Presented at GAU-PRII-IPI National Symposium on:
Balanced nutrition of groundnut and other field crops grown in calcareous soils of India
September 19-22, 2000, Junagadh, Gujarat, INDIA
Integrated Nutrient Management for Sustaining Crop Yields in Calcareous Soils
Patricia Imas - International Potash Institute, Coordination India. c/o DSW, Potash House, P.O.Box 75, Beer Sheva, 84100, Israel. E-mail: email@example.com
- Definition of Calcareous Soils
- Origin of Calcareous Soils
- Role of CaCO3 in plant nutrition
- Nitrogen Management in Calcareous Soils
- Phosphorus Management in Calcareous Soils
- Testing Calcareous Soils for Phosphorus
- Potassium and Magnesium Management in Calcareous Soils
- Iron Management in Calcareous Soils
- Zinc and Manganese Management in Calcareous Soils
- Copper, Boron and Molybdenum Management in Calcareous Soils
- Nutrients Availability in the Rhizosphere
- Sulfur Products Used as Soil Acidifiers
Calcareous soils have free calcium carbonate (CaCO3) in the profile. The carbonates, due to their relatively high solubility, reactivity and alkaline character, buffer the pH of most calcareous soils within the range of 7.5 to 8.5. These soils generally have 100% base saturation, and the exchange complex is dominated by calcium.
Nutrient management in calcareous soils differs from that in non-calcareous soils because of the effect of soil pH on soil nutrient availability and chemical reactions that affect the loss or fixation of almost all nutrients. Both native and applied P is tied up in highly insoluble Ca and Mg phosphates, rendering the added P only sparingly available for plant uptake. Iron, Zn, Mn and Cu deficiencies are common in soils that have a high CaCO3 due to reduced solubility at alkaline pH values.
Improved fertilizer management is required to grow crops successfully on calcareous soils. To avoid ammonia volatilization, fertilizers containing ammonium-N or urea should be moved into the root zone with rainfall or irrigation, or be incorporated into the soil. Band placement of P minimizes soil contact thus reducing or delaying the formation of insoluble Ca and Mg phosphates. Crops planted on calcareous soils may require above normal levels of K and Mg fertilizer for satisfactory nutrition.
Using tolerant rootstocks and varieties reduces the severity of lime-induced Fe chlorosis on calcareous soils. An effective but expensive remedy for lime-induced Fe chlorosis on non-tolerant rootstocks or varieties involves the use of chelated Fe. Zinc and MN deficiencies can be corrected through foliar application of chelates. Adequate K supply and organic matter application can improve the availability of microelements.
Sulfur products that act as soil acidifiers can potentially improve nutrient availability in calcareous soils by decreasing soil pH. In many cases, it is not economical, as it is required repeated applications at high rates, and a long interval to observe results.
A calcareous soil is a soil that has free calcium carbonate (CaCO3) in the profile, i.e. contains enough CaCO3 so that it effervesces when treated with hydrochloric acid. When free carbonates are present, the acid will produce bubbling due to the evolving of CO2 gas (Loeppert and Suarez, 1996):
CaCO3 + 2H+ Ca2+ + CO2 + H2O
Calcareous soils cover more than 30% of the earth surface, and their CaCO3 content varies from a few percent to 95% (Marschner, 1995). Hagin and Tucker (1982) define calcareous soil as a soil that its extractable Ca and Mg levels exceed the cation exchange capacity.
Calcareous soils occur naturally in arid and semi-arid regions because of relatively little leaching (Brady and Weil, 1999). They also occur in humid and semiarid zones if their parent material is rich in CaCO3, such as limestone, shells or calcareous glacial tills, and the parent material is relatively young and has undergone little weathering.
Some soils that develop from calcareous parent materials can be calcareous throughout their profile. This will generally occur in the arid regions where precipitation is scarce. In other soils, CaCO3 has been leached from the upper horizons, and accumulated in B or C horizons. These lower CaCO3 layers can be brought to the surface after deep soil cultivation (Brady and Weil, 1999).
In some soils, the CaCO3 deposits are concentrated into layers that may be very hard and impermeable to water. These caliche layers are formed by rainfall leaching the salts to a particular depth in the soil at which water content is so low that carbonates precipitate (Jackson and Erie, 1973).
Soils can also become calcareous through long period of irrigation with water containing dissolved CaCO3 (Hagin and Tucker, 1982).
A major characteristic of calcareous soils is that they develop in regions of low rainfall and must be irrigated to be productive. Marginal desert soils, with low content of organic matter and high concentration of CaCO3, can be of high agricultural value by supplying the nutrients in the soil solution through drip irrigation (Brady and Weil, 1999). For example, tomato grown on a virgin desert soil containing 85% CaCO3 and EC of 7 mmho cm-1 yielded 80 t ha-1 when providing N, P and K through the irrigation system (Kafkafi and Bar Yosef, 1980).
Calcareous soils are alkaline because of the presence of CaCO3, which dominates their chemistries. The carbonates are characterized by a relatively high solubility, reactivity and alkaline nature; their dissolution resulting in a high solution HCO3- concentration which buffers the soil in the pH range of 7.5 to 8.5:
CaCO3 + H2O Ca2+ + HCO3- + OH-
Usually, the pH is not in excess of 8.5 regardless of CaCO3 concentration, unless a significant quantity of sodium is present (Lindsay, 1979). Calcareous soils have 100% base saturation and calcium is the dominant cation in the exchange complex and in soil solution (Loeppert and Suarez, 1996).
Reported symptoms of impaired nutrition in calcareous soils are chlorosis and stunted growth. This is attributed to the high pH and reduced nutrient availability, as direct toxicity of bicarbonate ions (HCO3-) to physiological and biochemical systems is much less likely (Pearce et al., 1999).
Improved nutritional management is required to grow crops successfully on calcareous soils. Crops fertilizer management on calcareous soils differs from that on non-calcareous soils because of the effect of soil pH on soil nutrient availability and chemical reactions that affect the loss or fixation of some nutrients. The presence of CaCO3 directly or indirectly affects the chemistry and availability of nitrogen, phosphorus, magnesium, potassium, manganese, zinc, copper and iron (Marschner, 1995; Obreza et al., 1993).
Nitrogen fertilizers should be incorporated into calcareous soils to prevent ammonium-N volatilization. The availability of phosphorus and molybdenum is reduced by the high levels of calcium and magnesium that are associated with carbonates. In addition, iron, boron, zinc, and manganese deficiencies are common in soils that have a high CaCO3 due to reduced solubility at alkaline pH values (Marschner, 1995; Brady and Weil, 1999).
Particle size distribution, surface area and reactivity are important properties of soil carbonates which influence soil pedogenic, chemical and rhizosphere processes (Loeppert and Suarez, 1996). Calcium carbonate provides a reactive surface for adsorption and precipitation reactions, for example, of phosphate, trace metals and organic acids (Talibudeen and Arambarri, 1964; Amer et al., 1985). Carbonate reactivity influences the rate of volatilization of ammonia (Ryan et al., 1981). Carbonate affects also rhizosphere processes, especially those processes in which acidification is an important factor. For example, the Fe-deficiency response of dicotyledons involves the exudation of protons and acidification of the rhizosphere. The effectiveness of Fe-deficiency stress response is therefore negatively influenced by the neutralization of plant-produced acidity, which is influenced by the reactivity of the carbonate phase (Loeppert et al., 1988; Morris et al., 1990).
The alkaline pH values found in calcareous soils affect the rates of N transformations, which in turn can influence the efficiency of N use by plants.
Nitrification: is the conversion of ammonium (NH4+) to nitrate (NO3-) by soil bacteria. The nitrification rate is most rapid in soils with pH values between 7 and 8, and decreases with decreasing pH values. Ammonium fertilizers lower the pH during nitrification because protons are released in the process:
2NH4+ + 3O2 2NO3- + 8H+
Ammonium fertilizers are superior as compared to nitrate fertilizers in very slightly alkaline soils (pH 7-7.5) due to its side effect as a soil acidifier. However, in soils with an alkaline pH and high carbonate content, the excess CaCO3 provides such a large buffer capacity that the H+ ions produced do not alter the soil pH to any appreciable extent (Hagin and Tucker, 1982).
Ammonia volatilization: is the loss of N to the atmosphere through conversion of NH4+ to ammonia gas (NH3). When ammonium fertilizers are surface applied to calcareous soils, the following reaction occurs:
2NH4A + CaCO3 (NH4)2CO3 + CaA2
Where A represents the accompanying anion in the ammonium fertilizer. The (NH4)2CO3 product is unstable and decomposes as follows:
(NH4)2CO3 + H2O 2NH3 + H2O + CO2
NH3 + H2O NH4+ + OH-
In calcareous soils, with pH > 7, the equilibrium shifts to the left due the higher OH- concentration, and gaseous NH3 is formed and lost by diffusion into the atmosphere. The formation of (NH4)2CO3 and thus the extent of ammonia losses depends on the anion accompanying the NH4+ cation in the fertilizer, which forms the Ca salt. If the CaA salt is an insoluble one, then the reaction will proceed to the right causing more (NH4)2CO3 to be formed and thus more NH3 is generated and volatilized. But when the accompanying anion forms a soluble Ca compound, less (NH4)2CO3 will be formed. Therefore those sources which forms precipitates of low solubility with Ca such as ammonium sulfate and phosphate will suffer larger ammonia losses than ammonium nitrate or chloride, which form soluble reaction products with Ca (Hagin and Tucker, 1982; Wiezler, 1998) (Table 1).
Ammonia loss can also occur in the vicinity of hydrolyzing urea applied on the surface of high pH soils. Ammonium carbonate is produced upon urea hydrolysis, which dissociates to form NH4+, OH- and CO2. In alkaline conditions, NH4+ forms NH3 that may be lost by volatilization (Finck, 1982; Mortvedt et al., 1999).
Proper management of N fertilizer in calcareous soils involves practices that minimize its loss through ammonia volatilization. Following an application of ammoniacal-N or urea to the soil surface, the fertilizer should be moved into the soil profile through irrigation or mechanical incorporation if rainfall is not imminent (Finck, 1982; Mortvedt et al., 1999). Regarding NH3 losses from urea, additional practices include mixing the urea with KCl, CaCl2 or TSP, and the use of granular forms, urease inhibitors, and sulfur-coated urea (Wiezler, 1998).
Symbiotic N fixation: the infection by symbiotic Rhizobium bacteria, nodulation and subsequent symbiotic N fixation was impaired in Lupinus when grown on calcareous soils. Nodulation was usually decreased by high soil pH (Tang and Robson, 1995). Iron induced deficiency, which is common in calcareous soils, has been found to decrease nodulation and N fixation in soybeans (Tang and Robson, 1992).
Phosphorus availability in calcareous soils is usually restricted. Maximum availability to plants of both native and applied P is in the pH range of 6.0 to 7.5. At higher pH values, phosphate anions react with Ca and Mg to form phosphate compounds of limited solubility (Mortvedt et al., 1999)
After P fertilizer is added to a calcareous soil, it undergoes a series of chemical reactions with soil components that decrease its solubility (a process referred to as P fixation). The mechanisms of fixation are: phosphate adsorption on clay minerals and CaCO3 surfaces and precipitation of Ca phosphates. Consequently, P availability to plants is controlled by the application rate of soluble P and the dissolution and desorption of fixed P (Talibudeen, 1981).
The chemistry of calcium phosphates is of special significance to phosphate reactivity in calcareous soils. Added phosphate is precipitated as dicalcium phosphate dihydrate (DCP; CaHPO4 2H2O) or octacalcium phosphate (OCP; Ca8H2[PO4]6 5H2O). Hydrolysis of DCP to OCP increases with increasing pH of soil. OCP is converted over a long period of time to less soluble apatites (hydroxyapatite, Ca10[OH]2[PO4]6) and fluorapatite (Ca10F2[PO4]6) (Hagin and Tucker, 1982).
Soluble P fertilizers (triple superphosphate, ammonium phosphates) are the preferred source in calcareous soils. Insoluble P fertilizers (rock phosphates) may dissolve so slowly that dissolution would be more limiting than the rate of fixation, giving them a very low residual value in addition to low immediate effectiveness (Fuehring, 1973; Hagin and Tucker, 1982).
The main strategy of P fertilization is through the manipulation of its placement and timing, aiming to insure sufficient quantities of soil solution P at points of greatest root activity and at times of peak plant requirement. Application of P in bands instead of broadcasting and mixing with a large soil volume, and as large granules instead of a fine powder - decreases the reversion to less soluble forms by reducing the contact between fertilizer and soil (Fuehring, 1973; Hagin and Tucker, 1982).
In citrus grown on calcareous soils, P fertilizer should be applied each year in newly planted groves, and when trees approach maturity, P applications can be limited to only once every few years (Obreza et al., 1993). In tomato grown on highly calcareous soils under drip irrigation, P was applied in a basal band fertilization of single superphosphate combined with daily soluble P as KH2PO4 with the irrigation water, and no P deficiency was observed (Kafkafi and Bar Yosef, 1980).
The major methods used by soil testing laboratories to measure soil P include Mehlich 1 (double acid), Bray P1, Bray P2 and Olsen (sodium bicarbonate). The chemistry of the extraction methods is affected by carbonate reactions. Treating calcareous soils with ammonium fluoride solution (Bray) to extract soil phosphate leads to the formation of surface coatings of calcium fluoride that react with soil P (Talibudeen, 1981). Mehlich 1 is not appropriate for use on calcareous soils because its extracting ability is weakened by exposure to CaCO3.
The Olsen method has been consistently correlated to P uptake by plants growing on calcareous soils, giving particularly good results in the alkaline pH range. Therefore, this method is widely used to predict both deficiency levels and levels of adequacy or possible excess (Fuehring, 1973; Talibudeen, 1981).
Available K and Mg are usually found in an adequate supply in calcareous soils. This is due to native high levels of exchangeable K and Mg, which are hardly leached in low rainfall regions (Brady and Weil, 1999).
However, an imbalance between plant available Mg, Ca and K ions may lead to Mg and/or K deficiencies to crops. In calcareous soils, the proportion of Ca to other exchangeable cations generally exceeds 80%, and a low proportion of exchangeable Mg (less than 4%) may lead to Mg deficiency in plants (Hagin and Tucker, 1982). High Ca levels in soils suppress Mg and K uptake by crops in part, presumably, through the competition between Ca, Mg and K (Marschner, 1995). For example, in grapevine, the K-Ca antagonism impairs plant K uptake and results in decreased K concentration in the berries, leading to more acidic wines (Garcia et al., 1999). Therefore, crops growing on soils high in Ca often require above normal levels of Mg and K fertilization for satisfactory nutrition (Brady and Weil, 1999).
The K supplying power of calcareous soils was found lower than that of alluvial soils, even when their content of non-exchangeable K was similar. This may stem from the high Ca and Mg content in calcareous soils, since the activity ratio K/(Ca+Mg) was found to control the uptake of K, more than the K activity (Metwally and El-Damaty, 1973).
Although in calcareous soils of semiarid and arid regions K levels are generally quite high, K deficiencies have been identified on eroded soils and extremely sandy soils, specially those that have been heavily cropped (Hagin and Tucker, 1982). Soils that were exhaustively cropped with alfalfa, showed that after 16 cuttings, the exchangeable K levels declined by 33 up to 58% and the yields were increased by K application. The release of non-exchangeable K was higher for clay soils, indicating that these soils had a higher long-term supply of available K than the light-textured soils (Havlin and Westfall, 1985).
It is often difficult to increase leaf Mg and K levels with fertilizer applied directly to calcareous soils, which contain very high quantities of both exchangeable and non exchangeable Ca. Magnesium sources like dolomitic limestone or calcined magnesite (MgO) are not effective in alkaline soils. In these soils, readily soluble materials are required, such as MgSO4. However, fertilizing with soluble Mg salts may not always be effective, due to the rapid formation of low soluble Mg salts such as magnesite (MgCO3) (Hagin and Tucker, 1982).
In cases where soil-applied fertilizer is ineffective, the only way to increase leaf Mg or K concentration may be through foliar application of water-soluble fertilizers, such as magnesium nitrate [Mg(NO3)2] or potassium nitrate (KNO3).
In citrus, a spray solution of 25g KNO3 L-1 increased leaf K concentration, especially if applied several times during the year. Higher concentrations should be avoided, since high salt levels promote leaf burn. The amount of N applied as foliar KNO3 should be considered when determining annual N fertilization plans for citrus groves (Obreza et al., 1993).
Calcareous soils may contain high levels of total Fe, but in forms unavailable to plants. Visible Fe deficiency, or Fe chlorosis, is common in many crops. However, owing to the nature and causes of Fe chlorosis, leaf Fe concentration is not necessarily related to degree of chlorosis; in chlorotic plants Fe concentrations can be higher or lower than those in normal plants. Thus, this disorder on calcareous soils is not always attributable to Fe deficiency; this condition is known as lime-induced Fe chlorosis.
Iron is considerably less soluble than Zn or MN in soils with a pH value of 8; thus, inorganic Fe contributes relatively little to the Fe nutrition of plants in calcareous soils. Most of the soluble Fe in the soil is complexed by natural organic compounds. The primary factor associated with Fe chlorosis under calcareous conditions appears to be the effect of the bicarbonate ion (HCO3) in reducing Fe uptake and translocation to the leaves. In some other cases, the lime-induced chlorosis is related to a high Fe level in the chlorotic leaves, which has to be somehow unavailable or immobilized inside the leaf tissue (Bavaresco et al., 1999). This latter aspect has been called Fe chlorosis paradox, and it is still unclear the mechanism for Fe to be inactivated in the chlorotic leaves (Romheld, 1997).
Susceptibility to Fe chlorosis depends on a plant's response to Fe deficiency stress, which is controlled genetically. The easiest way to avoid lime-induced Fe chlorosis is to use tolerant varieties and rootstocks. In citrus, rootstocks vary widely in their ability to overcome low Fe stress. For example, trifoliate orange (P. trifoliata) and swingle citrumelo (C. paradisi x P. trifoliata) are very susceptible rootstocks, while rough lemon (C. jambhiri) and sour orange (C. aurantium) are tolerant rootstocks (Obreza et al., 1993).
Remedy of iron chlorosis in susceptible varieties grown on calcareous soils is very difficult. Inorganic sources of Fe such as ferrous sulfate (FeSO4) or ferric sulfate [Fe2(SO4)3] have a very limited effect unless applied very frequently at extremely high rates.
Different strategies based on improved management of nutrition can be considered for amelioration of Fe chlorosis:
Chelates: Iron chlorosis can be corrected through soil application of Fe chelates. Commercial chelates are synthetic organic compounds that contain Fe in a complex form and protect it from reacting in the soil and forming insoluble precipitates. Plants can take up the soluble chelate as complete molecules and then metabolize the metal (Finck, 1982; Hagin and Tucker, 1982).
The most common synthetic organically chelated forms of Fe include Fe-EDTA (iron ethylene diamine tetra acetate), Fe-HEDTA (hydroxy ethylene diamine triacetate), Fe-DTPA (iron diethylene triamine penta acetate), and Fe-EDDHA (iron ethylene diamine di hydroxyphenyl acetate). The effectiveness of these chelates varies greatly, depending on soil pH. Fe-DTPA may be used on mildly alkaline soils (with pH values of 7.5 or less), whereas Fe-EDDHA is the chelate of choice for use on highly calcareous soils, with a pH greater than 7.5 (Norvell, 1991).
Lachover and Ebercon (1972b) showed that groundnut yield response to Fe application in Israel was related to % CaCO3 in the soil. Papastylianou (1989) surveyed 35 groundnut fields in Cyprus and determined that plants were chlorotic when % CaCO3 >20-25% and Fe content <2.5 mg kg-1 (DTPA extractable). Lachover et al. (1970) applied Fe-EDDHA to a soil in Israel with pH 7.9 and 15% CaCO3 and measured a 50% increase in pod yield and a 40% increase in hay yield. They also showed that Fe chelate applied to a soil of pH 7.9 and 11% CaCO3 caused leaves to green up and increased yield. Yields were increased 359% by application of 11 kg Fe ha-l (as Fe-EDDHA) to a loamy clay with pH 7.9 and 31% CaCO3 (Lachover and Ebercon 1972a). In citrus, applications of 20-30 g of Fe chelate per tree have proved satisfactory (Mengel and Kirkby, 1987). Although agronomic results are positive, the high cost of chelated Fe makes this treatment economically impractical.
The differences between cultivars obtained in resistance to chlorosis and response to Fe chelates is a classical example of genetically controlled mineral nutrition (Marschner, 1995). The non-adapted groundnut cultivar Congo Red, originated from acid soils, became severely chlorotic when grown in a calcareous soil, and Fe chelate had to be applied to overcome chlorosis and to obtain a reasonable yield, with a response of 210% as compared to the yield without Fe application. In contrast, in the adapted cultivar 71-238, chlorosis was absent and the yield without Fe application was much higher. Application of Fe-EDDHA increased yield by only 9% (Fig 1.; Hartzook et al., 1974)
Potassium - Iron interaction: Barak and Chen (1984) found that K fertilization at rates of 135 to 405 mg K kg soil-1 ameliorated iron chlorosis in groundnut grown in an extremely calcareous soil (63% CaCO3). These results are attributed to the cation-anion balance of ion uptake: the plant takes up more cations than anions, there is an efflux of H+ to correct this imbalance, the rhizosphere is acidified and consequently iron is more available to roots. The antichlorotic effect of K fertilizers was quantified by measurements of chlorophyll content in the leaves, and K2SO4 was found to be more effective than KCl (Fig.2).
Iron chlorosis alleviation with KCl fertilization has also been observed in groundnut grown on calcareous soils in Junagadh, India (Field experiments conducted by Gujarat Agricultural University, International Potash Institute and Potash Research Institute of India).
An important factor in evaluating the agricultural potential of potassium-iron interaction is the fact that Fe chelates cost as much as 100 times more than K, and they are easily leached out of the root zone by irrigation and rainfall, unlike potassium.
Potassium - Ammonium interaction: Supplying ammonium sulfate in the presence of nitrification inhibitor (nitrapyrin) reduced Fe chlorosis in groundnut grown on very high calcareous soil (98% CaCO3) in Israel (Kafkafi and Ganmore Neumann, 1985). NH4+ nutrition resulted in higher cation uptake than anion uptake, and protons (H+) were released from the roots. This proton excretion reduced the pH in rhizosphere thus increasing Fe availability and correcting Fe chlorosis (Plate 1). In practice, an ammonium sulfate band with nitrapyrin is less expensive than application of Fe chelates in soils where iron chlorosis is usually observed.
Organic Manures and Sewage sludge: Applications of manure can help to correct Fe chlorosis (Mengel and Kirkby, 1987). Chen and Barak (1982) reviewed several works where mixtures of Fe salts and organic matter such as manure, compost, sewage sludge and peat have been used successfully in controlling Fe chlorosis in various soils.
Natural, organically complexed Fe exists in organic waste products such as sewage sludge, but at lower concentrations than in chelated Fe fertilizers. On calcareous soils in the western United States, sludge applied at 37 ton ha-1 was an effective Fe source for field crops severely deficient in Fe. Sludge can be potentially useful since it contains readily soluble forms of Fe that may remain in soil solution through organic complexation (Obreza et al., 1993).
Foliar fertilization: In citrus, foliar application of ferrous sulfate (FeSO4), ferrous sulfate heptahydrate (FeSO4 7H2O) or Fe chelates has not proven effective because of poor Fe translocation within the leaf. The use of foliar sprays also increases the possibility of fruit and/or leaf burn. For these reasons, foliar application of Fe is not recommended to correct Fe chlorosis of citrus (Obreza et al., 1993). For soybeans, usually more than one application of iron sulfate is needed. Treatment needs to begin as soon as chlorosis first becomes visible, and repeated at 7-10 day intervals until new growth is normal in color. Foliar treatment with an iron solution is a last option and is suggested only as a rescue treatment (Penas and Wiese, 1990).
Soil pH is the most important factor regulating Zn and MN supply in calcareous soils. At alkaline pH values, very low levels of soluble Zn are found, and therefore only a negligible amount can be in the form of exchangeable Zn2+, which is available to plants. Zinc and MN deficiencies are clearly pH-dependent, and both Zn and Mg concentration in solution decreases 100-fold for each unit increase in pH (Lindsay, 1972). At high pH levels precipitates are formed, such as Zn hydroxides [Zn(OH)2 and CaZn(OH)4] and Zn carbonate.
Soils that are slightly alkaline may not necessarily be deficient in Zn or MN In addition, Zn and MN can be chelated by natural organic compounds in the soil, a process that aids the movement of these nutrients to the plant root. On highly alkaline soils, however, Zn and MN deficiencies are not uncommon (Finck, 1982). The most common inorganic Zn and MN fertilizers are the sulfates (ZnSO4, MnSO4) and their oxide forms (ZnO, MnO). Broadcast application of these compounds to correct Zn or MN deficiencies in calcareous soils is not effective, since the alkaline pH renders the Zn and MN unavailable almost immediately.
Zinc is also available in chelated forms, including Zn-EDTA and Zn-EDDHA. Chelated Zn when applied to calcareous soils remains soluble and available to plants considerably longer than the inorganic forms (Hagin and Tucker, 1982). However, soil application of chelated Zn is rarely economical. Manganese chelates have limited effectiveness in calcareous soils and are not normally used (Murphy and Walsh, 1972).
The least expensive way to apply Zn and MN is through foliar sprays. In addition to the forms listed above, a number of other Zn and MN formulations are available for foliar spraying, including nitrates and organically chelated forms using lignin sulfonate, glucoheptonate, or alpha-keto acids. Preliminary research data indicate little difference in magnitude of foliar uptake, regardless of the form of carrier or chelate applied. Foliar applications of low rates of MN or Zn (e.g., 0.6-1.2 kg ha-1) are not adequate to correct moderate to severe deficiencies often found in soils with high pH values. Foliar application of Zn has to be repeated several times per year, due to the limited translocation from older to new leaves (Hagin and Tucker, 1982).
Copper: Copper solubility is pH-dependent and it decreases with increasing pH.
However, the appreciable changes in solubility occur in the pH range below 5, and above pH 5 the solubility does not change very much. Copper deficiencies are generally found in leached sandy soils (Hagin and Tucker, 1982). The most common source of Cu for soil and foliar application is copper sulfate (CuSO4 5H2O) and also Cu chelates (Mortvedt et al., 1999). In orchards, the crop Cu requirement is normally satisfied through foliar sprays of Cu fungicides (Obreza et al., 1993).
Boron: Soil pH affects B availability more by sorption reactions than by formation of less soluble compounds. Availability of B is highest in the pH range of 5.5-7.5. Boron is sorbed to Fe and Al oxides in soils and is lowest in the pH range of 6-9 (Mortvedt et al., 1999).
There is an interaction between B availability and the presence of Ca ions. High levels of Ca at high pH reduce the uptake of B. This may explain the fact that high B levels in calcareous soils, considered as toxic in other conditions, do not produce B toxicity in crops (Lucas and Knezek, 1972).
Responses to boron application in sandy calcareous soils have been reported for sugar beet and bitter orange (Hagin and Tucker, 1982). In deficient soils, B is most applied in the form of sodium borates (Na2B4O7 xH2O). Excess and toxicity of B usually occur in soils with high levels of B or due to additions of B in irrigation water (Hagin and Tucker, 1982).
Molybdenum: Mo deficiencies are not known in calcareous soils, as MO availability increases with pH (Lindsay, 1972). Molybdenum toxicity to plants has not been reported. However, high MO levels can accumulate in alkaline soils, which may be toxic to livestock this condition is known as molybdenosis (Hagin and Tucker, 1982)
Mobilization of P and micronutrients in the rhizosphere of plants grown on calcareous soils can be brought by different mechanisms: (a) Root-induced decrease in pH as a consequence of preferential cation uptake; (b) Exudation of organic acids by the roots; (c) Release by roots of photosynthates as substrate for rhizosphere microorganisms, which in turn affect pH, redox potential, and chelator concentrations (e.g. siderophores) in the rhizosphere (Marschner, 1995).
Roots can affect P and micronutrients availability by inducing pH changes in the rhizosphere. The pH at the root surface may differ from the bulk soil pH by up to two units (Marschner, 1995). Such differences in pH are caused by differential rates of uptake of cations and anions, which are associated with OH- and H+ effluxes. The form of N nutrition plays an important role. In plants supplied with NO3-, anions uptake exceeds cation uptake and OH- or HCO3- is released from the roots. This causes the rhizosphere pH to become more alkaline. On the other hand, plants supplied with NH4+ take up more cations than anions, release H+ into the soil, thus acidifying the rhizosphere and increasing P and micronutrient availability (Mengel and Kirkby, 1987). N- anionic nutrition (i.e. supply of NO3-) was found to cause Fe chlorosis in calcareous soils by inducing high pH levels in the free space of root and leaf tissue and thereby restrict the uptake of Fe into the cell (Mengel and Geurtzen, 1986). Shifting to N-cationic nutrition (i.e. application of NH4+) induce Fe chlorotic maize leaves to re-green without application of Fe (Mengel and Kirkby, 1987).
Plants roots also exudates low molecular weight compounds to the rhizosphere, such as sugars, organic acids, amino acids and phenolics. Organic acids, such as citric and malic acid, are of general importance in mobilizing P, Fe MN and Zn in the rhizosphere, and can increase their uptake rates and contents in the plants. In response to Fe deficiency, increased rates of root exudation of phenolics and amino acids (phytosiderophores) play a particularly significant role (Mengel and Kirkby, 1987; Marschner, 1995).
Enhanced exudation of organic acids by tomato roots was measured under P deficiency conditions (Imas et al., 1997). Exudation of organic acids can cause localized acidification of the rhizosphere; this is specially observed in species with clustered roots (proteoid roots) such as white lupin. Citric acid was the dominant compound in proteoid roots of white lupin, and increased Fe, MN and Zn mobilization from calcareous soils (Dinkelaker et al., 1989; Table 2). The increased availability of these nutrients by exuded organic acids stems either from the lower pH of the rhizosphere, or the chelation of Fe, MN and Zn, and by lowering the Ca2+ concentration by chelation and formation of sparingly soluble salts such as Ca citrate (Marschner, 1995).
In addition to supplying sulfur as a nutrient, S compounds are also used as soil amendments. These compounds act as soil acidifiers neutralizing CaCO3 with acid; this, in turn, may lead to a lowering of soil pH and improved nutrient availability. The rates of soil acidifiers required to cause a plant response depend on the amount of CaCO3 in the soil (Finck, 1982).
In soils with high content of CaCO3 it is not advisable and usually quite impractical to lower pH because of the vast amounts of acidifier required. Repeated applications at high doses are needed, and a long interval to observe results. Because plant response to broadcast application of an acidifier is unlikely in this instance, such applications are not recommended (Hagin and Tucker, 1982).
In contrast, soils containing little CaCO3, or those that have become alkaline from irrigation water with high levels of bicarbonate, require less acidulation and respond faster. It is feasible to acidify in this situation (Obreza et al., 1993).
Examples of S-containing acidifiers include elemental S, sulfuric acid (H2SO4), aluminium sulfate [Al2(SO4)3] and ammonium and potassium thiosulfate [(NH4)2S2O3, K2S2O3]. Elemental S is the most effective soil acidifier. Although not an acidic material itself, finely ground elemental S is converted quickly to H2SO4 in the soil through microbial action.
In theory, 16 ton ha-1 of elemental S is required to neutralize each 1% of CaCO3 in the soil to a depth of 30 cm. Broadcast application of S over the entire root zone is not practical because a large amount of S is required for acidification. A high rate of S, concentrated in a small volume of calcareous soil creates an acidic zone and increases the availability of P and micronutrients to roots growing in and near the acidic zone. Citrus on calcareous soil in Florida completely recovered from lime-induced chlorosis after application of 1.1 L of concentrated H2SO4 distributed equally into 6 holes dug within a tree root zone; however, this occurred only if Fe-EDTA was concurrently applied to the holes at a rate of 57 g of Fe per tree (Obreza et al., 1993).
The soil within the wetted pattern of a microirrigation emitter often becomes alkaline when the water contains bicarbonate. To lower the soil pH in this situation, acid or acidifying fertilizer must be applied to the wetted pattern only. Applying acid or thiosulfate fertilizer through the irrigation system can be effective in treating this problem. Dilute concentrations of H2SO4 can be applied safely with irrigation water in order to prevent Ca and Mg precipitates from forming in microirrigation. Levels of CaCO3 in the soil and of bicarbonate in the irrigation water determine the proper rate and frequency for injecting H2SO4. Repeated applications of H2SO4 with irrigation water will tend to lower soil pH within the wetted pattern of the emitter.
Ammonium thiosulfate and potassium thiosulfate are clear liquid fertilizers that can also be injected into a microirrigation system. Part of the S2O3 converts to elemental S, and further to H2SO4, which gives the thiosulfate its acidifying power. For example, fertigation of ammonium thiosulfate at a rate of 115 kg N ha-1 to an orchard containing 375 trees ha-1, each tree would receive the acidifying power of 0.5 kg of elemental S, which might be sufficient to correct a mild alkalinity problem (Obreza et al., 1993).
If soil acidifiers are used, a comprehensive program of soil pH measurement should be undertaken. Soil pH should be measured before and periodically after the application of an acidifier to monitor its effect. Decisions regarding the rate and frequency of subsequent applications of acidifier can be based on desired changes in soil pH and visible plant response.
Table 1: Cumulative NH3 volatilization (% of N applied) from soil columns in a controlled environment 8 days after application of urea, ammonium sulfate and ammonium nitrate to the surface of five soils (Wiezler, 1998).
|Soil||Soil properties||Urea||Ammonium sulfate||Ammonium nitrate|
|pH||CaCO3||CEC||NH3-N loss|| pH after
|NH3-N loss||pH after 24 hs.||NH3-N loss||pH after 24 hs.|
|(%)||(mmol 100g-1)||(%N applied)||(%N applied)||(%N applied)|
|Soil||pH||Citrate µg g soil-1||Micronutrients concentration
(µmol kg soil-1, DTPA extractable)
Plate 1: Correction of iron chlorosis in groundnut (cv. Shulamit) grown in a calcareous soil by ammonium sulfate and nitrification inhibitor. Left: chlorotic plants with application of ammonium sulfate. Right: No Fe-stress is observed when ammonium sulfate was applied along with a nitrification inhibitor (Kafkafi and Ganmore-Neumann, 1985).
|Fig 1: Effect of iron chelate (10 kg Fe ha-1 as Fe-EDDHA) on the pod yield of groundnut grown in a calcareous soil. The difference in resistance to chlorosis between cultivars is due to genetic potential (Hartzook et al., 1974).|
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