Soil Acidity

Acid Soils in Hawaii:
Problems and Management

N. V. Hue and H. Ikawa
Department of Agronomy and Soil Science,
College of Tropical Agriculture and Human Resources
University of Hawaii at Manoa

Introduction

Acid soils, by definition, are those with pH below 7.0. The lower the pH, the more acid is the soil. Each unit pH drop indicates ten times more acidity. For example, pH 5.0 has 10 times more acidity than pH 6.0, and 100 times more acidity than pH 7.0. Most Hawaii soils have pH ranging from 4.0 to 8.0, and many have pH below 6.0. Table 1 lists pH of some common items for comparison.

Table 1. pH of some common items.

Item	pH	Item	pH
Most acid soils	4.0 - 6.0	Lemon juice	2.2 - 2.4
Orange juice	3.4 - 4.0	Vinegar	4.0 - 4.5
Acid rain	3.0 - 5.0	Clean rain water	5.5 - 5.7
Fresh milk	6.3 - 6.6	Blood plasma	7.2 - 7.4
Mild soap solution	8.5 - 10.0

How Soils Become Acid

In Hawaii, many soils have become acid because of warm temperatures and high rain fall. Under such environmental conditions, soils weather quickly. Basic cations (e.g., Ca, Mg, K), which are essential to living organisms, are leached from the soil profile, leaving behind more stable materials rich in Fe and Al oxides. This natural weathering process makes soils acid and generally devoid of nutrients.

Soil Acidification by weathering
Man-made processes also contribute significantly to soil acidity. Applications of NH4+ producing fertilizers (e.g., urea, anhydrous NH3, (NH4)2SO4) acidify soils through a biological reaction by which NH4+ is oxidized to NO3- and H+. Sulfur dioxide (SO2) and nitrogen oxides (NOx) released primarily by industrial activities react with water to form acid rain, which acidifies soils, particularly forest soils with low buffering capacities.

Why Acid Soils are Infertile

Acid soils have many problems that adversely affect crop growth. Most important among these are:

Aluminum Toxicity.

Aluminum is more soluble under acidic conditions; and high Al levels are toxic to plants. Aluminum toxicity usually damages the root system first. Aluminum-affected roots tend to be shortened and swollen, having a stubby appearance.

Manganese Toxicity.

Unlike Al, Mn toxicity first shows up in plant tops. The symptoms vary among plant species, but often are specific for a given species. For example, stunted, crinkled, and chlorotic leaves are the Mn toxicity symptoms in soybeans.

Calcium Deficiency.

Unlike acid soils of the US mainland, acid soils in Hawaii are often Ca-deficient rather than Al-toxic. This problem is particularly severe in Kauai, the oldest island in the State. Since Ca is fairly immobile inside the plant, its deficiency symptoms first appear at the growing points. In corn and taro, Ca-deficient plants are stunted; young leaves are unable to fully unfurl then the leaf tips or margins soon die.

(Ca deficiency in Bun-Long taro; courtesy of Ms. M. Calisay)

Phosphorus Deficiency.

Phosphorus can react strongly with Fe and Al components of acid tropical soils, thereby becoming unavailable for plant uptake. Older leaves in P-deficient plants are often purple because of the accumulation of anthocyanins (purple pigments).

Managing Acid Soils

Liming.

Although planting acid-tolerant crops is a reasonable option for dealing
with acid soils, liming is traditionally used to correct soil acidity and to improve
soil productivity.

When lime (i.e., CaCO3) is added to a moist soil, the following reactions will occur:

(1) Lime is dissolved slowly by moisture in the soil to produce Ca2+ and OH-

CaCO3 + H2O (in soil) ==> Ca2+ + 2OH- + CO2 (gas)

(2) Newly produced Ca2+ will exchange with Al3+ and H+ on the surface
of acid soils

       2Ca2+ + soil-Al ===> soil-Ca + Al3+
              + soil-H         soil-Ca + H+

(3) Lime-produced OH- will react with Al3+ to form Al(OH)3 solid and with H+
to form water.

             Al3+ + 3OH- ===> Al(OH)3 (solid)
                H+ + OH- ===> H2O

Thus liming eliminates toxic Al3+ and H+ through the reactions with OH-. Excess OH- from lime will raise the soil pH, which is the most recognizable effect of liming. Another added benefit of liming is the supply of Ca2+ and Mg2+ if dolomite [Ca,Mg(CO3)2] is used.

Because soils differ widely in mineralogy, organic matter and clay content, they require different amounts of lime to raise soil pH to a given value. Thus lime requirement curves should be constructed for individual soils to be used in lime estimation. Figure 1 shows an example of such curves.

Figure 1. Lime requirement curves of the Halii soil (Kauai) and the Paaloa soil (Oahu).

Planting Acid-Tolerant Crops.

Different plant species (even varieties within a species) grow best at different pH ranges. For example, azalea and camelia grow well only at pH below 5.5, and appear to sufferfrom Fe and/or Mn deficiencies at higher pH. Also, pineapple can tolerate soil acidity much better than sugarcane. Table 2 lists soil pH ranges for optimum growth of selected crops.

Table 2. Soil pH range for optimum growth of some crops.

Crop	pH	Crop	pH
Alfalfa	6.5 - 7.5	Avocado	6.0 - 6.5
Azalea	4.5 - 5.0	Camelia	4.5 - 5.5
Ginger	6.0 - 7.0	Macadamia	5.0 - 6.5
Pineapple	4.7 - 5.7	Sugarcane	6.0 - 7.0
Taro	5.5 - 6.5

Conclusions

Acid soils are widespread in Hawaii as well as around the world. Their occurrences are caused by natural processes (weathering) and/or man-made processes (adding NH4 producing fertilizers to soils, releasing acid forming gases to the atmosphere). Acid soils are infertile because of (i) Al and/or Mn toxicities and (ii) Ca and/or P deficiencies. Acid soils can be managed by liming based on appropriate lime requirement curves or by growing acid-tolerant crops.

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