Irrigated Grain Sorghum Response to Long-Term Nitrogen, Irrigated Grain Sorghum Response to Long-Term Nitrogen, Phosphorus, and Potassium Fertilization Phosphorus, and Potassium Fertilization

Long-term research shows that phosphorus (P) and nitrogen (N) fertilizer must be applied to optimize production of irrigated grain sorghum in western Kansas. In 2020, N applied alone increased yields 60 bu/a, whereas N and P applied together increased yields up to 83 bu/a. Averaged across the past 10 years, N and P fertilization increased sorghum yields up to 82 bu/a. The application of 160 lb/a N (with P) produced the maximum yield in 2020, which is slightly less than the 10-year average (2011–2020). The application of potassium (K) has had no effect on sorghum yield throughout the study period. The 10-year average grain N content reached a maximum of ~0.7 lb/bu while grain P content reached a maximum of 0.15 lb/bu (0.34 lb P 2 O 5 /bu) and grain K content reached a maximum of 0.19 lb/bu (0.23 lb K 2 O/bu). At the highest N, P, and K rate, apparent fertilizer recovery in the grain was 33% for N, 69% for P, and 40% for K. Nitrogen fertilization increased soil organic matter and decreased soil pH. Phosphorus fertilization tended to maintain or increase soil test P levels.


Introduction
This study was initiated in 1961 to determine responses of continuous grain sorghum grown under flood irrigation to N, P, and K fertilization. The study is conducted on a Ulysses silt loam soil with an inherently high K content. The irrigation system was changed from flood to sprinkler in 2001.

Procedures
This field study is conducted at the Tribune Unit of the Kansas State University Southwest Research-Extension Center. Fertilizer treatments initiated in 1961 are N rates of 0, 40, 80, 120, 160, and 200 lb/a N without P and K; with 40 lb/a P 2 O 5 and zero K; and with 40 lb/a P 2 O 5 and 40 lb/a K 2 O. All fertilizers are broadcast by hand in the spring and incorporated before planting. The soil is a Ulysses silt loam. Grain sorghum (Pioneer 85G46, 2011;Pioneer 84G62, 2012Pioneer 84G62, -2014Pioneer 86G32, 2015;Pioneer 84G62, 2016Pioneer 84G62, -2017Pioneer 85P44, 2018Pioneer 85P44, -2019and Pioneer 86P33, 2020) was planted in late May or early June. Hail damaged the 2015, 2017, 2019, and 2020 crops. Irrigation is used to minimize water stress. Sprinkler irrigation has been used since 2001. The center two rows of each plot are machine harvested after physiological maturity. Grain yields are adjusted to 12.5% moisture. Grain samples were collected at harvest, dried, ground and analyzed for N, P, and K concentrations. Grain N, P, and K content (lb/bu) and removal (lb/a) were calculated. Apparent fertilizer N recovery Kansas State University Agricultural Experiment Station and Cooperative Extension Service in the grain (AFNR g ) was calculated as N uptake in treatments receiving N fertilizer minus N uptake in the unfertilized control divided by N rate. The same approach was used to calculate apparent fertilizer P recovery in the grain (AFPR g ) and apparent fertilizer K recovery (AFKR g ). After harvest in 2020, all plots were soil sampled (8 probes/ plot) to a depth of 6 inches, dried, and ground. Servi-Tech Laboratories analyzed the samples for soil pH, organic matter (OM), P (Bray-1 and Mehlich-3), K, zinc (Zn), manganese (Mn), and iron (Fe).

Results
Grain sorghum yields in 2020 were 5% lower than the 10-year average (Table 1). Nitrogen alone increased yields 60 bu/a, while P alone increased yields 9 bu/a. However, N and P applied together increased yields up to 83 bu/a. Averaged across the past 10 years, N and P applied together increased yields up to 82 bu/a. In 2020, 40 lb/a N (with P) produced about 75% of maximum yield, which is less than the 10-year average of 82%. The 10-year average for 80 lb/a N (with P) and 120 lb/a N (with P) was 93 and 94% of maximum yield, respectively. Sorghum yields were not affected by K fertilization, which has been the case throughout the study period.
The 10-year average grain N concentration (%) increased with N rates but tended to decrease when P was also applied, presumably because of higher grain yields diluting N content (Table 2). Grain N content reached a maximum of ~0.7 lb/bu. Maximum N removal (lb/a) was obtained with 160 lb N/a or greater with P. Similar to N, average P concentration increased with P application but decreased with higher N rates. Grain P content (lb/bu) of ~0.15 lb P/bu (0.34 lb P 2 O 5 /bu) was similar for all N rates when P was applied. Grain P removal was similar for all N rates of 40 lb/a or greater with P removal ranging from 19 to 23 lb/a. Average K concentration (%) and content (lb/bu) tended to decrease with increased N rates. Similar to P, K removal was similar for all N rates of 40 lb/a or greater plus K ranging from 23 to 27 lb/a. At the highest N, P, and K rate, apparent fertilizer recovery in the grain was 33% for N, 69% for P, and 40% for K.
After 60 years, pH of the surface soil was decreased up to 1 unit by N fertilization (Table 3). Nitrogen fertilization increased soil OM, Mn, and Fe concentrations while decreasing P, K, and Zn concentrations. Phosphorus fertilization increased P (both Bray-1 and Mehlich-3) and Zn concentrations with little effect on other soil properties. The original soil test P level (in 1961) was about 17 ppm (Bray-1), so annual applications of 40 lb/a P 2 O 5 tended to maintain or increase soil test P levels. Potassium fertilization only affected the K content of the soil.

Acknowledgment
The former International Plant Nutrition Institute and Servi-Tech Laboratories partially supported this research project.