing more nutrients to the soil. Root-secreted chemicals mediate multi-partite interactions in the rhizosphere, where plant roots continually respond to and alter their immediate environment. Increasing evidence suggests that root exudates initiate and modulate dialogue between roots and soil microbes. For example, root exudates serve as signals that initiate symbiosis with rhizobia and mycorrhizal fungi. In addition, root exudates maintain and support a highly speciï¬c diversity of microbes in the rhizosphere of given particular plant species, thus suggesting a close evolutionary link (Dayakar and Jorge, 2009; Sunita, 2017). Moreover, inclusion of legumes in crop rotations protects the fragile soil surface by restoring the organic matter content and organic fertility of these soils and this would also help to restore the natural fertility of these soils (Ahmad et al., 2010a; Liu et al., 2006).
Increased potassium level under intercropping and crop rotation of chickpea and dolichos was also reported by Ahmad et al. (2010b) who found out that use of green manure especially legumes in a cropping pattern could help restore crop productivity. In addition, Aziz et al. (2010) reported that manure application significantly increases soil K contents due to increased microbial activity in the soil. Another similar observation was made by Suge et al. (2011), who found that addition of organic fertilizers improves soil water holding capacity as well as the CEC and nutrients are released slowly to crop plants, thus impacting on nutrients availability. The inclusion in a rotation of cover crops or green manures can also enhance the efficient use of nutrients by plants, mainly owing to the increase in soil microbial population and activity (Watson et al., 2002).
Changes in potassium content Cmol+/Kg across the seasons (SRS 2012 and LRS 2013)
Changes in potassium content across the two seasons was observed with increase during the LRS (3.65 Cmol+/kg) and (3.39 Cmol+/kg) as compared to the SRS (3.37 Cmol+/kg) and (3.09 Cmol+/kg) under oxen plough in sorghum mono cropping and intercropping of sweet potato/dolichos with the application of MRP+FYMin sorghum and sweet potato plots, respectively (Tables 3 and 4). During the LRS of 2013, the soil moisture content increased as a result of prolonged rainfall as opposed to SRS of 2012. Soil moisture content affects the availability of K in soil, with greater efficiency of K fertilizer with increasing soil moisture (Kuchenbuch et al., 1986) since it influences microbial activities responsible for decomposition for release of potassium.
Decomposition of organic matter is chiefly carried out by heterotrophic microorganisms. This process is influenced by temperature, moisture and ambient soil conditions leading to the release and cycling of plant nutrients, especially nitrogen (N), potassium and phosphorus (Murphy et al., 2007; Sunita, 2017).
Available phosphorous
The soil available P level increased significantly (P ≤0.05) in plots with MRP+FYM compared to other treatments FYM, MRP and control. Accordingly, combined TR, intercropping sweet potato and sorghum/dolichos with application of MRP + FYM had highest P levels (51.45 ppm and 46.31 ppm), respectively in the SRS of 2012 (Tables 5 and 6). Increased available P with application of MRP+FYM was due to the enhanced release from MRP when mixed with FYM since decomposition of FYM releases humic acid which further promote the release of P from the rock. Organic exudates of soil microbes and roots of grain legume crops can mobilize phosphorus from unavailable soil-P pool and increase its availability for P inefficient plant species grown in inter-cropping or crop rotation. Legume crops adopt different strategies such as development of cluster roots, exudation of carboxylates, protons and acid phosphatase to render the P available from inorganic and organic P sources (Hawkins et al., 2005; Lambers et al., 2006; Sunita, 2017).
Thus, inter-cropping or crop rotation of cereal crops with such legumes that have improved mechanisms to gain access to this fixed P will contribute toward more sustainable agriculture (Sunita, 2017). In addition, it implies that MRP underwent considerable dissolution leading to release of P in the MRP applied. Addition of FYM results into an increased microorganism decomposition rates and thus release phosphorous into the soil. Organic manures after decomposition may also provide organic acids and increase P-bioavailability after dissolution of MRP when combined with FYM. Sunita (2017) reported similar findings in India. This also conforms to a study by Mengel and Kirkby (2001), and Marschner (2011) who noted an increase in P contents with addition of FYM and attributed it to mineralization and increased water holding capacity, thus making P readily available to crops. A study by Kari (1996) stated that application of FYM affect available P content considerably. In addition, FYM increase soil moisture content (Boateng et al., 2006), increase microbial activity and resultant biochemical transformations of P in soil. The added organic manures may lead to mineralization of more recalcitrant P fraction (Nziguheba et al., 1998) as also reported by Maerere et al. (2001) and Odedina et al. (2011) in their studies.
There was a significant difference (p≤0.05) across the tillage practices with increased available content under Tied ridges (51.45 ppm) compared to Furrows and ridges (48.24 ppm) and Oxen plough (38.59 ppm) under intercropping sorghum/chickpea and with the application of MRP+FYM during the SRS of 2012. The increased P under Tied ridges and Furrows and ridges was attributed to the increased soil moisture content harvested under Tied ridges and Furrows and ridges resulting in reduced runoff, hence less soil loss by erosion and then reduced P losses in soil are less mobile and most losses are due to soil erosion. Kaushik and Gautam (1997) found out that increased soil water retention reduces nutrients losses through erosion. Oxen ploughed plots may have had lower P due to increased loss through erosion and leaching. Use of the oxen plough tillage practice could increase erosion due to the inappropriate width adjustment on the plough which led to formation of plough furrows acceleration on the rate of rill erosion, especially in sloping lands causing nutrients losses as documented by Kaumbutho and Simalenga (1999).
There was also a significant difference (p≤0.05) across all the cropping systems with increased P content under the intercropping of chickpea (51.45 ppm) and dolichos (46.88 ppm) in Tied ridges with the application of MRP+FYM during SRS of 2012. This was due to enhanced release of the nutrients from the organic inputs as presence of the legumes enhanced release, fixation of nutrients and increased biological activity rotation (Suge et al., 2011; Larkin, 2008). Suge et al. (2011) attributed increased available P to crop while Larkin (2008) indicated that crop rotation help in pests and diseases control, thus increasing soil biological activity. Christen and Sieling (1995) observed increased water use efficiency, which in turn increased P content in the soil under crop rotation. This conforms to a study that crop rotation along with increasing soil organic matter increased biodiversity and soil biological community (Kamkar and Mahdavi, (2009).
Changes in phosphorous content ppm across the season (SRS 2012 and LRS 2013)
Changes in P content across the two season indicated an increase during the LRS (58.8 ppm) and (52.92 ppm) compared to the SRS (51.45 ppm) and (46.31 ppm) under OP in intercropping of sorghum/chickpea and sweet potato/ chickpea with the application of MRP+FYM in sorghum and sweet potato plots, respectively (Tables 7 and 8). The higher amounts of soil available P in the LRS 2013 than SRS 2012 was attributed to the residual effects of the organic inputs applied (MRP, MRP+FYM and FYM). According to Rowell (1994), the rapid adsorption of P onto soil particle is followed by a slower conversion into less available forms including mineral phosphates; thus, P in the MPR and most phosphate fertilizers is available in the first season after application but the rest remains over long periods of time, hence their residual effects.
Total nitrogen
Total N increased significantly (P≤0.05) through application of FYM in all the tillage practices and cropping systems compared to other treatments. Significant (P≤0.05) increased % TN (0.19) was recorded under FYM with the intercrop of dolichos/sorghum in furrows and ridges (Tables 7 and 8). The increase in soil TN after FYM application was due to addition of N by decomposing FYM. These results conform to the findings of Thamaraiselvi et al. (2012) who reported increases in soil TN with FYM application. Nyambati (2000) also reported that MRP and organics (FYM) combinations provide cheap N sources. Also, solubilization of MRP through formation of favorable acid environments that results when organics are in contact with MRP decompose in soils releasing N to the soil.
Percent TN content increased significantly (p≤0.05) across cropping system with crop rotation dolichos/sorghum at 0.21% and intercrop sorghum/chickpea at 0.19% under Tied ridges and application of FYM (Table 7). This same trend was observed under Furrows and ridges (0.19% and 0.17%) and OP (0.15% and 0.13%) under crop rotation of dolichos/sorghum and intercrop though the content was lower compared to sorghum planted plots. Increase in TN under dolichos intercrop and rotation was attributed to the legumes ability to fix Nitrogen and the amount obtained from the legumes residues which led to increased soil organic matter (SOM) as opposed to the mono-cropping. Aita and Giacomini (2003) observed that leguminous species have capacity to fix atmospheric nitrogen and narrow the C/N ratio, resulting in faster residue decomposition and consequent release of accumulated N and other nutrients such as P and K to the soil. Crop rotations usually increase organic matter and prompt changes in N sources, affecting their availability to plants and, as a consequence, the N efficiency is greater when a crop rotation is adopted (Montemurro and Maiorana, 2008). In this study, it was observed that ridges and furrows enhanced infiltration thus reducing runoff and consequently prevented nutrient losses, a fact consistent with FAO (1993). The lower TN content in OP compared to ridges and furrows and Tied ridges was attributed to higher soil erosion and runoff (Kaumbutho and Simalenga, 1999) leading to their loss.
Changes in total nitrogen across the season
Changes in % TN across the two season was observed to increase during the LRS at 0.23 and 0.21% compared to SRS at 0.19 and 0.19% under Tied ridges of intercropped sorghum/chickpea and crop rotation of dolichos/sweet potato with the application of MRP+FYM in sorghum and sweet potato plots, respectively (Tables 7 and 8). This implies that TN increased with increased soil moisture during the LRS of 2013; hence mineralization was dictated by soil moisture content. The study therefore shows a correlation between soil moisture and soil N mineralization, which agree with previous studies (Li et al., 1995; Zhou and Ouyang, 2001).
Organic carbon
The level of organic carbon increased significantly (P≤0.05) across all cropping system and tillage practices compared to initial soil analysis where FYM and MRP + FYM were added. An increased % OC (2.45) and (3.15) was observed in combined OP, intercrop of sorghum/chickpea and dolichos/sweet potato rotations, respectively where FYM was applied (Tables 9 and 10). This was due to high carbon content present in applied FYM as opposed to MRP application alone plus additional residue from legumes that further raised the carbon content. Across the cropping systems, increased % OC was observed under sorghum/ dolichos intercropping (2.45%), then rotation of sorghum/chickpea (2.26%) and rotation of sorghum/dolichos (2.19%) under oxen plough with the application of FYM. Cover crops are generally grown to provide soil cover, thus preventing soil erosion by wind and rainwater, which increases organic matter content in the long run (Karuku et al., 2014; Karuku, 2018). Komatsuzaki (2004) indicated that cover crop utilization is a technique that limits nutrient leaching, scavenging the soil residual N and making it available for subsequent cultivation.
Among the three tillage practices, a significant difference (p≤0.05) was observed with improved % OC under OP followed by furrows and ridges though under different cropping systems and application of FYM. The observed % OC level conform to the study by Bayu et al. (2006) who noted that FYM application increased soil SOC content by up to 67% over the control. The crop residues from the legumes further act as manures thus increasing %OC and other nutrients. This agrees with Knight and Shirtliffe (2005) where legume green manure increased benefits such as atmospheric N2 fixation and P mobilization from the soil, facts also observed in this study.
Changes in % organic carbon across the season (SRS 2012 and LRS 2013)
Data on % OC changes across seasons indicate an increase during the LRS (2.28%) and (2.27%) compared to the SRS (2.1%) and (2.51%) under Tied ridges with intercropping sorghum/dolichos and intercropping sweet potato/dolichos applied with MRP+FYM in sorghum and sweet potato plots, respectively (Tables 8 and 9). The higher % OC observed during LRS is attributed to higher biomass production, which upon decomposition, releases CO2, thus raising carbon levels. Devi and Yadava (2006, 2009) in earlier studies, had reported that high rate of CO2 release during the LRS could be due to a congenial environment for microorganisms dwelling in the soil decomposing organic matter. The low % OC in the SRS seen in the study is attributed to low soil moisture content, temperature and relative humidity, thereby inhibiting the microbial activity (Devi and Yadava, 2006; Kosugi et al., 2007). Ginting et al. (2003), for example, found that 4 years after the last application of FYM, the residual effects resulted in 20 to 40% higher soil microbial biomass C.
CONCLUSIONS
Soil organic inputs, MPR and FYM are viable alternatives to the expensive inorganic fertilizers for improving the soil nutrient status in Matuu, Yatta Sub County. Combined TR, intercropping of sorghum and sweet potato with dolichos and with application of MRP + FYM significantly increased soil K and P whereas combined TR, intercropping of dolichos with sorghum and sweet potatoes and with application of FYM led to an increase in soil % OC and TN. Moreover, the MRP and FYM are locally available, thus making it an ideal source of nutrients for smallholders economically.
CONFLICT OF INTERESTS
The authors have not declared any conflict of interests.
ACKNOWLEDGEMENTS
The authors appreciate the financial support by the McKnight foundation to the first author to undertake the research as part of her postgraduate studies along with the farmers who provided their land and other resources for the study
REFERENCES
|
Abiven S, Menasseri S, Chenu C (2009). The effects of organic inputs over time on soil aggregate stability a literature analysis. Soil Biology and Biochemistry 41(12):2357-2369. |
|
|
Ahmad G, Mehdi D, Barat S, Mahmoud R (2010a). Effect of maize (Zea mays L.) - cowpea (Vigna unguiculata L.) intercropping on light distribution, soil temperature and soil moisture in arid environment. Journal of Food, Agriculture and Environment 8(1):102-108. |
|
|
Ahmad W, Khan F, Naeem M (2010b). Impact of cropping patterns and fertilizer treatments on the organic fertility of slightly eroded Pirsabak soil series in NWFP, Pakistan. Soil and Environment 29(1):53-60. |
|
|
Aita C, Giacomini SJ (2003). Crop residue decomposition and nitrogen release in single and mixed cover crops. Brazilian Journal of Soil Science 27:601-612. |
|
|
Aziz T, Ullah S, Sattar A, Nasim M, Farooq M, Khan MM (2010). Nutrient availability and maize (Zea mays) growth in soil amended with organic manures. International Journal of Agriculture and Biology 10–070/RAS/2010/12–4–621–624 |
|
|
Bayu W, Rethman NFG, Hammes PS and Alemu G. 2006. Application of Farmyard Manure Improved the Chemical and Physical Properties of the Soil in a Semi-Arid Area in Ethiopia. Biological Agriculture and Horticulture 24(3):293-300. |
|
|
Belay A, Classens AS, Wehner FC and De Beer JM (2001). Influence of residual manure on selected nutrient elements and microbial composition of soil under long–term crop rotation. South African Journal of Plant and Soil 18:1-6. |
|
|
Blay ET, Danquah EY, Ofosu-Anim J, Ntumy JK (2002). Effect of poultry manure on the yield of shallot. Soil Plant Nutrition 42:105-111. |
|
|
Boateng PY, Adeji P, Asante DKE (2002). Effect of method Application of poultry manure on Growth, Yield and economic Returns of Okra (Abelmoschus esculentus l. moench). Growth in Forest Zone of Ghana. Ghana Journal of soils. Agronomy Journal 54:465. |
|
|
Borkert CM, Gaudêncio CDA, Pereira JE, Pereira LR, Oliveira A Jr. (2003). Mineral nutrients in the shoot biomass of soil cover crops. Pesquisa Agropecuária Brasileira 38:143-153. |
|
|
Bremner JM (1996). Total nitrogen. In Methods of Soil Analysis: Chemical Methods. Part 3. D.L. Sparks, editor. Soil Science Society of America. Madison WI.C. |
|
|
Cavigelli M, Thien S (2003). Phosphorus bioavailability following incorporation of green manure crops. Soil Science Society of America Journal 67:1186-1194. |
|
|
Christen O, Sieling K (1995). Effect of different preceding crops and crop rotations on yield of winter oil-seed rape (Brassica na-pus L.). Journal of Agronomy and Crop Science 174:265-271. |
|
|
Dayakar VB, Jorge MV (2009). Regulation and function of root exudates. Plant, Cell and Environment 32:666-681. |
|
|
Devi NB, Yadava PS (2006). Seasonal dynamics in soil microbial biomass C, N and P in a mixed oak forest ecosystem of Manipur, North-east India. Applied Soil Ecology 31:220-227. |
|
|
Devi NB, Yadava PS (2009). Emission of CO2 from the soil and immobilization of carbon in microbes in a subtropical mixed oak forest ecosystem, Manipur, North-east India. Current Science 96(12):1627-1630. |
|
|
Donovan G, Casey F (1998). Soil fertility management in Sub-Saharan Africa. World Bank Technical Paper No. 408. World Bank. Washington, D.C. |
|
|
FAO (1993). The state of food and agriculture. (FAO Agriculture Series, no 26) ISBN 92-5-103360-9 Rome Italy. |
|
|
Gichangi EM, Njiru EN, Itabari JK, Wambua JM, Maina JN, Karuku A (2007). Assessment of improved soil fertility and water harvesting technologies through community based on-farm trials in the ASALs of Kenya. In; A. Batiano (Eds). Advances in integrated soil fertility management in Sub-Saharan African: Challenges and opportunities. pp. 759-765. 2007 Springer.10.1007/798-1-4020-5760-1-71. |
|
|
Ginting D, Kessavalou A, Eghball B, Doran JW (2003). Greenhouse gas emissions and soil indicators four years after manure and compost applications. Journal of Environmental Quality 32:23-32. |
|
|
Hawkins HJ, Wolfe G, Stock D (2005). Cluster Roots of Leucadendron laureolum (Proteaceae) and Lupinus albus (Fabaceae) Take Up Glycine Intact: An Adaptive Strategy to Low Mineral Nitrogen in Soils? Annals of Botany 96(7):1275-1282. |
|
|
Itabari JK, Nguluu SN, Gichangi EM, Karuku AM, Njiru EN, Wambua JM, Maina JN, Gachimbi LN (2003). Managing Land and Water Resources for Sustainable Crop Production in Dry Areas. A case study of small-scale farms in semi-arid areas of Eastern, Central, and Rift Valley Provinces of Kenya. In: Crissman, L. (eds.) Agricultural Research and Development for Sustainable Resource Management and Food Security in Kenya. Proceedings of End of Programme Conference, KARI, 11-12 November 2003. pp. 31-42. |
|
|
Jaetzold R, Schmidt H (2006). Farm Management handbook of Kenya. Natural conditions and farmer management information. Part C, East Kenya. |
|
|
Kamkar B, Mahdavi DA (2009). Principles of sustainable agriculture. Ferdowsi University Press. In Persian 345 p. |
|
|
Kenya Agricultural Research Institute (KARI) (1996). Kenya Agricultural Research Institute Annual Report for 1996. Nairobi. |
|
|
Karuku GN, Mochoge BO (2016). Nitrogen forms in three Kenyan soils nitisols, luvisols and ferrallisols. International Journal for Innovation Education and Research 4(10):17-30. |
|
|
Karuku GN, Gachene CKK, Karanja N, Cornelis W, Verplancke H (2014). Effect of different cover crop residue management practices on soil moisture content under a tomato crop (Lycopersicon esculentum). Tropical and Subtropical Agroecosystems 17(2014):509-523. |
|
|
Karuku GN, Gachene CKK, Karanja N, Cornelis W, Verplancke H, Kironchi G (2012). Soil hydraulic properties of a Nitisol in Kabete, Kenya. International Journal of Tropical and Subtropical Agroecosystems 15:595-609. |
|
|
Karuku GN (2018). Soil and Water Conservation Measures and Challenges in Kenya; a Review. International Journal of Agronomy and Agricultural Research 12(6):116-145. |
|
|
Kaumbutho PG, Simalenga TE (1999). Conservation tillage with animal traction (Eds). A resource book of the Animal Traction Network for Eastern and Southern Africa (ATNESA). Harare. Zimbabwe 173 p. |
|
|
Kaushik K, Gautam RC (1997). Effects of tillage and moisture conservation practices on productivity, water use and water use efficiency of pearl millet (Pennisetum glaucum) on light soils under dry conditions. Indian Journal of Agriculture Science 67(6):232-236 |
|
|
Kibunja CN, Mwaura FB, Mugendi DN (2010). Long-term land management effects on soil properties and microbial populations in a maize-bean rotation at Kabete, Kenya. African Journal of Agricultural Research 5(2):108. |
|
|
Knight JD, Shirtliffe SJ (2005). Saskatchewan organic on-farm research: Part II: Soil fertility and weed management. Department of Plant and Soil Sciences, University of Saskatchewan, SK. |
|
|
Komatsuzaki M (2004). Use of cover crops in upland fields. Japanese Journal of Farm Work Research 39:157-163. |
|
|
Kosugi Y, Mitani T, Itoh M, Noguchi S, Tani M, Matsuo M, Takanashi S, Ohkubo S, Rahim AN (2007). Spatial and temporal variation in soil respiration in a Southeast Asian tropical rainforest. Agricultural and Forest Meteorology 147:35-47. |
|
|
Kuchenbuch R, Claassen N, Jungk A (1986). Potassium availability in relation to soil moisture. Plant and Soil 95:221-231. |
|
|
Lambers H, Shane MW, Cramer MD, Pearse SJ, Veneklaas EJ (2006). Root Structure and Functioning for Efficient Acquisition of Phosphorus: Matching Morphological and Physiological Traits. Annals of Botany 98(4):693-713. |
|
|
Larkin RP (2008). Relative effects of biological amendments and crop rotations on soil microbial communities and soil borne diseases of potato. Soil Biology and Biochemistry 40:1341-1351. |
|
|
Li ZA, Weng H, Yu ZY (1995). The impact of human activi¬ties on the soil nitrogen mineralization in artificial forests. Chinese Bulletin of Botany 12:142-148. |
|
|
Liu X, Herbert SJ, Hashemi AM, Zhang X, Ding G (2006). Effects of agricultural management on soil organic matter and carbon transformation – a review. Plant, Soil and Environment 52(12):531-543. |
|
|
Macharia P (2004). Kenya. Gateway to land and water information: Kenya National Report. FAO (Food and Agriculture Organization). |
|
|
Maerere AP, Kimbi GG, Nonga DLM (2001). Comparative effectiveness of animal manures on soil chemical properties, yield and root growth of amaranths (Amaranthus cruentus L.). African Journal of Science and Technology 1(4):14-21. |
|
|
Makinde EA, Ayoola AA (2008). Residual influence of early season crop fertilization and cropping system on growth and yield of cassava. American Journal of Agricultural and Biological Sciences 3:4. |
|
|
Marschner P (2011). Mineral nutrition of higher plants.3rd edition. London: Academic Press pp. 135-178. |
|
|
Mengel K, Kirkby EA (2001). Principles of Plant Nutrition, 5th Edition. Kluwer Academic Publishers, Dordrecht, Boston. London P. 849. |
|
|
Montemurro F, Maiorana M (2008). Organic Fertilization as Resource for a Sustainable Agriculture, Fertilizers: Properties, Applications and Effects. In: Mariangela Diacono Francesco Montemurro (eds), pp. 123-146. |
|
|
Mullins CE (2000). Hard setting soils. The handbook of soil science. Press; Edited by: Fraser, ME. G65–G85. New York: CRC Press P. 19. |
|
|
Murphy DV, Stockdale EA, Brookes PC, Goulding KWT (2007). Impact of microorganisms on chemical transformation in soil. In: Abbott L.K., Murphy D.V. (Eds.). Soil biological fertility – A key to sustainable land use in agriculture. Springer pp. 37-59. |
|
|
Nelson EW, Sommers LE (1996). Total Carbon, Organic Carbon, and Organic Matter. In Methods of Soil Analysis: Chemical Methods. Part 3. D.L. Sparks, editor. Soil Science Society of America. Madison WI. |
|
|
Nyambati RO (2000). Soil phosphorus fractions as influenced by Phosphorus and Nitrogen sources on two sites in western Kenya. M.Phil. Thesis, Moi University, Eldoret. Kenya Publishers. |
|
|
Nziguheba G, Palm CA Buresh RJ, Smithson PC (1998). Soil phosphorus fractions and adsorption as affected by organic and inorganic sources. Plant Soil 198:159-168. |
|
|
Odedina JN, Odedina SA, Ojeniyi SO (2011). Effect of types of manure on growth and yield of cassava (Manihot esculenta, Crantz). Researcher 3(5):1-8. |
|
|
Payne RW, Harding SA, Murray DA, Soutar DM, Baird DB, Welham SJ, Kane AF, Gilmour AR., Thompson, R, Webster R, Tunnicliffe WG (2005). The guide to GenStat Release 8, Part 2: Statistics. VSN Int., Oxford. |
|
|
Phan TC, Roel M, Cong SS, Nguyen Q (2002). Beneficial effects of organic amendment on improving phosphorus availability and decreasing aluminum toxicity in two upland soils. Symposium No. 13 Paper No. 1226 17th, W.C.SS 14-21, Thailand. |
|
|
Rees RM, Ball BC, Campbell CD, Watson CA (2000). Sustainable management of soil organic matter.CAB International Publishing, Wallingford, UK pp. 377-384. |
|
|
Rezig AMR, Elhadi EA, Mubarak AR (2012). Effect of incorporation of some wastes on a wheat-guar rotation system on soil physical and chemical properties. International Journal of Recycling of Organic Waste in Agriculture 1:1. |
|
|
Rowell DL (1994). Soil Science: Methods and Applications. Harlow UK. Longman. |
|
|
Smalling MA, Nandwa SM, Janssen BH (1997). Soil fertility in Africa is at stake. pp 47 - 61. In: Buresh RJ, Sanchez PA, Calhoun F (Eds.). Replenishing Soil Fertility in Africa. Soil Science Society of America Journal Special Publication Number 51, Madison, WI |
|
|
Suge JK, Omunyin ME, Omami EN (2011). Effect of organic and inorganic sources of fertilizer on growth, yield and fruit quality of eggplant (Solanum Melongena L). Archives of Applied Science Research 3(6):470-479. |
|
|
Sunita G (2017). Phosphorus Mobilization Strategies of Grain Legumes: An Overview. JAM 3(1):1-15. |
|
|
Thamaraiselvi T, Brindha S, Kaviyarasi NS, An-nadurai, B and Gangwar SK (2012). Effect of Organic Amendments on the Bio Chemical Transformations under different soil. International Journal of Advanced Biological Research 2(1):171-173. |
|
|
Walkley A, Black IA (1934). An examination of the Destjareft method for determination of soil organic matter and proposed modification of chromic acid titration method. Soil Science 37:29-38. |
|
|
Watson CA, Atkinson D, Gosling P, Jackson LR, Rayns FW (2002). Managing soil fertility in organic farming systems. Soil Use and Management 18:239-247. |
|
|
World Reference Base (WRB) (2015). World Reference Base for Soil Resources 2014 (update 2015). International soil classification system for naming soils and creating legends for soil maps. World Soil Resources Reports 106. FAO, Rome, Italy. |
|
|
Zhang S (2005). Soil hydraulic properties and water balance under various soil management regimes in the Loess Plateau of China. PhD Thesis. |
|
|
Zhou CP, Ouyang H (2001). Influence of temperature and moisture on soil nitrogen mineralization under two types of forest in Changbai Mountain. Chinese Journal of Applied Ecology 12:505-508. |
|
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