The range in soil environments has likely contributed to variation in the determination of that level of K availability at which no further yield increase to K is expected. Potassium response of soybean [Glycine max (L.) Merr.] from three different soil environments was studied in the field to determine if a single index of K availability could be found. Relationships between grain yield, R1‐R2 leaf K concentration, ammonium acetate exchangeable K, Ca, Mg, and water soluble K, Ca, and Mg were determined. Significant (P ⩽ 0.05) yield responses to added K were found at all three sites, as were increases in R1‐R2 leaf K. Water soluble K, K/Mg, and K/(Ca + Mg)1/2 were more stongly related to plant response than extractable K, K/Mg, or K/(K + Ca + Mg). All soils provided optimal K nutrition to soybeans at soil test levels ≤100 mg K kg−1 soil, as determined by neutral, molar ammonium acetate solution. Determinations of soil solution phase composition may improve our understanding of the role(s) of soil properties on divergent patterns of response to K nutrition.

Excessive fertilizer and manure phosphorus (P) inputs to soils elevates P in soil solution and surface runoff, which can lead to freshwater eutrophication. Runoff P can be related to soil test P and P sorption saturation, but these approaches are restricted to a limited range of soil types or are difficult to determine on a routine basis. The purpose of this study was to determine whether easily measurable soil characteristics were related to the soil phosphorus requirements (P(req), the amount of P sorbed at a particular solution P level). The P(req) was determined for 18 chemically diverse soils from sorption isotherm data (corrected for native sorbed P) and was found to be highly correlated to the sum of oxalate-extractable Al and Fe (R2 > 0.90). Native sorbed P, also determined from oxalate extraction, was subtracted from the P(req) to determine soil phosphorus limits (PL, the amount of P that can be added to soil to reach P(req)). Using this approach, the PL to reach 0.2 mg P L(-1) in solution ranged between -92 and 253 mg P kg(-1). Negative values identified soils with surplus P, while positive values showed soils with P deficiency. The results showed that P, Al, and Fe in oxalate extracts of soils held promise for determining PL to reach up to 10 mg P L(-1) in solution (leading to potential runoff from many soils). The soil oxalate extraction test could be integrated into existing best management practices for improving soil fertility and protecting water quality.

Few studies have examined the water quality impact of manure use in no-tillage systems. A lysimeter study in continuous corn (Zea mays L.) was performed on Maury silt loam (fine, mixed, semiactive, mesic Typic Paleudalf) to evaluate the effect(s) of tillage (no-till [NT] and chisel-disk [CD]), nitrogen fertilizer rate (0 and 168 kg N ha(-1)), and dairy manure application timing (none, spring, fall, or fall plus spring) on NO3-N, atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), and alachlor [2-chloro-2'-6'-diethyl-N-(methoxymethyl)acetanilide] concentrations in leachate collected at a 90-cm depth. Herbicides were highest immediately after application, declining to less than 4 mug L(-1) in about two months. Manure and manure timing by tillage interactions had little effect on leachate herbicides; rather, the data suggest that macropores rapidly transmitted atrazine and alachlor through the soil. Tillage usually did not significantly affect leachate NO3-N, but no-tillage tended to cause higher NO(3)-N. Manuring caused higher NO3-N concentrations; spring manuring had more impact than fall, but fall manure contained about 78% of the N found in spring manure. Nitrate under spring "only fertilizer" treatment exceeded 10 mg L(-1) 38% of the time, compared with 15% for spring only manure treatment. After three years, manured soil leachate NO3-N exceeded that for soil receiving only N fertilizer. Soil profile (90 cm) NO3-N after corn harvest exceeding 22 kg N ha(-1) was associated with winter leachate NO3-N greater than 10 mg N L(-1). Manure can be used effectively in conservation tillage systems on this and similar soils. Accounting for all N inputs, including previous manure applications, will be important.

A 3‐yr N‐source study was conducted on a site where urea was known to be less effective than NH4NO3 (AN) for no‐till corn (Zea mays L.) production. The purpose of the study was to compare two urea‐containing N sources thought to have lower N loss potential than AN and urea at two rates and times of application. Sources evaluated were urea, AN, Tennessee Valley Authority (TVA) experimental urea nitric phosphate (UNP) and TVA experimental urea ammonium phosphate (UAP). The soil was a Pope (coarse‐loamy, mixed, mesic Fluventic Dystrochrept), which is an acid soil with a C.E.C. of 12.1 cmol/kg and organic matter content of 3.7%. Nitrogen rates of 80 and 160 lb/acre were broadcast either at planting or 4 to 5 wk after planting. Experimental design was a randomized block with three replications. Results showed that AN and the TVA experimental materials performed comparably and that both were better than urea in grain yield and N content of total dry matter. Urea performed as well as the other sources in total dry matter accumulation. Delayed application of N increased grain yields from all N sources when compared with application at planting.

Utilization of seed proteins from cotton (Gossypium hirsutum L.) as an ingredient of food products for nonruminant animals may provide producers with a salable seed commodity in addition to oil and meal. It was important to confirm and more fully establish the effects of N fertilization on cottonseed protein concentration and protein quality (amino acid composition). In order to achieve this, seed was collected in 1976 from a N fertility experiment which had been established with the same N rates the previous 2 years. Cotton (‘DES 56’ was grown on a Dundee silt loam soil, a fine‐silt, mixed, thermic, Aeric Ochraqualfs, with four replications. The experimental design was a split‐plot with N rates (67, 90, 112, 134, and 157 kg ha‐1) as the main plots and time of application (preplant, 1/2 preplant‐1/2 sidedressed, and sidedressed) as subplots (the same plots received the same treatment each year). We measured yield of seed cotton, seed N concentration and amino acid composition. In 1976 the higher N rates increased seed N concentrations from 6.05% N up to 6.60% N, whereas time of application had no effect. Application of N decreased lysine and increased arginine concentration, whereas histidine, serine, tyrosine and phenylalanine were affected by time of application. Glycine was affected by both rate and time of application. Correlation of seed N concentration with amino acid concentration showed that arginine, cysteine and proline were positively related and that lysine, histidine, and threonine were negatively related to seed N. The effects produced by application of fertilizer N to cotton in a field situation result in seed N and amino acid concentration similar to results reported previously in an unusual field situation and allow for better prediction of N effects on cottonseed composition.