A great majority of the dryland winter wheat (Triticum aestivum L.) producers in the west central Great Plains currently do not use soil tests to determine N fertilizer needs although most producers have grain protein data from their fields. Field experiments with N rate as the variable were conducted throughout eastern Colorado over a 4‐year period with winter wheat grown under summer fallow cropping. The objective was to determine if grain protein content could be used as a post‐harvest indicator of sufficiency of N for maximum grain yield. The soil types were of the Paleustoll, Argiustoll, Paleargid, and Haplargid great groups. The yield and protein response data from the experimental locations were analyzed by an interaction chisquare procedure. Grain protein content was found to be an effective post‐harvest indicator of N nutrition adequacy for grain production. Both a critical level and a transitional zone of grain protein content were defined using interaction chi‐square analysis. The most critical level was determined to be 11.5% protein and the transition zone between the deficient and N sufficient populations of yield observations was between 11.1 to 12.0%. The amount of yield lost to N deficiency was not related to grain protein content. Workers in other geographic areas should be able to take existing N response data and develop similar guidelines appropriate for those areas.

Alfalfa (Medicago sativa L.) was established at two field locations in Colorado in 1976 and triple superphosphate applied at rates of 25 and 50 kg P/ha annually or 75 kg/ha in a single application at seeding. The initial P applications were incorporated while subsequent applications were topdressed. Soil profile changes in NaHCO3‐extractable P and in P solubility measured in 0.01M CaCl2 were determined over a three‐year period. The resulting calcium hydroxide and monocalcium phosphate potentials were related to known solubility products for distinct P mineral phases including octacalcium phosphate (OCP) and β‐tricalcium phosphate (TCP). Octacalcium phosphate controlled solution P if the NaHCO3‐extractable P rose above 35 mg·kg−1 while TCP or a similar mineral phase dominated P intensity in the range of 10 to 25 mg·kg−1. Relating these potentials to the changing NaHCO3‐P levels indicated that considerable Ca8H2(PO4)6·5H2O, octacalcium phosphate (OCP), accumulated in one soil. Solubility data from both soils also indicated significant buffering of soil P on or near the solubility isotherm of β‐Ca3(PO4)2, tricalcium phosphate.

Soils from the orders Alfisol, Inceptisol, Mollisol, Ultisol, and Oxisol contained exchangeable K (neutral 1N NH4OAc) and boiling 1N HNO3 extractable K varying from 31 to 358 ppm and 62 to 652 ppm, respectively. Total K varied from 1,780 to 14,200 ppm. Quantities absorbed from each soil by 7 cuttings of perennial ryegrass (Lolium perenne) in the greenhouse ranged up to 13 times that of exchangeable K and up to 5 times that of HNO3‐extractable K. Total plant uptake represented 3.5 to 29.7% of total soil K. In no soil did K become limiting in the first four cuttings. However, except for the Mollisols, uptake decreased abruptly in the later cuttings, indicating very little “slowly” available K. Even though exchangeable K and K removed by strong acids were highly correlated with plant uptake (r = 0.744** to 0.881**), all extractants greatly underestimated actual plant‐available K under intensive cropping. Total soil K did not significantly correlate with plant uptake (r = 0.211).

The production of high sugar yielding crops of sugarbeets (Beta vulgaris L.) is highly dependent on available N. Excessive N fertilization is frequently cited as a cause of low sugar containing beets, which has become a problem in todays beet industry. The objective of this study was to evaluate the usefulness of N soil tests to predict N fertilizer requirements for this crop. Fertilizer trials consisting of 4 N rates (0, 70, 140, and 210 kg N/ha) were conducted on 33 farm sites in central and eastern Colorado over a 2‐year period. Soils ranged in texture from sandy loam to clay loam and were generally calcareous at or near the surface having a pH > 7.0. The results show that soil NO3‐N levels measured before planting are useful in estimating the N requirements of the sugarbeet crop. Soil NO3‐N levels were closely correlated with crop yield and sucrose percentage. Regression analysis indicated that yield and recoverable sugar responses to fertilizer N are unlikely when soil NO3‐N levels (0 to 60 cm) are > 130 kg/ha. A linear decrease in sucrose percent resulted from increasing N levels. Soil NO3‐N had a greater effect on sucrose percentage and plant N content than did an equal amount of fertilizer N during both years. An assessment of the NO3‐N content of the soil to at least the 60‐cm depth was found to be an adequate index of soil mineral N availability.

Field experiments with irrigated alfalfa (Medicago sativa L.) were established in far eastern Colorado on a Keith soil and in far western Colorado on a Ravola soil in 1976. Concentrated superphosphate was applied at rates of 25 and 50 kg P/ha annually for 3 years or 75 kg P/ha in a single application at seeding. Muriate of potash was applied at rates of 140 and 280 kg K/ha annually for 3 years or 420 kg K/ha in a single application at seeding. Soil profile changes in NaHCO3‐P and exchangeable K were determined. At a location having low to medium available‐P status (crop response to P expected), 2.2 times the P removed by the alfalfa was required to maintain the initial NaHCO3‐P level. At a location with medium to high available‐P status (no P response expected), only 1.4 times the removed P was required for maintenance. The two locations required a fertilizer K rate of 0.75 and 0.22 times the K removed by alfalfa uptake to maintain their respective initial exchangeable‐K levels. The low maintenance requirement of the latter soil was apparently due to the K minerals present which buffered the exchangeable K near 100 mg·kg−1.