Thanks @Ecompost for this one. A gem
The Effect of CaCO3 On Nitrogen Transformations
Soil pH affects the rates of several reactions involving N and can influence the efficiency of N use by plants. Nitrification, or the conversion of ammonium (NH4+) to nitrate (NO3-) by soil bacteria, is most rapid in soils with pH values between 7 and 8. Nitrification approaches zero below pH 5. Ammonium-N fertilizers applied to calcareous soils are converted within a few days to nitrate, which moves freely with soil water. The acidity produced during nitrification is quickly neutralized in highly calcareous soils but may lower the pH value in soils containing low levels of CaCO3.
Ammonia volatilization is the loss of N to the atmosphere through conversion of the ammonium ion to ammonia gas (NH3). Volatilization of ammoniacal-N fertilizer is significant only if the soil surface pH value is greater than 7 where the fertilizer is applied. This condition occurs in calcareous soils, or where the breakdown of the N fertilizer produces alkaline conditions (e.g., urea decomposition). Nitrogen loss through ammonia volatilization on calcareous soils is a concern when ammoniacal N is applied to the grove floor and remains there without moving into the soil. Following an application of dry fertilizer containing ammoniacal N, the fertilizer should be moved into the root zone through irrigation or mechanical incorporation if rainfall is not imminent. Since urea breakdown creates alkaline conditions near the fertilizer particle, surface application of urea can cause N loss if the urea is not incorporated or irrigated into the soil, regardless of initial soil pH.
The Effect of CaCO3 On Magnesium and Potassium
Although low concentrations of Mg and K in citrus leaves are not uncommon in groves planted on calcareous soils, there is not necessarily a concurrent reduction in fruit yield or quality. If a low concentration of leaf K or Mg is found in a grove that produces satisfactory yield and quality, attempts to increase leaf levels with fertilizer are not necessary. However, if a detrimental condition such as low yield, small fruit, or creasing is observed, an attempt to raise the leaf K or Mg concentration with fertilizer is justified.
It is often difficult to increase leaf Mg and K levels with fertilizer applied directly to calcareous soils, which contain tremendous quantities of both exchangeable and nonexchangeable Ca. Leaf Mg and K concentrations are strongly influenced by soil conditions that control leaf Ca concentration, including high soil Ca levels. High Ca levels suppress Mg and K uptake by citrus trees in part, presumably, through the competition of Ca2+, Mg2+, and K+. Citrus growing on soils high in Ca often requires above normal levels of Mg and K fertilizer for satisfactory tree nutrition. In cases where soil-applied fertilizer is ineffective, the only means of increasing 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).
The Effect of CaCO3 On Phosphorus
Phosphorus availability in calcareous soils is almost always limited. The P concentration in the soil solution is the factor most closely related to P availability to plants. The sustainable concentration is related to the solid forms of P that dissolve to replenish soil solution P following its depletion by crop uptake. Many different solid forms of phosphorus exist in combination with Ca in calcareous soils. After P fertilizer is added to a calcareous soil, it undergoes a series of chemical reactions with Ca that decrease its solubility with time (a process referred to as
P fixation). Consequently, the long-term availability of P to plants is controlled by the application rate of soluble P and the dissolution of fixed P. Applied P is available to replenish the soil solution for only a relatively short time before it converts to less soluble forms of P.
Testing Calcareous Soils for P
Accumulation and loss of soil P can be evaluated through soil testing, but more information is required to make a fertilizer recommendation based on this method. The amount of extractable P must be related to crop yield or quality. An ideal P-extracting solution should remove from soils only those forms of P that are available to plants. This is difficult to achieve with the extracting solutions that are currently used.
The major extractants used by southeastern U.S soil testing laboratories to measure soil P include Mehlich 1 (double acid), Bray P1 and P2, and sodium bicarbonate. Mehlich 1 is not appropriate for use on calcareous soils because its extracting ability is weakened by exposure to CaCO3. While Bray and sodium bicarbonate have been consistently correlated to P uptake by plants growing on calcareous soils in other parts of the United States, these extractants have not been calibrated with citrus leaf P concentration or yield on Florida calcareous soils. Mehlich 3, a newer extractant with promising ability for Florida conditions, is not yet widely used and also will require calibration. Currently, no suitable extractant for soil P has an established, calibrated sufficiency level for use with citrus grown on Florida calcareous soils.
The Effect of CaCO3 On Zinc and Manganese
Soil pH is the most important factor regulating Zn and Mn supply in alkaline soils. At alkaline (high) pH values, Zn and Mn form precipitous compounds with low water solubility, markedly decreasing their availability to plants. A soil pH value of less than 7 is preferred to ensure that Zn and Mn are available to plants in sufficient amounts. The soil around a plant root (the rhizosphere) tends to be acidic due to root exudation of H+ ions. Therefore, 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. Soil applications of Zn and Mn fertilizers are generally ineffective in these situations, but deficiencies can be corrected through the use of foliar sprays.
The Effect of CaCO3 on Iron
Calcareous soils may contain high levels of total Fe, but in forms unavailable to plants. Visible Fe deficiency, or Fe chlorosis, is common in citrus. The term
chlorosis signifies a yellowing of plant foliage, whereas Fe
deficiency implies that leaf Fe concentration is low. Owing to the nature and causes of Fe chlorosis, however, Fe concentration is not necessarily related to degree of chlorosis. In chlorotic plants, Fe concentrations can be higher than, equal to, or lower than those in normal plants. Thus, this disorder on calcareous soils is not always attributable to Fe deficiency. It may be a condition 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. (Fe nutrition in plants has improved in response to the application of sewage sludge, which contains organically complexed Fe.) The primary factor associated with Fe chlorosis under calcareous conditions appears to be the effect of the bicarbonate ion (HCO3) on Fe uptake and/or translocation in the plant. The result is Fe inactivation or immobilization in plant tissue.
Susceptibility to Fe chlorosis depends on a plant's response to Fe deficiency stress, which is controlled genetically. Citrus rootstocks vary widely in their ability to overcome low Fe stress (see Table 2). The easiest way to avoid lime-induced Fe chlorosis in citrus trees to be planted on calcareous soils is to use tolerant rootstocks. Existing Fe chlorosis can be corrected by using organic chelates
http://edis.ifas.ufl.edu/ch086
