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Application of plant nutrients with drip irrigation systems is desirable for labor and energy savings and flexibility in timing nutrient applications. Evaluations of P movement in the soil and uptake by tomatoes (Lycopersicum esculentum) were made when orthophosphate and glycerophosphate were applied through a drip irrigation system and in comparison with 26 kg of P/ha banded below the seed at planting. A significantly higher P content was measured in seedling leaves when 26 kg of P/ha was applied by drip irrigation than when the same rate was banded. No differences in P content of whole tops of seedlings were measured at equal rates of inorganic or organic P applied through the drip system. There was a significant linear response of P uptake to P rate.With drip irrigation, orthophosphate moved a much greater distance into the soil than had been previously observed for comparable application rates per hectare. Glycerophosphate moved 5 to 10 cm farther through the soil at application rates of 6.5 and 13 kg of P/ha than did orthophosphate. At the relatively high rate of 39 kg of P/ha, orthophosphate moved 25 cm horizontally and 30 cm vertically in the soil profile. Movement of orthophosphate resulted from saturation of the soil reaction sites near the point of application with phosphorus and subsequent mass flow with the soil water. The distance of P movement was proportional to the application rate. Glycerophosphate must undergo enzymatic hydrolysis before releasing orthophosphate ions into the soil solution where orthophosphate can be removed from solution by soil reactions. Consequently, glycerophosphate moved farther than orthophosphate at low application rates.

Following nitrogen (N) transformations and movement in natural systems against a background of large amounts of this element requires use of isotopically labeled material which can be distinguished from that which is indigenous to the system. Methods are described for sampling and analyzing for labeled N in various components of the soil‐plant‐water system, and estimates of variability presented. The procedures described permitted construction of a balance sheet for long‐term field trials with corn (Zea mays L.) in two locations in California. Unaccounted for losses ranged from 1% on Yolo fine sandy loam at 90 kg N/ha to > 50% in the coarse‐textured Hanford sandy loam with high rates of N application. It was found possible to identify labeled N from 15N‐depleted fertilizer in the soil organic fraction even after extensive dilution by using the isotopic composition of N in the corresponding layer of soil from unfertilized plots as the reference value rather than an overall mean.