TOPIC OUTLINE. Importance of P 3/21/2016 UNDERSTANDING PHOSPHORUS IN PLANTS AND SOILS. Agronomy Department Iowa State University

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1 UNDERSTANDING PHOSPHORUS IN PLANTS AND SOILS Agronomy Department Iowa State University TOPIC OUTLINE Importance and Abundance Deficiency symptoms Phosphorus cycle in soils Organic soil P Inorganic soil P Factors affecting P availability Importance of P Essential element for all life forms Source of energy e.g. N 2 fixation 16ATP + N rhizobia 2 16ADP + 16P i + energy + 2NH 3 Adequate P enhances physiological processes: Photosynthesis Increased root growth Flowering Fruiting Grain production Earlier crop maturity Greater stalk strength Improve overall quality 1

2 Abundance of P In lithosphere is about 0.12% Present in soils, rocks, water, and plant and animal remains (No gaseous form) Inorganic or organic forms Total P is lbs/ac in furrow slice Available soil P concentration usually less than 0.1 mg/l in soil solution (0.2 to 0.4 mg/l needed to maximize plant growth) Deficiency Symptoms Stunting of the plant during early growth Shriveled grain Abnormal discoloration such as in corn. The plants are usually dark bluish-green in color with leaves and stem becoming purplish Reduced growth and yield Delayed maturity 2

3 Corn Deficiency Symptoms Deficiency Symptoms The Phosphorus cycle The concentration of P in solution is determined by three factors 1. adsorption/desorption reactions 2. mineralization/immobilization 3. precipitation/dissolution reactions 3

4 The Phosphorus cycle The Phosphorus Cycle Uptake: by plant roots Adsorption: removal of ionic P (H2PO4 -, HPO4 2- ) from solution by reaction with solid phase of soil. Desorption: occurs when the orthophosphates, H 2 PO 4- and HPO 4 2- release into the soil solution Dissolution: dissolve and increase the concentration of soil solution P Precipitation: as secondary minerals Ca, Fe, Al P Immobilization: available P is taken up by soil microbes Mineralization: conversion of organic P to inorganic P 15-80% of total P Soil Organic P Not directly available, must first be mineralized Inositol hexaphosphate (phytic acid) 2-50% Nucleic acids, phospholipids, sugar phosphates 1-5% Carbon to P ratio in organic matter is 100 to 200 Determines net mineralization or immobilization Nature of more than 50% of organic P is unknown 4

5 Abundant Organic P Forms in Soils O R O P OH OH Monoester-P O R O P OH OR 1 Diester-P Inositol Hexaphosphate or Phytic acid Dynamics of P in Soil slow fast Nonlabile Soil P Labile Soil P Solution P < 0.01 mg P L -1 in infertile soils 1 mg P L -1 in well-fertilized soils 7-8 mg P L -1 in recently-amended soils Inorganic P in Soils High ph P complexes with Ca and Mg ph below 7.5 P complexes with hydrous oxides of Al, Fe, and Mn Insoluble Al, Fe, and Mn P form below ph 5.5 5

6 Inorganic P in Soils Apatites (compounds of very low solubility that tend to be present more in nonacid soils than in acid soils): Hydroxyapatite Ca 10 (PO 4 ) 6 OH 2 Fluorapatite Ca 10 (PO 4 ) 6 F 2 Calcium phosphates (Forms of P that tend to form when P fertilizers are added to nonacid soils.): Monocalcium phosphate Ca(H 2 PO 4 ) 2. H 2 O Dicalcium Phosphate Ca(HPO 4 ). H 2 O Tricalcium Phosphate Ca 3 (PO 4 ) 2 Octacalcium phosphate Ca 8 H 2 (PO 4 ) 6 5. H 2 O Inorganic P in Soils Aluminum phosphates (Forms of P that tend to form when P fertilizers are added to acid soils that have Al.) Variscite AlPO 4. 2H 2 O Taranakite H 6 K 3 Al 5 (PO 4 ) 8.18 H 2 O Wavellite Al 8 (OH 3 (PO 4 )) 2 Iron phosphates (forms of P that tend to form when P fertilizers are added to acid soils that have Fe.) Vivanite Fe 3 (PO 4 ). 2 8H 2 O (reduced Fe) Strengite FePO 4. H 2 O (oxidized Fe) Factors Affecting P Availability Soil ph Highest near ph 6.5 Most important reason for liming in acid soils 6

7 Factors Affecting P Availability Type of Clay Oxide clays: contain a lot of Al 3+ and Fe 3+ that can absorb P ions. Low availability because P held very strongly Kaolinite (1:1) clays, have lots of OH - ions that can be replaced with P ions ---- low availability Montmorillonite, Illite, etc (2:1) clays: have few OH - ions and they are mostly inaccessible, low anion exchange capacity and adsorb little P Factors Affecting P Availability At ph of 7.21, H 2 PO 4- = HPO 4 2- (predominant P absorbed by plants) [Total P] = [H 3 PO 4 ] + [H 2 PO 4- ] + [HPO 4-2 ] + [PO 4-3 ] Mole Fraction of Total P H3PO H2PO4-1 HPO PO ph Factors Affecting P Availability Temperature The lower the temperature the slower the P used by plants Iowa soils are somewhat cold in the spring, so higher P levels are needed than warmer areas Organic matter decomposition Release organic P Organic acids formed can dissolve Ca-P Complexes Fe and Al 7

8 Factors Affecting P Availability Soil moisture The amount of P present is proportional to the amount of water present in the soil Too much water restricts root growth, reduces the amount of P that roots can reach Low P is more damaging in dry and poorly drained soils than in good air-water relations Time of reaction Fertilizer P is slowly converted into less soluble form Low solubility of P causes slow reaction TOPIC OUTLINE (NEXT CLASS) Phosphorus fertilization Fertilizer nomenclature P fertilizer Placement of P fertilizer Environmental implications Phosphorus Fertilization Soil Test Based on chemical principles that relate to inorganic P minerals When the solution P decreases with plant uptake, P minerals can dissolve and be released to resupply soil solution P The process simulated by chemical extractant 8

9 Soil P determination Mild Acid extractant (Bray P-1) Used in most eastern and midwestern States (acid soils) 0.025M HCl M NH 4 F Not recommended for soils with ph > 7.4 Stronger acids (Bray P-2) 0.1MHCl M NH 4 F Is not suitable for Iowa soils, too acid especially in high lime soils (Harps) Give high readings on these soils Mehlich III Used in various regions M NH 4 F + 0.2M CH 3 COOH M NH 4 NO M HNO 3 +EDTA The same manner as Bray P-1 Extracting Solution Distilled water Does not extract much low extraction Alkaline extractant (Olsen test) 0.5M NaHCO 3 at ph 8.5 Used in neutral to calcareous soils Used in most western states where most soils are alkaline Soil test Classifications Soil tests classifications indicate whether or not adding a nutrient is likely to result in a yield increase. % of maximum yield Soil test: Very low low medium/optimum high very high Fertilizer response likely. Response to fertilizer not likely. 9

10 Recommendations Recommendations for P are based on soil tests: Determine how much needs to be added to reach optimum soil test levels: the fertilizer uptake efficiency. P Recommendations for Various Crops Soil Test Category Corn (grain) Soybean Wheat Corn/Sorghum Alfalfa (silage) P 2 O 5 to apply (lb acre -1) Very Low Low Optimum High Adapted from ISU Extension PM 1688 Rev Oct

11 Phosphorus Fertilization P Fertility Evaluation Plant test Deficiency symptoms (stunting, purple color, deformation of grain shriveled grain) Sap test Can be used to verify deficiency suspicions: adequate vs. inadequate Tissue analysis Optimum for P approximately 0.3% for corn and many other crops Phosphorus Content of Fertilizer Usually expressed as %P 2 O 5 (old chemist nomenclature from Liebig) P 2O 5 = phosphorus oxide Amount of P in fertilizers Rate of P to apply in recommendations Lb P 2O 5/acre H 2PO 4 -, HPO = ionic forms of P that plants use Mined Phosphorus 90% of the world s estimated P reserves are found in five countries: Morocco, China, South Africa, Jordan and the United States Reserves remaining 40 years or 400 years? Peak P Nature

12 Cordell et al. 2009_ The story of phosphorus: Global food security and food for thought TED Talk A simple solution to the coming phosphorus crisis by Mohamed Hijri Manufacturing process for common solid and liquid P fertilizers from rock phosphate Symbol Compound Formula % TSP Triple superphosphate Ca(H 2 PO 4 ) MAP Monoammonium phosphate NH 4 H 2 PO TPP 1 Triammonium pyrophosphate (NH 4 ) 3 HP 2 O DAP Diammonium phosphate (NH 4 ) 2 HPO Main in liquid form P Fertilizers 12

13 Phosphorus Content of Fertilizer % P = 0.44 x % P 2 O 5 (2 x Atomic. wt. P)/(Mol. wt. P 2 O 5 )= (2 x 31)/([2 x 31]+ [5 x16]) = % P 2 O 5 = % P x 2.29 (Mol. wt. P 2 O 5 )/(2 x Atomic wt. P) = ([2 x 31]+ [5 x 16])/(2 x31)=2.29 Fertilizers are designed to have a known "grade" or concentration of nutrients. For example, the fertilizer shown below has a grade, meaning it is 20% N, 5% P 2 0 5, and 10% K 2 O by weight. The concentration of nutrients in the fertilizer is multiplied by the amount of the fertilizer material applied per acre to find the actual amount of N, P 2 0 5, and K 2 O applied per acre. For example, if 100 lbs of the fertilizer above were applied evenly over an acre, you would apply: 20% N x 100 lbs fertilizer/acre = 20 lbs N/acre 5% P lbs fertilizer/acre = 5 lbs P /acre 10% K 2 O x 100 lbs fertilizer/acre = 10 lbs K 2 O/acre On the other hand, if you wanted to satisfy a crop P fertilizer guideline of 15 lbs P /acre with that same fertilizer, you would... (15 lbs P /acre) / (0.05) = 300 lbs/acre of the fertilizer would need to be applied In this case, the crop P guideline is divided by the concentration of P in the fertilizer and the amount of fertilizer material to be applied per acre is calculated! 13

14 Phosphorus Content of Fertilizer Triple superphosphate (0-46-0) costs $280 per ton. What is the cost per pound of P 2 O 5? First, calculate the pounds of P 2 O 5 in the fertilizer: 2,000 lbs fertilizer x 0.46 = 920 lbs. Next, calculate the cost per pound of P 2 O 5 : $280 / 920 lbs = $.30/lb P 2 O 5. Classification of P Fertilizer Based on Solubility Water soluble Usually small amount, available quickly and may react quickly to unavailable forms Citrate soluble Dissolve in 1N ammonium citrate available (printed on the bag) Why citrate? Citrate is mild acid and high buffer capacity to approximate plant P availability Total P = all that is present, regardless of availability Competing Uses for Phosphorus Detergents Pesticides Explosives (matches to bombs) Nerve agents Water softeners Metals manufacturing (steel and bronze) 14

15 P Fertilizers Manure Beef manure = /ton wet Hog manure, the same or little higher in P Chicken and sheep manure, 2x as much P as cattle Manure management plans will continue to play a major role in managing P fertility for plant growth as rock phosphate reserves are depleted and transportation costs continue to rise Iowa DNR Animal Feeding Operations p?s= Late Winter Manure Application and Risk Angela Rieck-Hinz is an extension program specialist for Iowa State University Extension and is the coordinator of the Iowa Manure Management Action Group (IMMAG). Rieck-Hinz can be reached at (515) or by ing amrieck@iastate.edu. Placement of P Fertilizer To Broadcast or Band apply: That is the question? P is not mobile and not soluble (placement can be important): Most important if soil test P is low close to the seed As soil test P increases, placement close to the seed is less important P fixing soils - highly weathered soils Placement in a band, reduces surface soil/fertilizer contact, reduces the rate of fixation Banded Starter placement 2 below and 2 to the side of the seed Purpose: offset cool soils 15

16 Why Broadcast? Can apply large amounts conveniently Ideal for building soil fertility Minimizes risk of fertilizer injury On fields of low fertility, banded starter P may not be adequate for maximum yield Combinations of broadcast and band applications produce the highest yields on low testing soils Placement of P Fertilizer Band Application Overwhelms soil fixation capacity Places nutrients near the seedling Roots intercept early and proliferate near the band Including ammonium-n, slows P fixation and lowers ph near the root to improve both P and micronutrient availability Favors the crop over the weeds Keeps nutrients away from the surface Reduces runoff P concentrations 16

17 Phosphorus and Environmental Concerns P losses from cropland can cause surface water quality problems Soil P levels have increased Manure P is a major contributor to soil P buildup Land application of manure is often the only practical management option The Fate of Phosphorus Transport Phosphorus and Water Quality Phosphorus losses from agriculture can be a major source of P entering lakes and streams. Phosphorus additions to natural waters can stimulate weed and algae growth. Vegetative growth -> oxygen depletion (aka hypoxia) -> reduced water quality. 17

18 Eutrophication Caused by Excessive P 18