The effect of cow manure compost enriched with iron and zinc metal scraps on the available iron and zinc concentrations of calcareous soils with different textures

Introduction
Although micronutrients are required by plants in small amounts, they play an important role in plant growth and development. Plant structure, plant enzyme system, formation and degradation of proteins, and production of plant hormones such as gibberellic acid and chlorophyll are affected by micronutrients including iron and zinc. The soil of most regions of Iran has an alkaline pH and is poor in terms of micronutrients, which has occurred as a result of lack of rainfall, a lot of salts such as carbonates and bicarbonates, unfavorable management of irrigation, destruction of vegetation, fallow, indiscriminate plowing, etc. Therefore, organic fertilizers enriched with micronutrients are suitable alternatives due to their high amounts of organic matter content, and their role in improving soil properties. Organic fertilizers have high amounts of macronutrients such as nitrogen and potassium but the amount of their micronutrients is relatively low. Therefore, mixing iron and zinc metals with the organic fertilizer induces the increased available iron and zinc in the organic fertilizer. The increase in the available concentration of iron and zinc can be due to reduction conditions by the organic fertilizer induced. Adding enriched fertilizers with iron and zinc metal scrap to the soil, while eliminating iron and zinc deficiency, reduces the cost of producing chemical fertilizers and their import, and prevents environmental pollution caused by the consumption of these fertilizers and the accumulation of metal scraps. Considering that the effect of fertilizers enriched with metal iron and zinc on the soil has not been studied so far, in this research, the effect of cow manure compost enriched with iron and zinc metal scraps on the concentrations of iron and zinc in soils with different textures was investigated.
 
Materials and Methods
In order to investigate the effect of cow manure compost enriched with iron and zinc metal particles on the amount of iron and zinc in soils with different textures, an experiment was carried out in the form of split plots-factorial in a completely randomized block design. The main factor includes soil texture in three types heavy, medium, and light texture. The secondary factors include iron-enriched cow manure compost and zinc-enriched cow manure compost in two levels of zero and 30 t ha^-1. At first, the produced cow manure compost was passed through a two mm sieve to remove extra materials (gravel and straw pieces). For enrichment, two percent of iron and zinc scraps were added to 100 gr of dry matter of cow manure compost that was saturated in plastic containers and kept for 60 days under laboratory and moisture conditions. After the enrichment process, the cow manure compost with iron and zinc metal scraps of two percent was added to the soils with field moisture capacity in plastic containers after 60 days. Then, the soils were sampled and the available iron and zinc concentrations of the soils were measured.
 
Results and Discussion
The results showed that the use of iron-enriched cow manure compost led to an increase of available iron in different soil textures so that it reached 24.16 mg kg^-1 in the heavy soil texture, which was 3.8 times greater than the control treatment in the same texture. Also, the addition of zinc-enriched cow manure compost could increase available iron up to 21.51 mg kg^-1. Since this treatment contains highly soluble zinc, the soluble zinc can be placed on the surface of the soil colloids and thus increase the available iron. Cow manure compost treatment enriched with zinc also caused a 3499 % increase in available zinc in the heavy soil texture compared to the control treatment in the same soil texture. The reason for this increase in available iron and zinc can be related to the formation of chelate and complex with soil organic matter components. The interaction effect of cow manure compost enriched with iron and zinc caused a significant increase of available zinc in the soil compared to the application of these fertilizers alone, and the concentration of available zinc in the heavy soil texture reached 70.87 mg kg^-1. This can be attributed to the high amount of zinc in zinc-enriched fertilizer (4538.7 mg kg^-1) and the synergistic effect of iron-enriched fertilizer, which had a double effect on the chelation of zinc elements with organic matter in the soil.
 
Conclusion
Based on the results of this experiment, the application of 30 tons per hectare of cow manure compost enriched with iron and zinc metal scraps increased the amount of available iron and zinc in the soil, especially in the heavy soil texture. This issue is probably due to the higher content of organic substances and as a result of increasing their chelation rate with soil organic components. Therefore, it is recommended to use organic fertilizers enriched with iron and zinc along with metal scraps to solve the deficiency of these elements and improve the chemical properties of the soil according to environmental and economic considerations. Also, due to the addition of significant amounts of available iron and zinc to the soil by enriched cow manure compost, it is recommended to consume fewer amounts of enriched manure.