Conservation Agriculture In Dryland Maize Based Systems Biology Essay

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Maize is the most important grain crop in South Africa and is produced throughout the country under diverse environments. The crop occupies about four millions hectares of the country's arable land and about one third of South African farmers are maize farmers (Van Rensburg, 1978). Approximately 8,0 million tonness of maize grain are produced in South Africa annually on approximately 3,1 million ha of land (Du Plessis, 2003). ). The global production is approximately 600 million tons of grain, of which 50% is produced in developing countries (FAO, 2003).

Limiting factors for maize production in many developing countries include drought, insufficient levels of plant nutrients, especially of the major nutrients Nitrogen (N), Phosphorus (P) and Potassium (K). Fertilization and soil fertility management are among the most important farming practices to improve grain yield and water use efficiency towards sustainable maize production (Fan et al., 2005).

Small scale farmers face problems related to viability, small farm units, natural disaster, lack of infrastructure, water suppliers, fertilizers, transport, financial support, research and extension services. The incorporation of N- fixing legumes, whether used in sequential or intercropping with cereals crops, is a possible solution to the N problem (Elowad & Hall, 1987). In areas where maize, monocropping is practiced, soil fertility and crop yields decline rapidly if nutrients are not supplemented.

Agriculture is of extreme importance to the North West. About 64 000 people (1.7% of North West population) are working in the agricultural sector It contributes about 2.6% of the total GDPR and 19% of formal employment. (Statistics South Africa, 2007a). The province is an important food basket in South Africa. Maize and sunflowers are the most important crops, and the North West is the major producer of white maize in the country.

On average, the western region of the province receives less than 300mm of rainfall per annum. This may result in a serious problem of food shortage in North West Province because of low rainfall in the area. The solution to this problem lies in improving the water use efficiency in the agricultural sector. Improving crop water use efficiency will save water for environmental flow requirements and industrial consumption (Sally et al., 2003). Effective water conservation technologies may sustain the agricultural output in the semi-arid environments (Botha et al., 2003).

1.2 Problem statement

Most smallscale farmers in South Africa practice monocropping of maize and this result in rapid soil fertility decline. It is well known that maize production in is dominated by smallscale farmers mostly planting maize for home consumption and yield is relatively low. This is associated with inadequate water resources, poor soil fertility, low farm income, inappropriate land utilization and lack of know-how on soil fertility management practices. Cropping systems which lead to improved moisture conservation, crop yield and soil fertility should be considered to improve sustainability of smallholder farming in South Africa.

In the current situation where there is a problem relating to climate change, constant droughts, socio economic and farm management challenges, inclusion of legume crops and cover crops in maize based systems can improve yields and reduce reliance on external farm inputs. Soil fertility decline may be due to low soil organic matter content, which subsequently leads to poor soil moisture relations and low soil nitrogen content.

Soil moisture content is a critical resource that limits the growth and development of crops. The cropping system which results in moisture conservation should therefore be considered in the study. Use of winter cover crops is identified as an introduction to conservation agriculture on most small scale farmers as the land is usually not planted in winter.

1.3. Rationale of the study

The study is significant in the domain of agronomy. The new knowledge, which the study contributes to the field of agronomy, conservation agriculture in dryland maize based systems, in terms of soil moisture management, soil fertility improvement, carbon sequestration and nitrogen management. New knowledge will also be generated on appropriate cover crops for dryland systems in the North West.

1.4 Main Objective

The overall objective of the study will be to evaluate winter cover crop biomass production, soil fertility and yield of subsequent maize.

1.5 Specific objectives

The specific objectives of the study will be:

To investigate the beneficial effects of Lupin, Faba beans, Hairy Vetch and Rye grass on maize production

To evaluate the response of fertilizer on maize growth and N uptake in rotation with cover crops compared to maize monocropping

To evaluate the response of fertilizer on maize grain yield and yield components in rotation with cover crops compared to maize monocropping

To determine the effects of cover crops and fertilizer on biomass, carbon and N uptake by different winter cover crops.

To determine the effects of different cover crops mulches and fertilization on soil fertility, moisture content and weed growth

To determine the residue decomposition rate and release of nutrients from cover crops litter in laboratory incubation experiment

1.6 Hypotheses to be tested:

Winter cover crops (Lupin, Faba beans, Hairy Vetch and Rye grass) will improve maize yields

Fertilization on maize improve nutrient N uptake in rotation with cover crops compared to maize monocropping

Fertilization and rotation with cover crops can improve maize growth, grain yield and yield components in compared to maize monocropping

Biomass, carbon and N uptake differ between different winter cover crops.

Fertilization of cover crops improve soil fertility, moisture content and weed growth

Rate of residue decomposition and release of nutrients differ from cover crops litter in laboratory incubation experiment

CHAPTER 2: LITERATURE REVIEW

2.1 Maize production in South Africa

Maize is a primary food staple in southern Africa, and 50 percent of the total maize output in the area is produced in South Africa, where maize constitutes approximately 70 percent of grain production and covers 60 percent of the country's cropping area. It is an important grain crop under irrigation, which produces high yields and is one of the most efficient grain crops in terms of water utilization (Department of Agriculture, 2003).

In SA maize producing regions include the provinces of Free State (FS), North West (NW), Gauteng (GP), Mpumalanga (MP), Limpopo (L) and Kwa-Zulu Natal (KZN), with parts of NW, FS and MP being the major maize growing areas. The dry western areas of the country which make up the main maize producing regions are characterized by water-limited production practices. Maize crop is mainly planted at the beginning of the rainy season (around the middle of November) to reduce the need for irrigation.

Maize is of major importance for the South African economy. In the year 2000, maize yielded over 15% of the gross value of all agricultural products. S.A meets its annual maize consumption requirements entirely from domestic production, which has been steadily increasing over the years. Local consumption of maize is about 7.5 metric tons per year, but the country often produces surpluses that are exported, mainly to the neighbouring countries in the SADC regions (NDA, 2001).

In Southern Africa, crop production is not limited by shortage of water alone; it is also being limited by poor soil fertility. The poor soil fertility and scarce water resources inhibits farmers to prepare large portions of land to plant crops due to high risk of crop failure associated with arid and semi-arid environment. In most cases, farming in the arid and semi-arid environment is not limited by unavailability of land but due to poor soil fertility and water deficits (Kundhlande et al, 2004)

Although the maize plant is quite hardy and adaptable to harsh conditions, warmer temperatures and lower levels of precipitation could have detrimental effects on yields, thereby increasing food insecurity in the region. Dry land maize production in South Africa varies from year to year depending on the amount and distribution of rainfall. Yield reduction in most dry land maize growing areas is due to erratic seasonal rainfall distribution (Du Toit et al., 2002). Water availability is specifically the most limiting factor of dry land maize production in South Africa (PECAD, 2003).

2.2 Maize production by small scale farmers

Smallholder maize farmers in South Africa face numerous production challenges including land degradation, lack of tillage services resulting in late planting, scarcity of water, poor soil fertility and heavy weed infestations (Fanadzo, 2007; Mandiringana et al., 2005). There is an estimated 3 million smallscale maize farmers, located mainly in the commune areas of the former homelands that primarily produce to meet their families nutritional needs. Effectively, these small scale maize farmers, who depend for their survival on maize farming and related industries, comprise more than half of the country's provinces and about 40% of the country's total population (NDA, 2001). However, grain yields obtained by most smallholder irrigation farmers, are far below potential with an average of less than 3 tons per hactor being common (Machethe et al., 2004; Fanadzo, 2007).

Provided nutrients and moisture are not limiting, successful cultivation of maize depends largely on the efficacy of weed control. Weed induced losses are highest in smallholder farming and can be as high as 99% in maize (Fanadzo, 2007). Poor weed control decreases water and nitrogen use efficiency, the two most important inputs to achieving high yields under irrigation (Thomson et al., 2000). Use of both organic and inorganic fertilizers is low on these farms, leading to low maize yields (<3 t/ ha). The problem is exacerbated by weeds which compete with crops for scarce water and nutrient resources thus affecting productivity of the cropping systems.

2.3 Climatic requirement of maize

Zhiming et al., (2007) reported that after germination and up to tasseling, the maize crop uses less moisture. Maize requires more moisture during reproductive period and requires less moisture when developing towards maturity. Maize requires a mean temperature of about 22°C and night temperature above 15°C. These authors (Zhiming et al., 2007) revealed that cultivation of maize is not possible when night temperatures are less than 19°C and day temperature during the first three months fall below 21°C. Noon temperatures above 35°C for several days destroy pollen and yields are drastically reduced. Hassan (2006) reported that maize can be produced in areas where rainfall exceeds 350 mm per year. Production is dependent on an even distribution of rain throughout the growing season. It was revealed that maize in SA is planted from October to December

Zhiming et al., 2007 reported that after germination and up to tasseling, the maize crop uses less moisture. Maize requires more moisture during reproductive period and requires less moisture when developing towards maturity. Maize requires a mean temperature of about 22°C and night temperature above 15°C. These authors (Zhiming et al., 2007) revealed that cultivation of maize is not possible when night temperatures are less than 19°C and day temperature during the first three months fall below 21°C. Noon temperatures above 35°C for several days destroy pollen and yields are drastically reduced (Zhiming et al., 2007).

Hassan (2006) reported that maize can be produced in areas where rainfall exceeds 350 mm per year. Production is dependent on an even distribution of rain throughout the growing season. It was revealed that maize in SA is planted from October to December (Hassan, 2006). Due to variation in rainfall pattern, temperature and duration of the growing season, planting times vary from the eastern to western production areas. Milbourn et al., 1978 reported that when daily temperatures during the growing season are greater than 20°C, early grain varieties take 80-110 days and medium varieties 110-140 days to mature.

When mean daily temperatures are below 20°C, there is an extension in days to maturity of 10-20 days for each 0.5°C decrease depending on variety. These authors (Milbourn et al., 1978) revealed that for germination the lowest mean daily temperature is about 10°C, with 18 to 20°C being optimum. For maximum production a medium maturity grain crop requires between 500 and 800 mm of water depending on climate (Milbourn et al., 1978).

Belfield & Brown (2008) reported that the optimum temperature for maize growth and development is 18 to 32°C, with temperatures of 35°C and above considered inhibitory. The optimum soil temperatures for germination and early seedling growth are 12°C or greater, and at tasselling 21 to 30°C is ideal. Maize can grow and yield with as little as 300 mm rainfall, but prefers 500 to 1200 mm as the optimal range. Depending on soil type and stored soil moisture, crop failure would be expected if less than 300 mm of rain were received in crop (Belfield & Brown, 2008). Due to variation in rainfall pattern, temperature and duration of the growing season, planting times vary from the eastern to western production areas. Milbourn et al., 1978 reported that when daily temperatures during the growing season are greater than 20°C, early grain varieties take 80-110 days and medium varieties 110-140 days to mature.

When mean daily temperatures are below 20°C, there is an extension in days to maturity of 10-20 days for each 0.5°C decrease depending on variety. These authors (Milbourn et al., 1978) revealed that for germination the lowest mean daily temperature is about 10°C, with 18 to 20°C being optimum. For maximum production a medium maturity grain crop requires between 500 and 800 mm of water depending on climate (Milbourn et al., 1978).

Belfield & Brown (2008) reported that the optimum temperature for maize growth and development is 18 to 32°C, with temperatures of 35°C and above considered inhibitory. The optimum soil temperatures for germination and early seedling growth are 12°C or greater, and at tasselling 21 to 30°C is ideal. Maize can grow and yield with as little as 300 mm rainfall, but prefers 500 to 1200 mm as the optimal range. Depending on soil type and stored soil moisture, crop failure would be expected if less than 300 mm of rain were received in crop (Belfield & Brown, 2008).e s

2.4 Effect of soil water in cropping system

Jones et al., 2005 reported that when soil water content was relatively high at field capacity, mass flow of nutrients like nitrate, sulphate, calcium and magnesium is often sufficient for plant needs. The diffusion rates of phosphorus and potassium were often relatively high. Soil water content can be useful for monitoring changes over season, or for determining irrigation timing, while soil water potential can be useful in understanding where water will flow and how plants are responding to water content (Jones et al., 2005).

Gan et al., 2010 reported that crops grown in semi-arid rain-fed conditions were prone to improving cultural practices. In the tilled-fallow system, chickpea extracted 20% more water in the 15-30 cm depth, 70% more in the 30-60 cm depth and 156% in the 60-120 cm depth than when it was grown in the no till systems. It was further reported that water use efficiency increased from 4.7 to 6.8 kg ha-1/mm-1 as N fertilizer rate was increased from 0 to 112 kg N ha-1 when chickpea was grown in the no till barley or wheat systems. Chickpea inoculated with Rhizobium achieved a water use efficiency value similar to the crop fertilized at 84 kg N/ha-1 (Gan et al., 2010). Gao et al., 2009 reported that in the maize/soybean strip intercropping system, soil water content decreased in the order of maize zone, soybean zone and middle zone indicating that each strip intercropped crop preferentially absorbed the soil water in its strip and utilized the soil water in intermingled zone later (Gao et al., 2009.

Beets (1990) reported that having a variety of root systems in the soil reduces water loss, increases water uptake and increases transpiration. The increased transpiration may make the micro climate cooler, which along with increased leaf cover, helps to cool the soil and reduce evaporation. This is important during times of water stress, as intercropped plants use a larger percentage of available water from the field than monocropped plants (Beets, 1990). Ouda et al., 2007 reported that soybean/maize intercropping could be a way of irrigation water saving, especially in situation of limited water resources. Intercrops have been known to conserve water, largely due to early high leaf area index and higher leaf area.

These authors (Ouda et al., 2007) further reported that water capture by intercrops was higher by about 7% compared by sole crop. Water use efficiency was the highest under soybean/maize intercropping, compared with sole maize and sole soybean. Water utilization efficiency of intercrops was higher by about 18% compared to sole crops. It was also revealed that water stress during maize growing season resulted in reduction of plant height, leaf area index and total leaf area reduction. The most important times for soybean plants to have adequate water are during pods development and seed filling. These are the stages when water stress can lead to a significant decrease in yield (Ouda et al., 2007). Velykis & Satkus (2005) reported that minimum and no till production system can also be effective in conserving moisture since crop residues are allowed to remain on the soil surface. Crop rotation can also lead to greater efficiency in soil water utilization. For example, deep rooted crops following shallow crops can take advantage of the extra reserve of deep moisture, which was unavailable to the shallow rooted crops. The authors (Velykis & Satkus, 2005) further reported that, by increasing winter crops in the crop rotation reduced compaction of the topsoil from high to moderate, maintain up to 37.3% of higher productive moisture reserves, improves water to air ratio and increase the crop rotation productivity up to 44.7%.

DeAngelis (2007) reported that the soil moisture content may be expressed by weight as the ratio of the mass of water present to the dry to the dry weight of the soil sample, or by volume as ratio of water to the total volume of the soil sample. To determine any of these ratios for a particular soil sample, the water mass must be determined by drying the soil to constant weight and measuring the soil sample mass after and before drying. The water mass is the difference between the weights of the wet and oven dry samples (DeAngelis, 2007).

It was concluded that soil moisture conservation may be the most efficient and economical way of increasing net return over the long term (Velykis & Satkus, 2005). Ofori and Stern (1986) reported that water utilization can be increased when cowpea is grown with other crops. Cereal and legume intercrops use water equally, and that competition for soil water may not be a determining factor for efficiency in intercrop system (Ofori & Stern, 1986).

2.6 Water use efficiency in the small-scale farming

Crop productivity is usually measured in ratio to inputs such as capital, fertilizer, energy and labour. The concept of crop productivity has shifted to water productivity with the idea to manage water resources. The concept of water productivity is a useful water management tool because it provides farmers with an insight into the quantity of water required to acquire minimum, optimal, and maximum crop yield (Bennett, 2003). In semi-arid areas water is ussually the most important production limiting factor. Thus the basic principle that should be used to manage the soil water balance ensuring minimum water losses under dryland an even irrigation in order to increase the amount of water that can be transpired (Hensley et al., 1997).

Crop yield is a major output in water-productivity frameworks (Bastiaanssen et al., 2003). Water productivity, a concept expressing the value or benefit derived from the use of certain quantity of water, has been defined as the amount of output produced per unit of water involved in the production, or the value added to water in a given circumstance (Singh et al., 2006). Water productivity can be defined with respect to the different sectors of production involving water for example, crop production, fishery, forestry, domestic and industrial water use. Water productivity with respect to crop production is referred to as crop water productivity and is defined as the amount of crop produced per volume of water used (Igbadun et al., 2006).

2.7 Conservation farming

Conservation farming focuses on abandoning the detrimental practice of conventional soil inversion through ploughing so as to increase use of rainfall, contribute to dry spell mitigation and to increase crop yield. Research results indicate that conventional ploughing with mould board and disc plough on tropical soil contribute to soil degradation and erosion (Rockstrom, 2003). The emphasis is on soil management practices with less tillage and maintenance of more residues on the soil surface (Bowen, 2003). Improved tillage, where soil inversion is abandoned in favour of sub soiling, manual pitting, ripping and zero tillage systems, builds soil biology and improves soil fertility that contributes to immediate productivity benefits (Rockstrom, 2003).

Similarly, the effects of conventional tillage practice on soil, due to severe disturbance, further necessitate the adoption of conservation agriculture tillage practices such as ridging, minimum, zero and reduced tillage (Hellin, 2006). However, the low adoptions of conservation agriculture technologies may be related to its perceived poor understanding and non-adaptability to African farmers' prevailing practical realities (Gill et al., 2009). Conservation agriculture technologies have been reported to have tremendous potential for all sizes of farms and agro-ecological systems (Derpsch, 2005), resulting in reduced tillage costs, timely sowing, reduced land degradation, improved weed control and water conservation as well as maize yields (Hobbs & Gupta, 2004; Teasdale, 1996). On the other hand Conservation agriculture, combines minimal soil disturbance, a permanent soil cover through use of cover crops with crop rotations (Derpsch, 2005; Hobbs, 2007).

Soil water conservation is a priority activity in South Africa (Fanadzo et al., 2010) and mulching could address this and result in improved crop yields. Opinions also vary on whether CA benefits can be realized on SH farms in Sub-Saharan Africa (Giller et al., 2009. Conservation agriculture is a relatively new technology being actively promoted on small holder farms in SA (Allwood, 2006). Use of winter cover crops has been identified as an avenue of introducing CA on most SH farms as land is usually not planted in winter. Lack of tillage services, lack of technical knowledge and labour shortages

2.8 Use of legumes as winter cover crops

Cover crops can improve the soil by adding organic matter, nutrients, and stability and by acting as scavengers to trap leftover nutrients that otherwise might leach out. Cover crops are used as ground cover, mulches, green manure, nurse crops, smother crops, and forage and food for animals or humans. Cover crops can be annual or perennial species, including certain legumes, grasses, and nonleguminous dicots. There are benefits of using legumes as cover crops such as:

2.8.1 Nitrogen importance and its contribution in cropping system

Nutrient elements are not readily available for plant use. They become available for plant use through mineral weathering and organic matter decomposition. Nitrogen (N) nutrition is an important determinant of the growth and yield of maize. N fertilizer must be used judiciously to maximize profit, reduce the susceptibility to diseases and pests, optimize crop quality, save energy and protect the environment (Schroder et al., 2000).

Nitrogen limitations on maize productivity in smallholder farming systems in Southern Africa are widespread and endemic (Robertson et al., 2005). As fertilizer prices rose, organic sources of fertility became an increasingly important option for increasing soil fertility and maize yield (Palm et al., 1998). The amount of N2 fixed and the N contribution from leguminous crops are influenced by a number of environmental factors including soil type, nutritional status of soil, species and varieties, climate as well as management of crop residues (Rao & Mathuva, 2000).

Among the plant nutrients, nitrogen plays a very important role in crop productivity and its deficiency is one of the major yield limiting factors for cereal production (Shah et al., 2003). N deficiency is frequently a major limiting factor for high yielding grain crops in the tropics. The extent of the deficiency depends on many factors including inherent soil fertility, whether the crop is a legume or non-legume, the cropping system or rotation employed and the skills of the producer (Date, 2000).Success of a legume crop to N contribution to succeeding crop depends on the capacity to form effective nitrogen fixing bacteria. In many farming systems the use of leguminous green manures is traditional, and the inputs from BNF often promote significant increase in subsequent grain or other crops (Ramos et al., 2001).

Plant residues decomposing in soils is the most important source of N for plant growth in natural ecosystems, with the exception of those dominated by N2 -fixing plants. The environmental concerns related to the use of mineral fertilizers have raised new interest in nutrient recycling through plant residues in agriculture (Ehaliotis et al., 1998). Research studies have shown that regular and proper addition of organic materials (crop residues) are very important for maintaining the tilth, fertility and productivity of agriculture and controlling wind and water erosion, and preventing nutrient losses by run-off and leaching (Bukert et al., 2000).

The release of N from decaying plant residues has been clearly related to their structural and chemical characteristics (the residue quality), to biotic activity and abiotic characteristics of the soil environment (Jenkinson, 1981). The total amount of N released from high quality legume residues during the first cropping season is large. Up to 70% of legume N is released in temperate systems and even higher amounts under tropical conditions (Giller & Candisch, 1995). Legume residues, because of their quality (e.g. low C: N ratio), can decompose fast with residual soil moisture or after early rains. Returning residues into the soil may also moderate extremes of soil temperatures, improve soil organic matter levels, soil structure, infiltration storage and utilization of the soil (Doran et al., 1984 & Power et al., 1986).Returning crop residues after harvest is one way to improve water conservation and storage as well as stabilize soil fertility and crop yields (Shafi et al., 2007). Organic compounds help to improve soil by increasing water retention capacity, thus impeding nutrient loss by leaching, by decreasing erosion and surface drainage, and by helping control weeds and other pests (Anaya et al., 1987).

2.8.2 Biological nitrogen fixation (BNF)

Biological nitrogen fixation (BNF) is the process that changes inert N2 to biologically useful NH3. This process is mediated in nature only by bacteria. Other plants benefit from nitrogen fixing bacteria when the bacteria die and release N to the environment or when the bacteria live in close association with the plants. In legumes and a few other plants, the bacteria live in small growth on the roots called nodules. Within these nodules, bacteria do N fixation, and the plant absorbs the NH3 produced. N fixation by legumes is a partnership between a bacterium and a plant (Liedemann & Glover, 2003).

When legumes cover crops are used in cropping system, N availability in the soil may increase as a result of two effects. Firstly, the conservation of soil N through N2 fixing legumes in comparison to non- fixing plants (Giller & Wilson, 1991). Secondly, the enhanced mineralisation of soil organic nitrogen during the decomposition of legume residue 'primary effect' (Jenkinson et al., 1985). This nitrogen may be either released throughout the growing season as roots and nodules die or sloughed off or as exudates or during the decomposition of roots after harvest (Crawford et al., 1997; Jensen, 1996).

The net amount of symbiotically fixed nitrogen in legume residue returned to the cropping system depends on the amount of symbiotic activity, the amount and the type of residue left in the soil and the availability of soil-N to the legume (Hargove, 1986). Haynes & Beare (1997) suggest that some legume roots deposit material of higher nitrogen content, which enhances aggregate stability through greater exploration of those aggregates by fungal hyphae.

2.8.3 Improving and maintaining soil nutrients

Soil fertility decline has been described as one of the causes of declining food production. It has resulted due to continuous nutrient mining without sufficient external input for soil fertility replenishment and unsuitable production systems. It is an important factor affecting soil quality and long- term sustainability of agriculture (Doran & Parkin, 1994). The use of organic inputs such as leguminous green manure and crop residues could therefore be an alternative for maintaining soil fertility.Cover crops can improve soil quality by increasing soil organic matter levels over time. Soil structure is improved during the breakdown of organic matter in the soil when compounds, such as gums, waxes and resins, are formed that are resistant to decomposition. These compounds help bind together soil particles as aggregates. A well-aggregated soil is well-aerated and has a high water infiltration rate. Maintaining and improving soil quality is crucial if agricultural productivity and environmental quality are to be sustained for future.

Sustainable agriculture seeks to provide the needs of the present without compromising the potential in the future. Therefore, practices that produce sustainable yields and economic returns at the same time enhance and maintain soil quality, are preferred over those that degrade the soil as a resource base (Ferreira et al., 2000). An essential element of agricultural sustainability is the effective management of N in the environment. This usually involves the use of biologically fixed N2 because N from this source is used directly by the plants, and is thus less susceptible to volatilization, denitrification and leaching (Graham & Vance, 2000). In the agricultural setting, 80% of this biologically fixed N2 comes from symbioses involving leguminous plants and species of Rhizobium, Bradyrhizobium,Azorhizobium, Mesorhizobium and Allorhizobium (Vance, 1998).

According to Zoumana et al., 2000, research results found that organic inputs are needed not only to replenish soil nutrients but also to improve soil physical, chemical and biological properties. According to Armstrong et al., (1999), perennial legumes had a more beneficial effect on soil chemical and physical properties than annual legumes. Greenland (1975) suggests five basic principles of soil management essential for sustainable agricultural production. He suggest that chemical nutrients removed by crops must be replenished; the physical condition of the soil must be maintained; there must be no buildup of weeds, pest or diseases and there must be no increase in soil acidity or toxic elements and soil erosion must be controlled to be equal to less than the rate of soil genesis.

2.9. Maize pest

Bell (1993) reported black maize beetle as one of the pest of maize. Black maize beetle favour cooler areas and sandy soil. The beetles search for maize plants, crawl in to the soil when they are near the plant and begin to feed on it. A typical symptom is the dying off of the crown of the plant. Spotted maize beetle feeds on pollen, but will also attack the soft young kernel of maize cobs when the silks are wilting off. It was revealed that American boll worm attacks maize cobs and it is commonly called the cobworm. The moth of American boll worm measures about 30 mm across the wings and is variable in colour. Other pest includes the maize chafer beetle which attacks tender growth at night and causing damage to maize leaves. The maize root worm is becoming a significant pest in parts of South Africa (Bell, 1993).

Gouse et al. (2006) reported that maize stem borer and the chilo stem borer are the most harmful pests to maize and grain sorghum in South Africa. Damage caused by stem borer (Busseola fusca) has been estimated to result in a 5-75% yield reduction and it is accepted that Busseola annually reduces the South African maize crop by an average of 10% across years and regions. Songa et al. (2002) reported that agronomic practices that may influence stem borer infestation in maize, including cropping system, sowing time, fertilizer/manure use, stover storage, usage and disposal and maize varieties should be taken into consideration. Farmers use insecticides, wood ash, saw dust, chillie pepper powder, dry cell powder and Mexican marigold to control stem borers (Songa et al., 2002).

CHAPTER 3: MATERIALS AND METHODS

3.1. Description of experimental sites

Field trials will be conducted in a rain-fed experimental site at Agricultural Research Council-Institute for Industrial Crops (ARC-IIC) in Rustenburg, North West province. ARC-IIC is located between latitudes 25o 43'S and longitudes 27o 18'E, 1,157 m elevation.

3.2 Soil characteristics

Pre- plant chemical and physical soil properties will be analysed. Soil will also be analysed at the end of each planting seasons. Rainfall and temperature for long term average and planting season will be collected.

3.3 EFFECT OF MAIZE/LEGUME COVER CROPS ON MAIZE GRAIN YIELD AND N UPTAKE

3.3.1 Experimental design and treatments

The experiment will consist of four treatments as follows: T1: growing maize without mineral fertilizer application, T2: growing maize with fertilizer application, T3: growing maize in rotation with winter cover crops without fertilizer application and T4: growing maize in rotation with winter cover crops with mineral fertilizer application. The four treatments will be laid in Randomised Complete Block Design with three replicates. Winter cover crops will be rye grass, Hairy Vetch (Vicia villosa)and Faba beans (Vicia faba cv. Icarus) and Lupin (Lupinus angustifolius cv. Tanjil)) with and without fertilizer application. Control plots with no cover crops during winter season will be included. Fertilizer will be applied according to the recommendation after soil testing. Fertilizer will be applied by banding at planting and at six weeks after planting in non-legume cover crop. All legume cover crop seed, including in the no fertilizer treatments, will be inoculated with Rhizobium legunominosarium biovar viciae having 5 x 108 rhizobial cells/g (Stimuplant CC, Zwavelpoort 0036, SA) at planting. Seeds will be coated by mixing with slurry containing the inoculant, water and a sticker (methyl cellulose). Seeds will be air dried in a shade before sowing.

During the first planting season, all the plots will be ploughed, disked and maize will be planted at two fertilizer levels (0 N kg/ha and 60 N kg/ha) at spacing of 0.9 x 0.3 m. The cultivar of maize that will be used is PAN 6479 which is a medium maturing variety (Pannar, 2007). Subsequent to maize harvesting, maize stalks will be rolled and glyphosate (360 g/l) at a rate of 5 l/ha will be applied to kill maize stalk. This will be done to allow glyphosate to reach any weeds growing. Pest will be controlled during all planting seasons. During winter season, all cover crops will be planted at generally recommended seed rates. When cover crop reach the flowering stage or just starting the grain filling period, all cover crops will be rolled, glyphosate will be applied as for the previous maize crops and maize will be planted.

Fertilizer will be applied in four fertilizer regimes. The treatments where fertilizer will be applied in both winter and summer seasons, treatments were fertilizer will be applied for winter cover crops with no fertilization in the subsequent maize, treatments where only the summer maize crop will be fertilized with no fertilization on the follow up cover crop and treatment where fertilizer will not be applied in both winter and summer seasons. That means there will be two factors in the second summer season experiment with cover crop species and fertilizer regime giving a 4 X 4 factorial plus control plots laid out as a randomised complete block design with three replications.

3.3.2 Measurements

Two plants per plot will be sampled by cutting at their base near the soil surface at 30 days after planting (DAP) (vegetative stage), 55 DAP (tasseling) and at harvest. Measurements of maize leaf area (LA), plant height and above ground shoot dry mass will be measured by oven drying for 72hours at 65oC. The LA will be recorded using leaf area meter. Maize grain, stover yield (kg/ha) and yield components (grains/cob and one hundred seed mass) will also be measured. Days to 100% tasseling in each plot of maize will be recorded at vegetative stage. At harvesting, the net plot (3x4m) will be used for measuring maize yield. A length of 0.9 m will be discarded on each side of rows and remove 1m end row effects. Grain moisture will be corrected to 12.5 % moisture content using the following formula (Beuerlein, 2009):

(100-wet)/ (100-dry) x wet grain mass.

Where wet is the moisture percent of wet grain and, dry is the grain moisture is the grain moisture at the required percent, usually 12.5%. A potable grain moisture meter will be used to measure maize grain moisture content (MC-7825G Grains Moisture Meter, Pinegowrie 2123, South Africa). Cobs from the net plot will be bulked. Ten samples will then be collected from the bulk randomly irrespective of whether they were the primary or secondary cob to determine cob length. The N concentration of maize plant tissue will be determined by micro-Kjeldahl digestion (Parkinsson & Allen, 1975). N uptake will be assessed as the N content of plant dry matter at three intervals. At harvest grains and stover will also be analysed for nitrogen content.

3.3.3 Data analyses

Maize dry mass, plant heights, yield and yield components, soil temperature, soil moisture and weed dry mass will be analysed as a factorial design using analysis of variance (ANOVA) across seasons. The differences between treatment means will be separated using Least Significant Difference (LSD) test. Genstat Statistical Package will be used for the analysis.

3.4 EFFECT OF COVERCROPS AND FERTILIZER ON BIOMASS, CARBON AND N UPTAKE BY WINTER COVER CROPS

3.4.1 Materials and Methods

This experiment followed maize planted in summer season as described in Section 3.3.1. Temperature and rainfall will be recorded as stated in section 3.2.

3.4.2 Measurements

Two quadrats, measuring 30 cm x 30 cm, will be randomly placed in each plot and plants will be sampled by cutting them at the soil surface for determination of shoot cover crop and weed dry mass. Samples will be randomly collected from plots at three intervals in the both seasons. Weeds and cover crops will be separated and oven dried at 65oC for 72 hours for dry mass determination. On the last sampling date in both seasons, weeds will be identified, grouped into species and dry mass will be determined for each species. Cover crop and weed samples will be ground to pass through a 1 mm sieve and C and N content (%) for both cover crops and weeds will be determined using the automated C/N LECO analyser. Percentage of symbiotically fixed N will be estimated for rye grass by the total N difference method with N uptake from rye grass plots being used as reference biomass (Giller, 2001):

NdA (%) = (TNfix - TNref)/TNfix * 100

Where NdA is N derived from atmosphere; TNfix and TNref is total N accumulation by N2 fixing and reference plants, respectively. Atmospheric N2 fixation will be determined at termination of the N2 fixing cover crops.

3.4.3 Data analyses

Data will be analysed as a factorial using analysis of variance (ANOVA). Common treatments between seasons will be used to allow an across season analysis. The differences between treatment means will be separated using Least Significant Difference (LSD) test.

3.5 EFFECT OF COVER CROPS AND FERTILIZER ON SOIL MOISTURE AND SOIL FERTILITY IMPROVEMENT

3.5.1 Materials and Methods

This experiment followed maize planted in summer season as described in Section 3.3.1. Temperature and rainfall will be recorded as stated in section 3.2.

3.5.2 Measurements

Soil moisture will be measured at 15 cm, 30 cm, 60 cm, and 90 cm depths using a Mobi-check probe (AquaCheck Soil Moisture Management, 44 Oxford St, Durbanville, South Africa) at 7 days after sowing (DAS) and at flowering. Soil samples will be collected randomly at three sampling times: before maize planting, at flowering and at harvest from 0-15 cm and 15-30 cm depth. Four positions in each plot will be randomly selected for soil sampling. Soil samples taken will thoroughly be mixed in a bucket. When taking soil samples plant residues at the surface will be carefully removed and auger will be used to sample. Soil samples will be air dried and ground to pass through 2mm sieve and analysed for organic carbon, organic nitrogen, available phosphorus and pH. Soil inorganic nitrogen will be determined by extraction with 0.5 M K2SO4 (1:4, soil: solution) and analyzed spectrophotometrically as described by Okalebo et al., 2002. The sum of ammonium-N and nitrate-N will be referred to as total mineral N. Available Phosphorus (P) using Bray 1 method, Molybdenum reagent will be used to extract phosphorus from the soil at 1:5 soil water ratios. A spectrophotometer with light band will be used to determine the concentration of phosphorus in the soil extract and potassium was determined by means of an atomic absorption spectrophotometer (Jackson, 1967). pH (KCL) will be measured using a pH meter, Organic carbon will be measured using Walkey Black method.

3.6EFFECTS OF DECOMPOSITION, N AND P MINERALISATION FROM WINTER GROWN COVER CROP RESIDUES

3.6.1Measurements

3.6.1.1Litterbag experiment

Samples of cover crop biomass will be collected by cutting at ground level in unfertilized plots only after the winter trial. Plant materials will be dried at 65oC until the constant mass is achieved. A subsample of each treatment will be ground to pass through < 1 mm sieve for total C and N content analyses using the automatic LECO C/N analyser (LECO Corporation, 2003). Phosphorus will be determined by digesting the plant material in sulphuric acid-selenium digestive mixture and then a calorimetric procedure will be used to determine P concentration as described by Okalebo et al., 2002. Lignin, cellulose and polyphenols by the acid detergent fibre method (Goering & Van Soest, 1970).

For every plot, 10 litterbags will be filled each with 10 g oven dried biomass material. The litterbags measured and weighed. Plant materials were chopped to < 5 cm before they were put into litter bags. Litterbags will be placed on the soil surface and plant residues in the plots will be rolled on top of the litterbags to create a firm contact between the litterbags and the soil surface to allow maximum influence of meso and macrofauna. Litterbags will be placed in the field at the start of the summer season. Temperature and rainfall occurring during cover crop decomposition in the field will be recorded.Litterbags will be sampled at fortnight intervals, with one litterbag randomly selected from each plot. Un-decomposed material will carefully be separated from the litter bags and soil particles removed. The cleaned samples will be in paper bags and oven dried at 65oC to constant weight to determine the remaining mass. Ash free dry mass (AFDW) will be determined after the dried material is oxidized (ashed) in a furnace at 450oC for 5 hours and re-weighed.

For the purpose of estimating N and P contribution to maize growth by decaying cover crops, two maize plants will be sampled per plot by cutting at their base near the soil surface at 78 days after sowing (DAS), at 50 % pollen shedding. Maize shoot dry mass will be measured after oven drying to a constant weight at 65oC. Maize plants will then be ground to pass through < 1 mm sieve and analyzed for total N and P using methods described by Okalebo et al., 2002. Total N and P uptake by maize will be taken as the product of N or P concentration and maize dry mass. Nitrogen and P contribution to maize growth by the decaying cover crops will be estimated as the difference in N and P uptake by maize growing on cover crop residues and maize in the control plots.

3.6.1.2N and P mineralization under laboratory incubation

The soil that will be used for the laboratory incubation studies will be collected from the top 20 cm of the soil at the experimental sites. It will be air dried and sieved to pass through a 2 mm mesh before it will be used in the incubation experiments. Ground samples (< 1 mm) of the plant materials will be thoroughly mixed with 50 g air dry soil. Plant materials will be mixed separately with the soil. A control with no plant material will be included. The plant/soil mixtures will be placed in 150 ml plastic bottles; there will be 18 bottles for each treatment to allow weekly measurements for up to six weeks. The five treatments will be arranged in a randomised complete block design with three replications.

The plant/soil mixtures will be brought to 70 % field capacity and incubated at 27oC. Field capacity of the soil will be determined as described by Okalebo et al., 2002. Soil water will be maintained by periodic addition of deionized water. Three bottles for each treatment will be removed from the incubator weekly and analyzed for pH (2.5:1 water to soil suspension), inorganic nitrogen (NH4-N and NO3-N) as described by Okalebo et al., 2002 and extractable P by the Bray-1 method (NASAWC, 1990). Net mineralized nutrients will be obtained by the difference between values of the control and the treated soil.

3.7. Plot size and row number

Each plot per replicate will have the size of 4.5 x 6 m. Plots of maize and cover crop will have six rows planted at inter-row spacing of 0.9 m. The spacing between each plot will be one meter and between the replicate, it will be two meters.

3.8. Plant density and row spacing of each crop

Maize will be planted at spacing of 0.9 x 0.3 m under rotational and mono-cropping plots. Cover crops will be planted at spacing of 0.45 x 0.3 m.

3.9. Cultural practices

The management practices that will be carried out during experiment will be sowing of seeds in the plots, fertilization and weed control. Weeding will be done more than once as long as there are some interfering weeds. Weeds will be controlled manually. Maize stalk borer will be controlled by the application of pesticide combat at 4 kg/ha. This granular product will be applied manually to the funnels of the plants using perforated lid container.

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