UNDERSTANDING THE DILEMMA OF PHOSPHORUS IN AGRICULTURE
DR. MIDRAR UL HAQ
Hinning Brand is boiling Urine Krafft German Chemist
Phosphorus extraction from Phosphate Rock John Bennett Lawes Rothemsted Experimental Station
Phosphorus necessity for seed formation
Justus Von Liebig (1803-1873) was the first one, who developed the concept of fertilizer recommendations based on the chemical analysis of the plants and interpretation of the analysis.
Also highlighted the necessity of phosphorus for seed formation.
German Chemist Justus Von Liebig
waxy white (yellow cut), red (granules center left, chunk center right), and violet phosphorus
2. World Phosphate Reserves 18,000,000 World total (rounded) 1,200,000 77,000 260,000 25,000 6,600,000 100,000 90,000 180,000 900,000 5,700,000 200,000 50,000 1,500,000 100,000 30,000 100,000 800,000 United States Australia Brazil Canada China Egypt India Israel Jordan Morocco Russia Senegal South Africa Syria Togo Tunisia Other Countries (including Pakistan 7.45 million metric tons) Reserves (*000 Metric tons) Country
3. Why phosphorus is shown P2O5 on the fertilizer bag?
Before the invention of modern sophisticated instruments, chemists used to analyze P and K through gravimetric (weighing) ignition method. In that method, it was difficult to isolate P from fertilizer, so they used the method of expressing P and K as P2O5 and K2O.
Fertilizer companies prefer to use these notations because these show more concentration than that in the elemental form and hence attract customers.
4. When fertilizer is applied to soil, it dissolves in water and gives phosphorus ions, which have negative charges while the clay mineral in soil have also negative charges then how it is adsorbed and retained in soil?
Schematic diagram of the phosphate cycle
How Can organically complexed metals affect P adsorption? P is complexed with OM through metal bridges? OM interferes with Ca-P and metal oxide precipitation by coating the calcite surfaces?
Phosphorus in soil
P adsorption & precipitation in calcareous soils.
Traditional Concepts Adding P Fertilizer to High pH/ High Calcium Soils
First few weeks, P initially precipitates as MCP…. Then DCPD
After 3 to 5 months, octacalcium phosphate precipitates
After 8 to 10 months, tricalcium phosphate forms
Long periods (years) hydroxyapatite minerals form…
(mineral with lowest solubility controls P concentration- while intermediates are unlikely to persist)
5. How fertilizer P becomes unavailable?
When phosphatic fertilizer is applied to the alkaline soils of Pakistan, due to high calcium carbonates in the parent material of these soils, calcium activity is high, which makes the phosphorus in the fertilizer unavailable due to the formation of calcium phosphate compounds, some of which have low availability and some have very low availability.
6. How phosphorus moves to the plant roots?
Plant Availability of Soil and Fertilizer Phosphorus
Plant roots take up phosphorus from soil solution as orthophosphate:
H2PO4- (acidic soil)
2. HPO4-- (alkaline soil)
Movement of Phosphorus to Roots
Mass Flow (2% of available P )
2. Diffusion (97 % of available P)
3. Root Interception (1% of available P)
1. Mass Flow
Assume P in soil solution=10-5M=0.31mg P/lit=0.2kg P/ha.
If a crop uses 37cm of water during its growth, there will only be about 1kg P/ha dissolved in the soil solution, yet it may take up 20-40 kg P/ha during its growth season.
(10-6 M=0.031 mg P/ha)
Very low fertility level
7. Measuring soil and fertilizer phosphorus recovery and defining phosphorus –use efficiency
There are several methods for determining the efficiency of phosphorus but the most important are:
i. Agronomic efficiency (YN-YO)/FN*100
ii. Apparent efficiency (UN-UO)FN*100
Balance Method (UP/FP)*100
Difference Method (UN-UC)/FP*100
Measuring soil and fertilizer phosphorus recovery and defining phosphorus –use efficiency
Balance method has been used occasionally and is an appropriate method for calculating P recovery and efficiency.
Difference method is an appropriate method for calculating N efficiency because very little of an N application remains in the soil as mineral N to benefit a subsequent crop.
Percentage recovery of three amounts of applied P at two levels of Olsen P, sandy clay loam soil, saxmundham Data are the mean of two 4-year rotations, 1969-1972 and 1970-73. Rotation: sugar beet, barley, potatoes, barley Total P applied in 4 years was 55, 110 and 165 kg P/ha. Ditto Formula 50 39 Ditto Fromula 4 24 82.2 63.8 165 Ditto Formula 72 52 Ditto Formula 4 31 79.4 57.2 110 46.9/55 *100 140 85 (46.9-23.3)/55 *100 3 43 77.0 46.9 55 75.2 23.3 0 Formula used % recovery by the balance method Formula used % recovery by the difference method P uptake in 4 years (kg p/ha) 33 4 33 4 33 4 P applied (kg/ha) Olsen P (mg/kg)
P offtake 1856-2001 by arable crops growing on plots that had received no P or a total of 1410 kg/ha from 1865-1901 and none since, Exhaustion Land, Rothamsted 1 P in winter wheat and spring barley grain plus straw and in potato tubers. 2 Except 1902-1940 when no N was applied. 531 (56-2001) 170 (02-48) 308 (56-1948) 138 (56-01) Total of the variable period 5.45 4.18 42 1115 (56-2001) 9.63 96 Wheat 10 92-2001 4.38 4.06 65 8.44 135 Barley 16 1976-91 5.08 4.46 116 9.54 248 Barley 26 1949-74 4.1 4.9 39 307 (02-48) 9 72 Barley 9 1941-48 2.66 3.36 131 636 (56-1948) 6.02 235 Barley 39 1902-40 4.77 1.73 45 329 (56-01) 6.5 169 Potatoes 26 76-1901 3.35 4.65 93 8 160 wheat 20 1856-75 Per year Total Per year Total Difference in annual P offtake Plot 7 (N)2 p offtake Total of the variable period Plot 7 (NPK)2 p offtake Cropping No of Years Period
Cumulative recovery by arable crops of P applied between 1856 and 1901, Exhaustion Land, Rothamsted. 1115/1410 *100 636/1410 *100 329/1410 *100 Formula 41 23 14 Difference Method (1115-531)/1410 *100 79 1115 160+169+235+72+248+135+96 W+P+B+B+B+B+W 1856-2001 (636-308)/1410 *100 45 636 160+169+235+72 W+P+Barley+Barley (B) 1856-1948 (329-138)/1410 *100 23 329 160+169 Wheat (W)+ Potato (P) 1856-1901 Formula Balance Method Kg/ha Kg/ha Recovery (%) Total P offtake Individual offtake by crop Crops Period
Percentage recovery by the balance method of the residue of P applied between 1856 and 1901that remained in the soil in 1901 and 1948, Exhaustion Land, Rothamsted. Formula 62 28 Recovery (%) 479*100/774 248+35+96= 479 1410-479 774 1949-2001 307*100/1081 235+72= 307 1410-307 1081 1902-1948 Formula & Recovery % P offtake (kg/ha) Formula Used P residue (kg/ha) Period
Immediately High Low Very low Accessible Accessibility Accessibility Accessibility In Solution Readily extractable Low extractability Very low extractability Immediately Readily low Very low Available available availability availability Soil Solution Surface- adsorbed P Strongly-bonded Or Absorbed P Very strongly-bonded Or inaccessible or mineral Or precipitated P 8. Conceptual diagram for the forms of inorganic P in soils categorized in terms of accessibility, extractability and plant availability.
9. Strategies for improving P use efficiency
Modifying surface soil properties.
Managing surface soil and its P content
Managing P sources
Optimizing P use through the use of economically appropriate rates and timing.
Most of the inorganic P added to soils in fertilizers and manures is usually adsorbed
initially, which is held with a continuum of bonding energies on the surfaces of, or
within, soil components, and that this gives rise to the differing extractability of
soil P and its differing availability to plants.
The P use efficiency must be measured over an adequate period i.e. at least a decade
and must be estimated by using balance method. However, the difference method is
2. Inappropriate because it does not consider the residual effect of added P.
3. Residual P contributes to the readily plant available pool, but the rate of release may
not be sufficient to maintain the critical value required to meet the P requirement of
high yielding cultivars. In such situations, P must be added to maintain critical value
for optimal crop yields.
Phosphorus must be applied to plants through the most appropriate method to
minimize phosphorus conversion in soil to the least available forms.
6. On many soils, the added P is not irreversibly fixed in forms that are unavailable to