THE SUITABILITY of five chemical extractants (EDTA, DTPA, O-Ph, 2,2'Bip and HCl) to estimate the amount of active iron, is studied in greenhouse experiments, using tomato plants. Different levels of Fe (0, 5 and 10 ppm) in combination with various ratios of K: Ca fertilization (1:20, 1:10 and 1:5) were used. The shoots of the harvested plants after 60 and 75 days from planting were analyzed for their total Fe, K, Ca, P and active Fe content as extracted by the abovementioned extractants.
 
The highest dry matter yield of tomato shoots at the two stages of growth were associated with 0 ppm Fe and 1:20 (K: Ca) ratio treatment and the least yield was recorded at the case of 0 ppm Fe and 1:5 (K: Ca) ratio. Iron application significantly increased the Ca content of the plants but decreased both K and P, while total iron showed an insignificant increase. Increasing K: Ca ratio increased K, P and Fe but decreased Ca content.
 
Active iron extracted using the previous methods increased with iron application increment and decreased by increasing K: Ca ratio. The extractability power, of the five solutions has the following ascending order: EDTA < DTPA < O.Ph < 2,2'Bip < HCl.
 
The data illustrate that deficiency is expected at active iron levels less than 58, 67, 116, 146 and 221 (mg/kg), for the 60-days old tomato shoots, and 49, 56, 63, 84 and 105 (mg/kg) for the 75 days old tomato shoots in the same order.