Irrigated Corn Response to Long-Term Nitrogen and Phosphorus Fertilization

Long-term research shows that phosphorus (P) and nitrogen (N) fertilizer must be applied to optimize production of irrigated corn in western Kansas. In 2020, N applied alone increased yields by 85 bu/a, whereas P applied alone increased yields 10 bu/a. Nitrogen and P applied together increased yields up to 136 bu/a which is 11 bu/a less than the 10-year average of 147 bu/a. The application of 120 lb N/a (with highest P rate) produced 98% of maximum yield in 2020, which is greater than the 10-year average (2011–2020). The application of 80 instead of 40 lb P2O5/a increased average yields 1 bu/a. The 10-year average grain N content reached a maximum of 0.6 lb/bu while grain P content reached a maximum of 0.15 lb/bu (0.34 lb P2O5/bu). At the highest N and P rate, apparent fertilizer nitrogen recovery in the grain (AFNRg) was 43% and apparent fertilizer phosphorus recovery in the grain (AFPRg) was 63%. Nitrogen fertilization increased soil organic matter and decreased soil pH. Phosphorus fertilization at 40 lb/a P2O5 was not sufficient to maintain soil test levels.


Introduction
This study was initiated in 1961 to determine responses of continuous corn and grain sorghum grown under flood irrigation to N, P, and potassium (K) fertilization. The study is conducted on a Ulysses silt loam soil with an inherently high K content. No yield benefit to corn from K fertilization was observed in 30 years, and soil K levels remained high, so the K treatment was discontinued in 1992 and replaced with a higher P rate.

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 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. The treatments were changed in 1992; the K variable was replaced by a higher rate of P (80 lb/a P 2 O 5 ). All fertilizers were broadcast by hand in the spring and incorporated before planting. The soil is a Ulysses silt loam. The corn hybrids (Pioneer 1151XR, 2011Pioneer 0832, 2012Pioneer 0832, -2013Pioneer 1186AM, 2014Pioneer 35F48 AM1, 2015;Pioneer 1197Pioneer , 2016Pioneer 0801, 2017Pioneer 0801, -2018Pioneer 0339, 2019;and Mycogen 10Z29, 2020) were planted at about 32,000 seeds/a in late April or early May. Hail damaged the 2017, 2019, and 2020 crops. The corn is irrigated 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 15.5% moisture. Grain samples were collected at harvest, dried, Kansas State University Agricultural Experiment Station and Cooperative Extension Service ground, and analyzed for N and P concentrations. Grain N and P content (lb/bu) and removal (lb/a) were calculated. Apparent fertilizer N recovery 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 ). 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
Corn yields in 2020 were only 1% higher than the 10-year average (Table 1). Nitrogen alone increased yields 85 bu/a, whereas P alone increased yields 10 bu/a. However, N and P applied together increased corn yields up to 136 bu/a. Maximum yield was obtained with 200 lb/a N with 80 lb/a P 2 O 5. Corn yields in 2020 (averaged across all N rates) were 1 bu/a greater with 80 than with 40 lb/a P 2 O 5 .
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 0.6 lb/bu. Nitrogen removal (lb/a) was greater at the higher yield levels. Maximum N removal (121 lb/a), was attained with 200 lb N and 80 lb P 2 O 5 /a. At the highest N and P rate, AFNR g was 43% and AFPR g was 63%. Similar to N, the average P concentration increased with increased P rates but decreased with higher N rates. Grain P content (lb/bu) of about 0.15 lb P/bu (0.34 lb P 2 O 5 /bu) was greater at the highest P rate with low N rates. Grain P removal averaged 31 lb P/a at the highest yields.
After 60 years, pH of the surface soil was decreased up to 0.5 unit by N fertilization (Table 3). Nitrogen fertilization increased soil OM, Mn, and Fe concentrations while decreasing P and Zn concentrations. Phosphorus fertilization slightly increased OM and substantially 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 was not sufficient to maintain soil test P levels but 80 lb/a increased soil test P levels.

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