The magnitude of a seed yield increase that occurs when soybeans [Glycine max (L.) Merr.] are irrigated depends upon the phenologic timing of the irrigation in relation to the temporal sequence with which the components of seed yield are established and fixed. Eight soybean cultivars, varying in stem growth habit and maturity, were irrigated according to a factorial treatment design in which either no irrigation or one irrigation was applied at three reproductive stages: R1 to R2 flowering (F), R3 to R4 pod elongation (P), or R5 to R6 enlargement (S). An F irrigation increased the numbers of pods/plant (+ 2.8) and seeds/plant (+ 5.8), but an offsetting decrease occurred 100‐seed weight (−0.9 g), resulting in little change in seed yield. P irrigation had no effect on 100‐seed weight, but increased the numbers of pods/plant (+3.4) and seeds/plant (+7.9) resulting in a large increase in seed yield. An S irrigation resulted in only slight increases in the numbers of pods/plant (+1.0) and seeds/plant (+3.3), but greatly increased 100‐seed weight (+1.4 g), again leading to a large seed yield increase. These observations suggested that irrigation early in reproductive ontogeny greatly reduced flower and pod abortion, whereas irrigation later in ontogeny reduced ovule abortion within developing pods. The cultivar ‘Harcor’ possessed unusually high numbers of oneand two‐seed pods, while ‘Elf’ had substantially fewer numbers of three‐seed pods, and ‘Woodworth’ possessed larger numbers of fourseed pods. Irrigation timing differentially influenced the frequencies of the various pod classes relative to their contribution to the increase in total pods/plant, primarily because of effects on ovule abortion within developing pods. The effects of irrigation timing on number of seeds/plant and 100‐seed weight were thus consistent with the effects on seed yield reported earlier.

Successful irrigation management for soybeans [Glycine max (L.) Merr.] requires a knowledge of the effect of irrigations applied during specific stages of reproductive ontogeny. Eight soybean cultivars, varying in stem growth habit and maturity, were subjected to a factorial set of eight irrigation treatments involving either no irrigation or one irrigation applied at three reproductive stages: R1 to R2 flowering (F), R3 to R5 pod elongation (P), or R5 to R6 seed enlargement (S). The experiment was conducted for 3 years on a Sharpsburg silty clay loam (Typic Argiudoll). An F irrigation had little effect on ultimate seed yield (+20 kg/ha) and for some cultivars actually depressed yield below that of the nonirrigated check. A P irrigation consistently increased seed yield (+379 kg/ha), irrespective of cultivar or year. An S irrigation also enhanced seed yield (+ 384 kg/ha), but the degree of enhancement was inconsistent across years, being influenced by the amount of natural rainfall occurring during the seed enlargement stage. Although a dual F‐P irrigation treatment enhanced seed yield above that for both the nonirrigated check and single F irrigation treatment, the seed yield increase was significantly smaller (+166 kg/ha) than that achieved with a single P irrigation treatment. Soybean maturity was delayed 2 to 6 days with irrigation but the delay was greatest when irrigation was applied late in reproductive ontogeny. This senescence delay lengthened the seed‐fill period and was responsible in part for the seed yield enhancement effected by irrigation. Increases in soybean plant height and lodging were greatest with an F irrigation, intermediate with a P irrigation, and minimal with an S irrigation. Although irrigation effects on seed quality were variable depending on the cnltivar, there was an overall tendency for a P irrigation to improve and an S irrigation to worsen seed quality. An F irrigation had no effect on seed quality. Of the eight cultlvars evaluated, the determinate ‘Elf’ was the most yield responsive to irrigation and was relatively resistant to changes induced by irrigation in plant height, lodging, and seed quality. Based on these data, optimal yield enhancement in irrigated soybean culture can be achieved with the use of short‐stature or lodging‐resistant cultivars irrigated during pod elongation (R3 to R4) and seed enlargement (R5 to R6).

Nitrogen fertilization of soybeans (Glycine max (L.) Merr.) has given variable results in increasing soybean yields. The objectives of this study were to determine the extent and magnitude of yield increases of soybeans from N fertilization on soils of the Sharpsburg‐Marshall association in Nebraska, and to determine factors to assist in identification of conditions conducive to N induced yield increases. Experimentation with N rates of zero to 224 kg N/ha applied to soybeans was carried out at 13 sites on farmers' fields over a 3‐year period. Yield increases were observed at 9 of the 13 sites. Plant N concentration was increased at 7 sites. Increases in yields were not necessarily related to increase in plant N content; but were closely related to increase in seed size. Where effects were observed, seed yield, plant N content, and seed size were generally linearly related to rate of N application. Regression analyses were performed to determine factors which were related to the occurrence of N response. The factors found were soil pH, soil organic matter content, and yield level.

Degradation and its effect on atrazine leaching were observed in soil in the laboratory. Degradation of 1.5 ppmw (wt/wt) of ring labeled 14C‐atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine) over time periods up to 8 months was predominantly a nonbiological process in three nonsterilized soils at 0.1 bar moisture content, although there was evidence that the atrazine molecule was dealkylated simultaneously. Between 4 and 17% of the applied 14C was present as nonpolar compounds after 1 month of incubation at 30°C. The amount of 14CO2 evolved was cumulatively less than 0.05% of the added radiocarbon at 5 and 30°C. Desorption of atrazine from soil treated and incubated for 0.5 hour; 1 and 2 weeks; 1, 2, 4, and 8 months showed a steady increase in the unextractable fraction remaining in soil with time. In the soil extract, a tolueneinsoluble fraction increased as time passed, and made up between 24 and 62% of the extracted radiolabeled fraction of the three soils after 1 month of incubation at 30°C. The ratio of polar compounds to nonpolar compounds steadily increased with time of incubation. Major possible metabolites of the toluene‐soluble fraction were identified by thin layer chromatography as monodealkylated products. During elution of atrazine from soil columns saturated with water, decreasing the water flow rate from 2 cm/hour to 0.08 and 0.04 cm/hour greatly reduced the downward movement and enhanced degradation of the herbicide.

Field studies were initiated in 1971 and continued through 1974 to evaluate the influence of time and rate of fertilizer application for both seeded warm‐season and bluegrass (Poa pratensis L.) pastures. The warm‐season pastures consisted of a mixture of big bluestem (Andropogon gerardi Vitman), switchgrass (Panicum virgatum L.), indiangrass (Sorghastrum nutans L.) and sideoats grama (Bouteloua curtipendula L.). Prior studies had shown that fertilization would increase production of several forage species. Yet results were often conflicting and influence of time of fertilizer application had not been investigated. Rates of fertilization ranged from 0 to 180 kg N/ha and from 0 to 34 kg P/ha. Fertilizer was broadcast in both early and late spring on established stands on Crofton silt loam, a Typic Usthorthert. Applications were repeated each year. The relationship between yields of the bluegrass to rate of applied N and P was linear in each year of the study. Higher yields were recorded when fertilizer was applied in early (15 April) compared to late (30 May) spring. For the warm‐season grasses, the relationship of yields to rate of N application was linear in 1971 and 1972 but was curvilinear in 1973 and 1974. Yields were linearly related to rate of P applied in all years except 1972. Botanical composition of the warm‐season grass mixture was influenced by time of fertilization. Early spring application encouraged encroachment by cool‐season species, intermediate wheatgrass (Agropyron intermedium Host.), which dominated the stand after 2 years of fertilization. Application of N rates less than 90 kg/ha in late spring did not result in encroachment. At the lower N rates, higher yields of warm‐season grasses were observed when fertilizer was applied in late spring.