At the end of this section, you should know: 1. The effect of aluminum and hydrogen toxicity on plant growth. 2. The deficiency or toxicity of various plant nutrients due to low soil pH. 3. The effect of low pH on certain soil microorganisms.
Aluminum and Hydrogen ToxicityPoor plant growth on acid soils is generally associated with a low soil pH value. The effects of soil pH on growth are complex and it is difficult to separate the direct effects from the indirect effects associated with changes in the solubility and availability of various elements affecting plant growth. There are two main effects of Al and H on growth: (i) injury to roots, and, (ii) decreased uptake of cations (Ca2+, Mg2+, K+).
The toxic effects of Al and H are manifested quite differently in soils. In organic soils, Al is complexed very strongly by the organic fraction, therefore, there is very little Al3+ present. Consequently, poor plant growth resulting from low pH can be due primarily to H toxicity. In most instances this does not occur until the pH drops to pH 4.0 or less. In acid mineral soils (pH 5.0 or less) poor plant growth is due primarily to excessive levels of Al.
Root injury is observed at pH 5.0 and lower. At these low levels, lateral root development is suppressed and in some cases root tips are killed. The roots become discolored brown or a dull gray (similar to nematode damage). At low pH and low Ca concentrations, damage to root membranes is accentuated.
Aluminum Stunted Corn Roots
Much of the poor root development (and drought susceptibility) seen in soils with acid subsoil layers (pH less than 5.0) is generally due primarily to Al toxicity, which limits both rooting depth and the degree of branching
|Aluminum toxicity reduces root growth|
|Poor Corn Growth - Low pH|
At pH 5.0 and below, high Al concentrations suppress the uptake of cations such as Ca and Mg and results in deficiencies of these elements, particularly in soils with a very low cation exchange capacity (CEC). At pH 5.5 and above, however, there is little if any suppression of cation uptake.
A large portion of the effective CEC of very acid soils is occupied by Al. When the exchangeable Al saturation is greater than 60%, there is a large increase in soil solution Al3+. It is the Al3+ in soil solution that is toxic to plants, not the exchangeable Al3+.
The following table shows that the soil pH value at which maximum corn grain yield occurs is quite different for two soils, but comparable when Al saturation is considered.
|Wharton silt loam||Morrill silt loam|
|pH||Al sat., %||Yield, bu/acre (% of max)||pH||Al sat., %||Yield, bu/acre (% of max)|
|4.7||60||44 (50)||4.9||18||120 (94)|
|4.9||45||71 (81)||5.1||10||123 (97)|
|5.2||30||77 (87)||5.5||2||128 (100)|
|5.8||5||82 (94)||5.8||0||128 (100)|
|6.2||0||88 (100)||5.8||0||123 (97)|
Manganese ToxicityManganese toxicity is one of the factors that may limit plant growth on certain acid soils. Highly weathered soils that have a large sesquioxide clay content often contain high amounts of Mn.
The exchangeable Mn in acid soils increases markedly at soil pH values less than 5.0. Plants growing on soils with a high content of exchangeable Mn accumulate large amounts of Mn in their tissues and growth is reduced. Liming these soils to pH 5.5 and above decrease the solubility of Mn and uptake of Mn sufficiently to eliminate the toxicity and increase growth.
|Hiwassee loam soil from blue-ridge province.|
|pH||Water sol. Mn, ppm||Soybean leaf Mn, ppm||Grain yield, bu/acre|
Symptoms of manganese toxicity
- Cotton Leaf crinkling
- Wheat and other small grains Leaves show marginal chlorosis, curling, and necrotic spots
- Soybean Leaves are crinkled and cup down. Youngest leaves show first symptoms.
- Tomato Toxicity symptoms begin an lower leaves (oldest). Leaves die and hang from the stem. Necrotic streaking develops on the main stem.
- In general, crinkle leaf symptoms occur when leaf Mn is from 400 to 500 ppm
Soybean Manganese Toxicity
Calcium DeficiencyMany acid soils have very low amounts of exchangeable Ca and a low Ca saturation of the CEC. At these low levels of available Ca, the uptake may also be inhibited by Al.
Although it is often difficult to separate the direct effects of H and Al from those of Ca on plant growth, a number of points can be made about Ca in highly weathered soils. A Ca saturation of approximately 25 to 30% appears to be adequate for supplying the Ca requirements of most plants. Growth may not be maximum at this Ca saturation because some other factor is limiting yield.
Perhaps the single most important factor affecting the availability of Ca in acid soils is the level of exchangeable or soluble Al in relation to that of Ca.
Watermelon Blossom End Rot
Tomato Blossom End Rot
An important function of calcium in the plant is with the formation of cell walls. Since calcium is immobile within the plant, the fruit continues to grow but the growing tip is soft and dark due to poor cell wall formation.
Magnesium deficiencySandy soils at pH 5.0 are often very low in magnesium. At that pH value and below, plants begin to show magnesium deficiency symptoms.
Plant uptake of magnesium from nutrient solutions is influenced by the H ion concentration. Uptake of Mg by plants increases with increasing pH and reaches an optimum at approximately pH 5.5. The effect of the soil pH value on Mg availability is probably due to an antagonism of both Al and H on Mg uptake, particularly when the percent Al saturation is high. Neutralization of Al and H is necessary for optimum Mg availability. When soils are acid and low in available Mg, the use of dolomitic lime is the best approach to correct Mg deficiencies.
Magnesium Deficient Corn
Magnesium Deficient Cotton
Al toxicity is often manifested as a phosphorus deficiency (leaves -- purplish coloring on dark green, spindly, slow growth)
Mechanisms for the phosphorus deficiency include:
- Reduced root growth. Phosphorus uptake is highly dependent on a finely branched, extensive root system with as many root hairs as possible.
- Precipitation-adsorption reaction in soil and along the mucilaginous layer at the surface of the root and in the inter-cellular regions of the apical 3 to 4 mm of root tip.
- Interference with P metabolism in the plant.Neutralization of exchangeable Al by liming has a marked effect on the response of plants to additions of fertilizer P. Much lower rates of fertilizer P are required for optimum growth when exchangeable Al is neutralized.
Phosphorus Deficient Corn
Decreased availability of P fertilizers if Al is present Ceci1 soil
pH 4.6 6.1 dry wt, g 0 0.06 0.11 2 1.53 1.43 58 1.86 2.38 116 2.26 2.42
Al causes P to precipitate in the roots P uptake, mg pH Tops Roots 4.7 3.52 1.72 5.0 4.68 0.98 6.0 5.56 0.94
Potassium DeficiencyLiming acid soils that have a relatively large Al saturation on the exchange sites will more than double the soil buffering capacity after the exchangeable Al is neutralized. One benefit of liming acid soils, therefore, is the reduction in leaching losses of fertilizer K. High rates of lime, however, which raise pH levels above 6.0, may decrease the available K by displacement on the exchange sites and increase the need for K fertilization on certain soils.
Availability of Micronutrient Cations in Relation to LimingThe availability of Mn, Cu, Zn, and Fe to plants generally decreases as the soil pH increases. Liming soils that are inherently low in these elements may induce micronutrient deficiencies for a short or long time.
ManganeseIn the sandy soils of the Atlantic Coastal Plain of the U.S., liming to a pH value near neutrality often produces Mn deficiency in certain crops. Two examples are soybeans and peanuts. Soybeans develop Mn deficiency at pH 6.2 in Atlantic Coastal Plain soils.
Reduction in water soluble Mn pH Water Soluble Mn
5.0 2.5 5.6 0.9
Manganese Deficient Wheat
Manganese Deficient Soybeans
Manganese Deficient Corn
BoronThe liming of acid soils to pH 7.0 often causes B deficiencies. Fixation of the applied B is much greater at pH 7.0 than at pH values less than 7.0. Hydroxy Al and Fe materials are responsible for B fixation when acid soils are limed. Retention of the B by hydroxy Al and Fe compounds is pH-dependent.
The availability of Mo is very low in acid soils. Deficiency symptoms are most often observed with legumes. It is seen as a N deficiency since Mo is required by the nitrogen fixing rhizobia.
Soil pH is one of the most important factors affecting the availability of Mo and its uptake by plants. In contrast to the other plant micro nutrients, liming acid soils increases the availability of soil Mo. This might be due to OH- ions replacing adsorbed Mo2-4. The amount of Mo absorbed by soils and hydrous oxides is increased as pH is decreased. Molybdate ions replace the surface OH- ions of hydrous oxides of Fe and Al. As the pH decreases, surface OH- ions are held less tightly and can be replaced by MoO2-4 ions.
Molybdenum Deficient Soybeans
Adding Mo increases grain yield of soybean when pH is 6.0 or less, but not at higher pH. Soil pH minus Mo plus Mo -------------------------- grain, bu/acre ------------------------------- 5.6 32 41 5.7 34 43 6.0 37 40 6.2 40 42 6.4 42 41
Increased Mo availability due to an increase in soil pH
Microorganismsa. Nitrification is reduced at lower pH values
NH4+ and microbes NO3-
b. Rhizobium activity is increased as the pH value increases
pH Nodule Number (soybeans) 4.7 21 5.0 64 6.0 77
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