Influence of Ironite and Phosphorus on Yield of Oats, and Content of Lead and Arsenic at Different Stages of Growth 

P.J. Eberhardt and L.J. Clark



Ironite and phosphorus were applied to plots seeded to oats to find their effect on crop yield. The effects of both additives were positive, even though not statistically at the 90% level of confidence. The concern for lead and arsenic uptake by plants was also addressed as soil and plant samples were tested for these two elements. The results showed that no more arsenic nor lead were present in oat plants on Ironite plots than from check plots, as they approached maturity.



Ironite is a soil amendment commonly added to crops in alkaline soils to solve problems of iron uptake. Phosphorus is also fixed by alkaline soils and is many times added to help root development in cold season seeded crops. A randomized split plot experiment was designed to test two phosphorus containing materials in the presence and absence of Ironite to find their affect on yield. After the experiment was designed, concerns were expressed about the uptake of arsenic and lead by plants, where Ironite had been applied. The experiment was amended to determine the uptake of those two elements throughout the plant development cycle.

Materials and Methods

After the soil was prepared for planting, a small field on the Eberhardt farm in San Simon was divided into individual plots for treatment application. Soil samples were taken from the north and south halves of the field prior to treatment application and the analyses are listed in Table1 The following treatments were applied to four replicated blocks:

P-0 = No phosphorus applied
P-1 = Phosphoric acid applied preplant (50 pounds of P2O 5 per acre)
P-2 = Phosphoric acid applied preplant and tillering (50 pounds of P2O 5 per acre each time)
P-3 = Phosphoric acid applied preplant, tillering and jointing (50 pounds of P2O 5 per acre each time)
P-4 = Mono ammonium phosphate (MAP) applied preplant (50 pounds of P2O 5 per acre)

Ironite was applied across two of the blocks at a rate of 200 pounds per acre. Cayuse oats were seeded at a rate pf 100 pounds per acre on the 6th of December and watered up. Approximately 30 acre inches of water was applied by furrow irrigation throughout the growing season. No nitrogen was applied initially because the soil test indicated the presence of adequate nitrogen. A total of 210 pounds of nitrogen per acre were added in the first three irrigations following stand establishment.

Plant samples were taken at tillering (5 March), joint stage (4 April), boot stage (4 May) and milk stage (28 May) in the following manner: The entire above ground portion of the oat plant was taken from two square foot areas of each plot as two sub-samples taken near the center of the plots. The sub-samples were weighed and dried separately, but combined for the analysis for lead and arsenic. The plant samples were ground, dry ashed and dissolved in dilute acid prior to analysis on the ICP. The analyses were done with the EPA recommended spike for sensitivity.

Soil samples were taken from each plot at joint stage. With sub-samples taken in the same manner as the plant samples. The soil samples were dried, ground and dissolved in Aqua Regia according to EPA method 3050. Again, the EPA recommended spike was used for sensitivity on the ICP analysis.

Strips were harvested out of the center of each plot using a John Deere combine with a 30 foot header. Each strip was dumped into a weigh wagon with electronic scales where plot weights were determined and samples taken for moisture and bushel weight analysis. Grasshoppers were a problem in the extreme north and south plots, so these plots were eliminated from the statistical analysis for yield. 

Results and Discussion

A quick glance at Table 1shows that the south half of the field is slightly more sandy than the northern half and the iron (Fe) and zinc (Zn) values are noticeably lower. Several other elemental analyses varied slightly but were not considered to be significantly different. Ironite was applied to the south half of the field, where the analyses indicated the soil was slightly poorer.

Table 2 presents the oat yields in pounds per acre adjusted to 10% moisture, percent moisture and bushel weights. Because of the variability caused by the grasshoppers, no significant differences are seen between treatments at the 90% level of confidence, but strong trends seemed apparent. The 455 pounds difference between the Ironite treatment and no Ironite is statistically significant at the 85% level of confidence. Statistically significance is seen at the 87% and 78% levels of confidence for Ironite differences on % moisture and bushel weight, respectively. Differences in yield from phosphorus applications are seen at the 85% level of confidence. It is interesting to note that the additional phosphorus added after the initial 50 pounds per acre at planting did not produce an increase in yield. This would follow Ottmanís data (1) which indicates a bicarbonate soluble phosphorus test <6 is likely to have a response to 50 to 100 pounds of P2O5 per acre and a test of 6 to 12 would have a variable response to 0 to 50 pounds of P2O5 per acre. The soil tests in this field are near the break point of 6 ppm P. Another interesting observation is the difference in response between phosphoric acid and mono-ammonium phosphate applied at planting. 

Table 3 addresses the concern about arsenic uptake in the oat plant when Ironite is applied to the soil. Column one shows a slightly higher value of arsenic in the soil where it was applied then where it was not. It is not surprising that the difference is small considering the dilution effect of 200 pounds of Ironite being added to 4 million pounds of soil (weight of an acre furrow slice). At tillering the oat plants from the Ironite plots contained more arsenic than from the control plots. That difference diminished with stage of development after joint stage there was no difference between Ironite treated and non-treated plots.

Table 4 shows the lead concentrations in soil and plant tissues. The soil test for lead showed less lead in the Ironite plots than the control plots. This probably indicates the native variability of lead in the soil is greater than the lead added with the Ironite. Throughout the crop development lead concentrations were lower in the Ironite treated plots than the control plots, except at boot stage. Just prior to maturity, lead concentrations in the oat plants were one third as high in the Ironite plots as in the control plots.

Figures 1 and 2 graphically show the presence of arsenic and lead in the plant tissue at different stages of growth.

Uptake of several of the essential nutrients were tracked during the growing season comparing Ironite treatments with the control. This information will be provided in a subsequent publication.


1. Ottman, M.J., S.H. Husman and B.R. Tickes. 1997. Barley and durum response to phosphorus at Buckeye, Maricopa and Yuma, 1997. Forage and Grain, A College of Agriculture Report, The University of Arizona, Tucson, AZ. Series P-110, pp. 125-129

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This is a part of publication AZ1059: "1998 Forage and Grain Agriculture Report," College of Agriculture, The University of Arizona, Tucson, Arizona, 85721. 
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