Long‐Term Phosphorus and Potassium Application to Corn on Coastal Plain Soils

The effect of long‐term fertilizer applications on soil test levels, yield, and crop removal depends on soil texture and mineralogy. The objectives of this study were to measure the fluctuations in soil P and K in soils of varying texture and to evaluate the usefulness of the Mehlich‐1 soil extractant in predicting yield response. A field experiment was conducted for 11 yr on two soils: a Tifton loamy sand (fine‐loamy, siliceous, thermic Plinthic Kandiudults) and a Greenville sandy clay loam (clayey, kaolinitic, thermic Rhodic Kandiudults). Phosphorus treatments were 0, 92,183, and 275 lb P 2 O 5 lb/acre in the first year followed by annual applications of 0,37,73, and 0 lb/acre, respectively. Potassium treatments were 0, 48, 96, and 144 lb K 2 O/acre in the first year followed by annual applications of 0, 48, 96, and 0 lb/acre, respectively. Soil samples (0–6 in.) were collected each year at tasseling of the corn ( Zea mays L.) crop, P and K concentrations were determined in soil and grain samples, and yield was measured. Initial application rates influenced initial soil test levels, and annual application rates affected the rate of change of soil test levels with time. All significant yield increases on the Greenville soil were due to P application. Phosphorus moves rapidly from solution into less available forms in the Greenville soil due to the soil's high P fixing capacity. Yield increases on the Tifton soil resulted from K applications, presumably due to greater leaching losses on the sandier textured soil. The Mehlich‐1 extractant provides a questionable measure of soil P availability for corn. Particularly in areas where P runoff potential is high, P fertilizer should not be applied to irrigated corn in the Southeast unless the Mehlich 1 extractable soil P concentration ≤ 12 lb/acre. Data from this long‐term study show that accounting for soil K below the plow layer may improve the use of Mehlich‐1 extractant as a measure of K availability for corn. Research Question Fertilizer applications can result in buildup or decline of soil nutrients over the long term. This study was conducted to determine how long term P and K applications to corn affect soil nutrient levels, corn yield, nutrient removal in the grain, and nutrient storage in the subsoil of Coastal Plain soils of varying texture. Fertilizer recommendations from Georgia and Alabama based on Mehlich‐1 soil test levels were evaluated based on accompanying yield response. Literature Summary Fertilizer recommendations are based on philosophies of replacement of nutrients removed by the crop, maintenance of initial soil test levels, or building up soil nutrient levels. Environmental effects of excessive fertilization are stimulating new interest in establishing fertilizer recommendations that maximize economic yield while minimizing potential environmental hazards. Previous research has reported soil critical levels, maintenance fertilization rates, and yield response on various soil types; however, the influence of soil texture and subsoil storage of nutrients has largely been ignored in the development of state soil testing programs. Study Description A field experiment was conducted with corn for 11 yr on a Tifton loamy sand and a Greenville sandy clay loam. Phosphorus treatments were 0, 92, 183, and 275 lb P 2 O 5 /acre in the first year followed by annual applications of 0, 37, 73, and 0 lb/acre, respectively. Potassium treatments were 0, 48, 96, and 144 lb K 2 O/acre in the first yr followed by annual applications of 0, 48, 96, and 0 lb/acre, respectively. Soil samples were collected from the plow layer each year and were collected to 36 in. in years 1, 7, 9, and 10. Yield was measured, and grain samples were analyzed for P and K content in year 11. Applied Questions How does soil texture affect buildup/decline of soil P and K and fertilizer recommendations for soil maintenance? Soil P level in the plow layer decreased in the Greenville sandy clay loam, regardless of treatment. On the Tifton loamy sand, the 183‐73 treatment increased soil P level, the 92‐37 treatment maintained soil P, and the 0‐0 and 275‐0 treatments reduced soil P levels. More P was required for maintenance of soil P levels in the Greenville soil (188 lb P 2 O 5 /acre per year) than in the Tifton soil (44 lb P 2 O 5 /acre per year) due to the higher P fixation capacity of the Greenville soil. Soil K level was diminished in the 0‐0 and 144‐0 treatments of both soils. However, the 48‐48 and 96‐96 treatments increased soil K concentration of the Tifton soil and maintained soil K levels in the Greenville soil, due to differences in initial soil test levels. On the other hand, a higher K application rate was required for maintenance of soil K level on the Tifton soil (49 lb K 2 O/acre per year) than on the Greenville soil (36 lb K 2 O/acre per year), since the Tifton soil has greater K leaching losses due to its sandier texture. Are current P and K fertilizer recommendations for irrigated corn based on Mehlich‐1 soil test levels maximizing agricultural sustainability in the Southeast? Georgia and Alabama would have recommended P fertilizer in every year of this study at both locations; however, yield response only occurred in years 9 and 10 on the Greenville soil. The two location‐years that did get a P yield response have the distinction of fitting the Very Low soil classification category that Alabama uses in its fertilizer recommendations. This Very Low category is defined as having 0–12 lb P/acre. With agricultural P runoff concerns increasing and competition from other countries mounting, it is vital that farmers do not waste money on unnecessary P fertilization. Georgia and Alabama both would have recommended K fertilizer for irrigated corn in every year and location of this test, although the only yield responses to K occurred in years 9 and 10 on the Tifton soil. In these 2 yr, Mehlich‐1 soil K levels were 27 and 31 lb/acre; however, in four out of eight of the previous years soil K measurement was similarly low (ranging from 29–31). Therefore, two out of six cases with Mehlich‐1 soil K from 27 to 31 lb/acre responded to fertilizer K. To what extent does the subsoil provide and store nutrients for crop production? Long‐term fertilizer applications resulted in changes in soil P and K concentrations in the subsoils of both soils studied. The 96‐96 K treatment increased soil K level and the 0‐0 treatment reduced soil K level to a depth of 24 in. The soil P level was altered by P treatments to a depth of 12 in. in both soils. Recommendations Storage and uptake of K occur in Coastal Plain soils to depths of 24 in. or more. The K reserves below the plow depth should not be ignored in evaluating the amount of available K in the soil. Accounting for soil K stored below the plow layer may improve the use of Mehlich‐1 extractant in making K fertilizer recommendations for corn. Soil P can be determined satisfactorily by sampling the plow layer since P is concentrated in the soil surface, but the Mehlich‐1 extractant provides a questionable measure of soil P availability for corn. Phosphorus fertilizer is currently recommended for irrigated corn in Georgia and Alabama when Mehlich‐1 soil P level is too high to produce a yield response. Particularly in areas where P runoff potential is high, P fertilizer should not be applied to irrigated corn in the Southeast unless the Mehlich‐1 extractable soil P concentration 5 12 lb/acre.