Using tracers, the researchers confirmed earlier findings that roots within two or three days reached fertilizer placed less than two inches directly below seeds, but the roots tended to congregate there. When the fertilizer was two inches below and two inches to the side, roots reached it within a week and a better root system developed. With three inches between seeds and fertilizer, the desired seedling “boost” was delayed three or four weeks. (See [Fig. 1].)
Do Fertilizers Move Fast in Plants?
The movement of radioactive phosphorus from root to leaf was found to be remarkably fast, sometimes requiring less than twenty minutes. (See [Fig. 2].)
What Else Do Radioisotopes Tell Us?
Some plants take in chemicals that the plant probably cannot use: for example, the so-called locoweeds accumulate enormous amounts of selenium. With tracer techniques, we can see that the root uptake process has poor powers of discrimination.
Fig. 1—Soil tests tell how much of each fertilizer element is needed but not where to put it to give seedlings the much-needed “push.” With tracers it is found that:
(A) Fertilizer mixed throughout the soil gives the least benefit to seedlings.
(B) If placed in a band below and beside the seeds, the fertilizer gives high uptake and good root distribution.