Curtis E. Swift, Ph.D., High Altitude Lavender
Curtis.Swift@alumni.colostate.edu
Abstract
Phosphorus is an essential major element required by plants and while necessary for plant growth, an excess amount of plant-available soil phosphorus can have a negative impact on plant health and yield. It is therefore necessary to refer to the results of a soil test prior to applying any phosphorus to the lavender field.
Introduction:
Phosphorus is a component of certain enzymes and proteins, adenosine triphosphate, (ATP), ribonucleic acids (RNA), deoxyribonucleic acids (DN) and thus critical to plant and animal life (Jones, 2012) with plants absorbing dihydrogen phosphate (H2PO4) and monohydrogen phosphate (HPO42-) from the soil solution. Aluminum (Al), iron (Fe), and calcium (Ca) phosphates are the major inorganic sources of these two anions with the decomposition of organic matter and microorganisms such as bacteria and fungi providing another major source of P for plant utilization. The concentration of phosphates available to the plant are determined by soil analysis and added to the soil as needed using fertilizers in the inorganic & synthetic form or as organic matter such as manures, bone meal, rock phosphate, compost, etc.
Plant-available phosphorus is determined by various methods by different soil testing laboratories. Table 1 at the end of this document lists the most common testing procedures utilized and gives the correlation between these tests. The critical phosphorus levels recommended for lavender in this document are based on the ammonium bicarbonate DPTA (AB-DPTA) extraction method. Test results using other methods will need to be converted to AB-DPTA to use the critical numbers provided.
An excess of soil phosphorus restricts the plant’s iron (Fe) uptake. Excess phosphates in the soil tend to immobilize iron in roots and younger leaves. (Rogers, 1975; Tiffin, 1972; Swift, 1983). In some cases even though leaf Fe appears to be adequate, it has been found precipitated as iron phosphate (FePO4) in the leaf veins and therefore not available for plant use.
High levels of soil phosphorus have been shown to have an antagonistic effect on zinc (Zn), copper (Cu), and manganese (Mn).
High soil phosphorus levels also restrict the development of mycorrhizal-forming fungi and inhibit the effectiveness of acid phosphatase produced by the roots. Both of these are necessary for the proper development of lavender plants.
Phosphorus and mycorrhizal-forming fungi
Mycorrhizae are an integral part of most plants in nature (Giazninazzi et al., 1982) and occur on 83% of dicotyledonous and 79% of monocotyledonous plant investigated (Wilcox, 1996). Lavender requires its roots to be infected by arbuscular mycorrhizal fungi (AMF) and thus is described as a mycorrhizal-dependent species (Azcon and Barea, 1997), cataloged as obligatory mycorrhizal (Brundett, 1991), and reported to be “highly dependent on mycorrhiza” (Habt and Manjunath, 1991). Infection of the root system of the plant by these fungi creates a symbiotic (beneficial) relationship between the plant and fungus. Upon root infection and colonization, mycorrhizal fungi develop an external mycelium which is a bridge connecting the root with the surrounding soil (Toro et al. 1997).
One of the most dramatic effects of infection by mycorrhizal fungi on the host plant is the increase in phosphorus (P) uptake (Koide, 1991) mainly due to the capacity of the mycorrhizal fungi to absorb phosphate from soil and transfer it to the host roots (Asimi, et al. 1980). In addition, mycorrhizal infection results in an increase in the uptake of copper (Lambert et al., 1979; Gildon and Tinker, 1983), zinc (Lambert et al., 1979), nickel (Killham and Firestone, 1983), and chloride and sulphate (Buwalda et al., 1983).
Mycorrhizae also are known to reduce problems with pathogens which attack the roots of plants (Gianinazzi-Pearson and Gianinazzi, 1983), improve heavy metal and salinity resistance and stimulate plant growth (Zubek, et al., 2012).
The benefits listed above are greatest in P-deficient soils and decrease as soil phosphate levels increase (Schubert and Hayman, 1986). Very high and very low phosphorus levels may reduce mycorrhizal infection/colonization (Koide, 1991). It is well established that:
- infection by mycorrhizal fungi is significantly reduced at high soil phosphorus levels (Amijee et al., 1989).
- the addition of phosphate fertilization results in a delay in infection as well as a decrease in the percentage of infection of roots by mycorrhizae (DeMiranda et al., 1989; Asimi et al., 1980).
- an increase in the level of soil phosphate results in a reduction in chlamydospore production by the fungus (Menge et al., 1978). These spores are involved in root infection and spread of the fungus through the soil profile.
Research by Abbott and Robson (1979) concluded that levels of soil phosphorus greater than required for plant growth eliminated the development of the arbuscles of AMF types of mycorrhizae. Arbuscles are structures produced within the host plant cells by the fungus. These structures are responsible for the transfer of absorbed nutrients from the fungus to the plant. The arbuscles resemble miniature shrub-like trees (arbuscular = shrub in Latin). Mosse (1973) reports adding phosphate results in no arbuscles forming. This however depends on the amount of phosphorus already in the soil.
What level of P is critical?
When the soil level of bicarbonate-soluble phosphorus (using the AB-DPTA extraction technique) exceeded 140 parts per million (ppm) the rate of infection was found to decrease (Amijee et al., 1989). Abbott and Robson (1977 & 1978) found the mycorrhiza Glomus fasciculatum ceased to be effective when the soil level of phosphorus reached 133 parts per million (ppm). Schubert and Hayman (1986) found mycorrhiza was no longer effective when 100 mg or more of P was added per kilogram of soil (100 ppm).
Mycorrhizal infection virtually disappeared with the addition of 1500 ppm (1.5 grams or more of mono calcium phosphate per kilogram of soil) (Mosse, 1973). With small additions of phosphorus fertilizer, entry points and fungal growth on the root surface remained normal but arbuscles were smaller and fewer in number reducing the effectiveness of the fungus/plant relationship. Other researchers have reported mycorrhizal infections tend to die out in soils containing or given much phosphorus (Baylis, 1967; Mosse, 1967). The development of mycorrhizal relationships were found to be the greatest when soil phosphorus levels were at 50 ppm (Schubert and Hayman, 1986).
Phosphorus and acid phosphatase enzymes
Phosphatase is an acid exuded by lavender roots and supplement the uptake of phosphorus provided by arbuscular forming fungi (AMF). Root phosphatase activity decreases with an increase in phosphorus (Azcon and Barea, 1997). Lavender plants showing high levels of acid phosphatase activity may still grow poorly unless an organic source of P is available (Alexander and Hardy, 1981).
Summary and recommendations:
Lavender are dependent on Arbuscular forming mycorrorhizal-forming fungi (AFM). The benefits of mycorrhizae are greatest when soil phosphorus levels are at or below 50 ppm (50 mg kg -1). Mycorrhizal infection of roots declines above this level with little if any infection occurring above 100 ppm P even when soil is inoculated with a mycorrhizae mix.
The addition of phosphorus to the lavender field should be based on a soil test and only added if the P level is below 50 ppm (mg/kg) and then not to exceed 130 ppm. Additional research is needed to fine tune this number based on soil texture.
Acid phosphatase activity requires an organic phosphorus source such as wood chips, bark, manure, or compost to be effective.
Additional Notes:
The dissolution of Ca5(PO4)3OH in bone meal requires H+ ions, thus pH is an important factor influencing P release from bone meal (Jeng et al. 2006). According to Dr. Jessica Davis, Soil Scientist Colorado State University, a pH of less than 7.0 is required for P to be released from bone meal (personal communique). Any growth response resulting from the addition of bone meal to soils with a pH of 7.0 or greater is most likely due to the nitrogen (8%) content of the meal. Bone meal also contains phosphorus (5%) and calcium (10%) (Jeng et al., 2006).
Growers using rock phosphate for their phosphorus source need to realize this material will not release phosphorus for plant use unless the soil pH is 5.5 or less.
Table 1. Multiply the results provided in your soil test report (left column) by the number under the AB-DPTA column (far right column) to determine the phosphorus level resulting from the AB-DPTA extraction method.
| Results given as | Mehlich-l | Mechlich-lll; Olsen -P |
Bray-1; Mehlich-II |
NaHCO3 | AB-DPTA |
| Mehlich-1 | 1 | 2 | 2 | 0.67 | 0.33 |
| Mehlich-3; Olsen-P |
0.5 | 1 | 1 | 0.33 | 0.17 |
| Bray-1; Mehlich-2 |
0.5 | 1 | 1 | 0.33 | 0.15 |
| NaHCO3 | 1.5 | 3 | 3 | 1 | 0.5 |
| AB-DPTA | 3 | 6 | 6.7 | 2 | 1 |
For example: A grower with a soil test from a lab that uses the NaHCO3 phosphorus extraction method will multiply their results by 0.5 to obtain the AB-DPTA P2O5 level. A grower with results from the Olsen-P (O-P) method would multiply the results by 0.17 to obtain the AB-DPTA P2O5 level.
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