Agronomic potential of phosphatic fertilizer.
Phosphorus is involved in many of the energy yielding and energy utilizing reactions of plant from photosynthetic phosphorylation to respiratory metabolisms. Its role in energy storage and transfer is singly the most important. Phosphate compounds act as "energy currency" within plants. It is also constituent of the cell nucleus and is essential for cell division and for the development of meristematic tissue. Plants utilize monovalent phosphate ([H.sub.2]P[O.sub.4]) much more readily than divalent form (HP[O.sub.4]) as the pH of the solution decreases from 7 to 5. Phosphorus increases the strength of cereal straw, stimulates root development and shoot growth, promotes flower formation and fruit production. It is considered essential for seed formation. It hastens the maturity of crops growth on soil low in P. Adequate P fertilization may improve quality of certain fruits, forge, vegetable and grain crops and increases their resistance to diseases under adverse conditions.
Most soils of Pakistan are alkaline and calcareous and deficient in P. Farmers frequently supply P fertilizers for correcting P deficiency. Availability of P to plants is affected by soil texture, moisture, temperature, aeration, soil pH, clay content and CaC[O.sub.3] in soil. These factors also control chemical reactions of applied P resulting in its conversion into forms, which are not available to crops. The availability of P decreases due to adsorption and precipitation in the soil system. Lack of adequate P lead to retarded growth with the result that plants are sickly, stunted, spindly and weak. When P deficiency is severe, visual symptoms appear, such as dark as dark bluish green or dull grayish-green colours on mature leaves of leaf edges, or purpling of stems due to synthesis of anthocyanin pigment, abnormally dark green in some species and rusty brown lesions in potato turbers. As the plant mature, P is translocated into the seeds and fruit. Phosphorus deficiency causes delayed ripening, poor fruiting and retarded root growth, reduced tillering in cereals, reduced fruit quality and storage potential.
Plants take up P from soil solution, so water-soluble P fertilizers are generally more effective than properly soluble forms. The original sources of P used for agriculture were poorly soluble materials, including manures, bones, guano and phosphate rock. In contrast, highly soluble monocalcium phosphate and diammonium phosphate are the major compounds present in modern, manufactured solid fertilizers containing water-soluble P. Consequently, farmers have been gradually adding various forms of P to the soil.
Modern agriculture mostly relies on granulated fertilizers manufactured from phosphate rock. Most P fertilizers in these fertilizers is water-soluble and they are hereafter called water-soluble P fertilizers. Various P fertilizers are used in horticulture and agriculture in developed countries. These fertilizers are used in agriculture for many reasons. They are usually very effective in overcoming P deficiency and maximizing profitable production regardless of soil type, climate and plant species. The granules are easy to handle and apply by both machinery and by hand. The availability to farmers of diverse water-soluble P fertilizers has made it possible to develop large areas of land for agriculture in many parts of the world.
Forms and Transformation of P in Soils: The phosphates present in soil can be divided into three groups: (i) Phosphorus in soil solution (very small); (ii) Phosphorus present in organic matter; (iii) Inorganic P as definite compounds films of P held in organic particles.
Organic Phosphate: (i) Phytin, inositol hexaphosphates 940-50%); (ii) Nucleic acid and its nucleotides (0.6%). The number of times the soil becomes really dry between wettings and the temperature, are the major factors in the rate of decomposition of organic phosphorus.
Inorganic Phosphates: (i) Calcium Phosphorus:
a) Ca([H.sub.2]P[O.sub.4]) [H.sub.2]O = monocalcium phosphate
b) Ca(HP[O.sub.4]) 2[H.sub.2]O = dicalcium phosphate (hydrated). The dihydrate is metastable and goes over to the dehydrated form relatively easily.
c) CaH(P[O.sub.4])[.sub.3]. 2[H.sub.2]O = octaphosphate may exist, form by precipitation.
d) [Ca.sub.3](PO4)[.sub.2] = tricalcium phosphate--still doubt as to formation by precipitation from aqueous solution.
e) [Ca.sub.10](P[O.sub.4]) (OH)[.sub.2] = hydroxyapatite--bones and teeth
f) [Ca.sub.10](P[O.sub.4])[.sub.6] [F.sub.2] = fluoroapatites--principal phosphatic constituent of mineral phosphates.
Iron and Aluminium Phosphates:
a) In well drained soils, the principal compounds of Al and Fe phosphates are probably the variscite--barrandite--strengite group: (i) FeP[O.sub.4].2[H.sub.2]O strengite, (ii) ALP[O.sub.4].2[H.sub.2]O variscite;
b) In badly drained or waterlogged soils as [Fe.sub.3](P[O.sub.4])[.sub.2] 8[H.sub.2]O;
c) If granulated superphosphate is added to a soil then the following crystals are formed. (i) a trakanite [H.sub.6][K.sub.3][Al.sub.5](P[O.sub.4])[.sub.8] 18[H.sub.2]O, (ii) Hg K. (Al. Fe)[.sub.3] (P[O.sub.4])[.sub.6].6[H.sub.2]O may be formed.
These phosphates occur in many soils as films, a few molecules thick, held on the surface of hydrated ferric and aluminium oxide films; or in association with ferric and aluminium irons forming part of the surface layer of dry crystals. The main experimental facts on phosphate sorption and desorption form hydrous iron (Fe) and aluminium (Al) oxide filmsare: (i) Raising the solution pH releases P., e.g., of simple exchange with OH, hydroxides being insoluble at high pH; (ii) Silicate anions will displace some of the sorbed phosphates; may be anion exchange or formation of Si-Fe-Al complexes; (iii) Oxalate, citrate and tartarate will displace P by forming strong chelation compound with Fe and weaker one with Al; reduces power to absorb p; (iv) Fe phosphate films are more resistant to resolution than LP films.
Phosphate Reserves: Various chemical methods have been devised for determining soil P: (i) Dilute acids dissolve Ca phosphates except apatites; (ii) Concentrated acids dissolve apatites; (iii) Fluorides displace P from the surface of hydrated aluminium oxides and subsequent treatment with alkali displaces it from the surface of hydrated iron oxides; (iv) Reduction solution and iron chelating agent will remove P from below the surface of [Fe.sub.2][O.sub.3] films. Using these methods, it has been concluded that: (a) In strongly weathered calcareous soils--the p exists mainly as Ca phosphates; (b) In moderately weathered soils--P is mainly sorbed on Fe and Al films. In British soils, the apatites tend to be concentrated in the fine sand and silt and sorbed P and organic P in the clay fraction. These are the naturally occurring phosphates in soil.
Single or ordinary superphosphate (SSP about 9% P) was the first water-soluble P fertilizer to be manufactured on large scale by reaction of apatite phosphate rock with sulphuric acid. Apatite phosphate rocks are also used to manufacture phosphoric acid by adding sulphuric acid to apatite phosphate rock. The phosphoric acid is then used to manufacture more concentrated and compound fertilizers. Triple Super Phosphate (TSP) is made by reacting apatite phosphate rock with phosphoric acid, and contains about twice the P present in single superphosphate. Ammonium phosphate fertilizers are manufactured by reacting ammonia with phosphoric acid and contain about twice the P of single superphosphate, together with 11-18% N. The single superphosphate contains about 11% Sulphur (S), whereas the concentrated P fertilizers contain little S.
Phosphate Rock: When phosphate rock is applied to the soil, it needs to dissolve to produce water-soluble P for plant uptake. Dissolution is achieved by reaction between the phosphate rock and the soil, and partly through the direct action of plant rootes and associated mycorrhizae in phosphate rock. Different plant species utilize phosphate rock with different efficiencies. In p-deficient soils, some plant species can acidity the rhizosphere, enabling these species to more efficiently utilize phosphate rock than other species.
Superphosphate Fertilizers: Single or ordinary superphosphate is made by adding concentrated sulphuric acid to powdered phosphate rock containing about 14% total P. The single superphosphate fertilizer contains 9 to 10% total P, 80-90% of which is water-soluble. Triple superphosphate is made by adding concentrated phosphoric acid to phosphate rock. Triple superphosphate is made by adding concentrated phosphoric acid to rock phosphate. Triple superphosphate contains about 20% total P, 80-90% of which is water soluble. The water-soluble P in both single and triple superphosphate is monocalcium phosphate (MCP, Ca([H.sub.2]P[O.sub.4])[.sub.2]. [H.sub.2]O). The insoluble P includes unreacted phosphate rock, dicalcium phosphate (DCP, CaHP[O.sub.4] and CaHP[O.sub.4].2[H.sub.2]O) and various complex Fe and Al phosphates resulting from Fe and Al impurities in the phosphate rock. The calcium sulphate in single superphosphate is predominantly anhydrite (CaS[O.sub.4]) with minor amounts of hemihydrite (CaS[O.sub.4], 0.5 [H.sub.2]O) and gypsum (CaS[O.sub.4].2[H.sub.2]O).
Ammonium Phosphates: The ammonium phosphates are made by passing anhydrous NH3 through phosphoric acid. The most commonly available ammonium phosphate fertilizers are diammonium phosphate (DAP, (N[H.sub.4])[.sub.2] HP[O.sub.4]), containing about 20% P and 18% N, the mono-ammonium phosphate (MAP, N[H.sub.4] [H.sub.2] P[O.sub.4]), containing about 22% P and 11% N. Generally, more than 90% of the P in these fertilizers is water-soluble; with the insoluble compounds being N[H.sub.4+], Al and Fe phosphates compound NPK fertilizer: These fertilizers are commonly made by adding K sals, usually KCI (muriate of potash), during manufacture of superphosphate and ammonium phosphate fertilizers. Generally, the constituents are granulated or physically mixed in the required properties of monocalcium phosphate.
Reactions of Fertilizer in Soil: The monocalcium phosphate and ammonium phosphate (N[H.sub.4]-P) compounds present in granulated water-soluble P fertilizers are soluble and hygroscopic so that even in soil with moisture tension of bout 3 bars sufficient water moves from the soil into the granule to initiate dissolution. The dissolved P reacts with dissolves Ca, Al and Fe in soil solution. Some reactions of the dissolved P can occur in the granule before the concentrated fertilizer solution enters the soil. Dicalcium phosphate as DCP (CaHP[O.sub.4]) and DCP dihydrate (CaHP[O.sub.4].2[H.sub.2]O) my precipitate in superphosphate granules. The Fe and I impurities in the phosphate rock used to make the water-soluble P fertilizers may also precipitate in the granule, forming complex Fe and Al phosphate, either during manufacture and storage of the fertilizer or during dissolution in the soil.
Phosphate Adsorption by Soils: Much P is adsorbed by reacting with Fe, Al and Ca and other irons coordinated with oxygen and hydroxide irons exposed at the surface of soil constituents. In acid soils, the soil constituents that adsorb P include crystalline Fe and Al oxides and oxyhydroxides, day minerals, amorphous compounds of Fe and Al that may exist as coatings on soil constituents and Al that is associated with organic matter. Alkaline soils commonly contain carbonates which adsorb P, P is also commonly precipitated as Ca-phosphate from the alkaline, Ca-rich soil solution. Those constituents that adsorb P in acid soils also adsorb P in alkaline soils. The phosphate irons replace surface irons, including hydroxide, sulphate, molybdate and citrate irons as well as water.
Agronomic Effectivenaess of Water-Soluble P Fertilizers: Agriculture plants need abundant P very early in their growth so as to maximize their growth potential. All water-soluble P fertilizers are equally effective per unit of P. Thus, the agronomic effectiveness of the water-soluble P fertilizers is most strongly influenced by the capacities of the soil to retain and release P. In general, the greater the capacity of the soil to retain P, the relationship between yield and the level of P applied. Therefore, as the P retention capacity of the soil increases, a larger amount of fertilizer P is needed to produce the desired yield. In acid soils, increasing amounts of [Al.sub.+3] irons that exist in soil solution reduce root growth, thereby decreasing the ability of plant roots to explore the soil and take up phosphorus.
Similarly, when plants are under stress, due to disease, pests, water logging and other constraints, their capacity to utilize soil P is reduced. Banding method of application increases the agronomic effectiveness of P fertilizers as developing roots are in intimate contact with P-enriched soil adjacent to fertilizer granule. Fertilizer P applied to the soil surface, most of the P is retained at the surface therefore my be inaccessible to plant root. When surface soil dries between rains during the growing season, roots are unable to utilize soil P and the agronomic effectiveness of the fertilizer is consequently decreased. P-fertilizer banded at depth in the moist soil for longer period, increasing absorption of P by plant roots. Field experiment have consistently shown that the agronomic effectiveness of water-soluble P fertilizer is greatest when the when fertilizer is banded with, below or to the side of the seed, while sowing crops. Annual plants need abundant P during very early growth. The initial source of P is seed.
Residual Value of P Fertilizer: Only 10-30% of P applied as water-soluble P fertilizers is usually utilized by plants in the year of application. Most P from the fertilizer granule is retained by the soil as adsorbed-P reaction products or residual fertilizer compounds. Some of the P that is taken up by crops or pasture is returned to the soil as organic matter. In addition, the P taken up by soil animals and micro-organisms is also returned to the soil as organic matter. The P compounds not taken up by plants can be utilized in future years. That is, P fertilizers have a residual value that is of economic significance to farmers because it reduces the need for fertilizer in subsequent years. The various forms of residual P provide P to soil solution that is utilized by plants for many years after application of water-soluble P. The effectiveness of the sum of residual P forms for plant production can be determined using either plant yield or chemical soil tests for available P. For most soils and plant species, the residual value of water-soluble P fertilizer decreases markedly in the first and second years after application. This is due to the rapid conversion of P in soil solution to more stable, less soluble compounds.
It was observed that the availability of fertilizer P to plants can be adequately explained on the basis of the chemical reactions of soils and fertilizers, together with the interaction of seasonal conditions on the capacity of plant roots to exploit soil P.
--Dr. S.M. Alam and Dr. M.H. Naqvi