- Agronomic: millet, rye, wheat
- Vegetables: beans
- Crop Production: application rate management, intercropping, multiple cropping, nutrient cycling, tissue analysis
- Education and Training: demonstration, display, extension, farmer to farmer, networking, on-farm/ranch research, participatory research
- Pest Management: mulches - living, mulching - vegetative
- Production Systems: agroecosystems, holistic management
- Soil Management: nutrient mineralization, organic matter, soil analysis, soil quality/health
Based on a farmer’s observation in 2002 that pinto beans intercropped with annual ryegrass did not exhibit iron-deficiency chlorosis and outperformed bean monocultures, the objective of this project was to determine if interplantings of annual ryegrass can mitigate iron deficiency in dry beans in high pH calcareous soils in Wyoming and zinc deficiency in acidic soils in western Kenya. The project also investigated recent observations that iron deficiency chlorosis can be overcome by more resistant cultivars of dry beans, temperature, close bean plant spacing and low levels of nitrate nitrogen in these soils.
Iron deficiency chlorosis in dry edible beans is common in the high pH, calcareous soil prevalent in the western Great Plains (Stevens and Belden, 2005). In general, this condition is not due to absolute lack of soil iron, as total iron concentration in the soil is usually adequate (Aktas and Egmond, 1979). The deficiency is normally due to the conversion of the more bio-available ferrous (Fe2+) iron into less soluble ferric (Fe3+) oxides, hydroxides, oxihydroxides and ferrihydrites (Cornell and Schwertmann, 2003).
Higher plants have developed two different strategies that help them to increase iron availability in soils (Alcaniz et al., 2005). Strategy I is developed by dicot and non-graminaceous monocot species in response to iron deficiency and involves acidification of the rhizosphere root zone by the plants through proton (H+) extrusion, increasing Fe3+ – chelates’ solubility and the concomitant reduction by a ferric reductase to ferrous iron which can then be up-taken by plants (Robinson et al., 1999). In Strategy II plants developed by graminaceous (Poaceae family) species such as wheat (Triticum aestivum), barley (Hordeum vulgare), rice (Oriza sativa) and maize (Zea mays), iron-induced morphological and physiological changes adopted by Strategy 1 plants are absent. Instead methionine derivatives belonging to the mugineic acid family (phytosiderophores) are synthesized by the plant roots which are secreted in the rhizosphere root zone, where they chelate ferric iron and make it more bioavailable (Marschner and Romheld, 1994; Marschner, 1995; Vansuyt et al., 2000). Strategy I and II plants can also be referred to as iron-efficient plants. In high pH calcareous soils with high bicarbonate content, the activity of Strategy I can be neutralized causing iron deficiency chlorosis in the plants (Alcaniz et al., 2005). Studies by Venkat Raju and Marschner (1972) showed that reducing substances released by iron-efficient Strategy I plants under iron deficiency declined when the pH of the medium was sustained at high level during the growth of sunflower in nutrient solution.
Many microorganisms can also produce Fe3+ – chelating compounds known as hydroximate siderophores (HS) with high affinity for Fe3+and capable of transporting iron (Sylvia et al., 2005). Studies by Powell et al. (1980; 1983) found that HS occur in the soil at levels that can be sufficiently high enough to promote absorption of iron by plants. Uptake of iron by iron-efficient plants such as oats can therefore be due to the release of phytosiderophores by their roots or their ability to obtain iron chelated by microbial HS. Annual ryegrass has not yet been established in literature as an iron-efficient graminaceous plant capable of producing iron-chelating phytosiderophores. The role of microorganisms in alleviating iron deficiency in dry edible beans has also not yet been conclusively reported in literature.
Studies by Aktas and Egmond (1979), Mengel (1994), and Mengel et al. (1994) have shown that high nitrate nitrogen levels in the soil can also induce iron chlorosis. Besides the proton extrusion strategy developed by plants in response to iron stress, nitrogen nutritional status considerably influences proton or hydroxyl (OH-) ion excretion from plant roots (Aktas and Egmond, 1979). Plant species growing in complete nutrient solution with nitrate-nitrogen exude OH- or HCO- into the nutrient medium as long as there is enough nitrate in the medium (Kashirad and Marschner, 1974; Aktas and Egmond, 1979). Iron efficient plants secrete H+ ions into the rhizosphere root zone when iron stress develops, regardless of the nitrate status of the soil, and continue to excrete protons even after soil nitrate supply is depleted (Aktas and Egmond, 1979). Iron inefficient plants on the other hand continue to secrete OH- and HCO-ions into the rhizosphere root zone when nitrate is sufficiently available even when iron stress develops (Aktas and Egmond, 1979; Romheld et al., 1984). Excretion of protons by these plants only begins after nitrate supply in the soil has been depleted.
Practiced worldwide for many generations, mixed cropping – especially of legumes and grasses – can enhance on-farm biodiversity, promote biological nitrogen fixation, increase dry matter production and grain yield and enhance resource use efficiency (Agboola and Fayemi, 1972; Willey, 1979; Searle et al., 1981; Aggarwal et al., 1992; Fujita et al., 1992; Okereke and Anyama, 1992; Shaxton and Tauer, 1992; Kim and Rees, 1994; Sylvia et al., 2005). Different plant cultivars and species exhibit different susceptibility to iron chlorosis (Aktas and Van Egmond). Plant cultivars that are tolerant to iron deficiency selectively intercropped with susceptible plant cultivars can alleviate iron chlorosis in the latter. In comparative studies between iron-efficient sunflower plant species and iron-inefficient corn species, Venkat Raju and Marschner (1972) and Kashirad and Marschner (1974) showed that under iron deficiency conditions, sunflower plants lowered the pH of the nutrient solution resulting in increased uptake of inorganic Fe3+ evidenced by re-greening of the sunflower plants. In contrast, corn plants were not able to lower the pH of the nutrient solution as a result of which they were unable to utilize Fe3+ as their source of iron. When the two plant species were intercropped in the nutrient solution under iron-stress, the iron efficient sunflower lowered the pH of the nutrient solution, enabling corn plants to also re-green with Fe3+ as the source of iron. The role of annual ryegrass in a similar cropping system to mitigate iron deficiency in dry edible beans has not yet been reported in literature.
The purpose of this study was to investigate the role of interplantings of annual ryegrass, temperature, soil microorganisms, and nitrate-nitrogen in mitigating iron deficiency in susceptible dry beans in calcareous soil.
One of the most widespread nutritional constraints in crop plants (Sillanpaa;, 1982), zinc is an essential constituent of many vital enzymatic systems in plants (CPHA, 2002; Ludwick et al., 2002). Zinc is usually taken up by plants as a divalent cation (Zn2+), which can function as a metal component of enzymes or as a “functional, structural, or regulatory cofactor of a large number of enzymes” (Kickens, 1995). Among other important enzymatic activities, zinc plays a key role in the synthesis of indoelacetic acid, which is an important plant growth regulator (CPHA, 2002). Zinc deficiency in dry beans results in interveinal chlorosis starting at the terminal growth areas, decrease in stem length, rosetting of terminal leaves, reduced fruit bud formation and dieback of twigs after the first year (Ludwick et al., 2002). Low availability of zinc in soils can reduce crop yields by up to 50% (Cakmak, 2002).
The concentration of zinc in the soil between 0.5 and 2.0 mg kg-1, depending on the methods of extraction, are considered deficient for plant development (Jacobsen et al., 2003; Singh et al., 2005). While the total concentration of zinc in the soil is mainly due to parent rock materials, its application in fertilizers, pesticides and waste materials can also determine its content (Halina, 2003). Zinc deficiency commonly occurs on neutral and calcareous soils, intensively cropped soils, paddy soils, poorly drained soils, sodic and saline soils, peat soils, soils with high available phosphorus and silicon, sandy soils and highly weathered acid and coarse-textured soils (Singh et al., 2005). Factors such as topsoil drying, disease interactions and high cost of fertilizer also contribute to zinc deficiency (Sillanpaa;, 1982). Zinc deficiency is common in the old highly weathered acidic soils of western Kenya, where dry beans are a major source of protein for most low-income communities in Kenya (Sombroek et al., 1992; Itulya and Aguyoh, 1998).
Although zinc is thought to be adsorbed on cation exchange sites of clay minerals under acidic conditions, this adsorption is reduced by other cations competing for the exchange sites resulting in more bioavailability of zinc under low pH conditions (Kabata-Pendias, 2001). However, bioavailability of zinc in acid light soils is reduced through immobilization in soils rich in Ca and P, in well-aerated soils with S compounds and in soils containing substantial amounts of certain Ca-saturated minerals such as allophone, imogolite, and montmorillonite, as well as hydrous oxides (Kabata-Pendias, 2001).
Although zinc is thought to be adsorbed on cation exchange sites of clay minerals under acidic conditions, this adsorption is reduced by other cations competing for the exchange sites resulting in more bioavailability of zinc under low pH conditions (Kabata-Pendias, 2001). However, bioavailability of zinc in acid light soils is reduced through immobilization in soils rich in Ca and P, in well aerated soils with S compounds and in soils containing substantial amounts of certain Ca-saturated minerals such as allophone, imogolite, and montmorillonite, as well as hydrous oxides (Kabata-Pendias, 2001).
In Kenya, dry beans are commonly interplanted in rows between rows of maize to increase productivity per unit of land, given that land sizes among most farmers are quite limited, averaging 1.5 hectares per family of six (Woomer and Mukhwana, 2004). Interplanting dry beans with maize not only provides farmers with two staple food crops within the same growing season, but some of the nitrogen fixed by dry beans through its association with nitrogen fixing bacteria can be transferred to the maize growing in close proximity, thereby increasing the cropping system’s yield and efficiency of nitrogen use (Fujita et al. 1992). The role of annual ryegrass in such a cropping system to mitigate zinc deficiency in legumes has not yet been reported in literature.
The purpose of this study was also to investigate the role of annual ryegrass intercropped with dry beans in mitigating zinc deficiency in the acidic soils of western Kenya.
1. To determine the effectiveness of intercropping annual ryegrass with pinto beans in mitigating iron deficiency in calcareous soils. This was compared to interplantings of wheat as a grass known to exude compounds capable of chelating iron. Within this objective included an investigation into the following:
1.1 The role of temperature and high bean plant density/close spacing
1.2 The role of bean cultivar susceptibility/tolerance and microorganisms
1.3 The role of nitrate-nitrogen on iron deficiency
2. To determine the effectiveness of intercropping annual ryegrass with pinto beans in the field in mitigating zinc deficiency in more acid soils. This was compared to interplantings of wheat as a grass known to exude compounds capable of chelating zinc and interplantings of millet, a hardy cereal food crop popular in western Kenya.