Ahmad, N., Rashid, M., and Vaes, A. G. (1996). Fertilizers and their use in Pakistan: an extension guide (No. 2. ed.).
Baruah, T. C. and Barthakur, H.P. (1999). A text book of soil analysis. Printed at Vishal printers. New- Delhi.
Black, C.A. (1965). Methods of soil analysis. Part2. American Society of Agronomy.lnc. Publishers Madison, Wisconsin. U.S.A.
Cate, R. B., and Nelson, L. A. (1971). A simple statistical procedure for partitioning soil test correlation data into two classes. Soil Science Society of America Journal, 35(4): 658-660.
Cate, R. B., and Nelson, L. A. (1965). A rapid method for correlation of soil test analyses with plant response data. NC State University Agricultural Experiment Station.
Dahnke, W. C. and Olson, R. A. (1990). Soil test correlation, calibration, and recommendation. Pp 45-71., Madison, Wisconsin 1990.
Darwesh, D. A. and Esmail A. O. (2008). The role of supplemental irrigation and fertilizer treatments on yield componentof wheat. Mesopotamia Journal of Agriculture .36 (1): 30-38.
Hama-Amin, C.O.(2012).Using differend methods to determine critical level of potassium for wheat plant (Triticum durum L.)in Erbil plain soils .Univ. of Salahaddin, Erbil ,College of Agriculture.
Halverson, A.R. (2001). Soil sampling accuracy. Manual 19. Pacific. North West Fertilizer Conference .Pullman. Washington.
Havlin, J.L., Tisdale, S.L., Nelson W.L., and Beaton J.D (2014). Soil fertility and nutrient Management: an introduction to nutrient management. (8th edt) Pearson (p 505), Upper Saddle River, New Jersey. U.S.A.
Havlin, J. L., and Soltanpour, P. N. (1981). Evaluation of the NH4HCO3-DTPA soil test for iron and zinc. Soil Science Society of America Journal, 45(1): 70-75.
Hodgson J.M. (1976). Soil survey field handbook. Technical monograph (5): Pp56-57.printed in England.
Jackson, M. L. (1973). Soil chemical analysis. Prentice Hall, Inc. London.
Jarallah, Kh. A. (2005). Evaluation of iron fertility status and response of wheat in some of Mesopotamian plain soils. PhD Thesis, Dept. of Soil and water Science, College of Agriculture, Baghdad University.
Khdir, S.A. (2017) The role of some soilphysicochemical properties in limiting potassium critical level for Bitwen plain soils –Kurdistan region.MSC. thesis Univ. of Salahaddin, College of Agriculture.
Kumar, M. (2002). Effect of manganese and iron on the soil properties, nutrient uptake and performance of wheat (L.) and barley (L.) M.Sc. (Ag.) Thesis, Maharana Pratap University of Agriculture and Technology, Udaipur (Raj.).
Lindsay, W. L., and Schwab, A. P. (1982). The chemistry of iron in soils and its availability to plants. Journal of Plant Nutrition, 5(4-7):821-840.
Loeppert, R. H. and Iskeep, W. P. (1996). Iron methods of soil analysis, Part 3, chemical methods, C.F: Sparks, D. L., Page, A. L., Helmke, P. A., Loeppert, R. H., Soltanpour, P. N., Tabatabai, M. A., and Sumner, M. E. (1996). Methods of soil analysis. Part 3-Chemical methods. Soil Science Society of America Inc., Pp. 437-474.
Meena, R.S., Mathur, A.K. and Sharma, S.K. (2013). Determination of critical limit of iron for wheat in soils of sub-humid southern zone (IV-B) of Rajasthan. International Journal
Mohsin, H.A.A, (2013). Effect Different Levels of Chelated Iron on the Growth and yield of Wheat (Triticum aestivum L.) under Thi- Qar and Al-Muthanaa conditions. International Journal of Science and Research, 4(9):1741-1743.
SAS Institute, (2001). The SAS system for Windows, The SAS Inst. Cary, NC.
Shekh bzayni, I. T. A. (2005).The critical level of phosphorus for wheat (Triticum durum L.) in Arbil plain soils. MSc. Thesis Univ. of Salahaddin,Erbil ,College of Agriculture.
Sims, J.T and Johnson, G.V. (1991). Micronutrient soil tests, C.F: Mortvedt, J.J., Cox, F. R., Shuman L. M., and Welch, R. M. (2ndedt.), (1991). Micronutrients in Agriculture. Soil Science Society of America Journal. 4: 427-476.
Soltanpour, P. A., and Schwab, A. P. (1977). A new soil test for simultaneous extraction of macro‐and micro‐nutrients in alkaline soils 1. Communications in Soil Science and Plant Analysis, 8(3):195-207.
White, P. J., and Broadley, M. R. (2009). Bio fortification of crops with seven mineral elements often lacking in human diets–iron, zinc, copper, calcium, magnesium, selenium and iodine. New Phytologist, 182(1): 49-84.
Whitney, D. A. (1988). Micronutrient soil test zinc, iron, manganese and copper. Pp.20-22. C.F: Dahnke, W. C. (1990). Recommended chemical soil test Procedures for the North Central Region. North Dakota Agric. Exp. Stn. Bull, 499. Review