evaluation of the increased rates of water super absorbent and humic acid application under deficit irrigation condition on some agroecological characteristics of zea mays using response surface methodology

introduction water super absorbents are water absorbing natural or synthetic polymers (they may contain over 99% water). they have been defined as polymeric materials which exhibit the ability of swelling in water and retaining a significant fraction (> 20%) of water within their structure, without dissolving in water content. the applications of hydrogels are grown extensively. these materials do not have any harm to the environment. development of using super absorbent hydrogels to reduce crises such as soil erosion, frequent droughts or providing food security requires knowledge of their behaviors and performances in the soil. humic substances are a mixture of different organic compounds that extract from various sources such as soil, humus, peat, oxidized lignite and coal. they are different in molecular size and chemical structure. a little amount of humic acid increase soil fertility by improving the physical, chemical and biological characteristics of soil. increase in agricultural production and productivity depends, to a large extent, on the availability of water. hence, the importance of irrigation is however, the availability of irrigation facilities which is highly inadequate in iran. determining the optimal amount of irrigation water has always been a main goal of researchers. among the problems of excessive irrigation can be pointed to leach the nutrients especially nitrogen from the soil, the pollution of groundwater and environment and reduce fertilizer use efficiency, especially water-soluble fertilizers. to determine the optimal irrigation water and fertilizer, the use of mathematical models is inevitable. one of the most common methods used to optimize these factors is the central composite design. a central composite design is an experimental design, useful in response surface methodology, for building a second order (quadratic) model for the response variable without needing to use a complete three-level factorial experiment. despite of many researches on the effect of water super absorbent, humic acid and irrigation on different crops, information on simultaneous optimization of these factors for many crops is scarce, therefore, in this study optimum levels of water super absorbent, humic acid and irrigation in maize were determine. materials and methods in order to estimate the optimized application rates of water superabsorbent, humic acid and irrigation in cultivation of maize (zea mays l.), an experiment as box-behnken design using response surface methodology, was conducted at research farm of ferdowsi university of mashhad, during 2013-14 growing season. the experimental treatments were designed considering of the high and the low levels of water superabsorbent (80 and 160 kg ha-1), humic acid (4 and 8 kg ha-1) and irrigation (200 and 400 m3 ha-1 in each irrigation) using minitab 17 statistical software, as the central point in every treatment replicated 3 times, so 15 treatment combinations were provided totally. results and discussion the results showed that linear effects of humic acid and irrigation and quadratic effects of superabsorbent and irrigation were significant on seed yield, as the highest grain yield obtained in treatments of 120 kg ha-1 super absorbent, 8 kg ha-1 humic acid and 300 m3 ha-1 irrigation. the middle level of humic acid (6 kg ha-1) led to the highest dry matter yield and increase of humic acid from 4 to 8 kg ha-1 improved grain protein by 7%. in economic scenario, using 126.06 kg ha-1 superabsorbent, 7.19 kg ha-1 humic acid and 347.47 m3 ha-1 irrigation, resulted in 26710 kg ha-1 grain yield. in eco-environmental scenario, using 123.63 kg ha-1 superabsorbent, 7.19 kg ha-1 humic acid and 323.23 m3 ha-1 irrigation, resulted in the highest grain yield (26309 kg ha-1) and the highest water use efficiency (5.23). it seems that application of 120 kg ha-1 water super absorbent increased grain yield by improving soil physical properties. jahan et al., (2011) reported that the highest and the lowest grain yield of bean (phaseolus vulgaris l.) obtained in treatments of 80 kg ha-1 water super absorbent + humic acid and non-applicaion of water super absorbent and humic acid, respectively. yazdani et al., (2012) evaluated effect of different amounts of water super absorbent and irrigation on yield and yield components of soybean (glycine max l.) and reported that with increasing amounts of water super absorbent, grain yield was increased. conclusions according to the ecological perspective, eco-environmental scenario is a priority compared two other scenarios, therefore by using the optimized values of resources in this scenario can be achieved to the sustainable production of maize, meanwhile can be minimized environmental pollutions.