Phosphorous Dynamics in salt-affected soils and its Management
Ayesha Irum and Muhammad Ehetisham-ul-Haq
Sustainable agriculture depends on efficient consumption of nutrients to avoid nutrient losses. Since last 4 decades; latest production technology has been adopted by farmers to maximize their profit by producing crops only but not quality crops. According to an estimate, one in nine and total 805 million people in developing countries are not leading a healthy life as they are under-nourished. In developing countries 13.5% of total population is not having enough food to full fill their daily intake of calorie
Abiotic stresses are major constrain to plant growth and crop productivity. Among abiotic stresses, in arid and semiarid region salinity is the major threat. Worldwide, 20% of total cultivated land and 50% of irrigated areas are under salt stress. In Pakistan, about 14% of irrigated lands have deteriorated with salinity while 64% yield losses are reported due to salinity
Land degradation caused by salinity, only about 23 mha area is left suitable for agriculture. Sustainable crop production on marginal land is of the great concern to ensure food safety and security, wise utilization of such land is necessary. In recent years, the science of salt affected soils has become very important due to steady increase in population and expected food shortages in the world.
Most of the crops are very sensitive to even moderate salinity especially at three stages i.e. germination, seedling establishment and grain filling. In salt affected soil application of phosphatic fertilizers are seems ineffective to some extent.
Phosphorous is deficient in our soils that reduce crop yield and productivity. With ever increasing population fertilizer especially P and N use has been increased by 3.5 and 7 folds respectively in past 60 years; and with the conversion of intensive agriculture current fertilizer use expected to increase upto 3 folds by 2050.
The major problems related to the phosphate in soil are very low solubility of phosphate mineral and its strong binding or adsorption on to the particle surfaces causing low phosphate concentration in the soil. As soon as soil is fertilized with phosphatic fertilizer only a small portion of it became available for plant uptake while rest of it gets adsorbed or form precipitate. As a result, phosphate deficiency in crops is very common.
Phosphorous availability is influenced by various factors like soil pH, clay lattice and organic matter contents. In alkaline soils, calcareousness leads to the low recovery of P. Fixation of P in silicate clay is also responsible for deficiency. Soil salinity hinders the availability of essential nutrients to plants, amongst which the phosphate uptake by the plant in the saline soil has broadened the extent of the hazard. The interaction between salinity and phosphorus uptake of plants is a complex or more confusing phenomenon.
Phosphorus buildup and its role in sustainable soil fertility have been identified so long by agriculturists throughout the world, but we are still lagging behind in understanding the phosphorous behavior and its dynamics in soil-plant system. Although scientists have investigated the phosphorous transformation dynamics, release and fixation pattern in soil. However, actual amount of labile phosphorous to be made available in a growing season is still in question.
Strategies to enhance P availability in salt affected soils
In intensive agricultural system decline in soil available nutrient pool is showing fatigue in essential micro and macro nutrients. Deteriorated salt affected soils require special handling for a successful farming. Therefore, necessary measures are needed to understand phosphorous dynamics in soil plant system in order to maximize PUE, proper management practices to lessen the chemical fertilization, optimization of biological potential of microbial community in P mobilization, assimilation of soil phosphorous by plants and its recycling from organic manure. In this regard, 4R strategy i.e. right source, right rate, right time and right method is of great importance. Since prices of the commercial phosphate fertilizers have been increasing in the world and use of fertilizers has become a necessary evil for the sustainable crop production that corrects nutrient deficiencies. Here are some strategies that can be adopted to overcome phosphorous deficiency in salt affected soil.
Manipulation of rhizosphere pH
In rhizosphere plant root plays the vital role in protonation and de-protonation to produce localized acidity or release of OH−/HCO−3 to balance cations or anions entering the roots, respectively. Under P limiting condition, phosphorous transporters can significantly enhance P uptake by plants through root hairs, cell present in root cap, epidermis and outer layers of cortex. These root cells play an important role in solution P up take by plants However, rhizospheric acidification by organic acids released from plant roots is a controversial debate. Because in addition to root- induced changes, there are many factors that influence rhizospheric pH. Several authors suggested that, organic acid dissociation in the cytosol exuded anions rather than acids
It is also worth noted that rhizospheric pH alteration induced by plant roots may not always give the correct measure of H+ or OH−/HCO−3, more specifically in neutral and alkaline soils due to strong buffering capacity of these soils. Thus lowering in pH alone is not cost-effective option for most of the crops and hence other strategies must be used to enhance P uptake by the plant. Thus, different chemical and biological amendments including gypsum, farm yard manures, press mud, green manures are being used.
Effect of organic matter on P dynamics
Addition of organic matter not only sequester carbon in soil but as soil conditioner it improves soil health and fertility status by allowing sufficient release of essential nutrients in soil solution. In addition, organic amendments are proven to be effective in reclamation of salt affected soils. In dry conditions, uptake and in saline conditions availability of P is reduced. Release of humic substances upon mineralization of OM, improves solubilization of hardly available P thus increase its uptake due to acidification. As per as effectiveness of P absorption is concerned, organic sources are more effective than inorganic.
Sewage sludge and domestic waste plays a vital role in recycling of P. For sustainable crop production adequate supply of P is necessary as, rock P reserves of the world is becoming deficient therefore use of other sources i.e. sewage sludge and domestic waste as fertilizer should be encouraged. Many studies have proved sewage sludge as a good source of plant available phosphorous.
The major problem in P deficient soil is the transformation of available P to the unavailable organic residues. Mixing of the crop residues enhances the P availability, total contents and adsorption while decreasing its desorption rate in the soil. On decompositions, crop residue release significantly higher amount of available P in soil.
Gypsum is a common mineral composed of hydrated calcium sulfate (CaSO4·2H2O). Gypsum application in soil not only used as an amendment for correcting the sodic soil but it also decreases the soil pH due to presence of sulphate and enhances the efficiency of soil phosphate to plants.
Biochar, “Black Gold” is a carbon rich byproduct of pyrolysis of biomass. It acts as a potential source of phosphorous to mitigate phosphorous depletion. Number of factors influence behavior of biochar phosphorous in the soil i.e. pH and ionic concentration. However, neutral to high pH i.e. (pH 6.0-9.0) and low ionic strength merely affect P release. Previously effect of physical factors i.e. time and chemical factors such as nutrient concentration on phosphorous release from biochar surface has been studied. Under natural conditions, it was found that phosphorous release was <50% from biochar surface
Amongst the various organic fertilizers, animal manure is widely used material that can supply ample amount of organic P to the soil. The P content of the manure varies with animal species, age, and diet, but typically varies between 5 and 25 % of total P
Biological inoculation/ tools
Microbial communities are an integral component of the soil P cycle and act as bridge between transfer of labile and nonlabile P. Various mechanisms are involved to mineralize and solubilize organic P through phosphorous solubilizing bacteria. And make available for plant uptake. Microbial population plays a vital role in release of phosphorous from the organic amendments. Pantoea agglomerans, Burkholderia vietnamiensis. Pseudomonas aeruginosa, Citrobacter sp. Acinetobacter rhizosphaera, Aspergillus niger, Saccharomyces cerevisiae, Pseudomonas fluorescens are some microbial species that can make inorganic P available for plants uptake as labile P (HPO4-2). However, it is important to have better understanding of all the mechanisms involving P solubilization by phosphorous solubliers thus to optimize their use in field. The better understanding of plant- microbe interaction and P cycle the more beneficial will be its use to reduce reliance on chemical fertilizers.