Understanding Soil Acidity Brady and Weil 2002 Neutral

Understanding Soil Acidity Brady and Weil (2002) Neutral

p. H = - log (H+ ion concentration) p. H =7 neutral increases… As p. H decreases… Brady and Weil, 2002

Optimum p. H ranges have been identified for many crops

Collective term for the challenges faced by crops growing in acid soils The acid infertility complex

For Nutrient most soils, nutrient availability is optimized availability between p. H 5. 5 and 7. varies with

Molybdenum becomes more available as p. H goes up ! most ^ http: //www. farmtested. com/research_pp. html

Understanding aluminum toxicity Toxic forms Aluminum of Al are toxicity is bioavailable at lowminimal p. Hs above p. H 5. 5 http: //www 2. ctahr. hawaii. edu/tpss/research_extension/rxsoil/alroot. gif

Multiple forms of soil acidity Soil p. H is primarily a measure of active acidity Reserve acidity Active acidity Brady and Weil, 2002

Understanding reserve acidity Very little Reserve lime isacidity needed resupplies to neutralize the active acidity in soils Δp. H Reserve acidity High CEC soil Active acidity Reserve acidity Active acidity Low CEC soil

Each charge depicted on this diagram represents 1 centimol of charge per kg of soil K+ Ca+2 Mg+2 H+ ---Humus H 2 0 Exchangeable exchangeable acidity cations Clay SO 4 -2 ++ -Al - K -- +3 H 2 0 soil solution H 2 0 + H 2 O ↔ Al(OH)3 + 3 H+ + Ca+2 What is the “base” saturation ?

Is p. H related to base saturation ? It is probably more accurate to say that p. H is related to acid saturation 100 80 60 40 Acid Saturation, % 20 0

p. H dependent charge The dominant clay minerals in IL have mostly permanent charge

The charge on humic substances (and low activity clays) is very p. H dependent t n de n e p e d H e p g r a h c H+ ions dissociate when the soil p. H increases and reassociate when the p. H drops. Brady and Weil (2002)

Soil acidity increases when H+ producing processes exceed H+ consuming processes. s in sse e c o g pr H+ es H+ ing c u d pro ess proc um cons

Many processes add H+ ions to soils 1) Carbonic acid forms when carbon dioxide dissolves in water. H+ ions are released when carbonic acid dissociates: H 2 CO 3 → HCO 3 - + H+ VERY IMPORTANT PART OF SOIL FORMATION 2) Organic acids form during the decomposition of organic matter. H+ ions are released when these organic acids dissociate. 3) Sulfuric and nitric acids form during the oxidation of reduced forms of N and S (e. g. , NH 4+ from fertilizer, elemental S). NH 4+ + O 2 → NO 3 - + 2 H+ S 0 + O 2 → SO 4 -2 + 2 H+ 4) Sulfuric and nitric acids form when sulfur oxides and nitric oxides (released into the atmosphere by automobile emissions, industry smoke stacks, volcanoes, forest fires) dissolve in precipitation. H 2 SO 4 and HNO 3 are strong acids and fully dissociate in water. 5) Roots release H+ to balance internal charge when cation uptake exceeds anion uptake.

Many processes consume H+ ions in soils 1) Weathering of most minerals (e. g. , silicates, carbonates…) 2) Decomposition of organic anions 3) Reduction of oxidized forms of N, S and Fe. 4) Roots release OH- or HCO 3 - to balance internal charge when anion uptake exceeds cation uptake 5) Inner sphere adsorption of anions (especially sulfate) which displaces hydroxyl (OH-) groups

Acidity What is liberated and what is left behind when plant biomass is burned ? Oxides of C, N and S Alkalinity Oxides of Ca, Mg and K Elements that have traditionally been called “bases”

C, N and S oxides cause acid precipitation Brady and Weil, 2002

Sources of p. H buffering in soils Carbonates Chadwick and Chorover ( 2001)

K+ H+ NO 3 ? - The p. H of a plant’s rhizosphere changes as the plant regulates its internal charge balance.

Which plant received nitrate ? Which plant received ammonium ? http: //departments. agri. huji. ac. il/plantscience/topics_irrigation/uzifert/4 thmeet. htm

Acid inputs promote leaching of non-acid cations Why does leaching of these anions cause soil acidification ? Brady and Weil, 2002

Complete N cycle (no net acidification) released into the soil 1 H+ consumed Nitrification is an acidifying process, right? ? NH 3 1 H+ consumed The 2 H+ produced during nitrification are balanced by 2 H+ consumed during the formation of NH 4+ and the uptake of NO 3 - by plants

Excellent but focused on Australian soils

Standard values for the quantity of lime needed to neutralize the acidity generated by specific N fertilizers Assumes: 1) all ammonium-N is converted to nitrate-N and 2) half of the nitrate is leached. Composition Lime required (lb Ca. CO 3 / lb N) Anhydrous ammonia 82 -0 -0 1. 8 Urea 46 -0 -0 1. 8 Ammonium nitrate 34 -0 -0 1. 8 Ammonium sulfate 21 -0 -0 -24 5. 4 10 -52 -0 5. 4 18 -46 -0 3. 6 Nitrogen source Monoammonium phosphate Diammonium phosphate

Harvest of crop biomass removes alkalinity from agricultural fields Cation : N ratio in plant biomass Lime required to replace alkalinity removed in harvest (lb Ca. CO 3 /100 lb of N harvested) Corn grain 0. 14 25 Corn stover 0. 73 131 Soybean 0. 14 25 Oats grain 0. 14 25 Oats straw 0. 94 169 Alfalfa 1. 41 254 Crop http: //www. ianrpubs. unl. edu/epublic/pages/publication. D. jsp? publication. Id=111

Scenario Corn/soybean rotation 200 bushels, 50 bushels All P supplied as DAP N applied as DAP and AA Acidity from N fertilizer 3. 6 x 52 lbs of N in DAP required to supply P removed in harvest 1. 8 x 150 lbs of N in AA Acidity from grain harvest 25 x 180 lbs of N harvested/100 25 x 200 lbs of N harvested/100 ~ 190 lbs of lime ~ 270 lbs of lime ~ 45 lbs of lime ~ 50 lbs of lime Projected lime requirement ~ 0. 3 tons/rotation

Alfalfa field with dead strip where lime was not applied How should lime rates be determined?

Lime rates should be guided by soil testing

Pocket p. H meters can be very useful but require regular calibration !!!

Sources of variation in soil p. H measurements 1. The soil to solution ratio used when measuring p. H. 2. The salt content of the diluting solution used to achieve the desired soil to solution ratio. 3. The carbon dioxide content of the soil and solution. 4. Errors associated with standardization of the instrument used to measure p. H.

Why measure soil p. H Water p. H > Salt p. H ? using a salt solution Brady and Weil, 2002

Soil p. H depends on method used to measure it !! As a result, the method of measurement should be reported whenever soil p. H data is discussed.

The amount of lime needed to bring about a 1 unit change in p. H varies widely between soils

When a soil is limed, Ca+2 from the lime displaces exchangeable acidity from the soil colloids. The active acidity that is generated reacts with the carbonate ions from the lime, producing water and carbon dioxide. H+ soil colloid + Ca. CO 3 H+ Ca+2 soil colloid + H 2 O + CO 2

“Illinois method” of determining lime requirement How do you know which line to use ? http: //iah. aces. uiuc. edu/pdf/Agronomy_HB/11 chapter. pdf

Choosing the right line Line A: Dark colored silty clays and silty clay loams (CEC > 24) Line B: Light and medium colored silty clays and silty clay loams, dark colored silts and clay loams (CEC 15 -24) Line C: Light and medium colored silt and clay loams, dark and medium colored loams, dark colored sandy loams (CEC 8 -15) Line D: Light colored loams, light and medium colored sandy loams and all sands (CEC < 8) Line E: Mucks and peat (organic soils). Light colored (< 2. 5% OM) Medium colored (2. 5 -4. 5% OM) Dark colored (4. 5% OM)

Not all limestone is the same ! Pure calcium carbonate has a calcium carbonate equivalency (CCE) of 100 and is the standard against which all liming materials are compared. A ton of material with a CCE of 90 % can neutralize 10% less acid than a ton of pure calcium carbonate. Liming materials that are finely ground, have more surface area in contact with the soil solution than coarser ground materials and thus will neutralize soil acidity more rapidly. Fineness of grind is rated according to the percentage of material that will pass through 8 -, 30 -, and 60 -mesh screens.

http: //www. agr. state. il. us/news/pub/2007 Lime. Book. pdf

Page from the 2008 IL Lime book Multiply by these factors

Adjusting for differences in lime particle size distribution

Lime requirements determined using the “Illinois method” assume the following: A. A 9 -inch tillage depth. If tillage is less than 9 inches, reduce the amount of limestone; if more than 9 inches, increase the lime rate proportionately. In no-till systems, use a 3 -inch depth for calculations (one-third the amount suggested for soil moldboard-plowed 9 inches deep). Rates of lime should be adjusted if any of these B. Typical fineness of limestone. Ten percent of the particles are greater thanassumptions 8 -mesh; 30 percent pass an are 8 -mesh not and are held on 30 mesh; 30 percent pass a 30 -mesh and are held on 60 -mesh; and 30 percent pass a 60 -mesh. accurate C. A calcium carbonate equivalent (total neutralizing power) of 90 percent. The rate of application may be adjusted according to the deviation from 90.

It takes time for lime to react in soil

p. H measurements on the fly Soil p. H and lime requirement can vary widely within fields

Both past management and inherent soil properties affect soil p. H and lime requirement Why is variable rate lime more likely to pay than variable rate N, P or K?

Insufficient lime is applied to neutralize total acid inputs to IL soils South eastern IL has few quarries and the greatest lime deficit http: //iah. aces. uiuc. edu/pdf/Agronomy_HB/11 chapter. pdf

Barak P, Jobe BO, Krueger AR, Peterson LA, Laird DA 1997. Effects of longterm soil acidification due to nitrogen fertilizer inputs in Wisconsin. PLANT AND SOIL. 197(1): 61 -69 Abstract: Agroecosystems are domesticated ecosystems intermediate between natural ecosystems and fabricated ecosystems, and occupy nearly one-third of the land areas of the earth. Chemical perturbations as a result of human activity are particularly likely in agroecosystems because of the intensity of that activity, which include nutrient inputs intended to supplement native nutrient pools and to support greater biomass production and removal. At a long-term fertility trial in South-Central Wisconsin, USA, application of ammoniacal N fertilizer resulted in significant increases in exchangeable acidity accompanied by decreases in cation exchange capacity (CEC), base saturation, and exchangeable Ca 2+ and Mg 2+. Plant analysis shows that a considerable portion of the alkalinity generated by assimilation of N (and to a lesser extent by S) is sequestered in the above-ground plant parts as organic anions and is not returned to the soil if harvested. Elemental analysis of soil clays indicates a loss of 16% of the CEC. The reversibility of this change is doubtful if the changes are due to weathering of soil minerals.

Summary of common soil fertility problems that rarely occur in soils with p. Hs between 5. 5 and 7 p. H << 5. 5 p. H >> 7. 0 Al toxicity to plant roots Fe deficiency Mn toxicity to plant roots Mn deficiency Ca and Mg deficiency Zn deficiency Mo deficiency in legumes *Osmotic stress from salts P tied up by Fe and Al P tied up by Ca and Mg Slow N transformations Potato scab