Mounting concern about NO3 pollution of the nation's ground and surface waters make accurate N‐fertilizer recommendations increasingly important. A study was initiated in 1984 to determine if presidedress soil or corn stalk (Zea mays L.) NO3 concentrations could be used to predict soil N availability and the need for sidedress N fertilizer. Nitrate concentration was determined in the surface 30 cm of soil and in the lower 10 cm of corn stalks 22 to 37 d after emergence (corn was in the 5‐ to 6‐leaf stage). Eighty‐seven N‐response experiments were conducted over a 4‐yr period in central and southern Pennsylvania. A number of these sites were on fields that had received high rates of manure in the past. Corn stalk NO3 concentration was not an accurate predictor of soil N availability nor of the probability that a site would respond to N fertilizer. The NO3 concentration in the surface 30 cm of soil 4 to 5 wk after emergence was a good indicator of whether a response to sidedress N fertilizer would be attained. The best critical level for separating responsive from nonresponsive sites was 21 mg NO3‐N kg−1. No sites responded to N fertilizer when the soil NO3‐N concentration was greater than 25 mg kg−1. Soil NO3 concentration and soil N‐supplying capability (NSC) at soil NO3 levels below the critical level were not correlated well enough (r2 = 0.34) to use the predicted NSC for adjusting N‐fertilizer recommendations. As a result, the best use of this test in Pennsylvania appears to be in identification of nonresponsive sites rather than predicting N‐fertilizer rates. Use of this test by farmers in Pennsylvania will help minimize the practice of applying “insurance” N when N contributions from manure and legumes in rotations are uncertain.

A late‐season chlorophyll meter test for corn (Zea mays L.) could provide growers with a convenient evaluation of that year's N management program, along with information useful for N fertilizer rate decisions in subsequent growing seasons. Chlorophyll meter (SPAD 502) readings were taken late in the growing season on corn ear leaves in replicated plots of 357 N treatments from 93 N‐response experiments conducted in Pennsylvania. At sites not affected by severe late‐season drought or disease, chlorophyll meter readings taken at the 1/4 milk line stage separated N‐deficient from N‐sufficient treatments with 93% accuracy using a critical level of 52.0 SPAD units. If meter readings were normalized at each site by comparison with a site‐specific high‐N reference reading, chlorophyll meter readings of ear leaves made from the late milk to the middent stage of corn could separate N‐deficient from N‐sufficient treatments with 92% accuracy using a relative SPAD reading critical level of 0.93. For treatments with meter readings below the critical level, relative grain yields and the additional N that would have been needed for economic optimum grain yield were correlated with chlorophyll meter test values. For treatments with a meter reading above 52.0 SPAD units, the amount of excess N that may have been applied could not be accurately predicted. For corn tested at the 1/4 milk line stage, we suggest an N‐sufficiency range of 52.0 to 56.0 SPAD units, above which there is a 70% probability of an excess of >50 kg N ha−1 having been applied.

The increasing use of both no-tillage corn (Zea mays L.) production and urea-containing fertilizers makes it imperative that we develop management strategies that maximize economic return from applications of these fertilizers to no-till com. A 3-yr experiment was conducted on a Murrill silt loam (fine-loamy, mixed, mesic Typic Hapludult) to determine the relative efficiencies (yield and N uptake per unit of N applied) of three common N sources (NH 4 N0 3 (AN), urea, and urea-ammonium nitrate solution (UAN)J used on no-till corn. They were applied either as a surface band or as an injected band at or near the time of planting, or as a sidedress 26 to 31 days after plant emergence. Surface-banded AN, injected AN, and injected urea all produced similar no-till com yields and N uptakes. The apparent NHJ volatilization loss from surface-banded urea was strongly influenced by the number of days after application until a total of 10 mm of rain fell. Apparent losses ranged from less than 50 g kg-• of applied N with 10 mm of rainfall within 2 to 3 days after application, to over 300 g kg-• with S to 7 days of dry weather after application. Both surface-banded and injected UAN were less efficient than AN or injected urea. Sidedress applications of N resulted in more efficient N fertilizer utilization with all sources, but particularly with urea-containing sources. Shorter rain-free periods after sidedress applications may have contributed to this effect. Surface applications of an experimental fertilizer, 3:1 urea/urea phosphate, produced greater ear-leaf at silking N concentrations than did surface-banded urea. Amending urea with KCI, or UAN with ammonium thiosulfate, did not result in yields, ear-leaf N concentrations, or N uptakes significantly different from those obtained with their unamended surface-applied counterparts in the one year that each was tested.-----------------

A 6‐yr field experiment was conducted in central Pennsylvania to determine the fertilizer N equivalent value of three forage legumes managed as hay prior to 3 yr of succeeding corn (Zea mays L.). The forage component of the rotations were 3 yr of alfalfa (Medicago sativa L.), 3 yr of birdsfoot trefoil (Lotus corniculatus L.) (BFT), and 2 yr of red clover (Trifolium pretense L.). There was no significant grain yield response to N fertilizer for first‐year corn following any of the legumes. The 3‐yr total fertilizer N equivalences of alfalfa, BFT, and red clover were estimated to be 167,151, and 131 lb/acre, respectively. The proportion contributed each year averaged 70% the first, 20% the second, and 10% the third for alfalfa and red clover, and 55%, 30%, and 15% for the three years with BFT. The beneficial effect of legumes on subsequent corn yields appeared to be due primarily to their residual N rather than to a crop sequence or rotation effect.

in Pennsylvania (Beegle et al., 1990; Piekielek et al., 1997), their usage by farmers and farm advisors has We compared six late-season diagnostic tests for determining N been limited. Reasons given for this include: the narrow adequacy in corn (Zea mays L.) in a 3-yr study in Pennsylvania. The sampling-time window when farmers are often busy with six tests were: (i) the NO Ϫ 3 -N concentration of stalk sections at black layer; (ii) the NO Ϫ 3 -N concentration of stalk sections at the one-fourth other field work, difficulty of taking soil samples to 30 cm, milk line growth stage (MLGS), which allows corn grown for silage purchase price of the CM, and the reluctance to establish to be tested; (iii) the chlorophyll meter (CM) test at the one-fourth high-N reference strips needed to obtain the relative MLGS; (iv) the relative CM test (normalized values) at the one-fourth readings required for accurate estimates with the early MLGS; (v) a visual test based on the number of green leaves below season CM test. and including the ear leaf at the one-fourth MLGS; and (vi) a relative In an attempt to provide more convenient N managevisual test (normalized values at the one-fourth MLGS). We found ment tools for farmers and crop consultants, we have that with a critical level of 250 mg kg Ϫ1 NO Ϫ 3 -N, the stalk NO Ϫ 3 test been assessing end-of-season N tests. An accurate evaluseparated N-sufficient from N-deficient sites with approximately 93% ation of the N status of a corn crop at the end of a growaccuracy when sampling was done at either the one-fourth MLGS oring season could help a producer or consultant make within several weeks after black-layer formation. It appears that the 250 mg kg Ϫ1 NO Ϫ 3 -N critical level can be used to accurately predict PA 16802.