Abstract

Tillage and N management are important in dryland crop production of the west central Great Plains (area between the 99^th meridian and the eastern edge of the Rocky Mountains) because of frequent periods of limited soil moisture. Therefore, judicious use of N fertilizer is a management priority in wheat (Triticum aestivum L.)-sorghum [Sorghum biocolor (L.) Moench]- fallow (W-S-F) rotations. The objectives of this study were to: (i) determine the long-term effects of N fertilization (0, 20, 40, and 60 lb N/acre) on grain yields of winter wheat and grain sorghum under three tillage systems, (ii) investigate the effect of soil moisture at or near planting on grain yields, and (iii) evaluate the residual profile soil inorganic N after 20 yr of N fertilization in the three tillage systems. The study involved a W-S-F rotation under three tillage systems on a nearly level Harney silt loam soil (fine, montmorillonite, mesic Typic Argiustoll). The three tillage systems were clean-till (CT), reduced-till (RT), and no-till (NT). Nitrogen was broadcast preplant as ammonium nitrate on each crop at rates of 0, 20, 40, and 60 lb N/acre. As the level of soil moisture increased in each tillage system, there was a corresponding larger yield increase of wheat and sorghum to applied N. The correlation of grain yields of wheat and sorghum with soil profile N at all depths was highest for nitrate N and lowest for ammonium and total inorganic N. For all three tillage systems, sampling deeper than 6 in. resulted in little improvement in the coefficient of determination (R²) for grain yields regressed on soil nitrate N. Residual soil nitrate N was highest in the top 6 in., dropped significantly in the 6- to 12-in. depth, and remained relatively low thereafter throughout the 72-in. sampling depth. Data from this long-term study showed the optimum broadcast N rate was approximately 60 lb N/acre applied on each crop grown in a W-S-F rotation with the exact rate depending on soil moisture, fertilizer, and crop prices. Yields from CT were comparable with RT on this nearly level upland soil but failed to meet the residue requirements mandated in conservation compliance plans. Poorer stands, increased weed competition, and drier soils resulted in generally lower yields from NT plots. Considering all factors, RT systems for dryland wheat and sorghum production are recommended on upland fertile soils in the west central Great Plains.

Research Question

Nitrogen management is important in crop production on dryland cultivated soils in the west central Great Plains, a region that often experiences one or more periods of limiting soil moisture per year. Therefore, judicious use of N fertilizer is a management priority in wheat-sorghum-fallow (W-S-F) rotations. The objectives of this study were to: (i) determine the effects of long-term N fertilization on grain yields of wheat and grain sorghum in a W-S-F rotation under three tillage systems, (ii) investigate the effect of soil moisture at or near planting on grain yields of winter wheat and grain sorghum, and (iii) evaluate the residual profile soil inorganic N after 20 yr of N fertilization in the three tillage systems.

Literature Summary

The results from studies of tillage systems involving W-S-F rotations have been mixed. The management system that worked best in one area did not always work well in another. However, nearly all published tillage studies, with or without fertilizer, were of short duration and did not account for the long-term effects of depth of moist soil, precipitation, or surface moisture on grain yields or residual soil N.

Study Description

The study was conducted at the Kansas State University Agricultural Research Center near Hays, KS. The tillage phase (clean-, reduced-, and no-till) of the study was initiated in 1965. Starting in 1975 four N rates (0, 20, 40, 60 lb N/acre) were applied on each tillage system where a crop was grown. This four replication study was conducted on a nearly-level, fertile soil (Harney silt loam) using a split-plot design. Grain yield response to tillage and N rate was measured each year from 1975 through 1996. Soil profile nitrate and ammonium-N concentration (in increments to 72 in. depth) was determined prior to starting the study in 1965 and in the fall of 1994.

Applied Questions

What tillage system proved best for this long-term study?

Reduced-till in most comparisons was as good as or better than clean- or no-till systems. The reduced-till system also left sufficient crop residue to meet the requirements of conservation compliance plans. Therefore, we recommend a reduced-till system.

For the reduced-till system, what effect did soil moisture at or near planting and applied N have on grain yields?

Years with high soil moisture at or near planting resulted in significantly higher yields for both wheat and sorghum than years with medium or low moisture. Also, soil moisture had a greater effect on yields than applied N. Wheat and sorghum yield increase from applied N was greater with higher soil moisture at planting. The optimum N rate was approximately 60 lb N/acre for both crops, with the exact rate being dependent on soil moisture, fertilizer, and crop prices. Therefore, we recommend a 60 lb N/acre application rate for both crops in this rotation.

What sampling depth and form of inorganic N was best related to grain yields?

Sampling to a 6-in. depth for all forms of inorganic N correlated as well or better with grain yields than deeper samplings. Nitrate N correlated better with grain yields than did ammonium N or ammonium N plus nitrate N.

What effect did 20 years of N fertilization have on the profile nitrate N?

Most of the residual nitrate N was in the top 6 in. of soil. Nitrate N decreased to a low level in the 6- to 72-in. increments and buildup of nitrate N in the soil below 6 in. has been small. This means that over the 20 yr, N rates (highest rate of 60 lb N/acre) used in this W-S-F rotation study were not in excess of crop needs.

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