Winter wheat (Triticum aestlvum L.) was grown continuously and in rotation with alfalfa (Medicago sativa L.) or grass on Pond Creek and Grant silt loams in the 64-cm rainfall area of north central Oklahoma. Studies were conducted from 1956 through 1967. Stubble mulch and clean tillage were compared in continuous wheat and alfalfa-wheat systems. The principal effect of alfalfa on wheat yield was to supply N. Wheat in rotation with alfalfa (3 years alfalfa, 3 years wheat) yielded 2,020 kg/ha, while continuous wheat fertilized annually with 45 kg N/ha yielded 2,180 kg/ha. Yields of wheat in a grass-wheat rotation were inferior to those of continuous wheat plus N because of N deficiency. Without N fertilizer, clean-tilled continuous wheat outyielded stubble-mulched continuous wheat by 23% (clean tilled: 1,790 kg/ha; stubble-mulched: 1,460 kg/ha). With 45 kg N/ha, the advantage for clean tillage was reduced to about 8% (clean tilled: 2,160 kg/ha; stubble-mulched: 2,000 kg/ha). The 45 kg/ha N fertilizer rate was sufficient for near maximum wheat yields under both clean and stubble-mulch tillage.

Applications of 56 and 112 kg N ha⁻¹ (as KNO₃) on April 2 and 23 and May 9 and 224 kg N ha⁻¹ on April 23 to Arthur and Blueboy (soft red winter) and Parker (hard red winter) wheats increased yield of grain, grain protein and grain protein percentage without exception. The average increases for all treatments and varieties for grain yield, grain protein and percentage grain protein were 21, 63 and 36% over the respective control values. Based only on amino acid composition, quality of the grains was unaffected by the nitrogen treatments. Depletion of soil nitrogen estimated by removal of grain alone for all varieties occurred not only in the control plots but in plots receiving 56 and 112 kg N ha⁻¹. Only the 224 kg N ha⁻¹ treatment plots gave an estimated net gain in soil nitrogen. The favorable results obtained for grain and grain protein production, the increasing availability of lodgeresistant wheats, and the concern over nitrate pollution of ground water and streams, all indicate that the practice of split applications of nitrogen made late in the spring to wheat has merit.

Hard red winter wheat (Triticum aestivum L.) was grown with and without wheat straw mulch on the soil surface both in a growth chamber with controlled soil temperatures and in the field where soil temperatures were measured at the crown depth. Concentrations of N, P, K, Ca, Mg, Fe, Zn, Mn, and Cu in the plant were determined at the end of tillering and at heading. Wheat plants grown with soil temperatures below 10C for 50 days, whether in the growth chamber or in the field, had 2 to 3 fewer tillers per plant and 0.5 to 1.2 fewer heads per plant than plants grown with soil temperatures below 10C for only 15 days. Soil temperature seemed not to affect weight per head or total plant weight. Cooler temperatures reduced the concentrations of only K and Fe in the growth chamber at heading and N in the field at heading. When mulched and bare soil temperatures were identical, mulch did not reduce numbers of tillers or heads per plant, plant nutrient concentration, total uptake of any nutrient, or final grain yields in growth chamber or field. The concentration of all nutrients but Cu changed as the plant developed showing the necessity of knowing growth stages of plants if nutrient concentrations are to be compared.

Studies were conducted near Pendleton, Oregon, from 1962 through 1965 to compare the effect of no-tillage and various times of stubble mulch tillage operations during the fallow season on moisture storage, nitrate accumulation, and winter wheat (Triticum aestivum L.) yields. Weeds were controlled during the spring and summer of the fallow year with nonpersistent herbicides. Plots which had received no tillage tended to store less moisture and to accumulate less nitrates than tilled plots. During the second crop year, yields from nontilled plots were significantly reduced compared to yields from tilled plots. Conventional stubble mulch tillage including two or three rod weedings tended to be the optimum tillage treatment. Delaying the first tillage until June decreased moisture storage in comparison to other tillage treatments.

The effectiveness of potassium azide (KN₃) as a nitrification inhibitor for field-applied anhydrous NH₃ was evaluated on a Naff silt loam in the winter wheat (Triticum aestivum L.) area of eastern Washington. Formulations of KN₃ in liquid NH₃ of 0, 2, and 6% (w/w) were applied to fallow soil in August 1969 at a rate of 112 kg N/ha (81-cm spacing) with a special field applicator. The 2% formulation was also applied at a rate of 224 kg N/ha. The NH₃ retention zone was sampled periodically through February 1970 and analyzed for NH₄⁺-N, NO₂⁻-N, and NO₃⁻-N. Both levels of KN₃ were effective in reducing the nitrification rate during the early post injection period, but the higher level was more effective at later dates. Two months after application, the amounts of NO₃⁻-N recovered from the retention zone as percent of total extractable N were 67, 48, and 36 for NH₃ alone, NH₃ + KN₃ (2%), and NH₃ + KN₃ (6%), respectively. Percentage amounts of NO₃⁻-N decreased with increase in N application rate. Nitrification inhibition due to KN₃ was still evident 6 months after application, particularly at the high N rate. The inhibitory effect of low soil temperature during the winter months was also apparent. These results indicate that KN₃ formulated with anhydrous NH₃ was an effective nitrification inhibitor for this N source.