**3. Phosphorus problems in the soils**

Phosphorus (P), being the second most important macronutrient after nitrogen, has a critical role in biological nitrogen fixation (BNF). It has been a renowned fact that legumes require more phosphorus (P) compared to non-leguminous crops as they perform the process of BNF through nodulation which is a characteristic property of legumes. Nodulation occurs in almost all the legumes. However, physiology and efficiency of nodules to conduct the process of nitrogen fixation is species specific. So for the process of BNF, P serves as an ultimate source of energy in the form of ATP [5, 41, 42]. It is also needed for signal transduction, membrane biosynthesis, and nodule development and function [8]. Moreover, nodules under P-sufficient conditions have a higher P concentration (up to 1.5% of the total plant P) as compared to shoots

There are many reports about the influence of P on nodulation; ultimately the amount of N fixed by the plants e.g. nitrogen contents of the legume, *Crotalaria micans*, were increased about 4 folds due to increased nitrogen fixation with the application of P at 90 kg ha−1 [43]. Similarly Israel [15], studied the effect of P on accumulation of nitrogen in soybean (*Glycine max* L.). It was found that the concentration of nitrogen was increased with increasing the supply of P that was suggested to be due to increase in the symbiotic nitrogen fixation as P serves as an energy source. Plant dry matter, nodule number, and mass was also increased with increasing P supply. Enzyme activity of nitrogenase was also enhanced with increasing P. Earlier legumes are well renowned as P exhaustive crop plants due to the formation of nodules for symbiotic nitrogen fixation. In another experiment on different legumes (soybean, clover), similar effect

Another soil and sand culture experiment to study the interaction of N and P and their effect on growth, nodulation, nitrogen fixation, activity of nitrate reductase, and on the accumulation of nitrogenous compounds (ureides, amino acids, nitrate) in the xylem sap of common bean was conducted [3]. Both the soil and sand culture experiment showed that with increasing levels of N, nodulation parameters such as nodule number and mass, nitrogenase activity, and xylem ureides were decreased, while the concentration of asparagine in the xylem sap increases. Symbiotic nitrogen fixation was only increased at low N concentration with increasing P application. Similarly it was also found that the effect of N on the inhibition of nodulation including nodule number and biomass was systemic, while high dose of P had a systemic stimulatory effect on nodulation parameters as mentioned above. The systemic effect was confirmed by its direct effect on nodulation and not on the plant growth overall. There is still lack of information on whether there is effect of N and P on both the nodulation and

Similarly, another Leonard jar experiment was conducted to study the impact of P (0–2 mM P) on growth, symbiotic nitrogen fixation, N and C metabolism, as well as on the concentration of ATP, N and P contents of common bean (*Phaseolus vulgaris*). With the application of medium to high-P, not only the nodulation (nodule biomass: 4-fold) and growth parameters but also the P contents of the harvested plants were increased at the onset of the flowering. In the case of total soluble sugar and amino acid contents of leaf, root, and nodules of the plant, these were decreased with increasing the level of P application. Moreover, an increase of 20-fold in nodule-ARA and 70-fold in ARA per plant was observed with the application of 1.5 mM P [45]. Another

of P on growth and nodulation parameters was recorded [44].

nitrogen fixation or not and needs to be explored in future studies [32].

and roots [6].

112 Organic Fertilizers - From Basic Concepts to Applied Outcomes

Plants depend almost exclusively on the availability of nutrients from the soil as these are fixed in the soil. As far as P is concerned, its availability in the form of PO4 3-, HPO4 2-, and H2PO4 depends on the soil pH and is a significant determinant of plant growth [41]. There are two forms of P present in the soil: the organic (20–85%), which is the large proportion, and the inorganic. Phosphates (PO4 3-, HPO4 2-, and H2PO4 - ) are the most available forms of inorganic P in the soil. While the organic portion of P, phytin and its derivatives constitute about 50%, lecithin and glycerol-phosphate are present in minor fraction, and these organic compounds could serve as a good source of P if mineralized [47].

Normally, the amount of total P present in the soil is about 0.05% (w/w) but the available fraction of P to the plants is very small (Reference). To overcome this problem and to maximize the production of crops per unit area, farmers have to apply costly inorganic synthetic phosphatic fertilizers, but their availability to the crop plants hardly reaches 20% [48]. This has led to the addition of 2–4 times more than the required amount of phosphatic fertilizers and has led to the maximization of cost to benefit ratio of growing crop plants per unit area [49]. Another issue with the phosphatic fertilizers, most available form of P, is that their cost has gone sky high and unaffordable for the poor farmers of the world especially of the developing countries like Pakistan. Moreover, it becomes an environmentally ill practice if we consider the production of chemical phosphatic fertilizers, which include the use of sulfuric acid [50]. Moreover, environmental problems such as eutrophication has also emerged which has led to the destruction of the habitat of the aquatic life and has caused environmental degradation. In conclusion, these problems have pessimistically posed a problem not only on the environment but also on the economics of growing crops. There are two reasons, which are mostly quoted for acidic and basic/alkaline soils depending on the soil pH. In acidic soils such as oxisols, inceptisols, and utisols, the added P quickly reacts with Fe and Al oxides as these are the dominant cation oxides in these types of soils and make it unavailable to the plants. In case of alkaline soils, Ca and Mg salts are abundantly present, so these cations react with phosphatic compounds thus making them unavailable to the plants [41].
