Of all living organisms, only green plants and many micro-organisms contribute to the mineralisation and N fixation of raw materials (Epstein, 1971). As mentioned earlier, leguminous plants have a symbiotic relationship with rhizobial bacteria which fix N. In return, the plant supplies mineral elements and other organic compounds to the symbiotic bacteria. The amount of N fixed depends on the species of N-fixing bacteria, plant growth conditions, and the type of plant (Figure 2.1) (Espinoza et al., 2005). For example, the bacterial species that are very effective with soybeans to fix N2, are not effective with alfalfa. The process of N fixation occurs in the root nodules that form on the root system (Espinoza et al., 2005). The process of biological N fixation is achieved by stimulating the nitrogenase enzyme, a process that is influenced by many soil and weather factors. The process of biological N fixation can cease or fall to minimal levels as a result of very low soil pH levels, or the presence of large quantities of available mineral N. This is because the symbiotic associations between the bacteria and the host plants do not function very well at these levels. The use of an inappropriate bacterial inoculum will reduce the formation of nodules, thereby impeding the process of biological N fixation. In fact, there are other organisms in the soil that are able to fix N2 via non-symbiotic associations. However, the output of these organisms, with the exception of blue-green algae, is of minor importance (Espinoza et al., 2005).
Figure 2.1 Efficiency of symbiotic N2 fixation by some temperate legumes in the orchard, the data
source is (Phillips, 1980). Methods were used to estimate N2 fixation: (1)
for “Medicago sativa L. used
Plant species Medicago s ativa L. M. sativ a L. Phaseol us vul garis L. Vicia ben ghale nsis L. V. faba L. Pisum s ativum L. Trifoliu m sub ter-rane um L. T. hirtum L. T. pra tense L. kg N 2 f iXed / h a·yr a 0 100 200 300 400 500 600 700 N2 Fixed
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soil and crop N balance for 10-yr continuous culture. (2) M. sativa L. and T. pratense L. used soil and crop N balance for 10-yr rotating culture (legumes rotated in alternate years with Hordeum vulgare L.
or Secale cereale L.). (3) Phaseolus vulgaris L. used N difference between plots with and without
Rhizobium. (4) Vicia benghalensis L. used 15N A-value correction of total-N difference. (5) V. faba L.
and Pisum sativum L. used C2H2 reduction.
(6)
Trifolium subter-raneum L. used 15N A-value with
Bromus mollis L. as a reference crop. (7) T. hirtum L. used 15N A-value correction of total-N
difference” (Phillips, 1980).
2.5.2
Phosphorus
Along with N and K, P is also a key nutrient for plants. Phosphorus forms part of the nucleus of the plant cell, and the plant needs it for the production of bio-energy and for cell division. It is also necessary for opening of the stomata (Lyle et al., 2006), which may be due to the changes in the balance between cytokinins and abscisic acid, as result of the change in the level of P leaf (Radin, 1984). Because atmospheric returns of P are low compared with carbon and N, and biological P recycling is slow, agricultural fields are typically supplied almost entirely with artificial P (Walker and Syers, 1976, Ezawa et al., 2002, Tipping et al., 2014). Phosphorus is less mobile within both plants and soil compared to K; therefore, its deficiencies show up in young leaves. Also, unlike K, it is less easily leached from the soil (Lyle et al., 2006). Fernandez and Rubio (2015) reported that a deficiency of P enhances a steady increase in root aerenchyma and root porosity of sunflower, maize, and soybean. Fernandez and Rubio (2015) stated that the formation of aerenchyma causes a decrease in or modifies root length/unit root biomass, which in turn leads to a decline in foraging by the roots (Fernandez and Rubio, 2015). It was also observed that the deficiency of P led to a decrease in the density of roots in some pasture species (Fernandez and Rubio, 2015). However, because P can easily be locked away in a form that is inaccessible to plants due to its ability to form complex molecules with other nutrients in the soil, Lyle et al. (2006) believe that P deficiency is related more to soil pH rather than to deficiency in the soil itself. We can conclude from this that the pH of soil plays a pivotal role in facilitating or fixing a range of nutrients, just as it does in the biological life of soil. As has been stated previously, soil pH is directly affected by the type of agricultural practices used, particularly conventional practices. Therefore, for soil to be a suitable medium for nutrients, pH must be maintained at an appropriate level. Hence finding suitable horticultural applications that will improve the soil pH and enhance productivity and plant growth is important.
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Walker and Syers (1976) and Gyaneshwar et al. (2002) made two findings: firstly, that biological N fixation ceases under normal circumstances when available inorganic P had vanished, and secondly, that in the absence of inorganic P, non-nitrogen fixing organisms, either plants or microbes, compete with each other for the N and P mineralised from organic matter in the soil; as a result available inorganic P disappears or becomes limiting. However, these two issues can be solved if inorganic P is made available. This suggests that industrial input cannot be completely dispensed with. However, it is still feasible to minimise the negative effects of synthetic fertilisers by integrating specific rates of recommended applications with organic, biological alternatives.
It is well known that bacteria play a significant role in dissolving P in the soil for uptake by plants. However, studies by Banik and Dey (1982), Kucey (1983) and Turan et al. (2006) found that since most of these bacteria are able to solubilise calcium phosphates and few can solubilise iron phosphates and aluminium phosphates, they would be more effective in calcareous soils than in alfisols where phosphates are complexed with Fe and aluminium (Al). Further research is therefore needed to identify microbes that can solubilise these iron and aluminium phosphates and to mobilize phosphate reserves in the soil, so that more P is available to plants.
Fixed P in the soil requires several factors to become optimised, including sufficient moisture within the soil, appropriate soil pH, and biological activity in the soil. Gyaneshwar et al.
(2002) reported that mycorrhizal fungi and P-solubilising bacteria produce organic acids such as acetate, lactate, glycolate, tartarate, oxalate, succinate, gluconate, citrate and ketogluconate which, in turn, affect the ability of phosphate to be solubilised. Most bio-dissolution of phosphate occurs for phosphate that has formed a complex compound with calcium (Ca), while small amounts of phosphate solubilise from P when the latter is combined with Fe and Al. Gyaneshwar et al. (2002) also suggest that mycorrhizal fungi may not be able to form strong colonies on plant roots in situations where high concentrations of P are present in both plants and soil. They also discovered that, in some cases, there was a reduction in growth of plants which had been inoculated with mycorrhizal fungi in the presence of high levels of available phosphorous. This may be attributed to the direct uptake route that plants use which may be prevented by AMF colonisation or there are few benefits to outweigh the impact of the C-sink. Both of these findings must be taken into account when trying to improve soil fertility to maintain biological sustainability.
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