Groundwater provides about 30% of water requirements in Ontario, but farm families depend almost entirely on private wells. Major potential contaminants on farms are nitrate (NO ), pathogenic micro-organisms, pesticides and petroleum derivatives. A survey of farm 3 drinking water wells was conducted throughout the Province of Ontario, Canada, in 1991 and 1992 and tested for these contaminants. Main objectives of the survey were to determine the quality and safety of drinking water for farm families, and determine the effect of agricultural management on groundwater quality at a provincial scale.Four farm wells were chosen in each township where >50% of the land area was used for agricultural production. Elsewhere one well per township was usually sampled. Within each township the types of farming activity and dominant soils were additional criteria for selection.The network comprised 1292 of the estimated 500,000 water-wells in Ontario, and the study conformed to a stratified random survey. A subset of 160 wells, chosen by farm type, soil, and the presence or absence of a fuel storage tank, was investigated for the presence of petroleum derivatives: benzene, toluene, ethyl benzene, and xylene.About 40% of farm wells tested contained one or more of the target contaminants above the maximum acceptable concentration; 34% of wells had more than the maximum acceptable number of coliform bacteria, 14% contained NO -N concentrations above 10 mg L limit and 3 -1 about 7% were contaminated with both bacteria and NO . Only six wells contained pesticide 3 residues above the interim maximum acceptable concentration (IMAC), but pesticides were detected in 7% of wells in winter and in 11% in summer. No wells contained detectable petroleum derivatives. These results for NO contamination were not significantly different from 3 3 those reported for a survey of Ontario wells for the period 1950-1954, but the frequency of contamination by Escherichia coli was greater in the present study.None of the point sources investigated contributed significantly to the NO contamination. The 3 percentage of wells contaminated by coliform bacteria decreased significantly with increasing separation of the well from the feedlot or exercise yard on livestock farms. A full statistical model including the type of well construction, depth, age and soil hydrologic group was developed to describe the frequency of NO contamination.3

Contamination of groundwater under agricultural land by NO3 is influenced by the kind of farming system. One possible method of selecting farming systems that result in less NO3 leaching is to calculate whole farm N budgets, that are simplified by assuming soil‐N remains constant from one cycle of a rotation to the next. This method was applied to two model crop rotations using average crop yield data for two regions of Ontario, and to a cash‐crop farm and a dairy farm using information on purchases, sales, and crop yields, for these farms. The model rotations were corn (Zea mays L.)‐soybean [Glycine max (L.) Merr.]‐wheat (Triticum aestivum L.) and corn‐soybean‐wheat‐hay (mixture of timothy, Phleum pratense L. and alfalfa, Medicago sativa L.)‐hay‐hay. Atmospheric deposition (18.4 kg N ha−1 yr) was obtained by literature review. Symbiotic N2 fixation by legume crops with different yields was estimated from regression equations. A net surplus in the N balance was converted to maximum mean NO3‐N concentration in groundwater by assuming a groundwater recharge rate of 160 mm yr−1, and no denitrification. Predicted NO3‐N concentrations in leachate for the model corn‐soybean‐wheat rotation were greater for southwestern Ontario (22.4 mg L−1) than western Ontario (8.5 mg L−1), probably because more fertilizer was recommended in the southwest. Including hay in the model rotation increased the amount of N leached by a factor of two in western Ontario, but only by 9% in the southwest. Predicted NO3‐N concentration in groundwater for the cash crop farm was 6.7 mg L−1, compared with an average measured value of 9.5 mg L−1 in the tile drainage water. For the dairy farm the predicted value was 58 mg L−1 and a measured value was not available. The simplified N balance method provided useful estimates of potential NO3 leaching losses even though it relied on some major assumptions. A major uncertainty was atmospheric deposition of ammonia volatilized from on‐farm sources. Denitrification could be as much as 62 kg N ha−1 yr−1 under continuous production of grain corn, based on differences between N present after harvest and amount of N leached.

In a maximum yield experiment in Ontario, Canada, response of maize (Zea mays L.) to greater than recommended P fertilizer applications seemed to result mainly from improved P nutrition of seedlings. A 2-yr study was conducted to determine the effect of P nutrition during the seedling stage on final yield. An outdoor, hydroponic system was used to vary P nutrition during growth and provide adequate levels of all other nutrients and water. Plants receiving the greatest or an intermediate P concentration solution bad P concentrations of 7.2 and 4.4 g kg-•, respectively, at the 6-leaf stage, whereas plants receiving the least P concentration solution had a shoot P concentration of 2.8 g kg-•. Plants that received the intermediate or least P solutions until the 6-leaf stage but then received the greatest P solution to maturity attained shoot P concentrations of at least 5.0 g kg-• 8 d after increasing the P solution concentration. Grain yield was greater on treatments receiving the greatest P solution from the beginning than from the 6-leaf stage (8.2 vs. 7.2 t ha-•), mainly because of a greater kernel number. Kernel number was most sensitive to P nutrition during the period from planting to the 6-leaf stage. A shoot P concentration of at least 5.0 g kg-• at the 6-leaf stage was required to obtain maximum yield.

No abstract

. 1987. Effects of nutrient availability and subsoil bulk density on corn yield and nutrient absorption.Can. J. Soil Sci. 67:281-292.A 4-yr study was conducted to determine the extent to which soil fertility and subsoil bulk density limit corn yields at the Elora Research Station. In one treatrnent, soil was replaced to a depth of I m with a highly fertilized soil:peat:perlite artificial medium (AM). Two other main treatments consisted of the natural soil with either a recom-. mended fertility (RF) or a very high fenility (vHF) treatment. plots were irrigated using a trickle system . Corn (Zea mays L.) yields on the artificial medium were the same as those on the VHF treatment in all 4 yr. This was unexpected because of a high subsoil bulk density (1.5 Mg m-r in the Bm and 1.8 Mg m-: in the Ck horizons). Grain dry matter yields on the VHF treatment were considerably higher than those on the RF treafinent (7.8 compared to 5.3 t 1tu-t) and were higher than the average yield (6.1 t ha-t) on the best treatrnent in a previous long-term fertility experiment at Elora. The response appeared to be due to all three macronutrients. Some response to N and K was expected, but the response to P, which was a major portion of the total response, was unexpected. It is concluded that subsoil bulk density is not a significant limitation on this soil when adequate water and nutrients are provided. The marked response to phosphorus application above recommended rates may indicate a need to re-evaluate the fertility requirements of high-yielding corn hybrids. (Miller 1981(Miller , 1983