Handbook of Soil Acidity

Zdenko Rengel, Zdenko Rengel

• Publisher: CRC Press
• Year: 2003
• ISBN:
  9781135546151
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

This handbook offers effective strategies to modify and adjust crop production processes to decrease the toxicity of soil contaminants, balance soil pH, improve root growth and nutrient uptake, and increase agricultural yield. The Handbook of Soil Acidity provides methods to, measure soil acidity, determine the major causes of soil acidification, c

1: Soil Acidification: The World Story

Malcolm E. Sumner

University of Georgia, Athens, Georgia, U.S.A.

Andrew D. Noble

Land and Water, CSIRO, Townsville, Australia

1 INTRODUCTION

For decades, soil acidity has been a major constraint to crop production throughout the world. However, in developed nations, the use of lime to counteract acidity in high-input agriculture over the past 50 to 100 years has led to a marked decrease in the area of acid soils under cultivation and to spectacular increases in yields. Still, in the case of deep naturally acid profiles, little amelioration of subsoil acidity has occurred, and in some cases (e.g., in Australia), neutral to alkaline subsoils have actually been acidified as a result of the failure to correct topsoil acidity [1]. In contrast, in developing nations with largely low-input agriculture and farmers able to afford only minimal applications of lime, very little amelioration of soil acidity has taken place. In fact, the condition has probably worsened in many areas.

Experimental results [2] show that counteracting subsoil acidity can result in substantial (20–100%) yield responses, particularly under rain-fed conditions. There are a number of reasons for this situation. Many soils in developing nations are naturally very acid and infertile to great depths in the profile. Cultivation of such soils without inputs results in very low yields, which has maintained the farming population in poverty. As a result, farming has mined what little resources the soils have had to offer and yields have tended to decrease rather than increase with time for lack of adequate inputs.

Without access to financial loans to facilitate the purchase of inputs, it is extremely difficult for these resource-poor farmers to break out of this cycle of poverty. The small loans required by such farmers are seldom available from traditional lending institutions. Despite the results of several years of research which have shown that yields can be more than doubled by the application of lime to acid soils, this practice has not been widely adopted in developing nations because of the unavailability or high cost of lime, the lack of access to loans, and poor infrastructure capacity.

Poor crop growth on acid soils is usually a direct result of aluminum toxicity. When soil pH_H2O drops below 5, aluminum becomes soluble and causes severe root pruning that results in reduced water and nutrient uptake (see Chapter 10). Thus, one of the first manifestations of the harmful effects of soil acidity is drought stress. Many of the acid soils in developing nations are deep and well drained and have high yield potentials if the roots could penetrate and extract water from the acid subsoils, normally beyond their reach [3]. The limiting factors associated with poor root penetration are the lack of Ca²⁺ and/or excess Al³⁺. The efficient use of subsoil water again requires inputs such as gypsum and lime, which have been shown to promote yields on soils with acid highly weathered subsoils throughout the world [4]. A simple test is available to identify subsoils likely to respond to gypsum [3]. Thus, the solution to the problem of soil acidity in developing nations does not require more research but rather an aggressive extension program to promote the use of lime, gypsum, and other inputs (phosphorus, nitrogen, potassium, etc.) whose benefits have been amply demonstrated. This should be coupled with an appropriate system for making small production loans to farmers [5]. It is often not appreciated that soil acidification is a severe degradation process. Consequently, its remediation should be seen as a capital cost, which can be amortized over a number of years.

2 AREAL EXTENT OF ACID SOILS

Estimates of the total area of topsoils affected by acidity throughout the world vary from 3.777 × 10⁹ [6] to 3.950 × 10⁹ [7] ha, representing approximately 30% of the total ice-free land area of the world. The distribution of acid soils by region in relation to cultivated and total ice-free land area is presented in Table 1. The largest areas of acid soils are in South America, North America, Asia, and Africa. In most regions, the area of acid soils far exceeds the area under cultivation, indicating that large areas of acid soils are still under natural forest or grassland vegetation. The total area affected by subsoil acidity is estimated as 2.918 × 10⁹ ha [6], meaning that approximately 75% of the acid soils of the world suffer from subsoil limitations due to acidity.

TABLE 1 Areal Extent of Acid Soils in Relation to Total Ice-Free Land Surface and Area Under Cultivation for the Regions of the World

Using a scale for acid intensity based on pH_H2O ranges of <3.5 (extreme), 3.5–4.5 (high), 4.5–5.5 (moderate), and 5.5–6.5 (slight), the global distribution of these acid topsoils and subsoils is depicted in Figs. 1 and 2. Areas associated with each of these subdivisions are tabulated by region in Table 2. On a global basis, only a relatively small proportion of acid topsoils fall in the extremely acid category, with South America accounting for the lion’s share. The remaining acid soils are fairly evenly distributed among the other categories. Except for the extremely acid category, a similar pattern is exhibited for subsoils. In the Americas, Africa, and Asia, a large proportion of soils with topsoil acidity also exhibit marked subsoil acidity of similar intensity, indicating the strong linkage between top- and subsoil acidity under pristine conditions. Because Al³⁺ becomes soluble and toxic below pH ~ 5.0 to 5.2, the categories of moderate and high acidity are of particular interest in agronomic terms, accounting for 67 and 79% of the world’s acid topsoils and subsoils, respectively.

In Tables 3 and 4, the areal extent of acid topsoils has been broken down according to the Food and Agriculture Organization (FAO) Soil Groups [8] and Soil Taxonomy [9] Orders. As one would expect, the largest areas of acid soils occur in Soil Groups that have been either intensely weathered [Acrisols, Nitosols, Ferralsols] (Ultisols, Oxisols) or formed on basic cation-poor parent materials [Podzols, Histosols, Arenosols, Podzoluvisols, Cambisols] (Entisols, Spodosols, Inceptisols, Histosols).

The preceding estimates (Tables 1–4) should be treated with some caution as they are derived from databases that contain significant omissions, possibly inappropriate extrapolations and assumptions, and unspecified methods of measuring pH and have not been georeferenced [6]. However, they are the best estimates available at present.

3 TYPES OF ACID SOILS

3.1 Naturally Occurring Acid Soils

3.1.1 Due to Intensive Weathering

The natural weathering processes for acid soils involve leaching of the parent material by acidic rain due to the presence of H₂CO₃ that provides protons and removes basic cations in the leachate. As a result, parent rocks weather to form acid soils with the rate of acidification depending on the nature of the parent material, effective rainfall, and temperature. Highly basic parent materials weather more rapidly than highly siliceous substrates, with both increased rainfall and temperature promoting the process. Under extreme conditions such as occur in the humid tropics, most silicate minerals in the parent material are weathered away by desilication, leaving little other than the oxides of iron and aluminum. Such soils (Oxisols and Ultisols) are usually weathered to great depths. Thus, in nature, one finds a range of soils in different stages of weathering exhibiting different degrees of acidity often reflected in the systems used to classify soils.

FIGURE 1 Global distribution of soils with acid topsoils. (Reprinted with permission. Copyright Courtesy Natural Resource Conservation Service, USDA, USA.)

FIGURE 2 Global distribution of soils with acid subsoils. (Reprinted with permission. Courtesy Natural Resource Conservation Service, USDA, USA.)

TABLE 2 Areal Extent (× 10⁹ ha) of Acid Topsoils and Subsoils of Varying Intensity for Various Regions of the World

TABLE 3 Areal Extent (× 10⁶ ha) of Acid Topsoils for the FAO Soil Groups

TABLE 4 Aerial Extent (× 10⁹ ha) of Acid Topsoils for Soil Taxonomy Orders

In terms of soil taxonomy [9], acid soils in this category fall mainly into four Soil Orders, namely Oxisols (Ferralsols), Ultisols (Acrisols, Nitosols, Planosols), Andisols (Andosols), and Alfisols (Podzoluvisols). The nearest FAO Soil Group equivalents have been placed in parentheses. Some highly acid soils (acid sulfate) also occur in the Inceptisol Order.

Oxisols are the most highly weathered but not necessarily the most acidic soils (Table 5) because, in the final stages of weathering, soil pH increases due to the high point of zero charge (>pH 7) of Fe and Al oxides. They have very low basic cation status and effective cation exchange capacity (ECEC), but exhibit appreciable variable charge indicated by the difference [cation exchange capacity (CEC) – ECEC]. In this context, CEC refers to the value obtained with 1 M NH₄OAc (pH 7), and ECEC is the sum of exchangeable cations (Al³⁺ + Ca²⁺ + Mg²⁺ + K⁺ + Na⁺). Often they are heavy textured with a more or less uniform distribution of clay with depth. They are extremely poor in available phosphorus but usually have adequate to excellent physical properties.

Ultisols are less highly weathered but often more acid than Oxisols and usually contain appreciable amounts of silicate clay minerals (mainly kaolinite). Clay content increases with depth often abruptly, giving rise to a Bt horizon. In general, their ECEC values are higher than for Oxisols but they also exhibit considerable variable charge. They contain appreciable levels of toxic exchangeable aluminum to depth associated with low basic cation status (Table 5). Unfortunately, the NRCS-USDA database [13] does not contain information on exchangeable Al³⁺ and, consequently, many values are missing in Table 5.

TABLE 5 Chemical Properties of Representative Oxisols, Ultisols, Andisols, and Alfisols

Andisols, which are most commonly not very acidic, are dominated by amorphous minerals (allophane and imogolite), organic matter, and variable charge. They have very high contents of organic matter in the surface horizon, with good to excellent physical properties down the profile. The acidic Andisols are found mainly in hot, humid regions and can become quite acid, with low basic cation content (Acrudoxic Hapludand) (Table 5).

In contrast to Ultisols, Alfisols are much less highly weathered, with only some being acidic. They usually exhibit increasing silicate clay content (mainly 2:1 clay minerals), basic cation status, and pH with depth. In Australia, many naturally nonacidic Alfisols have become acidic as a result of anthropogenic sources of acidity (see Chapters 4 and 5).

3.1.2 Acid Sulfate Soils

These soils, which cover 24 million ha worldwide, usually fall in the Inceptisol (Gleysols, Rankers, Cambisols) and Entisol (Fluvisols, Gleysols, Regosols, Arenosols) Orders and are found mostly in the delta areas of the great rivers. Prior to drainage, they have a neutral reaction as unripe sulfidic clays, but they become extremely acidic when drained (raw acid sulfate soils) due to the oxidation of reduced sulfur compounds (FeS_x) to sulfuric acid. After complete oxidation of the S compounds and dissip...