Botanists have learned in recent years that the foliage of plants can take in some nutrients much as roots can. With tracers they discovered that many nutrients are readily taken up by foliage, including bark of dormant trees, even at temperatures below freezing. As shown by isotopic tracers, elements such as phosphorus, nitrogen, and potassium move both up and down from the point of application at rates similar to those following root absorption. Urea (a nitrogen compound) is now used as a nutrient foliar spray for many fruit and vegetable crops in this country. Where Should Fertilizer Be Placed? Even before the use of tracers, agronomists realized the inefficiency of spreading fertilizer uniformly over a seed-bed. They know the fertilizer should be placed somewhere near the seed, but where? Above? Below? Beside? Below and beside? How far away? They had conducted some research, but the methods were slow and tedious. 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: [Illustration: (A) Fertilizer mixed throughout the soil gives the least benefit to seedlings.] [Illustration: (B) If placed in a band below and beside the seeds, the fertilizer gives high uptake and good root distribution.] [Illustration: (C) If the fertilizer is placed directly beneath the seeds, highest uptake occurs but roots tend to “bunch”—a handicap to later growth.] Fig. 2—Radioactive plant nutrients injected in soil roots [Illustration: have not reached above ground parts in 5 minutes] [Illustration: but have, as indicated by Geiger counter, reached these parts in 20 minutes.] [Illustration: ]