Lecture 18. Soil Acidity, Alkalinity, and Socidity

Transcription

1 Lecture 18 Soil Acidity, Alkalinity, and Socidity 1

2 Questions ow can acidification occur in soils? ow does p affects availability of N, P, K? ow can acidic soils be managed? Define a saline and sodic soil. ow can soils become saline or sodic? What effects do salts have on plant growth? ow does sodium affects soil structure? ow are saline and sodic soils managed? 2

3 What is p? p is a measure of the + activity in solution: p log 1 [ ] log [ ] + ydrogen ion For example, a [ + ] of 10 5 is equal to a p of 5 3

4 Why is soil p important? Soil p affects: Nutrient availability, biological activity, mineral solubility Cl 4

5 Measuring Acidity Field observations nutrient deficiencies crop production Measuring p Soil paste 1:1 soil water or salt solution 6

6 Why Does Acidification Occur? Carbon dioxide dissolves in water: CO 2 O C O O + O C + _ O O + Carbon dioxide Water Bicarbonate ydrogen ion p of a solution at equilibrium with atmosphere is 5.7 CO

7 Why Does Acidification Occur? Mineralization Microbes R C O + O + O O R O C O Organic matter Water Oxygen Strong organic acid O O R C O R C _ O + + Strong organic acid 8

8 Why Does Acidification Occur? Oxidation reactions Example, Nitrification Nitrosomonas + O O N + O N O autotrophs + O + + Ammonium Oxygen Nitrite Water ydrogen ion 9

9 Why Does Acidification Occur? Al and Fe hydrolysis p dependent O Al 3+ O + O Al 3+ + Aluminum ion Water Aluminum hydroxide O + ydrogen ion 11

10 Why Does Acidification Occur? N + Root hairs O O N _ + O _ + O _ 13

11 Why Does Acidification Occur? Acid rain from air pollution Distribution of acidic precipitation 14

12 Leaching Acidification: mechanism Anions of acids (SO 4 2, etc.) A exchange for one Ca +2 Ca K + Na + Al 3+ Mg Al 3+ Ca +2 Mg +2 Al3+ + Ca Ca +2 Exchangeable cations A 2 Ca +2 Leaching loss along with anion 15

13 Effects of p on Nutrient Availability 4 Microbial inactivity Aluminum and iron 5 6 Soil p 7 N P Calcium Microbial inactivity Leaching K Calcium Leaching Ca and Mg Carbonates S Fe and Zn Mn (and Al) Oxides Oxides (and Silicates) Leaching Cu Oxides Leaching B Insolubility Insoluble Molybdates Mo 22

14 Favorable p Ranges for Common Crops Soil p Crop Corn Alfalfa Soybeans Wheat Oats Barley Rice 23

15 Managing Acid Soils Liming Adds a base to react with + Soil + + Ca +2 + CaCO O + CO 2 Soil CO O + CO 2 24

16 Must Consider Reserve Acidity Need more lime to manage acidity in Soil A (e.g., high clay, humus content) Solution p Solution p Reserve Acidity Reserve Acidity Well Buffered Soil A Weakly Buffered Soil B 25

17 Calculating Lime Requirement Example for soil: CEC = 15 cmol c / kg soil Initial p = 5.0 and base saturation = 40% Target p = 6.5 and base saturation = 80% Need to neutralize 40% of the acidity on the CEC: 1 kg of CaCO 3 gives 2000 cmol c 15 cmol (40%) kg soil c 6 cmol c kg soil 6 cmol c kg soil x kg CaCO 2000 cmol 3 c kg CaCO kg soil 3 One hectare of soil (15 cm depth) weights about 2 x 10 6 kg: kg CaCO 3 2x10 kg soil 6x10 kg CaCO 3 x kg soil ha ha 28

18 Salinity and Sodicity Salinity accumulation of soluble salts Sodicity accumulation of exchangeable sodium Calcareous igh calcium carbonate content 29

19 Types of Salts Salt Cation (+) Anion () Common name NaCl sodium chloride table salt Na 2 SO 4 sodium sulfate Glauber s salt MgSO 4 magnesium sulfate epsom salt NaCO 3 sodium bicarbonate baking soda Na 2 CO 3 sodium carbonate sal soda CaSO 4 calcium sulfate gypsum CaCO 3 calcium carbonate lime 30

20 ow Soils Become Saline or Sodic Evaporation Mildly saline/sodic drainage Mildly saline tap water Saline/Sodic groundwater and soils Evaporation Evapotranspiration Salt accumulation on ridge salty soil Water movement from furrows Evaporation Elevated water table Salt on soil surface Accelerated upflow carries salt Sources of Salt Irrigation water Weathering Dust Fertilizer 31

21 Effects on plant growth Water imbalance in the plant Imbalanced nutrient uptake Na + competes with K + Toxicity from Na + and Cl Non Saline Solution Cell Wall Cell Plasma Saline Solution Cell Plasma Cell Wall 32

22 Barley/ Wheat in Saline Soil 33

23 Saltaffected plants Plants may exhibit stunted growth, darker green leaf color, and leaf burn 34

24 Rice Saltaffected rice 35

25 Sodic Soil 36

26 Sodium Effects on Soil Structure + Ca + + Ca + + Ca + strong weak Attraction (Ca, Mg) Flocculation Repulsion (Na) Dispersion 37

27 Soil Response to Sodicity Rain or irrigation water low in salts Ponded water Nonsodic: Structure good Infiltration not hindered Sodic: Structure destroyed Pores clog at surface 38

28 Classification of SaltAffected Soils Classification Electrical Conductivity (ds/m) Exchangeable Sodium % (ESP) Soil Physical Condition Saline > 4 < 15 Normal < 8.5 Sodic < 4 15 Poor > 8.5 SalineSodic > 4 15 Normal < 8.5 p ESP Na CEC x100% 39

29 igh p and Sodic Soils Extremely high p (>8.5) occurs in sodic soils when sodium carbonates are present Sodiumclay, bicarbonate (CO 3 ), and carbonate (CO 3 2 ) react with water to form hydroxyl (O) ions: Na + Clay or humus Na + Clay or + 2 O + 2Na + + 2O humus + + CO O 2 CO 3 + O CO O CO 3 + O Sodium carbonates are relatively soluble 40

30 Electrical Conductivity A measure of the amount of salts present in a soil EC x 10 = total soluble cations (meq/l) EC x 640 = total dissolved salts (ppm or mg/l) 41

31 Examples of EC EC(ds/m): 03 ppm: EC(ds/m): 1.3 ppm: 850 Irrigation Water Colorado River EC(ds/m): 47 ppm: EC(ds/m): 46 ppm: 35,000 River in Negev Desert Pacific Ocean 42

32 General Interpretations for EC EC (ds/m) Salinity class Crop Response 0 to 2 Non saline Almost negligible 2 to 4 Very slightly 4 to 8 Slightly 8 to 16 Moderately > 16 Strongly Yields of very sensitive crops restricted Yields of most crops restricted Only tolerant crops yield satisfactorily Only very tolerant crops yield satisfactorily 44

33 Salt Tolerance for Selected Crops Crop Rating Threshold EC (ds/m) % Yield Decrease/ Unit EC increase Almond S Rice S Apricot S Corn MS Grape MS Tomato MS Wheat MT Cotton T

34 Management of SaltAffected Soils Scrapping Drainage Used in soils to lower water table and/or salts from the soil profile 46

35 Management of SaltAffected Soils Leaching requirement: Total applied water: LR ECw 5( ECse) ( ECw) AW ET 1 LR LR = minimum leaching requirement EC w = salinity of irrigation water (ds/m) EC se = threshold EC for crop (ds/m) ET = annual crop water demand (mm/year) AW = applied water needed (mm/year) 48

36 Leaching Requirement Example Corn crop: EC w = 1.2 ds/m Crop water demand = 800 mm/year Irrigation efficiency = 65% Applied water needed? 941 mm Total water needed? 941 mm/0.65 = 1448 mm LR ECw 5( ECse) ( ECw) 1.2 LR 5(1.8) (1.2) LR 0.15 AW ET 1 LR 800 AW AW 941 mm 49

37 Management of SaltAffected Soils Na + removal (for sodic soils) Gypsum Na + Clay or humus Na + Sulfur/Sulfuric acid Clay or + CaSO 4 humus Ca +2 + Na 2 SO 4 (leachable) Na + Clay or humus Na + humus SO Clay or 4 + Na 2 SO 4 + (leachable) Could you use lime? 51

38 Calculating Gypsum Requirements Exchangeable Sodium % = 25% CEC of 18 cmol c /kg soil Reduce ESP in top 15 cm to 5% Determine cmol c needed to be replaced on CEC: 18 cmol kg soil 1 kg of CaSO 4 gives 1162 cmol c : x 0.20 c 6 3. cmol kg soil c 3.6 cmol kg soil c kg CaSO 4 x 1162 cmol c kg CaSO kg soil 4 One hectare of soil (15 cm depth) weights about 2 x 10 6 kg: kg CaSO 4 2x10 kg soil 6x10 kg CaSO 4 x kg soil ha ha 52

39 Management of SaltAffected Soils Irrigation management can affect accumulation of salts 53

40 Management of SaltAffected Soils Modification of seedbeds permits germination of seeds for good stand establishment 54

41 Questions ow can acidification occur in soils? ow does p affects availability of N, P, K? ow can acidic soils be managed? Define a saline and sodic soil. ow can soils become saline or sodic? What effects do salts have on plant growth? ow does sodium affects soil structure? ow are saline and sodic soils managed? 55