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13Scientific Journal Impact Factor (SJIF 2022=4.63) Passport: http://sjifactor.com/passport.php?id=22230
SOIL PHOSPHORUS AND PLANT ADAPTATION MECHANISMS
Dilnoza Israeli kizi Masobirova
Assistant at the Fergana Polytechnic Institute
ABSTRACT
Phosphorus is one of the seventeen essential nutrients required for plant growth. Despite its importance, it is limiting crop yield on more than 40% of the world's arable land. Moreover, global P reserves are being depleted at a higher rate and according to some estimates there will be no soil P reserve by the year 2050.This is a potential threat to sustainable crop production. Most of the P applied in the form of fertilizers may be adsorbed by the soil, and is not available for plants lacking specific adaptations. Available soil P and hence crop yield can, however, be increased through applying P containing fertilizers to feed the ever increasing world population. The P contained in crop residues if left in the field can be recycled by incorporating the residues into the soil whereas part of P in crop residues fed to livestock can be returned back to the soil in the form of manure and as bone meal.
Keywords: adaptation mechanism, nutrient stress, phosphorus efficiency, soil phosphorus management.
Phosphorus is one of the seventeen essential nutrients required for plant growth. It is the second most important macronutrient next to nitrogen in limiting crop growth. Plant dry weight may contain up to 0.5% phosphorus and this nutrient is involved in an array of process in plants such as in photosynthesis, respiration, in energy generation, in nucleic acid biosynthesis and as an integral component of several plant structures such as phospholipids. Despite its importance in plants growth and metabolism, phosphorus is the least accessible macro-nutrient and hence most frequently deficient nutrient in most agricultural soils because of its low availability and its poor recovery from the applied fertilizers. The low availability of phosphorus is due to the fact that it readily forms insoluble complexes with cation such as aluminum and iron under acidic soil condition and with calcium and magnesium under alkaline soil conditions whereas the poor P fertilizer recovery is due to the fact that the P applied in the form of fertilizers is mainly adsorbed by the soil, and is not available for plants lacking specific adaptations. Moreover, global P reserves are being depleted at a higher rate and according to some estimates there will be no soil P reserve by the year 2050.
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More than 40% of the world soils are deficient in phosphorus and the acid-weathered soils of tropical and subtropical regions of the world are particularly prone to P deficiency. On the other hand, in order to cope with the ever increasing world population agricultural production and productivity need to parallel increase with the increasing population. One option to enhance soil P availability and hence crop yield is to apply P containing fertilizers. However, there is scarcity, particularly of chemical fertilizers, in tropical and subtropical regions where most of the earth's population is concentrated. Moreover, lack of fertilizer infrastructures, financial constraints by farmers, and poor transportation facility in the rural areas all make P fertilization unattainable for these areas. Sustainable management of P in agriculture requires that professionals in the area of crop sciences discover mechanisms that either enhance plant P acquisition ability and/or efficient P utilization ability and further exploit these adaptations to make plants more efficient to thrive under P limiting conditions.
Soil phosphorus status and its availability
Despite its importance for normal plant growth and metabolism, P is one of the least accessible nutrients. Many soils are inherently poor in available phosphorus content although the total amount of P in soil may still be high. This is evident from the extremely low soil solution P concentration (<1 ^M) in sandy soils, alkaline soils and highly weathered soils of tropics and sub-tropics. Moreover, a large fraction of total soil P is in organic form in many soils and these forms are not directly available to plants. Many of the agricultural soils in the developing countries in particular are P-deficient and have an unfavorable condition for P availability. It is estimated that crop productivity is limited by P deficiency on more than 40% of the world arable lands. Additionally, world's resources of P are limited.
Unlike nitrate, which readily moves in soil towards the roots via both mass flow and diffusion, phosphate ion is highly immobile in mineral soils. Thus, mass flow delivers only little phosphate ions (1-5% of plant demand) and the greater portion of required phosphate ions reach the root surface via diffusion. However, the diffusion coefficient for phosphate ion in soil is very low compared to those for other nutrients.
Phosphorus is commonly bound to iron and aluminium oxides and hydroxides through chemical precipitation or physical adsorption. As a result of adsorption, precipitation and conversion to organic forms, only 10-30% of the applied phosphate mineral fertilizer can be recovered by the crop grown after the fertilization. The rest stays in the soil and may be used by crops in the following years. Because of low P solubility and desorption, only a small proportion of phosphate ions exist in the soil solution for plant uptake even under optimum P fertilization making P fertilizer recovery to be lower compared to other nutrient containing fertilizers. This suggests that
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chemical fertilizer application alone is not a cost effective way of increasing crop production in many P-limiting soil. Therefore, the use of genotypes/cultivars with improved root traits able to unlock and absorb P from bound P resources and/or effectively utilizing the absorbed P is of paramount importance for enhancing the efficiency of P fertilization.
Soil phosphorus management options for sustainable crop production
Sustainable crop production aims at maintaining high crop yield without adversely affecting ecosystems to meet the need of current as well as future generations. Since phosphorus in agriculture is the second most growth limiting macronutrient after nitrogen, its proper management in soil contributes significantly to sustainable crop production. In such soils where yield is limited because of inherent low P concentration (P deficient soils), application of relatively higher amount of mineral P fertilizers is the only way to enhance soil available P status to a target value in a long run that can sustain high crop yield. However, once the target value is reached, the available soil phosphorus concentration can be kept at a level that can sustain high crop yield through maintenance fertilization.
The P contained in crop residues left in the field can be recycled by incorporating the residues into the soil whereas part of P in crop residues fed to livestock can be returned back to the soil in the form of manure and also as bone meal. The mineralization of such organic P sources can occur through the action of microorganisms and plants exuding phosphatases and phytases. However, the P removed along with cereal grains, other edible vegetable parts and livestock products such as cow dung, milk and meat used for human consumption need to be replaced through mineral P fertilizer application. Therefore, under condition where P removed from the soil by harvested crop can be returned as crop residues and manures, the amount of mineral P fertilizer required for maintenance fertilization becomes less. In a nutshell, regular application of maintenance P fertilizers, incorporation of crop residues and application of organic manures can reduce nutrient mining and contribute to sustainable crop production.
Mechanism of Phosphorus efficiency in plants
Phosphorus efficiency is a term that generally describes the ability of crop species/genotypes of a given plant species to give higher yield under P-limiting condition. Plant species as well as genotypes within the same species may differ in P efficiency. The ability of a crop/genotype to give higher yield under P-limiting condition may be related to: the ability to take up more P from the soil under P-limiting condition (uptake efficiency) or the ability to produce higher dry matter per unit of P in the plant tissue (utilization efficiency) or a combination of both.
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Conclusions
Global P reserves are rapidely being depleted. Moreover agrciultural soils especially of tropical and subtropical regions of the world are inherently poor in available P content thus ultimately affecting crop yield. This problem is further aggravated in the aforementioned regions by the absence or sub-optimal application of P fertilizers due lack of financial resources and lack of access to P fertilizers by the farmers making the available soil P content in these regions to be far below the optimum amount that can sustain higher crop yield. Therefore, maintenance of soil P at a target value through either of the following ways: application of P fertilizers, periodic incorporation of crop residue, application of organic manures would be very essential for sustainably higher crop yield. Under conditions where all these soil P management options might not be possible, the use of P-efficient crops or crop genotypes, having desirable traits that enables them to have better performance under P stress conditions, might serve as an alternate option for sustainable crop production.
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