Sweet tomato crops

Source: Israel Agri

December 5, 2017

In recent years, desalinated sea water is a primary source for irrigation water in many regions of Israel. These water lack mineral nutrients, particularly calcium (Ca), magnesium (Mg), and sulfur (S) that are essential for plant growth and development. As a result, Solanaceae crop species, including sensitive tomato cultivars, may express Ca and Mg deficiency symptoms such as typical yellowish leaves and increasing rates of fruit blossom-end rot (BER) (Fig. 1) with a consequent reduction in the marketable yields.

Figure 1. Blossom-end rot in tomato – a typical physiological disorder resulting from Ca deficiency

Polysulphate (polyhalite), a new mineral fertilizer, is mined by ICL in the UK from deep underground layers. It contains four important plant nutrients: sulfur (SO3, 48%), potassium (K2O, 14%), magnesium (MgO, 6%) and calcium (CaO, 17%), marketed in the forms of powder or granules, and suitable for a broad range of crops, including open-field and greenhouse vegetables. Polysulphate retains low environmental impact as its production processes involve only mining, grinding, screening and packaging.

 Polysulphate has been authorized for the organic agriculture and is already marketed in an increasing number of countries such as the UK, France, Germany, Italy, Holland, China, Brazil, and the USA. Polysulphate gains special importance where the nutrients Ca, Mg, and S are available at levels lower than the minimum threshold securing normal crop development.

 The fertilizer is available to plant roots as it is easily dissolved into the soil solution upon irrigation and during time. Polysulphate provides S in the form of sulphate, which is available to plants without any need for its breakdown by microorganisms. However, this fertilizer is not suitable for fertigation, as its dissolving rate is too slow for this use.


Standard Polysulphate application in the greenhouse before planting


Considering optimum mineral nutrition, two goals should be reached: 1. supplying adequate levels of available nutrients throughout the growing season; and, 2. maintenance of balanced cation saturation ratios in the soil solution.

 Winter greenhouse tomatoes require no less than 550, 110, 660, 350, and 110 kg ha-1 of nitrogen (N), phosphorus (P), K2O, CaO, and MgO, respectively, in order to produce yield of about 350 ton ha-1. The recommended cations ratio, as a percentage of the soil’s cation exchange capacity (CEC) are: Ca – 65-75%; Mg – 10-15%; K – 2-5%; and sodium (Na) – 0-5%. Practically, applying the required Ca and Mg rates via fertigation is costly; hence, these two nutrients should preferably be applied directly to the soil.

 The new Polysulphate fertilizer enables the application of Ca, Mg, and K (partially) at the pre-planting stage, with no need of additional application during the growing season.
The objective of the present observation was to examine the potency of Polysulphate, applied as a pre-planting fertilizer, to prevent Ca and Mg deficiencies and to ensure considerable yield and produce quality of winter season (September to June) greenhouse tomatoes.

Materials and methods
The observation was carried out at a farmer’s greenhouse on light to medium sandy loam soil (CEC – 15 meq 100g-1 soil) in Beit-Ezra, located in the coastal plain of Israel. The greenhouse size was 0.8 ha, 0.25 ha out of which was employed for the trial. The soil was solarized before, and during 8 weeks after a chemical disinfection (Condor and Edigan, 200 and 400 L ha-1, respectively, with 40 mm of irrigated water) took place.

A pre-planting fertilizer (40 mha-1 chicken manure [250 kg m-3, 85% dry weight] containing 3.0, 1.4, 2.1, 1.2, 0.3, and 0.3% of N [organic form], P, K, Ca, Mg, and S, respectively) was applied to the entire area to prevent nematodes. Before planting, soil was irrigated with 70 mm water. Polysulphate fertilizer was spread and embedded along the planting rows according to treatments (Table 1).

Cluster tomato (cv. Ikram, Zeraim Gedera Syngenta, Ltd., grafted on Arnold rootstock) seedlings were planted on 11-Sep, 2016, at density of 22,000 branches ha-1. Desalinated water (650 mm during the season, at ECw range from 0.35-0.45 dS m-1, and with Ca, Mg, and Cl concentrations of 35-40, 1-5, and 45 ppm, respectively) was used for irrigation. Liquid fertilizer Sarit Super 5-2-7+0.5+6 (N-P-K + microelements) was applied through fertigation at about 1.5 L m-3.


Table 1. Nutrient supply vs. anticipated requirements of greenhouse tomatoes under four Polysulphate treatments: PS0

The observation included four treatments: Control, where no additional Polysulphate was applied; PS1, PS2, and PS3, where 1000, 1500, and 2000 kg Polysulphate ha-1, respectively, were applied pre-planting (Table 1).
The plants were monitored visually. Plant vigor was evaluated by measurements of stem diameter below the uppermost inflorescence. Soil was sampled and analyzed for nutrients content every two months, as well as diagnostic leaves. Marketable fruit yield was weighed and summarized weekly.

Plants establishment and growth were normal, however, symptoms of Mg deficiency, expressed as typical yellowing of lower leaves, occurred in the control plants already on mid-November, two months after planting. Plants applied with Polysulphate remained green, healthy, and properly functioning (Fig. 2).

Minor Mg deficiency symptoms were observed also among plants fertilized with Polysulphate at the beginning of harvest, mid-December 2016. In February, after the regular practice of intensive removal of old leaves had taken place, the symptoms disappeared completely among PS treatments, while only slight signs could be observed in Control plants. Clear Mg deficiency symptoms returned in the lower leaves of Control plants as the weather warmed up during the spring, whereas similar signs were absent among the PS plants.


Figure 2. Magnesium deficiency symptoms in tomato leaves, as noticed on 10-Nov, 2016. A, Control plants; B, PS3 (Polysulphate at 2000 kg ha-1) plants; C, D, E, and F, representative leaves sampled from Control, PS1, PS2, and PS3 plants, respectively.



At the end of May, the PS plants seemed more vigorous than Control plants. Measurements of the stem diameter below the uppermost inflorescence showed that Control stems were significantly thinner than those of PS plants (Fig. 3).

Harvest began on 5-Dec, 2016, when the early fruit clusters were ripen. During the winter months, no differences between treatments were observed, and the accumulating yields averaged at 130 ton ha-1 at the end of March. Nevertheless, from April to the end of the season on 18-June, 2017, the marketable Control yield was consistently lower than those of the Polysulphate treatments. At the end of the season, the marketable yields of the Polysulphate treatments were higher than Control yield by 5-7% (Table 2).


 Figure 3. Tomato stem diameter below the uppermost    inflorescence on 23-May, 2017, as influenced

by pre-planting application of Polysulphate




 Table 2. Effects of pre-planting Polysulphate application on yield distribution between the winter and spring periods 

 Soil analyses conducted during the season showed a slight increase in Ca, Mg, K, and S where and in accordance to Polysulphate applications. High soil sulphate levels did not affect soil pH.

Conclusive remarks
Generally, the crop was sufficiently supplied with N, P, K, and Ca through fertigation alone throughout the growing season (Table 1). The basal application of chicken manure, aimed to reduce nematodes problems, significantly increased all nutrient doses far beyond requirements; however, its real contribution to the crop’s nutrition was unclear. Even though, Polysulphate pre-planting application prevented visually determined Mg deficiencies that usually occur during rapid growth periods of greenhouse Ikram cluster tomatoes. Polysulphate application promoted plant vigor, as expressed in stem diameter, and brought about a 5-7% increase in the marketable yield.

The opportunity of replacing large portion of the liquid fertilizer with a basal application of Polysulphate is very promising. This way, <a “=”” href=”http://www.polysulphate.com/”>Polysulphate can provide 33% of the K dose, as well as N-free Mg, thus reducing K-Mg competition and avoiding surplus N nutrition with its environmental consequences. Given the advantages observed in yield and produce quality during the warmer phase of the growing season, further research is required in order to optimize Polysulphate application in greenhouse tomatoes and hence, enhance the grower’s benefit