Long‐Term P and K Applications: I. Effect on Soil Test Incline and Decline Rates and Critical Soil Test Levels

Abstract

Phosphorus and K fertilization during the last 30 to 40 yr has led to increased levels of soil test P (STP) and K (STK). Soil test incline rates with continued fertilizer application, decline rates in the absence of fertilizer application, and the critical soil test level for yield response have to be known for long-term economically and environmentally sound crop production. A 20-yr study was conducted at two sites to determine: (i) the incline rates of STP and STK during a 12-yr application period, (ii) the decline rate of STP during an 8-yr residual period and during a 20- yr period on unfertilized plots, and (iii) the critical soil test value corresponding to no yield response to broadcast P or K. Initial Bray P 1 and exchangeable K soil tests were 22 and 150 ppm, respectively, for a Webster clay loam (fine-loamy, mixed, superactive, mesic Typic Endoaquolls) located at Waseca, MN, and 10 and 228 ppm, respectively, for an Aastad clay loam (fine-loamy, mixed, superactive Pachic Udic Haploboroll) located at Morris, MN. Phosphorus and K were broadcast annually for 12 yr at rates of 0, 50, and 100 lb P 2 O 5 /acre and 0, 50, and 100 lb K 2 O/acre. For the Webster soil, STP increased by 0.42 and 1.92 ppm/yr with the 50- and 100-lb P 2 O 5 rates, respectively, and by 0.69 and 2.49 ppm/yr for the Aastad soil Decline rates of STP ranged from as high as 3.3 ppm/yr at an initial STP of 40 ppm to 0.4 ppm/yr at initial STP of ≤10 ppm. Corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields were not increased by broadcast applications of P when STP was >13 ppm on the Webster soil and >19 ppm on the Aastad soil. Year-to-year STK variability was very high and precluded calculation of incline or decline rates or the critical STK concentration. These 20-yr results suggest that crop producers with very high STP levels could refrain from applying P and should use periodic soil testing to monitor the drawdown of STP. Soil test P can be maintained at 18 to 20 ppm for optimum profitability by applications of 40 to 50 lb P 2 O 5 /acre per yr.