A number of soil tests have been proposed to predict crop response to added P or to assess potential for soil P loss to runoff waters. A series of four separate experiments were conducted over a 10‐yr period to evaluate soil test methods on a total of 163 Vermont and New York field soils. The experiments included the following: (i) a pot study with alfalfa grown in the greenhouse with 31 soils either unfertilized or fertilized with 18 mg P kg−1; (ii) routine chemical analysis on 54 soils; (iii) a 360‐d incubation study with 24 soils receiving either 0, 20, or 40 mg P kg−1 as CaH2PO4, in which soils were analyzed for desorption and adsorption and the equilibrium P concentration (EPC0); and (iv) another set of 54 agricultural soils incubated with 0 or 40 mg P kg−1 and analyzed for CaCl2, distilled water, and ammonium acetate (Vermont 1)–extractable P (VT1P) and EPC0 Although P extracted by VT1 was significantly correlated with P removed by F extractants, it was better correlated with the ratio of F‐extractable P/Al extracted by either acetate or F. Phosphorus additions increased VT1P, as well as P extracted by acetate + F (Vermont 2 [VT2]), and they decreased reactive soil Al (VT1Al) and P adsorption. The amount of P needed to increase VT1P by a certain amount was directly related to the amount of Al in the VT1 extract. Phosphorus availability to plants, CaCl2‐extractable P, and the EPC0 were all more closely related to VT1P than P extracted by solutions containing F, such as Mehlich 3 (M3), Bray and Kurtz 1 (BK1), and VT2. In a number of instances the ratio VT2P/VT1Al had a better relationship with CaCl2P and EPC0 than did VT1P. Thus, the fraction of reactive Al that has reacted with P (as estimated by VT1P or the ratio of VT2P/VT1Al) appears to be a better indicator of P availability and potential P desorption to runoff water than is P extracted with F.

Root growth and development can affect N uptake. In turn, N additions may affect corn (Zea mays L.) root growth. Prolific (multiple ear) hybrids may need a more active root system to satisfy ear demand. To determine the extent of the effect of N fertilization on root growth in the field, three corn hybrids (Pioneer 3320, and prolific hybrids I202×Mol7 and I117×B73) were grown under fertilizer application rates of 56, 140, and 224 kg N ha−1 to determine (i) whether application of N fertilizer stimulated root growth of corn in the zone of application in the field 20 d before silking, at silking, and at physiological maturity and (ii) whether prolific hybrids have enhanced root development before silking compared with nonprolific hybrids. All three hybrids were grown at a uniformly low plant‐population density. Root length density to a depth of 60 cm, averaged over hybrids, was 1302, 2238, and 1184 m plant−1 in 1987, and 2195, 2833, and 3317 m plant−1 in 1988 for the three sampling dates. There were few genotypic differences in root length density. Root weight averaged over hybrids was 20.4, 40.5, and 21.5 g plant−1 in 1987, and 44.6, 61.7, and 58.9 g plant−1 in 1988 for the three sampling times, respectively. Root weight at harvest declined as N application increased, but increased in response to N at two earlier sampling times. The second year of the study, when rainfall was more plentiful than in the first year, root growth continued after silking for all hybrids. This coincided with higher yields. The data suggest that N stimulated root length in the area of application, without affecting total root length.

No abstract

Experiments with two maize (Zea mays L.) hybrids were conducted to determine (a) if the inhibition of nitrate uptake by aluminium involved a restriction in the induction (synthesis/assemblage) of nitrate transporters, and (b) if the magnitude of the inhibition was affected by the concurrent presence of ambient ammonium. At pH 4.5, the rate of nitrate uptake from 240/zM NH4NO 3 was maximally inhibited by 100/zM aluminium, but there was little measurable effect on the rate of ammonium uptake. Presence of ambient aluminium did not eliminate the characteristic induction pattern of nitrate uptake upon first exposure of nitrogen-depleted seedlings to that ion. Removal of ambient aluminium after six hours of induction resulted in recovery within 30 minutes to rates of nitrate uptake that were similar to those of plants induced in absence of aluminium. Addition of aluminium to plants that had been induced in absence of aluminium rapidly restricted the rate of nitrate uptake to the level of plants that had been induced in the presence of aluminium. The data are interpreted as indicating that aluminium inhibited the activity of nitrate transporters to a greater extent than the induction of those transporters. When aluminium was added at initiation of induction, the effect of ambient ammonium on development of the inhibition by aluminium differed between the two hybrids. The responses indicate a complex interaction between the aluminium and ammonium components of high acidity soils in their influence on nitrate uptake.

Using the soil‐based pre‐sidedress nitrate test (PSNT) rather than the yield‐goal‐based cropping and manure history (CMH) to recommend N for corn (Zea mays L.) frequently results in less N applied. Our objectives were (i) to determine if this reduction in N application substantially reduces the potential for N leaching and (ii) to compare dairy manure and commercial fertilizer as N sources, both within PSNT‐based systems. The 4‐yr field study included (i) a control (no manure or N fertilizer), and three N fertilizer rates based on (ii) PSNT, no manure added (PSNT−M); (iii) cropping and manure history (CMH), no manure added; and (iv) dairy manure application with additional sidedress N if recommended by the PSNT (PSNT + M). The PSNT − M treatment received 112 kg N ha−1 in 1990 and 123 kg N ha−1 in subsequent years, while the CMH treatment received 168 kg N ha−1 in all 4 yr. Corn yield, N uptake (defined as net aboveground N accumulation), spring and fall soil NO‐3 levels, and overwinter NO3‐N losses were measured. Silage yields of PSNT + M generally were greatest (average of 16.5 Mg ha−1), followed by PSNT−M and CMH treatments, which were similar (14.8 Mg ha−1). The CMH system resulted in the highest soil NO3‐N levels (to 120 cm) at harvest and greatest overwinter net profile NO3‐N loss (average 66 kg ha−1). The results indicate (i) a reduced residual N after harvest and therefore reduced potential for N leaching when using the PSNT for N recommendations and (ii) similar or higher yields using manure compared with commercial fertilizer.

Previous studies have demonstrated higher corn (Zea mays L.) yield potential for prolific corn hybrids (more than one ear) compared to nonprolific hybrids (one ear). However, little is known about the relative contribution of the second ear to total yield at various N levels. This study compared yield responses to N rates of 56, 140, and 224 kg N ha−1 of prolific Hybrids A (I202 × Mo17) and B (I117 × B73) with responses of nonprolific Hybrid C (Pioneer 3320) at a uniform low plant population density. All three hybrids gave a positive yield response to N applications. The prolific hybrids increased both in apical ear weight and in subapical ear weight and number in response to N rate. Averaged over N rates, Hybrid A yielded 24% and Hybrid C 14% higher than Hybrid B in 1987. In 1988, they yielded 13 and 10% higher than Hybrid B. Yield of single ears of nonprolific hybrid was higher than that of the apical ears in the prolific hybrids. Total potential yield in response to N application appears to be highest in Hybrid A because of second ear formation. Factors contributing to to yield increases from N applications in prolific Hybrid A included, in order of importance, an increase in the number of subapical ears, an increase in apical ear weight, and an increase in the subapical ear weight. Contrary to previous reports, apical and subapical ear weight increased simultaneously, with increasing N rates

Aluminium toxicity has been shown to decrease NO 3 uptake in Zea mays seedlings during the first 30 minutes after addition of A13+. This suggests that NO 3 uptake inhibition could be a primary response to A1 addition. We therefore tested the hypothesis that NO 3 uptake and root elongation are affected differently by A13+. Eight-day old seedlings were exposed to 100 #M A13+ in the presence of 1 or 10 mM Ca 2+, added as either CaSO4 or CaCI2. In the presence of A13+, cumulative uptake of NO 3 -N during an 8 h period was not affected by Ca 2+ level (1 or 10 mM). Root elongation at 1 mM Ca 2+ was decreased to 63% of the control by the presence of A13+. Raising ambient Ca 2+ from 1 to 10 mM in the presence of AI 3+ restored elongation rates to 78% (CaCI2) and 88% (CaSO4) of elongation without A 1. Because reductions in root elongation were partially overcome by added Ca 2+, but lowered uptake of NO~-was not, it was concluded that AI 3+ toxicity decreased root growth and NO~ uptake by different mechanisms.