Effect of saline irrigation water and application methods of iron and zinc on yield and quality of corn silage in a calcareous soil

Salinity stress affects plant growth and productivity, especially in calcareous soils in the central area of Iran. In these areas, increasing soil and water salinity has decreased yield and quality production of corn silage (Zea mays L.) (Dai et al., 2011). Response by corn to salt stress is a complex network affecting almost all processes, including nutrient uptake and metabolism, ion accumulation and photosynthesis. Likewise, corn is known to be very sensitive to micronutrient deficiency such as iron (Fe) and zinc (Zn). This deficiency commonly occurs in calcareous soils. It is believed that applying micronutrient elements such as Fe and Zn which can increase crop production to cope with declining saline condition (Eizadi et al., 2012). In this regard, Taher et al. (2008) found the foliar spray method of Zn and/or Fe increased corn silage compared to soil application method. Therefore, the purpose of the experiments was to evaluate the effect of application methods of Fe and Zn fertilizers on yield and quality of corn silage grown on calcareous soil under saline irrigation water.

A 1-year field experiment was conducted on an agricultural farm located in Arsanjan, Fars province (53° 15' E, 29° 40' N and 1638 m), Iran in 2014. The region represents semi-arid climatic conditions with relatively warm summers and cold winters. The soil was silty clay loam with a pH of 7.7 and an EC of 0.62 dS m-1. Mean Fe and mean Zn were 4.5 and 0.7 ppm, respectively. The experiment was lay-out as factorial based on randomized complete block design in three replications. The treatments consisted of three levels of saline irrigation water (2, 4, and 6 dS m-1) as the main factor and sub-factor was 12 application methods of Fe (source: Fe-Chelate: Sequestrene 138, and Fe-Sulfate) and Zn (source: Zn-Sulfate) fertilizers: T1: No-fertilizer (Control), T2: soil application of Zn-Sulfate, T3: soil application of Fe-Chelate, T4: soil application of Zn-Sulfate accompanied by Fe-Chelate, T5: soil application of Zn-Sulfate accompanied by Fe-Chelate+foliar spray of Zn-Sulfate, T6: soil application of Zn-Sulfate accompanied by Fe-Chelate+foliar spray of Fe-Sulfate, T7: soil application of Zn-Sulfate+foliar spray of Fe-Sulfate, T8: soil application of Fe-Chelate+foliar spray of Zn-Sulfate, T9: soil application of Zn-Sulfate accompanied by Fe-Chelate+seed treatment with Fe-Sulfate, T10: soil application of Zn-Sulfate accompanied by Fe-Chelate+seed treatment with Zn-Sulfate, T11: soil application of Zn-Sulfate accompanied by Fe-Chelate+seed treatment with Fe-Sulfate+foliar spray of Zn-Sulfate, and T12: soil application of Zn-Sulfate accompanied by Fe-Chelate+seed treatment with Zn-Sulfate+ foliar spray of Fe-Sulfate.

Yield and quality of corn silage were significantly (p≤0.01 and 0.05) influenced by saline irrigation water, application method of Zn and Fe fertilizers, and their interactions. Using saline irrigation water decreased corn height by 40%. The highest corn height (141.35 cm) was recorded by soil application of Fe-Chelate accompanied by foliar spray of Zn-Sulfate treatment at 2 dS m-1 treatment. Under saline irrigation water at 2 dS m-1 treatment, foliar spray of Zn-Sulfate accompanied by soil application of Fe- Chelate maximized corn silage (71.77 ton ha-1) up to 34% compared to no-fertilizers method (Table 3). At 6 dS m-1 treatment, the highest Sodium (Na) concentration (4.22%) was obtained by no-fertilizer method. At 2 dS m-1 treatment, the highest potassium concentration (9.55%) was achieved by soil application of Zn-Sulfate, and increased 25.33% compared to No-fertilizer method (Table 4). Likewise, the highest Fe and Zn concentration were achieved by combined fertilizer application method such as soil application of Zn-Sulfate accompanied by Fe-Chelate+seed treatment with Fe-Sulfate, and soil application of Zn-Sulfate accompanied by Fe-Chelate+seed treatment with Fe-Sulfate+foliar spray of Zn-Sulfate. Meanwhile, a strong relationship was observed between Na and chemical composition of corn silage (phosphorus (r=-0.84**), calcium (r=-0.61**), and potassium (r=-0.58**).
It has been reported that high concentrations of salinity caused lower shoot fresh and dry weight in corn. Under saline conditions, the reduced growth of crops has been mainly attributed to the toxic effects of excessive ions such as Na, unbalances of the plant ion problems on nutrient uptake and disturbances, damages on photosynthesis, respiration and physiological functions (Yarami and Sepaskhah, 2015). Hence inhibit translocation of produced assimilates to vegetative phase that finally could result into the growth reduction (Karmollachaab and Gharineh, 2013). On the contrary, corn growth parameter was enhanced by Fe and Zn fertilization under saline irrigation water. The same results were reported by Jokar and Ronaghi (2015) on sorghum (Sorghum bicolor L.) and Bostani et al. (2015) on corn. Zinc and Fe are essential micronutrients, involved in production of auxin, a natural plant growth hormone, and many other important plant functions. Therefore, application of Zn-Sulfate accompanied by application of Fe- Chelate and Fe-Sulfate fertilization might enhance silage quality through the increase of its content on yield components and decrease stress intensity.

Salinity and micro-nutrient deficiency are the most important factors limiting yield and quality production of corn silage in calcareous soils. It is believed that obtaining improvement of silage quality in many experiments was often the result of applied micro-nutrient such as Zn and Fe elements. Results showed that combined application of Fe and Zn elements compared to separate application method efficiently increased quality and quantity of silage corn under saline irrigation water at 6 dS m-1. Overall, selection and use of the correct fertilizer application methods are important for garneting the silage yield and quality of corn under saline condition and to decrease non-saline water use.