Meta-Analysis of the Effect of Super-Absorbent Application on Crops Yield in Iran

The most fundamental agricultural obstacle particularly in arid and semi-arid regions is water shortage. To mitigate this problem, application of some soil amendments such as water super absorbent polymers (SAP) for increasing water-holding capacity in the soil has been under attention at commercial level in recent years.
Despite numerous studies in Iran and the apparent positive and various functions have been mentioned for SAP and their advantageous in agriculture, there has not been a precise conclusion about these studies. On the other hand, variations in consumption rate, conditions and experimental treatments, makes an impossibility or hardiness to receive a conclusive result for the researchers studying one or few numbers of relevant articles.Meta-analysis is the statistical procedure for combining data from multiple studies. When the treatment effect (or effect size) is consistent from one study to the next, meta-analysis can be used to identify this common effect. When the effect varies from one study to the next, meta-analysis may be used to identify the reason for the variation. Decisions about the utility of an intervention or the validity of a hypothesis cannot be based on the results of a single study, because results typically vary from one study to the next. Rather, a mechanism is needed to synthesize data across studies (Rosenthal, 2001). Narrative reviews had been used for this purpose, but the narrative review is largely subjective (different experts can come to different conclusions) and becomes impossibly difficult when there are more than a few studies involved. Meta-analysis, by contrast, applies objective formulas (much as one would apply statistics to data within a single study), and can be used with any number of studies.
Therefore, this study was conducted with the aim of doing meta-analysis on the results of conducted researches in Iran and to answer a general question that whether the application of SAP has been successful or not, and if yes, how much of SAP is recommended. To conduct this research, articles were published during 2006-2016 with the subject of the effect of different rates of SAP on yield and yield components of crops (including cereals, legumes, medicinal and rangeland plants) were investigated. Amongst 32 articles were checked, 13 articles have enough data to conduct the meta-analysis for seed yield, dry matter yield and harvest index. The result of meta-analysis showed that the mean consumption rate of SAP for cereals, legumes, medicinal and rangeland plants were 83, 322, 1031 and 210 kg ha-1, respectively, and with this consumption rate of SAP, the mean seed yield in cereals, medicinal plants and legumes increased by 13.2, 11.2 and 27.5% (equal to 1059, 345 and 452 kg ha-1), respectively, compared with control.
Generally, the results showed that the effect of SAP on seed yield and dry matter was significant but was not significant for harvest index. The effect size of the application of different SAP rates on dry matter yield and at the whole levels was greater than its effect size on seed yield. The most effect size on seed yield was considered for the application of 100 kg ha-1 SAP and the application rates greater than 200 kg ha-1, unlike the dry matter yield, had no significant effect on seed yield. According to the results, it seems that application of 100 kg ha-1 SAP is the most appropriate rate on seed yield and dry matter increment and economical aspects. Design and implementation of the researches focused on estimation and comparison of economic benefits about water consumption as the result of SAP application can justify the relatively high cost of this ecological input and provide the necessary ground for its widespread use by farmers and producers involved in agronomy and horticultural sections.