PNW 599-E· September 2007

CIDIFYI G

OIL FOR

CROP

PRODUCTIO ••

Inland Pacific Northwest

Donald Horneck; Extension agronomist, Oregon State University; Donald Wysocki, Extension soil scientist, Oregon State University; Bryan Hopkins, Brigham Young University; John Hart . Extension soil scientist. Oregon State University; and Rohert Steven s. extension soil scientist, Washington State University.

This publication replaces Oregon State University Extens ion publication EM 8917-E.

H igh soil pH limits nutrient supply and plant growth. The goal of soil acidification is to reduce soil pH to improve crop pe rformance and increase economic returns.

Fruit trees, alfalfa, grass seed. corn . onions, and potatoes are a few crops that may ben fi t fro m soil pH reduction to maxi mize crop production. Both commercial producers and homeowners may need to acidify soil for opti mum production and/or appearance of maple trees ; azaleas, rhododendrons. or blueberries. Some plants, such as blueberries, cannot survive in neutral to high pH soi l. Table 1 (page 2) lists soil pH requirements for selec ted crops.

Soil acidification usually is a long-term and expensive process. The long- and short-t 1'111 eco­nomics of soil acidification can be diffi cult to assess and should be considered on a site-specific basis. Unfortunately, where soil acidi fication is necessary. no single soil test is adequate to determine what type of soil amendment is needed or how much to apply.

This publication is a technical and practical guide for soil acidification in commercial fields. It explains the soil chemistry involved . how to determine whether pH adjustment is feasible. and methods for acidifying soi l. This guide is di vided into five sections:

• Understanding soil pH

• The problem-iron chlorosis

• Causes of iron chlorosis

• Solutions to iron chlorosis

• Methods for acidifying soil in the inland Paci fic Northwest

APacific Northwest Extension publicati on Oregon State University • University of Idaho • Washington State University

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Crop production practices, from seedbed preparation to harv est, must be performed in an appropriate and timely manner for optimum quali ty and yield. Poor weed or disea se control, high levels of salts, poor drainage , or other production problems can be sig nifica nt limiting factors in obtaining high yields . A soil acidifi ca­tion program w ill not correct or substitute for these lim iting factors.

Understanding soil pH Soil pH is measured on a scale of 0 to 14. A

pH of 7.0 is neutral. A pH below 7.0 is acidic, and above 7.0 is basic (alkaline) . Examples of pH for some co mmon material s are: milk (6.4), lime j uice ( 1.9), carbonated beverages (3.0), drinking water (6.5 to 8.5), cheese (5.6), and egg s (7.8).

Table I. Optimum soil pH range for some .omm on crops.*

Crop pH range

A lfa lfa 6.5- 8.4 As parag us 6.5-9.0 Aza leas and rho dodendrons 4 .5- 5.5 Beans 5.8-8.1 Blueberr ies and cranberr ies 4.5- 5.5 Corn (field o r s ilage) 5.5-8.4 Fruit trees 6.0-8.0 Garl ic 6.5-8.4 Grass for seed or pastures 5.5-8.2 Onions 6.5-8.4 Potatoes 5.0-8.3 Sma ll gra ins 5.5-8.4 Sugarbeets 6.0-8.5 Turf and pasture grass 5.5-8.0 Vegetables 6.5-8.2

Origin ' of soil pH Soil pH is a product of parent material and

the en vironment. Rainfall and tem perature co ntrol the processes that determine soi l pH.

Soil acidifica tion occurs naturally in high­rainfall areas such as the eastern U.S. and some mountainous and coasta l reg ions of the west­ern U.S. Water from ra infall s lowly dissolves minerals containing exchangeable bases. T hese bases are calcium, magnesium, potassium. and sodium. They are call ed bases because their presence causes soil pH to be alkaline. As the bases are dissolved, rainfa ll leaches them fro m the soil and replaces them with hydrogen from rainwater, making the soil acidic.

The primary base leached, and the one with the largest influence on so il pH, is calcium. T he ca lcium either is leached into the groundw ater, caus ing "hard" well water, or is moved into surface streams and ev entu ally to the ocean. As a result of thes e processes, soils in high-rainfall areas are acidified.

What is soil pH?

Mathem atically, soil pH is the negat i e logarithm of hydrogen in the so il so lution [-log(W)]. A neutral pH (7.0) occurs where hydrogen (W ) and hydroxide (OH-) are equal (H ' =OH-).

A soil pH above 7 indicates alkaline soil, with hydroxide greater than hydrogen (OB- > H+). As hydroxide incre ases and hydro­gen decreases, soil pH increases.

A soil pH below 7 indicates acidic soil, with hydro gen grea ter than hydroxide (H+> OH-). As hydrogen increases and hydroxide decreases, soil pH decreases.

Becau se pH is a logar ithm ic scale, a so il with 6.0 pH has 10 times the amount of acidity as a 7 .0 pH soil; a 5.0 pH so il has 100 times the acidity of a 7.0 pH so il.

" Detcnn ined using deioni zed water and a 1:2 soil .water ratio.

In precipitation-li mited climates , a cemented layer rich in ca rbonates, known as caliche, is common be low the soil surface (Figure 1). A caliche laye r or oth er calcium-rich horizon develops when rainfall is not sufficient to leach calcium and magnesium carbonates from the soi l surface into groundwater. Depth to cali­che depends on depth of leaching, which is regulated by annual rainfall , season of rainfall , and soi l texture. As rainfall and sand content increase , the depth to caliche increases.

Land leve ling or ero sion commonly exposes caliche or oth er cal cium-rich horizons. These areas have alkal ine pH and, compared to

Figure 1.-A caliche layer approximately 24 inches below the soil surface at a road cut southwest of Pendleton, OR.

adjacent so ils, are lighter in color. lower in soil o rganic matter, and more difficult to manage. These carbonate-rich layers commonly cause pH-related problems.

Farming practices contribute to so il ac idi­fication. T he addition of acidifying fer tili zers and pure irrig ation wat er acidifies soil. For example, urea and ammonium nitrogen ferti liz­ers are acidifying (Table 2) . Proteins and oth r nitrogen-containing compounds in organic fer­ti lizers con vert to ammo nia, then to ammon ium, and finally to nitrate . When ammonium nitro­gen changes to nitrate, it releases fo ur acidi fy­ing hydrogen ions into the soil.

Irri gation water that is low in dissolved min­erals can have an effect similar to tha t of rain; it leaches exchangeable bases, acidifying the soil. Surface waters in the Pacific Northwest contain few dissol ved minerals. Conversely, irrigation water from wells is commonly high in calc ium and magnesium carbonates, which rep lace the leached exchangeable bases and kee p soi l pH alkaline.

Soil N transformations and acidity

organic N ~ ammonium =ammonification

ammonium ~ nitrate + 4H = nitrifi ca tion

organic N ~ plant-available N = minera lization

Table 2. Lime requirement of se lected nitrogen fertilizers,

Li me required to neutra lize Nitrogen sou rce (pound lime/pound N)*

Ammonium sulfa te 5.4 Anhydrous ammo nia 1. 8 Urea 1.8 Calc ium n itrate o Mono-ammonium phosphate (MAP) 5.0 Manure, compost, and other organic sources variabl e

*A highe r lime requi rement indicates a more acidi fying material.

Adapted from Tisdale et a I., 1985.

When to sample

Collect annual soil samples at the same time each year to minimize seasonal fluctuations and to accurately monitor soil acidification.

easonal soil pH change Soil pH changes seasonally. Higher soil pH

or lower acid ity is measured in late winter or early spring before fertilization. Soil pH can decrease more than 1 unit (for example, from 7.2 to 6 .2) fro m spring to fall in sandy, low­organic-matte r soi ls. In areas of low rainfall, soil pH is lowest or most acidic as the growing season beg ins. Soil pH is lowered by fertilizer application and by the increase in biological acti vity as so il warms.

Soil buffering capacity determines the extent of seasonal pH changes. Soils with substantial amounts of clay and/or organic matter are buff­ered agai ns t pH change and are likely to have a small seasonal pH change, about 0.3 unit. Soils with relati vely little silt, clay, and/or organic matter, and a pH below 7.5, can change I to 2 units. Soi ls containing carbonates are buffered against pH change and will not have as large a seasonal difference.

Seasonal pH fluctuations are an important consideration when monitoring the progress of a so il aci dification program.

The problem-iron chlorosis Plants are excellent indicators of the need

for soil acidification. They commonly exhibit symptoms of iron deficiency, but sometimes show the need for zinc or manganese. The most common symptom is a yellowing of leaves (chlorosis). This yellowing sometimes is con­fused with yellow leaf color caused by nitrogen or sulfur deficiency.

Symptoms of iron deficiency are unique as they generally are most dramatic on new growth. An iron-deficient plant often shows " interveinal chlorosis," or green veins with

yellowing between the veins (Figure 2). Severe iron deficiency results in smaller than normal leaves that are yellow ancl might have "burnt" leaf edges. Iron deficiency causes plants to per­form poorly or even die.

Plant species, varieties, cultivars, or hybrids can differ dramatically in their tolerance to iron deficiency. Some corn hybrids exhibit iron chlo­rosis, while other vari eties planted in the same location do not.

When nitrogen, potassium, zinc, manganese, or sulfur fertilization do not change ye llow leaves to the green color expected in healthy plants, an iron deficiency might be the problem. Tissue analysis sometimes , but not always, helps to confirm iron deficiency. In so me cases, tissue analysis shows adequate iron, but the iron is bound in the spaces between cells and is not functional. Consult a plant nutrition specialist before sampling and to help with interpretation.

Causes of iron chlorosis Iron chlorosis is caused by a variety of fac­

tors, but the primary reason is high soil pH. Other factors, such as fertilizers, irr igation water, soil conditions, and weather, can in flu­ence pH or interact with pH to create or exacer­bate iron chlorosis.

Figure 2.-An iron-deficient swe et gum tree from eastern Oregon showing yellow leaves with green veins.

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High soil pH Iro n has limited plant availability when soil

pH is near or abo ve 7. Low iron availability is a re sult o f de creased iron solubility when soi l pH is above 7 .0 (Figure 3). Zinc availability is also affected by soil pH, but les s so than iron avail ­ability (Figure 4) . Lo wering the soil pH will make iron mo re available to plants.

You mi ght ask, "If iro n is lacking, why not apply iro n to the so il?" Soil already contains tens of thousands of pounds of iron per acre, but most of it is not in a plant-available form when soil pH is h igh. When iron is added to an a lka line so il, it is rendered insoluble and unavailable to p lants in a relatively short time , usually befo re the plants can use it. Soil ac idifi­cation programs dissolve enough existing plant­unavailable iro n to permit nonnal plant growth.

Al though the primary problem with alkaline soil usua lly is iron deficiency, plant avail abil­ity of many o ther nutrients (phosphorus, z inc, manganese , copper, and boron) also is low in alkaline soils (Figures 4-5). The severity of the nu trient deficiency depends on plant species and the mine ra l composition of the soil. Iron chlorosis may not be expressed for a variety or species du e to the plant's genetic ability to disso lve and obtain iron across a wide soil pH range. Soil ac id ificatio n may still benefit these plants by inc reasing solubility and availability of iron and o the r nutrients.

Factors related to iron chlorosis

• Hi gh so il pH

• Lime appl ications

• Use of urea fert ilizer

• Irrigation water high in carbonate or bicarbonate

• Wet, coo l soil co nd itions leading to poor root growth

• Plastic mu lches or other soil conditions that limit gaseous exchange

0.05

'20.04. 0)

E ';;;'0.03 LL

20.02 :=J

e5S 0.01

0.00 I-----~---~----~

4 5 6 7 Soil pH

Figure 3.- The relationship of soil pH and iron (Fe) in solution extractedfrom a Woodburn soil .

0.12

'20.10 0)

.sO.OS c

N

.Q 0.06

~ 0.04 o

Cf) 0.02

0.00 -I----~----~---~

4 5 6 7 Soil pH

Figure 4.-The relationship ofsoil pH and zinc (Zn) in solution extracted from Bash aw and Woodburn soils.

6 '25­0)

.s4 ~ 3 .

6 2 :;::: :=J

1e5S o

4 5 6 7 Soil pH

Figure 5.-The relationship of soil pH and manganese (Mn) in solution extra cted from a Bashaw soil.

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For example, potatoes have a very high nutrient req uirement. Phosphorus, zinc, and manganese can be deficient without high fertili zer rate s. Research from Idaho shows that the amount of phosphorus and other nutrients needed to adequately fertilize a potato crop increases as so il pH increases.

Lime-induced chlorosis from fertilizer

Ch lorosis can be cau sed by lime and fertil­izers that incre ase bicarbonate (HC0

1") con­

centrat ion in so il solution. Carbonate (lime) or bicarbonate raises pH, inhibiting plants' ability to obtain iron from the soil solution (Figure 3, page 5). Bica rbonate can also reduce a plant's capability to use iron already within the plant (Marschner, (986). Lime-induced chlorosis has been observed in blueberries.

Lime-i nduced chlorosis is temporary and may last a few months or a few years. It will be corrected when soil pH is reduced. The short­term effect of lime-induced chlorosis usually is not detrimental for homeowners with perennial plants such as maple trees, but it may be a sig­nificant problem in the nursery industry, where appearance is critical. Lime-induced chlorosis in blueberries will not correct itself quickly; therefore. acidi fication may be necessary.

To avoid lime-induced chlorosis when apply­ing lime to correct low (below 4.5) soil pH, add small annual applications. Lime applications should be based on soil testing. Test soil after 6 to 8 months and add more lime if pH is still low.

Conversion of urea to bicarbonate

~ 2NH3 + cO2(NH2)lCO + Hp (urea + water ~ ammonia + carbon dioxide)

CO + H 0 ~ H ~ H+ + HCO,"2 2 2C03

Nitrogen fertilizers such as urea (46-0-0) can cause lime-induced chlorosis because they raise the bicarbonate level in the so il so lution. The conversion of urea [(NH1)2COJ to ammo­nia releases carbon dioxide (C02) into the so il solution, thus increasing carbonate concentra­tions (sec "Conversion of urea to bicarbonate ," below).

Lime-induced chlorosis is most often observed in the Vacciniwn species, such as blueberr ies, and in rhododendrons and azaleas. For these plants, apply nitrogen as ammonium phosphate, ammonium sulfate, or some similar fo rm; avoid urea if possible. When urea is used, make several small appli cations rather than a single large application.

Lime-induced chlorosis from irrigation water

Irrigating with water high in carbonate (rare) and/or bicarbonate (common) can cause excess bicarbonate in the soil solution and high soil pH . Some crops irrigated with high -bi carbonate water exhibit lime-induced chlorosis. T he expression of lime-induced chlorosis depends on factors such as drainage, climate, so il pl-l, and plant variety.

The best way to predict potential iro n chloro ­sis problems from irrig ation water is to monitor water chemistry, soil pH , and crop performance. (See "Calculating calcium carbonate in irrigation water," page 7 .) Chlorosis is most likely to occur when so ils with pH greater than 8.0 are irrigated with high-pH and high­bicarbonate water.

(carbon dioxide + water ~ carbonic acid ~ hydrogen + bicarbonate)

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A common way to reduce carbonates in irri­ by a combination of soil and climatic factors. gation water is to inject an acid. (See "Acidify­ Wet, cool soil conditions, ei ther from win­ing irrigation water with sulfuric acid," below). ter rains or irrigation during the spr ing, limit Irrigation water commonly is acidified to pH root growth and root function . Impaired root 6.5 or below. Acidification of high-pH irriga­ function decreases grapes ' ability to extract tion water usually is the most cost-effective iron from the soil, increasing the risk of iron treatment. chlorosis.

Where poor-quality irrigation water is used, soil acidi ficat ion requires continual applications Solutions to iron chlorosis of acidify ing materials and monitoring of plant The best way to avoid iron chlorosis in high­

health and so il pH. pH soil is to choose species or va rieties known to be resistant to iron deficiency. If this is not oil and climatic feasible, iron fertilization and soil acidification lime-induced chlorosis are possible solutions to the problem. This sec­

Recen t resea rch with Concord grapes indi­ tion discusses these topics. Ensuring adequate cates that lime-induced chlorosis can be caused drainage also helps with iron availability.

Calculating calcium carbonate in irrigation water

Pounds of calc ium carbonate equivalent (C0 + HC0 ) in I acre-foot of water = meq /liter 3 3

bicarbonate x equivalent molecular weight of calci um carbonate (50) x [weight of 1 acre-foot of water (2.7 mill ion) -;- I mi llion]

For example , if irrigation water has 5 meg bicarbonate per liter (305 ppm), and it is all present as calcium bicarbonate, then :

5 meq x 50 x 2.7 = 675 Ib lime (calcium carbonate) per acre-foot of irrigation water applied

You would need to apply 203 pounds of elemental S to the soil for every acre-foot of this water applied. To neutralize with concentrated sulfuric acid (36 N), 170 gallons per acre-foot woul d be required (see below) .

Acidifying irrigation water with sulfuric acid

H S0 + Ca t 2 + 2(HC0 - ) => Ca+2 + sO/ + 2C0 + 2H p 2 4 3 2

(sulfuric ac id + calcium + bicarbonate =>calcium + sulfate + carbon dioxide + water)

H SO + 2Na' + CO _2 => 2Na+ + SO -2 + CO + H ° 2 4 :I 4 2 2

(sulfuric ac id + sodium + carbonate => sodium + sulfate + carbon dioxide + water)

Acidify irrigation water by adjusting water pH. Add acid until water pH is between 6.0 and 7.0. Water pH below 6.0 will dissolve metal in irrigation systems, while water pH above 7 .0 will leave too much bicarbonate in the water. Water pH can be easily checked using paper pH sticks . The accu­racy is adequate.

Iron fertilization Adding iron fertilizer to alkaline soil by

broadcast appl ication generally is not effective. Iron fertili zer d isso lves in the soil solution, but quickly forms insoluble minerals that plants are unable to absorb. Iron deficiency sometimes can be corrected with the following materials:

• Chelated iron fertil izers

• Acidify ing iron fer tilizer materials (when placed in a concentrated band)

• Fo liar iron sprays

• Iron drenches

Chelated iron. Although most iron fertiliz­ers generally don ' t work well to supply iron to plants, applying iron in a chelated form greatly enhances its availabi lity. This material is rela­tively expensive and generally not used for lower value crops. Chelates are short lived, as microbes degrade them. Chelate stabil ity is also a function of soil pH. DTPA, a common che­late, is not stable when soil pH is greater than 8.0, making it unsuitable for high-pH situations. Other chelates are stable over a broader soil pH range.

Banded fertilizer. Ferrous sulfate fertilizer (about 100 Ib per acre) adequately supplies iron to field com when applied in a concentrated band near the seed (2 inches below and to the side). For orchards , injecting iron ferti lizer material s I2 to 24 inches deep into the soi I every few feet around the drip line corrects iron deficiencies .

What is a chelate? The word chelate is derived from the Greek

word for claw. A chelated iron molecule is wrapped by anot her larger molecule such as EDTA or DTPA. A good analogy is to visualize a baseball and a mitt. The baseball represents the iron, and the mitt is the chelate. The chelate slows interaction with the soil and prolongs availability of the iron. Soil pH affects how well an indiv id ual chelate will work.

Foliar sprays. Foliar iron sprays are used to correct iron deficiency where crop appearance is critical or where soil amendments are not fea­sible or do not produce de sired res ults. Foliar sprays usua lly are short lived because iron is not translocated to new growth. They mu st be applied at least every other week du ring the growing season. Foliar iron sprays, combined with a soil acidification program, may be the only way to co rrect a severe iro n deficiency.

A foliar iron spray can be made for "spot" use by dissolving 2 oz ferrous sulfate in 3 gal water and adding 2 Tbl mild household deter­gent. The de tergent acts as a wetting agent. Apply foliar iron sprays late in the day when temperatures are lower to avoid leaf burning.

For commercial application, add 50 oz fer ­rous sulfate and I pt surfactant to 100 gal water. Alternative recipes for iron sprays ca n be found in Iron Deficiency in Plants (Agricultural Research Service, 1976). Iron chelates ca n also be used in foliar sprays.

Soil drenches. Iron can be applied di rectly to the soil as a drench. Th is method usually is not suitable for large acreages. The most likely use for this application is in a landscape, nursery, vineyard, or orchard where only a few trees or vines need to be treated. To apply an iron drench for dormant trees, vines, or shrubs, dissolve 1 Ib ferrous su lfate in 1 gal water. To determine how much to apply, measure the

Recommendations­Iron fertilization

• Test soil pH annually.

• If using soil-applied iron fertilizers, apply about 100 Ib per acre as a band.

• Foliar fertilization will help minimize iron chlorosis symptoms. Mix 100 gal water, 50 oz iron sulfate, and I pt surfactant.

• For individual trees, apply 1 Ib ferrous sul­fate for every foot diameter of the leaf drip line as an iron drench.

diameter of the tree 's drip line . Apply 1 ga l of pre pared so lution for every foot of drip line d iameter, wi th a minimum of 1 gal per plant.

To apply the drench, dig a 3- to 6-inch-deep trench aro und the leaf drip line and pour so lu­tion into the trench. If the drip line is too large to co nveniently excavate a trench, dig evenly spaced holes around the drip line, making each ho le large nough to accommodate a ga llon of liquid. Aft I' the solution has soaked into the so il, rep lace the soil in the holes or tren ch .

Acidification Lowering so il pH of alkaline soi ls is more

d if ficult and expensive than raisin g the pH of ac id so ils with lime. Soil with pH greater than 8,4 requires the addition of eJem ental sulfur to low er pH and the add ition of gy psum (ca l­cium sulfate) to lower soil sodium co ntent. Soil with pH less than 8,4 generally requires only eleme ntal sulfur to lower pH.

Soil wi th high pH and carbonates is ex treme ly difficult to acidify and solve iron defi ciency pro blems. Poorly drained so ils or areas that arc continually wet also are extremely

Elemental S reactions in soil

Acidification Sulfur is ox id ized by bacteri a to form sulfuric ac id.

So+ 0 2+ H20 =:> H2S0 4

d ifficult to acidify. In these so ils, high wa ter tables lim it roo t grow th . reduc ing a plant' s abil­ity to take up iron. Co nsider ar tificial d rai nage before attempting soiJ acidi fication. Without adequate soil drainage, loweri ng pH is not economical.

The following section d iscusses mate rials used for soil acidification . See "Methods fo r ac idifying soil in the inland Pacific Northwest" (page JJ) for specific recommendations and me thods.

Sulfur as an acidifying material. The pri­mary material used to ac idi fy soi l is e lemen tal sul fur (S) . Sul fur in the form of sulfate (SO/). inc luding gy psum, is not an acidi fying material.

SUlfate-containing mater ial s are added to the so il for a variety of reasons, incl udi ng aci difica ­tion (see "Fert ilizers as acidi fying materials," page 11), but the sulfate in these materials does not acidify soil. For example, gypsum (CaS ) is add ed to soils that have both high pH and high levels of sodium. It is an imp ortant mate­rial used in reducing soil sodium (see "Elemen­tal S Reactions in Soil ," below). Th e soi l pH is lowered as a result of decreasing sodium.

(e lemental S + ox ygen + water + thiob acillus + time =:> sulfuric acid )

Production of gyp .um and leaching of sodium After e leme ntal S has changed to sulfur ic acid and reacted with lime in so il, gypsum is produce d. This reaction is fa irly rapid . On e ton of elemental S is equivalent to 5 ton s of gy psum.

H S0 + CaCO =:> CaSOj + CO + Hp 2 4 j 2

(sulfuric ac id + lime =:> gypsum + carbon dioxide + water)

Gy psum reacts with sodic so il, produ cin g sodium sulfate , which is rem oved by leaching.

CaS04 + NaHC0.l=:> CaC0.l + Na 2S04

Na SO =:> 2Na+ + SO -22 4 ..

(gy psum + sodic so iJ =:> ca lcium so il + sodi um su lfate) (sodium sulfa te =:> sodiu m + su lfate)

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Sulfate is also present in ammonium sulfate [(NH

4)lSO .J fert ilizer, a common nitrogen fer­

tilizer tha t acidifi es soi l. Ho wever, the ammo­nium in fertilizers-not sulfate-is responsible fo r acidifi cation.

Only e lemental S or reduced sulfur materials such as ammonium thiosulfate produce acidity. Acidificat ion from elemental S occurs when elemental S is oxidized by bacteria to form sul­furic acid (see "Elemental S reactions in soil," page 9). This reaction is temperature-dependent, and the process requires several years to com­plete. Incorporation of elemental S will increase the rate of conversion to sulfuric acid.

Reduci ng soi l pH may take many years and se veral elemental S additions. Check soil pH per iod ically to confirm the need for additional elemental S. For most agronomic crops, contin­ual elemental S additions generally are econom­ical only as sulfur fertil izer. Some high-value horticultural crops can warrant high elemental S additions to acidify soil.

Note that OSU fertilizer guides recom­mend a maximum of lOa Ib of elemental S for S nutrition. Although this rate will supply S to

the plant, it will not be enough to acidify the soil. If you apply enough elemental S or gyp­sum for soil acidification, additional S for plant nutrition will not be needed.

Elemental S reactions add so luble sa lt (sul­fate) to the soil (Figure 6). This salt will need to be leached. A large single elemental S appli­cation, or multiple applications in I yea r, can increase soluble salts to a level detri mental to plant growth. Monitor so luble salts after adding elemental S.

Usually, elemental S and gypsum rates are de termined by economics rather than by the amount needed for fu ll pH reduction. These materials can cost as much as $240 per ton. Generally, a minimum broadcast rate is 500 Ib per acre. A singl e broadcast rate can easily exceed several tons per ac re. Elemental Sand gypsum rates depend on pH, soil text ure , crop, and a grower's desire to correct pH. For ex am­ple, 6 inches of soil with I percent calcium carbonate contains 10 tons lime per acre and requires 6.5 tons elemental S ( J00 percent) to neutralize.

Figure 6.- Reaction of elemental S with calcium bicarbonate produced salts that were not leached from the root zone of these blueberry plants. The soluble salt concentration stunted or killed plants (left). Surviving plants continue to exhibit iron deficiency (right).

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An alternative to elemental S is the applica­tion of acid to the soil (for example, sulfuric acid) to quickly correct alkaline soil tempo­rarily. Unfortunately, acids are very danger­ou s to use and must be handled with special eq uipment.

Growers commonly inject or apply small quant ities of ac id, but these rates rarely have a sig nificant effec t on soil pH. Injecting acid may increase water infiltration and improve soil physical properties. Most soils require several hundred or thousand pounds of acid ifying mate­rial per acre in a single app lication to effec­tively lower pH for a single growing season.

Fertilizers as acidifying materi als. Ammo­nium-containing fertilizers can acidify soil. T his process is slow compared to acidification by elemental S. In a long-term study in dryland wheat in Pendleton, Oregon, ammonium sulfate was applied at the rate of 135 Ib N/acre. Soil pH decreased at the rate of 0.03 to 0.05 unit per year (Rasmussen and Dick, 1995; Rasmussen and Rhode, 1989).

In this example , 10 years of ammonium­containing fe rtilizer applications were neces­sary to attain the same degree of acidification as that achieved with a single application of e lemen tal S. The soil in this study was acidic (pH 6.3), with little or no carbonate. When carbonates are present, significant acidification with ammon ium-containing fertilizer is not pract ical. Columbia Basin soils are sandy with litt le natural carbonate. Farming and nitrogen applications over the past 50 years have acidi­fied many irriga ted fields.

Recommendations-Soil with pH between 6.5 and S.O

• Acidification usually is not req uired .

• If acidification is needed, follow recom­mendations in Acidifying Soil for Crop Production West of the Cascade Mountains, EM 8857-E.

Methods for acidifying soil in the inland Pacific Northwest Acidifying soils with pH between 6.5 and s.n

Soil with a pH between 6.5 and 8.0 rarely needs to be acid ified. Most agronomic and hort icultural crops grow well in th is pH rang e, although iron chlorosis may occasionally occur. Irrigation water quality. particularly bicarbon­ate content, will increase chlorosis and the need for acidification. Where acidification is needed for soil with pH between 6.5 and 8.0, follow instructions in the publication Acidifying Soil for Crop Production West of the Cascade Moun ­tains (http://extension. oregonstate.edu/catalogl pdflem/em8857-e.pdf).

Acidifying soils with pH greater than 8.0

Soil pH east of the Cascade Mo untains is commonly above 8.0. Before attempti ng acidi­fication, determine whether the soil is ca lca re­ous or sodic. Ca lcareous soils contain free lime (calcium carbonate) and generally have a pH of 8.0 to 8.4. Sodic, or sodium-affected, soils have a soil pH of 8.4 to 10.0 or even higher.

High-pH soils with f ree lime produce a fizz­ing reaction when a drop of weak aci d, such as household vinegar, is added to the soil. Car­bonate content can be estimated by obser ving the fizz reaction. Hold soil at normal reading distance and apply a few drops of vinegar. A

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vigorous fizz usually means the soil has more than 3 percent ca lc ium carbonate. A mild fizz means the soi l has approximately 1 to 2 per­cent carbonate. A fizz that can only be heard means the soil probably has less than I percent carbonate.

Consult a local soil expert if you are unsure whether you are dealing with calcareous or sadie so il.

cidificatiun of calcareous soils To aci d ify so ils that co nta in cal cium car­

bonate as determined by the fizz test , apply 1 ton /acre elemental S per year for at lea st 2 ca n .ecutive years (see Table 3) . Monitor soil pH and continue applying the same amount of material an nually until the desired soil pH is reached. Wait a year fo r the soil to equilibrate, check the soil pH, and add more S if pH is too high.

So ils containing calcium carbonate are dif ­ficult to acidify. For example, let's calculate the

Recommendations­Calcareous soils

• Soil test.

• Try bandi ng aci d fert ilizers .

• Broadcast elemental S ba sed on Table 3.

amount of sulfur needed to treat a soil that has a calc ium carbonate level of 5 perce nt. Assuming we treat the top foot of so il, 5 percent calcium carbonate represents 100 to ns of lime pe r ac re. O ne ton of elemental S will neutralize 3 ton s o f calcium carbonate. Therefore, 33 tons o f ele­mental S would be needed to neutra lize the ca l­cium carbonate present in the top foot o f so il. This assumes no additio nal carbonate is added by irrigation water.

This calculation is fo r demonstra tion pur­poses only. The addition of 33 to n/acre of elemental S is impractical and is not reco m­mended. Success can be achieved, howe ver. with smaller additions. In stead o f ac id ifying an entire foot of soil, treat a portio n of the rooting environment so that plants can gro w without pH-induced chlorosis. Localized ac idification can be accomplished by banding or creating aci d ifying ho les depending on the cro p grown and individual circumstances.

Reclamation of . odic soil Sodic, or sodium-affected, soils usually ha ve

a pH between 8.4 and 10.0. Soil pH greater than 8.4 is caused by the presence of sodium bica r­bonate (alkali) in the soil profile.

Sodium bicarbonate (NaHCO ) accumulates J

where soi l is poorly drained and wh ere the water table allows evaporation to co ncentrate sodium salts at or near the soil surface . T his

Table 3, Recommended elemental sulfur treatment needed to overcome iron chlnro. is in a calcareous so il, based on vinegar' t"fizz test") assessment of calcium carhonate ill the soil.

Estimated carbonates Annual addition

present* of elemental sulfur Duration Fizz test result (%) (ton/acre) (years)

None a none no ne Heard (bare ly aud ible) 0-1 0.5-1 1

light (few bubbles) 1-2 I 1-2 Moderate (se ve ra l bubbles) 2-3 1 2- 3 Vigorous (many bubbles) >3 1 3+

*An analysis for calcium carbonate equivalent is a more preci se way of determining calc ium carbon ates present.

situ ation often is referred to as subirrigation, or the movement of water from lower in the soil profile toward the surface. As water at the soil surface evaporates , dis solved salts and/or sodium remain. Sodium and/or salts accumu­late, soil pH rises, and iron chlorosis and other problems may occur. These soils are called sod ic soil s if only sodium is a problem or sod ic- saline so ils if both sodium and salts are a problem.

Shallow groundwater resulting in subini ­cati on is found in many areas of the inland e Pacific Northwest, including floodplains, near waterways, adjacent to leaky irrigation canal s, and where the water table has risen due to local irrigation or dam con struction.

The amou nt of subirrigation and the time of year the water table is high influence the sever ity of a sodic and/or salt problem and the expression of iron chlorosis (Figure 7).

Soils with salt and alkali accumulation need improved dra inage before a pH reduction pro­gram will be successful. Without correcting drainage, a saline (high electrical conductiv­ity) prob lem will become more severe with the addition of soil amendments, which are salts.

In addi tion to improved drainage, reclama­tion of sodium-affected soil requires both acidi­fi cation and leaching of sodium (and other salts, if present). For sal ine-sodic soils, the sodic problem must be corrected before reducing salinity. To reduce sodium, either apply calcium as gyp sum or apply an acid to dissolve calcium already in the soil and follow with leaching. Ap ply elemental S for acidification. Initial rates of 1 ton elemental S/acre and 1 ton gypsum! acre may be needed.

Fruit tree and ornamentals Acidificati on of so il around an established

tree does not require a broadcast application of acidifying material. The objective is to acidify a small zone from which the plant can obtain nutrients. Dig a minimum of 4 holes (prefe rably 8 to 12) even ly spaced around the drip line of

Increasingsoil pH

Figure 7.- Hypothetica f influence of soil drain­age on soil pH and the likelihood that iron chlo­rosis will be seen in susceptible plants.

Recommendations-Sodie soils • Soil test for pH , sod ium, carbonates, calc ium,

and elec trical co nductivity.

• Make sure water can drain through the root zone.

• Apply 1 ton/acre gypsum or other calcium source.

• Broadcast elemental S based on Table 3 (page 12) if adequate calcium is present.

Figure 8.-A Large, cemented clod of soif taken from an appLe orchard in eastern Washington. Before the center of the clod was treated with elemental sulfur and Nrphuric, the soil pH was 8.9. After S application, the soil pH at the clod slllfa ~ e was 2.5, thus demonstrating how small, localized changes can impact pH.

the tree. T he holes should be 8 to 12 inches deep. Mix 1 cup elemental S with the soil removed from each hole or add 112 cup N-phuric to eac h hole. Return the soil to the hole. Alternatively, you co uld use Yz cup sul­fur ic or phosphoric ac id per hole. but these mater ials are much more hazardous to handle. A small amount of iron and manganese sulfate can be added to each hole to help increase iron and manganese avai lability. An iron drench also may be used (see pages 8-9) .

When elemental S is added to the soil in concentrated areas such as holes around a tree, the pH of the treated soil can differ dramati­cally fro m that of the urrounding soi l. Bulk soil pH can be above 8.0, while pH in the soil immediate ly surrounding the hole may be as low as 2.5. Figure 8 (page 13) shows how small, local ized applications can affect soil pH without amending the whole soil. Apple roots were growing in the pic tured clod even though the pH was we ll be low what is considered suit­able for fru it trees.

Remember, you are changing a small area; don 't be shocked if soil pH in that area is well be low what wou ld be considered adequate for plant growth. Iron will be supplied to the plant from the treated area, while the rest of the roots continue to take up water and other nutrients from the untreated bulk soil.

Trunk injections of an iron solution may work as a last resort measure. This method should be tried on an experimental basis prior to treating a large acreage.

Other con iderations Sodium can accumulate even in well-drained

soils when rainfall is inadequate to leach it deeper into the soil profile. In the case of well ­drained soil that has never been farmed, the application of low-bicarbonate irrigation water and commerc ial fertilizer will facilitate leach ­ing of the naturally accumulated sodium. Most surface waters in the inland Pacific Northwest are relatively pure and low in bicarbonates. Soil

I.:J­

Recommendations-Fruit trees and ornamentals

• Soil test fo r pH, sodium, carbonates, and electrical conductivity.

• Apply elemental S in four to eight holes around each tree. Appl y 8 oz ( I cu p) of ele ­mental S in each hole.

amendment may not be needed in thi s situation or, if needed, only initially.

Irrigation water quality As discussed on pages 6-7, irrigation water

that is high in bicarbonate/carbonate can cau. e high soil pH and lime-induced chlorosis. See page 7 for possible solutions to this problem.

Irrigation water high in sodium bicarbon­ate/carbonate and low in calcium also can lead to sodic soils when leaching is not ad quate to remove the sod ium . Injecting gypsum into the irrigation water can be suc cess­ful in maintaining adequate soil pH and soi l infiltration rate and in keeping sod ium from accumulating. Gypsum injection allows sodium to leach because calcium from the gy psum rep laces the sodium. An even applicat ion with gypsum injectors through pivot systems is dif­ficult, however. See Irrigation Water Quality for Crop Production in the Pacific Northwest, PNW 597-E, for more info rmation.

Soil amendments and fertilizer Some soi l amendments, such as compost and

manures, can be detrimental if they are high in pH, salt, or sodium. Always ana lyze soil amendments for pH, EC, and sodium before application.

Soil amendments that are neutral in pH and low in sodium can aid in reclamation of sodic soils by improving aeration and wa ter infil­tration , which facilitates leaching of sodium. Amendments can improve crop performance and increase the rate of acidification.

15

Lime-i nduced chlorosis can be increased by the addition of a poor choice of amendment, lime, or urea fertilizers. See pages 6-7 for addi­tional explanation and solutions.

References Agricultural Research Service . 1976. Iron Defi­

ciency in Plants: How to Control It in Yards and Gardens. Home and Garden Bulletin Number 102.

Doergc. T.A. and E.H. Gardner. 1985. Reacidifica­tion of Two Lime Amended Soil s in Western Oregon. Soil Sci. Soc. Am . Journal , 49(3 ):6IW­6R5 (May-June 19R5).

Hem phill, D.D., Jr. and T.L. Jack son . 1982. Effect of So il Acidity and Nitro gen on Yield and Elemental Concentration of Bush Bean, Carrot, and Lettuce. 1. Arner, Soc. Hort . Sci. 107(5):740-744 .

Hopkins, B.G. ct al. 2007. Irrigation Water Qual­ityfo r Crop Production in the Pacific Northwest. PNW 597-1-:. Oregon State University.

Horneck, D.A. 1994. Nutrient Management and Cycling ill Grass Seed Crops. Ph.D. thesis, Oreg on State University. Chapter 5 and appendix .

Horne ck, D.A. e t al. 2004. Acidifying Soilfor Crop Production West of the Cascade Mo untains. EM 8857-E. Oregon State Unive rsity.

Mars chner, Horst. 1986. Mineral Nut riti on of High er Plant s. Academi c Press, Inc.. London, England. pp. 514-517.

Peterson, P. W. 1972. Liming Requirement of Selec ted Willam e//e Valley So ils. Master 's thes is, Ore gon State Un iversity.

Petrie, S.E. J982. N Fertilizer Effects on Soil Solu ­tion Mn and Mn Response of Barley and Oats. Ph.D. thesis, Oregon State Uni versity.

Rasmussen, P.E. and R.P. Dick. 1995. Long-Ter m Management Effects on Soil haracteristics and Producti vity. In Proceedings of the Western Nutrien t Managem ent Conf erence, Salt Lake City, U'I, March 9- 10, 1995. Potash and Phosph ate Institute.

Rasmussen, P.E. and c.R. Rohde. 19R9. Soil Acid i­fi cation from Ammonium-Nitrogen Fertiliza tion in Moldboard Plow and Stubble-Mulch Wheat ­Fallow Ti llage. Soil Sci. Soc. Am. J. 53(1)119­122 (January-Fehruary 1989) .

Tisdale, S.L. , W. Nels on , and J. Beaton . 1985. Soil Fertility and Fertilizers, 4 th ed. Mac millan Pub­lishers, New York .

umma y Acidification of soil sometimes is neces­ • Proceed cautiously, as so il pH can be low­

ered beyond the desired level , especially sary for optimum plant growth. Plants with yellow leave s-similar to the symptoms for in sandy soils.

nitrogen and sulfur deficiency-may be suf­ • Monitor the change through annual soil fering from iron deficiency. Iron deficiency testing. Sample soil at the same time each occurs when soil pH is higher than a plant year. can tolerate. Adding iron fertilizer to the • Keep complete records of the amount of so il rarely is successful in correcting an iron material added , the amount of mixing, and deficiency. The most common solution is to the time of year the acidifying material acidify the soil with elemental sulfur. If you was added. attempt to acidify your soil, keep in mind the

• Do not treat sodium-affected soils without following.

correcting drainage problems.

.<) 2007 Ore gon State Uni versity. T his publica tion may be pbotocopied or reprinted in its e nt ire ty for noncommercial purposes . Pub lished and distribu ted in furthe rance of the Ac ts of Co ng ress of May 8 and Jun e 30 . J914, by the Orego n State Un iver sity Ext ens ion Serv ice, Washington State Universit y Ext ension, Universi ty of Idah o Co opera tive Extension System, and the U.S . Departm ent of Agri culture coopc mtiu g.Thc three participa ting Ex tension Ser vices offe r educatio nal progra ms, activities. and mat erials-withou t regard to age . co lor , disab ility, gender iden tity or express ion, mar ita l sta tus, na tional or igi n. race . relig ion, se x. sex ua l o rientation, or vete ran's statux-s-as requ ired by Tit le VI of the C ivil Rights Act of 1964 , Ti tle IX of the Education Amendments of 1972 , and Section 504 of the Rehabilita tion Act o f 1973 . T he Oregon Sta le Unive rsity Extension Se rv ice . Wash ington State Unive rsi ty Extensi on, and University of Idaho Extension a re Equ al O ppo rtu nity Employ ers.

Pub lished Se ptember 200 7 .

Changing pH in Soil Soil pH d irectl y affects the life and growth of plants because it affects the availab ility of all plant nutrients . Between pI-I 6.0 and 6.5. most plant nutrients are in their most available state. A nu trient mu st be so luble an d rem a in soluble long enough to successfully trave l through the soil so lution into the roots. Nitroge n, fo r example, has its greatest solubility between soil pH 4 an d soil pH 8. Above or be low that ra nge, its solubil ity is serious ly restricted .

Soil acid ity or alkalinity (pH) is extremely important because it has an effect on the deco mpos ition o f mineral rock into essential elements that plants can use. It also changes fertilizers from their form in the bag to a form that plants can easily uptake. Soil microorganisms that change organic nitrogen (a mino acids) to the ammoni um form of nitrogen to the nitrate form that plant can use also depends on the soil pH. So il pH should be checked periodically and consistent testing wi ll ind icate whether your pH-contro l prog ram is working.

Raising pH The ideal pH range for soil is from 6.0 to 6.5 because most plant nutrients are in the ir most available state. Ir a soil test ind icates a pH below 6.5, the usual recommendation is for the application of gro und limestone. In add ition to having the ability to raise pl-I, limestone contains cal c ium. Some pre fer do lom it ic limeston e because it conta ins both calc ium and magnesium, however soils high in magnes ium (serpentine) do not nee d more magnesium. Table I indicates the number of tons per acre of ground limestone requ ired to ra ise the pH of a g iven soil to based on the original pH, desired pH. and soil type.

In order to se lec t the correct application rate use a soil test to determine both the so il texture gro up and the current pH. As the percentage of clay in a soil increases, it requires proportionate ly mor e limes to ne to rai se the pH. T his means it is much harder to change the pH of clay soil than sandy so il. Cons ider that limesto ne moves very slowly , taking years to move down a few inches in the soil. This is wh y it is so important to test so il ea rly in th e planning process. Limestone should be tilled into the soil root zone (top 7 inches).

Table 1. Ap proximate Amount of Finely Ground Li mestone Needed to R'arse th H of -IDCh L 01e Pi a 7' aver 0 f S '1

Lime Requirements (Tons per Acre) From pH 4.5 to 5.5 From pH 5.5 to 6.5Soil Texture

Sand and loamy sand 0.5 0.6 Sandy loam 0.8 1.3

1.2 1.7Loam 2.01.5Silt loam 2.3C lay loam 1.9 4.33.8Muck

L' . M . IT a ble 2 Com mon rmmz ateria s

Name Shell meal Limestone Hydrated lime

Chemical Formula

Equivalent % CaC03 Source

CaCo, 95 Natural shell deposits CaC0 3 100 Pure form, fin ely ground

Ca(OH)2 120-135 Steam burned

Burned lime CaO 150-175 Kiln burned Do lomite Sugar beet lime

CaC03 - MoCO:; 110 Natural deposit CaC0 3 80-90 Sugar beet by-product lime

Ca lci um s ilicate CaSi03 60-80 Slag

Lowering pH Some soils are alka line and have a pH above 6.5. So me fertilizers (ammonium sulfate, urea . and ammonium nitra te) creat e an acid reaction in the soil, so they aid in lowering or mai ntaining a spec ific pi !. Certa in acidifying organic materials such as pine needles or peat moss can lower so il pH gradua lly ove r man y years. In nature this takes thousands of years. For mo re rapi d results in lowering pH, sulfur is used. ulfuric acid form s when sulfur is add ed to the soil, the small er the particles of su lfur, the faster the react ion . Lowering the pH is a slow process and will take I -2 years to see a reaction.

Tabl ur nee e per acre toe 3 T ons 0 f suIf d d ower pIH to 65 Original pH

8.5 Sandy Soil Clay Soil

0.7- \. 0 \. 0 - 1.3 8.0 7.5

0.5 -0.7 0.7 - 1.1 0.4 - 0.5 0.2 - 0.3

om momy err ~q Ulva en d a ues Table 4 C I UsedMa tena. Isand Th . E tAmen men tV I Tons of Amendment Equivalent to

Material 1 Ton of 1 To n of (100'\10 Basis)* Chemical Formula Pure Gypsum Soil Sulfur Gypsum Ca S0 4 • 2H2O \.00 5.38 Soil sulfur S 0.19 \.00 Su lfuric ac id (cone.) H2SO4 0.6 1 3.20 Ferric su lfate Pel(So.')3 • 91-hO \. 09 5.85 Lime sulfur (22% S) Ca S 0.6 8 3.65 Calcium chloride CaC 12 • H2O 0.86 -­

Aluminum sulfate Ab(S0 4) 3 -­ 6 .34 * The percent purity is given on the bag or identification tag

Common Amend ment Reactions in Soil

• Gypsum (calcium sulfate) + sodic soil ~ ca lcium soil + sod ium sulfate (leachable w ith wat r) Sodium sulfate is then leached out ofthe soil by rainfall or heavy irrigations. The removal ofsodium lowers the sodium permeability hazard allowing for soil aggregation and impr oved drainage. Gypsum does not change p H nor improve drainage in non-sodic situations. Gyps um is used to add calcium to soils such as serpentine with velY high or toxic Mg levels.

• Sulfur (e lementa l) + oxygen + water ~ sulfur ic acid + soil calcium ~ gypsum Gyps um then acts as above. Sulfur and sulfuric acid also lower pH

• Lime (ca lcium or magnesium carbonate) + water ----:; calcium soil + OH Lime neutralizes the (acidity) - 11 ion concentration and adds calcium to so il

Pau l Vossen University of California Cooperative Extension

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