Research Projects

Research Projects

Availability Indices of Mg and optimal management of Mg fertilization for high yield and tolerance to soil borne-diseases

Dr. Asher Bar-Tal, Dr. Abraham Gamliel, Dr. Uri Yermiyahu
Start: 2018 –  End: 2021

(Mg) is one of the macro-nutrients of plants. The amount consumed and the concentrations in the plant tissue in normal condition are similar to those of calcium (Ca). The main roles of Mg in the plant are involvement in photosynthesis and transport of organic anions (Cakmak and Yazici, 2010). Like Ca and potassium (K), Mg is absorbed as a cation and like Ca it is a two-valence cation. Magnesium is a common element in many minerals. It accounts for about 2% of the Earth’s crust and 0.05-0.5% of the soil, but most of the magnesium in the soil (95-98%) is found in the lattice of the minerals in the soil, and it is not available (Gransee and Fuhrs, 2013; Senbayram et al., 2015).

The awareness of its importance to crops yield and quality has increased in recent years (Cakmak and Yazici, 2010; Gerendas and Fuhrs, 2013; Senbayram et al., 2015) and researchers have warned that for years Mg has been the forgotten nutrient element (Cakmak and Yazici, 2010) and that Mg deficiency in plants becomes an urgent problem (Guo et al., 2016). The dramatic change in the quality of water in agriculture in Israel following the massive entry of desalinated water into Israel necessitates an examination of the need to add Mg to the fertilizer management.  Magnesium, found in high concentrations in natural water sources, while its concentration in desalinated water is very low (Yermiyahu et al., 2007). In a study on irrigation of greenhouse tomatoes with desalinated water, we showed competition between Ca and Mg in terms the nutritional status of tomato nutrients that had a strong impact on fruit quality (the incidence of Blosson End Rot, BER) (Bar-Tal et al., 2017). Eisenstadt and his colleagues (2012) reported disorders in Ranunculus flowers irrigated with desalinated water and positive response to Ca and Mg fertilizers. According to the Israeli Agricultural Training Service (“Shaham”) data, Mg deficiencies are common in table grapes, anemones, Ranunculus flowers, peony, pomegranate, citrus, tomatoes, pepper (Asher Eizenkot, personal communication). The relationships between Mg concentration in diagnostic leaves and the fruit yield of various orchard were determined by Motti Peres (SHAM, The Field Service Laboratories). However, in this database there is no information on the relationships between soil’s Mg to the concentration of Mg in leaves.

Intensive fertilization with the main nutrients (nitrogen (N), K and phosphorus (P), while ignoring Mg, may cause depletion of Mg in the soil and turn it into a limiting nutrient. For example, in New Zealand fertilization of pasture soil without Mg for several decades (until the 1980s and 1990s) increased reports of Mg deficiency in coarse soils (Edmeades, 2004). We estimated the expected decline of Mg in typical sandy soil in Israel following several years of intensive production of pepper. We found that in coarse soils as Hamra and sand the Mg reservoir available in a layer of 0-60 cm is 47-55 and 29-36 kg ha-1, respectively, while intensive consumption of greenhouse peppers is 6.5 kg ha-1, so that without fertilizing the Mg reservoir in the soil will be depleted within 4.5 to 8.5 years.

Magnesium in solution is the immediate source for uptake by plants and it is in equilibrium with Mg in the cations exchangeable complex (CEC) of the soil (Seyfried et al., 1989). Cations exchange between the solid and water phases in soil is a very rapid process and can be treated as a simultaneous equilibrium process (Seyfried et al., 1989; Eick et al., 1990; Bar-Tal et al., 1995). Many studies have reported a high correlation between the exchangeable Mg released by sodium acetate and the absorption of Mg by plants (Rice and Kamprath, 1968; Kidson et al., 1975; Lombin and Fayemi, 1976; Edmeades, 2004). Non-exchangeable Mg was found as a significant source of Mg in soil containing swelling clay (Rice and Kamprath, 1968); In a study on the availability of K and Mg in calcareous soils (range of CaCO3 content: 16-36%) and with pH values ​​of 7.1 to 7.4, a significant contribution to Mg was made by the non-exchangeable Mg (which is part of the lattice of the clay mineral or magnesium trapped between the clay plates of vermiculite). However, in other soils there was no correlation or low correlation between non-exchangeable magnesium and Mg uptake by plants (Kidson et al., 1975; Gransee and Fuhrs, 2013). Simulations of Mg and K uptake by plants from soil showed that for Mg the transport with water to the roots was the main mechanism in contrast to K where diffusion was the main mechanism (Granse and Fuhr, 2013). These results indicates that in soils that do not contain fixed Mg and the dominant fraction of Mg is the exchageable complex, the release of Mg to the solution is very rapid compared to that of K, which is more dependent on the non-exchangeable source. Ortas et al. (1999) reported that Mg was not the limiting factor for growth of plants in pots even after several growth cycles. But it should be noted that in this experiment the limiting factor was K. In a comparison between different extraction methods in soil in Latvia, a very high correlation was found between the Mg values ​​extracted in 0.0125 M CaCl2, 1M KCl, 1M NH4OAc, Melich 3 whereas the correlation with water extraction were low (Staugaitis and Rutkauskiene, 2010). In a long-term fertilization experiment, it was found that the concentration of Mg in the wheat leaf, the plant index for wheat minerals status, was dependent on Mg available according to CaCl2 extraction, soil pH and availability of K (Jaskulska et al. 2015). Many studies have shown that Ca, K and ammonium (NH4) have a strong effect on Mg absorption by different plants (Bar-Tal et al., 2017). However, there was no direct correlation between the amount of chalk in the soil and the concentration of Mg in the soil solution (Farhat et al., 2015). In addition, a review of the literature on the relationship between plant nutrients status and the ratio of cations in the CEC has shown that the total concentration of the exchangeable cation is usually in high correlation with its uptake by plant rather than its relative fraction in the CEC (Kopittke and Menzies, 2007). In Australia, a threshold value of 0.2 meq Mg / 100 g of soil was determined to be deficient in Mg, whereas the ratio of Ca/Mg (between 20 and 1) had no effect on Mg uptake by plants. Jacoby (1961) reported that the values of Ca/Mg in the CEC in Mg deficient and healthy citrus orchards were 8.0 and 4.3, and the total concentration of exchangeable Mg were 0.76 and 1.28 meq/100 g, respectively (to the best of our knowledge, no later studies on soil indices for Mg fertilization were published in Israel). In both natural and agricultural soil systems in addition to Mg and Ca other cations like sodium (Na) and NH4 participate in the exchange processes (Feigenbaum et al., 1991, Levy and Feigenbaum, 1996). In a K fertilization experiment, we showed that when Na concentration is high it affects the availability of K through exchange reactions with exchangeable K and Ca and this effect can be estimated (Bar-Tal et al., 1991). There is an urgent need to develop a protocol for determining the threshold of available Mg in the soil, especially in Israel soils, and to develop a quantitative method for estimating the required dose of Mg fertilizer in soils where magnesium availability is lower than these threshold values.

Although Mg is one of the main nutrients, unlike the other nutrients, standard laboratory methods have not been developed to determine its availability in different soils. There are no protocols of fertilization recommendations using soil tests. The only indicator currently available for Mg nutritional status and for fertilization recommendations is the plant index, Mg concentration in a diagnostic leaf. The entry of desalination plants into use in Israel led to a dramatic change in the quality of the water; on one hand the total concentrations of salts is low removing salinity stress and on the other hand the concentrations of nutrients like Mg and Ca are too low. Phenomenon of Ca and Mg deficiencies were reported in intensive cultivation of flowers and vegetables grown in soilless culture or sand and irrigated with desalinated water. It is also expected that the concentration of Mg in treated waste water in Israel will decrease due to the domestic use of desalinated water with low Mg concentration and the low contribution of Mg by the urban use. We estimate that long term irrigation with very low Mg water without Mg fertilization will lead to soil Mg depletion and Mg deficiencies in intensive crops mainly in coarse soils. The growing supply of desalinated water to agriculture combined with the intensive fertilization of other minerals causes the spread of Mg deficiency, some of which are manifested by symptoms of plant deficiencies and some hidden deficiencies in which Mg is the undetermined limiting factor. There is currently no accepted method and measures to determine the availability of Mg in the soil in the field service laboratories in Israel.

Numerous studies reported that mineral nutrition and fertilization influence the resistance/tolerance of plants to various diseases (Datnoff et al., 2007). In most cases the mechanisms of these effects are unknown but the empirical findings showed correlations between the elements nutritional status of plants, and the rate of their impact of plants performance and response to diseases (Duffy and Defago, 1999, Triky et al., 2005). In a previous research we found relationship between soil salinity and the level of damage to tomato by the crown rot disease (Triky et al., 2005). A follow-up study showed increased plant tolerance to the disease with increased level of Mg. Recently, reports are increasing regarding mature tomato plant decline caused by severe infection by soil borne-pathogens (Fusarium, Pythium). This fungal pathogen establishes in many soils and crop country-wide. The high impact of these fungi on the tomato plants is probably due to presence and infection of the plants by new strains of the TMV (Tobacco Mosaic Virus). Additionally, the low level of Mg in the irrigating water which are originated from desalinization plants. In a previous study we found that fertilization with Mg and high ratio of Mg/Ca in the irrigating water reduced the level of tomato plants infected by crown rot (Yermiyahu, unpublished data).

The research hypotheses: (A) the availability of Mg in the soil and the change in availability in response to Mg fertilization depend on the capacity of the cations in the soil, the composition of the CEC, the content and composition of the carbonates in the soil, pH and soil organic matter (SOM) content and the history of the agricultural interface in the soil; (B) There is a relationship between the amount of Mg available in the soil and its concentration in the solution and the ratio of Mg/Ca or Mg/Total cations in the soil; (C) The Mg nutritional status of plants effect the level of the plant infected by soil borne disease.