International Program in Agricultural Development Studies (IPADS) 20 January 2016 IPADS Soil Fertility Management 6 Phosphorus Department of Global Agricultural Sciences IPADS Kensuke OKADA (akokada@mail.ecc.u tokyo.ac.jp) ipads.jp/soil 1 (Llanos Orientales in Colombia, photo by KO) Topics 1. P cycle 2. Forms and functions of P in plants 3. Forms of soil P 3 1. Solution P 3 2. Mineral solubility 3 3. Adsorption reactions 3 4. Factors influencing P fixation in soils 3 5. Organic soil P 4. P sources 5. Management Phosphorus cycle 1. P CYCLE http://www.sumanasinc.com/webcontent/animations/content/p hosphorouscycle.html (SOURCE: Cain, et al., Discover Biology, Third Edition, W. W. Norton & Co. 2006 W. W. Norton & Co.) P cycle in soil P amount of P pools and their flows in agricultural soil (Randall et al 2001 after Richardson 1994) SOIL ORGANIC MATTER (Nonlabile P) Labile P) 6 1
Forms and Functions of P in plants 2. FORMS AND FUNCTIONS OF P IN PLANTS P 0.1 0.5% Absorption : H 2 PO 4 or HPO 4 2 Forms in plants Energy storage and transfer (ATP/ADP) DNA, RNA cell membrane Visual Deficiency Symptoms Overall stunting Darker green color Purple leaf color (corn and grasses) Important! Phosphorus deficiency symptoms NPK trial of sorghum on an Alfisol fields Less tillering (rice) Mobile in plants (lower leaves symptoms) Delayed flowering and maturity NPK N K P 4 Sep 1986 ICRISAT, Panancheru, India Sorghum in NK plot on Alfisols (RP16: A1) at ICRISAT S o r g h u m P i g e n o n e a 2
Sorghum in NK plot on Alfisols (RP16: A1) at ICRISAT S o r g h u m Top total dry weight (g/m2) P i g e n o n e a 3. FORMS OF SOIL P 3 1 Solution P Soil solution P concentration to support crop growth Influence of ph on the distribution of orthophosphate species in solutions Influence of inorganic P in soil solution on corn grain yield Mass flow vs. diffusion Case of Mass flow : Transpiration ratio (H 2 O (g)/dry matter (g)) = 400 Tissue concentration of P = 0.2% in crop, average P conc. in soil solution is 0.05 ppm P? How much does mass flow P contribute? Diffusion P diffusion is the primary mechanism of P transport especially in low P soils 3
P fixation / P retention 1. Adsorption ( 吸着 ) to mineral surface 2. Precipitation( 沈殿 ) as secondary P compounds Factors 1. ph (next figure) 2. P concentration adsorption low conc. precipitation high conc. 3 2 P MINERAL SOLUBILITY ph is the important factor for the type of P fixation Common P minerals found in acid, neutral, and calcareous soils Important! Soil solution P concentration is ultimately controlled by P mineral solubility. e.g. Solubility of Ca, Al and Fe phosphate minerals in soils Please note: 1. effect of ph on Al P, Fe P 2. ph 6 6.5 3. effect of ph on Ca P 4. CaCO3 precipitates at >ph7.8 5. P minerals that support the lowest P conc. are the most stable 6. When fertilizer is added... 3 3 P ADSORPTION REACTIONS 4
Acidic soils Adsorption mechanisms Adsorption equation Freundlich Langmuir Calcareous soils 1. low P small quantities adsorb CaCO3 surface 2. high P Ca P minerals precipitate on the CaCO 3 surface Factor : Soil minerals Andepts reactive amorphous Al oxides 3 4. FACTORS INFLUENCING P FIXATION IN SOILS Oxisols Fe/Al oxides amorphous Al hydroxy compounds Calcareous CaCO 3 surface Other factors affecting adsorption Soil ph : P availability if highest at 6.5 Cation effects : Exchangeable Al 3+ precipitate P Anion effects : Inorganic and organic anions compete P adsorption site and decrease adsorption P saturation Soil OM : anions compete P (citrate, oxalate, tartarte and malate) Time and temperature Flooding Fertilizer P management considerations broadcast and incorporation vs. band placement 3 5. ORGANIC SOIL P 5
Proportion of organic P in total P Organic P in the soils Phytic acid (myoinositol hexaphosphate) 10 50% About 50 %, ranging 15 to 80%. Phospholipids (1 5%) Nucleic acids (0.2 2.5%) 4. P SOURCES Indices to show P content/availability of fertilizers water soluble citrate soluble 1N ammonium citrate soluble citrate insoluble available water soluble + citrate soluble total P Rock phosphate Egyptian rock phosphate CO 3 F apatite 0 in water soluble, but 5 17% citrate soluble Available low ph soils (2 3 times ) Extensive plantation (e.g. rubber, oil palm, cacao on very acid soils) Warm climate, moist soils, long growing seasons High initial dose (10 30 t/ha) and 5 10 years repetition Common phosphorus fertilizers Single Super Phosphate (SSP) Triple Super Phosphate (TSP) (Source: IPNI) Diammonium Phosphate (DAP) 6
5. MANAGEMENT Low mobility of P in the soil Residual effect of P fertilizer Residual effect and P management Effect of bandapplication on soil P 7
Phosphorus Summary Low P concentration in soil solution P fixation (adsorption and precipitation) ph and forms of inorganic P Forms of organic P Initial intensive application, repeat after certain years Broad application and band application 8