Agriculture and Environment, 4 (1978/1979) 207--225 207 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

NITRATE POLLUTION OF GROUNDWATER FROM FARM USE OF NITROGEN FERTILIZERS - - A REVIEW

BIJAY SINGH and G.S. SEKHON

Department of Soils, Punjab Agricultural University, Ludhiana 141004 (India)

(Accepted 5 September 1978)

ABSTRACT

Singh, B. and Sekhon, G.S., 1978/1979. Nitrate pollution of groundwater from farm use of nitrogen fertilizers -- a review. Agric. Environm., 4: 207--225.

The fears that the use of fertilizer nitrogen on farms is contributing considerably to nitrate pollution of groundwater have increased in the past few years. Investigations have indicated that nitrate is accumulating in the shallow groundwaters of some irrigated areas with intensive agriculture using fertilizers. In certain areas, natural geologic deposits of nitrate contribute a large percentage of nitrate leached to groundwater formations. Soil organic matter, animal wastes and plant residues also contribute, but their relative inputs are difficult to determine. The amount of fertilizer nitrogen leaching as nitrate below the root zone and the stability of nitrate in the unsaturated zone and in aquifers are the factors that determine the extent of nitrate pollution of groundwater from fertilizer N. The amount and distribution of rain and irrigation affect the leaching of nitrate below the root zone. However, exactly to what extent nitrate leaching occurs is determined by the amount of water percolating down the profile, which in turn is affected by growing plants. Vegetative cover is the most important factor affecting nitrate leaching by utilizing water (as transpiration) and fertilizer nitrogen. Nitrate in the unsaturated zone and aquifers is generally stable because there is insufficient supply of oxidizable carbon for denitrifiers to utilize. But the possibility of such an occurrence is not ruled out. For com- puting nitrogen application rates which can ensure both optimum crop yields and permis- sible nitrate leaching loss, the fertilizer efficiency factor needs careful consideration. Maximizing the efficiency of fertilizer nitrogen can reduce the risk of nitrate pollution from fertilizers.

INTRODUCTION

The groundwater bodies of any area reach a chemical equilibrium with the natural conditions of the area. Man's activities tend to upset this equilibrium and generally result in an increased flux of nutrients into the water bodies. Application of nitrogen fertilizers and the frequent increase of nitrate nitrogen (NO3--N) in groundwater has recently focused attention on the deep percolation of soluble forms of nitrogen. That a portion of fertilizer N is lost from the soil root zone is an undeniable fact. Improved soil fertility nearly always means increased soil nitrogen levels and greater potential for

208

nitrate to escape in soil leachates. However, the extent o f nitrate leaching and possible consequences of these losses are issues of considerable controversy. Comments by Viets and Hageman (1971) regarding nitrate enrichment of groundwater indicate the seriousness of the problem:

"The rate of water recharge from deep percolation is so slow that the possible nitrate pol lu t ion of aquifers from our modern technology will take decades. However, once nitrate gets into the aquifer, decades will be required to replace the water with l o w nitrate water. Fifty to 100 years might be required to establish a time trend, con- s idering the heterogeneity of aquifers. By the time the trend was established, a dangerous situation coultl be in the making that cou ld n o t be corrected in a time shorter than it took to create it."

According to Parr (1973), there is very little indication how far we are already into this time trend for certain aquifers.

Nitrate originating from fertilizer N applied in the surface soil layer has to pass through the soil profile to the aquifer to pollute the latter. Since the factors that affect deep percolation of nitrate in the profile act differently at different depths, the major objective of this paper is to review the results of numerous investigations on the transport o f NO3--N in different soil zones. A discussion of the recent evidence of enrichment of groundwater by fertilizer N and the relation between fertilizer N use efficiency and nitrate leaching is pert inent to this objective.

EVIDENCE OF NITRATE ENRICHMENT OF GROUNDWATER BY FERTILIZER NITROGEN

Since ecologists (Commoner, 1968; VoUenweider, 1968) have expressed their concern about fertilizers being serious pollutants of the environment, the problem of N loss from the soil profile has been reviewed by many scientists including Wadleigh (1968), Miller and Nap (1971), Tomlinson (1971), Viets and Hageman (1971), G~ichter and Furrer (1972), Thomas (1972), Amberger (1972), Czeratzki (1972, 1973), Cooke (1973) and in the reports of EPA, FAO, OECD, IIQE and ECE (Anonymous, 1971, 1972, 1973, 1974a, b). These reviewers have admit ted that at present there is a paucity of reliable experimental data to evaluate the real or potential contri- but ion of fertilizers to nitrate pollution of natural waters. In fact, only recently have experiments specificially designed to estimate loss o f fertilizer N to groundwater been carried out. Not only have the studies been under way for too short a time, but also they are too few in number to assess ade- quately the magnitude of nitrate loss in different situations. The pert inent data obtained so far are discussed in the following subsections.

Pollution of shallow groundwaters from fertilizer nitrogen

Direct evidence that fertilizers can pollute shallow groundwater has been obtained by Miller and Nap (1971) in an experiment at Guelph, Canada.

209

Urea was applied to corn at the rate of 224 kg N/ha in a Guelph loam in the spring of 1965. This was excess N, since the yield of corn was not affected. In December 1966 the groundwater under the fertilized plot contained 66 ppm NO3--N, well above the generally accepted nitrate standard (10 ppm NO3--N) for potable water. In a similar experiment Peele and GiUingham (1972) applied ammonium nitrate to an uncropped Dunbar sandy loam at the rates of 0, 112.1,336.3 and 672.6 kg N/ha in June 1971. The ground- water NO3--N concentration under these treatments in August 1972 was 21.4, 32.8, 101.3 and 88.5 ppm, respectively. Depth to water table was only about 1 m.

Chemical analysis of groundwater samples from Illinois (Walker, 1969) indicated tha~ nitrate pollution of surficial aquifers was widespread, especially in rural parts of the state. Many samples contained 22.6 to 225.8 ppm NO3--N. In Washington county of Southern Illinois, samples from 263 farm wells had a median NO3--N concentration of 32.3 ppm, and more than 73% of the wells had a NO3--N concentration exceeding the critical level (Smith et al., 1970). The primary sources of excessive nitrates in most of the groundwater supplies were found to be animal and human wastes and N fertilizers (Walker et al., 1972).

Convincing evidence that nitrate originating from fertilizers accumulates in shallow groundwaters is available from the studies in Grover City, Arroya Grande Basin of California (Stout and Burau, 1967), in Fresno Clovis area of the San Joaquin valley of California {Nightingale, 1970), and in the central coastal region of Israel (Gruener and Shuval, 1970). Sturrn and Bibo (1965) tested samples of groundwater in the Rheingau, an important wine growing area in Germany, and discovered that water beneath vineyards had a NO3--N concentration of over 9 ppm. Groundwater under arable land and forest yielded values of only 2.3 and 1.1 ppm, respectively. Schwille (1969) also reported high nitrate levels in wells in the Moselle valley (Germany) and attributed these mainly to N fertilizers used in vineyards. Vines do not have very extensive rooting system and there are high leaching losses from soils of vineyards (Pfaff, 1960).

Nightingale (1972) applied probability analysis to data pertaining to crop N fertilization rates, soil NO3--N concentrations and groundwater NO3--N concentrations in an intensively irrigated area within Fresno county, California. The results indicated that soil NO3--N below the root zone (below 4.27 m) and groundwater NO3--N concentrations are closely related. Variations were associated with types of crops and N management practices. Following a similar approach, Singh and Sekhon (1976a) found that in the Ludhiana district of India, where N fertilizer rates are highest in the country, 90% of the well water samples contained less than 10 ppm NO3--N. The nitrate concentration of well water decreased significantly with depth to water table and correlated positively with the amount of fertilizer N added per unit area per year.

210

Pollution hazards from N fertilizers in irrigated regions

Viets and Hageman (1971) reviewed several experiments and concluded, like Stout and Burau (1967), that all fertile soils located in a humid or in an irrigated region contribute nitrates to groundwater. In India, Singh and Sekhon (1976a) inferred that NO3--N tends to reach groundwater during the season of monsoon rains. In New Mexico, USA, Taylor and Bigbee (1973) observed that in irrigated and fertilizer N treated areas the NO3--N content of aquifers fluctuated with the irrigation season.

Evidence cited by Olson et al. (1973) and Muir et al. (1973, 1976) suggests that, except at sites of intensive irrigation development on very sandy softs and with irrigated crop production in valleys having a shallow water table, the quality of Nebraska groundwaters has not deteriorated as a result of agricultural use of N fertilizers. Statewide, during a 10-year period, groundwater NO3--N concentration increased on average by 24%, when fertilizer N consumption quadrupled; but there was also a 50% increase in the irrigated area.

Stewart et al. (1967, 1968} analyzed soil core samples from the South Platte Valley of Colorado and found that the mean NO3--N concentration to a depth of 6.7 m as related to land use was: alfalfa, 70; cultivated dry land, 233; irrigated fields (excluding alfalfa), 450, and feedlots, 1282 kg/ha. They estimated that 28 to 33 kg N/ha/y were lost to the water table from irrigated fields. Ward (1970) reported on five problem areas of California where, from 1953 to 1968, the nitrate content of water pumped for domestic use exceeded the safe level. Enough evidence was obtained in at least two areas to conculde that nitrate enrichment was related to the increased use of fertilizer N under irrigation.

Fertilizers do not pollute under all conditions

Inferences drawn from the foregoing studies are not applicable to all situa- tions. There are several reports which suggest a negligible effect of fertilizers in polluting groundwater. For example, Smith (1965, 1967) after analyzing about 6,000 rural water supply samples from Missouri obtained a non-signifi- cant correlation coefficient of --0.029 between the amount of fertilizer N used and the nitrate content of groundwater. Hedlin (1971) in Manitoba, Canada, Seim et al. (1972) and Muir et al. (1973) in Nebraska, and Gilliam et al. (1974) in North Carolina Coastal Plain also obtained negative data for contamination of groundwaters with NO3--N originating from fertilizers. Kolenbrander (1972) arrived at a similar conclusion by comparing the increase in concentration in chloride and nitrate in groundwater at various locations in The Netherlands. He calculated that a dressing of 100 kg C1-/ha/ yr can increase the C1- ion content in groundwater by a maximum of 9 mg C1-/1. But the increase since 1920 has been only 3 mg C1-/1. Concurrent with a total increase of 150 kg N/ha in the mean annual quantity of fertilizer N

211

applied during the period 1920--1967, the nitrate content of groundwater increased only by 0.37 mg NO3--N/1 and that increase was confined to only 33% of the wells. Groundwater was, however, extracted from a depth of 25 to 125 m.

In several cases where nitrate accumulates in groundwater, an intensive agriculture coexists with an urban and suburban population. At some places sources of NO3--N other than fertilizers could be contributing to a much more significant extent. These sources include treated and untreated sewage, animal wastes, food processing wastes, industrial effluents, biological N2 fixation, mineralization of soil organic N and movement of water through geological formations of high NOs--N content. Support for this thesis can be found in papers by Feth (1966), Stout and Burau (1967), Smith (1967), Stewart et al. (1968), Aldrich (1970), Viets (1970), Viets and Hageman (1971), and Boyce et al. (1976). Particularly stressing the role of organic N, Keeney and Gardner (1970) stated that if a soil contains 0.2% organic N, and 1--3% in a year is mineralized (Bremner, 1965), from 22.4 to 67.3 kg N/ha is released. They further stressed that the release is stimulated markedly by cultivation, and the high NO3--N levels in some aquifers may simply be the reflection of the onset of farming a century or more ago.

There is little doubt that the importance of non-agricultural sources of N will increase as nations become more urbanized. In view of the reports discussed in this section, comments by Parr (1973) seem appropriate. He states that

"The time has come for agriculturists to assume a less defensive and indeed, more positive attitude in their approach to the plant nutrient--water quality issue. We may have no alternative but to accept the fact that in some cases agriculture is contributing to environmental pollution."

Agricultural scientists are thus facing a dual challange to produce maximum crop yields and to prevent pollution of soil and water resources. This can be achieved by gaining a better insight into the processes and mechanisms involved in the transformations and transport of nitrogen, not only in the soil root zone but also in the unsaturated zone (the layer between the root zone and the aquifer) and in tl~e aquifer.

NITRATE LEACHING IN THE SOIL PROFILE AND TRANSPORT TO GROUNDWATER

Water is the solvent and the transporter of NOs--N from the surface soil layers to the unsaturated zone and aquifers. The penetration of rain or irrigation water into the soil results in soil--water interactions that involve not only the chemistry, physics and biology of the soil, but also the effects of both soil air and atmospheric air. Evapotranspirational loss of water is also one of the main processes that cause groundwater to differ from the water originally falling on or applied to the soil.

212

From the environmental point of view, it is of great interest to know the fate of NOs--N lost below the root zone. The total path of the nitrate ions from their origin in the surface layers to the aquifer, can be divided into three distinct zones, viz. roo t zone, unsaturated zone and saturated zone or aquifer. Each zone plays an important role in determining the amount of NO3--N reaching the groundwater.

Root zone

The root zone may be defined as the maximum depth to which roots penetrate into the soil. The thickness of the root zone depends on the kind of crop, soil and climate, and can be considered constant for a particular cropping system in an area.

Plants extract both water and soluble nutrients including nitrate from the soil solution in the root zone. To maintain an adequate supply of nutrients, fertilizers are applied in surface layers of this zone. However, a fraction of the applied fertilizer N leaves the root zone with percolating water. Part of it ult imately reaches the groundwater. An estimate of the quanti ty of NO3--N leaving the root zone by leaching is thus a measure of the pollution potential associated with N fertilizers.

The concentrat ion of NO3--N in the soil solution as it leaves the root zone is not necessarily the same as that in the soil solution when it reaches the groundwater. This is because of the mixing which goes on during the flow process. This latter phenomenon (dispersion) has been studied in detail theoretically (Gardner, 1965), bu t little application has been made to the determination of NO3--N concentration. The monograph Soil Nitrogen edited by Bartholomew and Clark (1965), discussed in detail the behaviour of nitrogen and nitrate in the root zone.

Downward movement of water and dissolved nitrate is controlled by gravity and differences in soil water potentials and chemical forces. The direction o f water movement is predominantly vertical. Because of dispersive and diffusive mechanisms (Biggar and Nielsen, 1967), nitrate movement through the zone of aeration (root zone + unsaturated zone) is slower than bulk fluid movement. This action is the result of water movement at dif- ferent rates through pores of different sizes. Warrick et al. (1971) found that maximum nitrate concentrat ion occurs at a depth above which the total water in the profile is just equal to the cumulative infiltration.

Two conditions are essential for nitrate to move below the root zone: the presence of nitrate and downward water movement. The latter condit ion is dependent on infiltration, and thus leaching can occur only when excess water is applied to the soil. For example, Olsen et al. (1970) observed that on irrigated sands, loss of NO3--N is closely related to the amount of irriga- tion and fertilizer N. Thirty centimetres of rain or irrigation water during the 5 weeks following application of NH4NO3 to the fallow Plainfield sand caused most of the NO3--N to move to a depth of 75 to 150 cm (Fig. 1).

213

0 0

0 u} 2 4 0

300

Soil 5 10 15 2.0

i i i

A \ s PT . ,sR 6 0

0 v

1 2 0 .E

180

NO3-N (ppm) 2 5 0 5 10 1.5 0

F P

5 10 15

A P R I L 10

1 9 6 9

t Fig. I. Movement of NO 3--N in a fallowed Plainfield sand over an 8-Month period following the application of N as NH4NO 3 on 1 August 1968. The plot area received 30 cm of rain or irrigation between 1 August and 9 September 1968, 14 cm of rain between 9 September and 28 October 1968, and 16 cm of precipitation between 28 October 1968 and 10 April 1969. o, 0 kg N/ha; a, 112 kg N/ha; a, 336 kg N/ha (adapted from Olsen et al., 1970).

Recovery of the applied N from the soil by analysis for the September sampling ranged from 80 to 96%. An additional 14 cm of rain from Septem- ber 9 to October 28 reduced N recovery in the profile to 26 to 46% and to about 25% at the end of 8 months with an additional 16 cm of precipitation. The low recovery values for the second and third sampling dates were apparently due to the leaching of NO3--N beyond a depth of 300 cm.

The amount and distribution of rainfall have a considerable effect on the translocation of NO3--N from the surface soil layers. Allison (1965) stressed the importance of considering precipitation and evapotranspiration together with the soil storage capacity for evaluating the potential for nitrate leaching. However, in examining the precipitation--evapotranspiration data, only current statistics are pertinent (Viets and Hageman, 1971). The long- term averages fail to explain nitrate accumulation below the root zone in situations where average rainfall is much less than average evapotranspira- tion.

Vegetation retards nitrate leaching from the root zone by absorbing nutrients and water. The absorption of nutrients by the crop precedes leaching by removing the ions from the soil solution and is a factor which should be considered when applying fertilizer several weeks prior to major absorption to row crops {Welch et al., 1970). Studies carried out by Gass (1969) using K'SNO3 revealed that the nitrate in the upper part of the pro- file is generally more available for absorption because the root systems of most plants decreases in density with increase in depth. Rooting habits of plants, in fact, exert a profound influence upon nitrate mobility in the root zone. Stewart et al. {1968) and Muir et al. (1976) observed that alfalfa with its deep rooting system is an effective scavenger of inorganic N that may

214

have accumulated under prior annual crops. Singh and Sekhon (1977) showed that maximum leaching loss of NO3--N occurred from crop rotations with heavily fertilized shallow rooted crops like potato. Wheat and maize, when grown in rotation, absorbed a large fraction of the applied N due to their deep and extensive rooting systems. Since maximum leaching o f NO3--N in the Punjab (India) occurred in the monsoon season (Singh and Sekhon, 1976a), Singh and Sekhon (1977) were able to show that, in the rainy season, growing deep rooted crops like maize instead of shallow rooted ones like groundnut and soybean can significantly reduce leaching of NO3--N below the root zone.

It had been shown in lysimeter trials that as a result of increased plant growth, NO3--N leaching losses were smaller in a N fertilized plot than in the PK control (Jung, 1972). Allison et al. (1959) showed that cropping with fertilization resulted in a much lower NO3--N loss than fallowing during a 5-year study period. Similarly Singh and Sekhon (1976c) were able to show that balanced application of N, P and K can significantly reduce the amount of unutilized nitrate in the root zone. Fig. 2 shows soil nitrate profiles for seven fertilizer treatments in two long-term experiments after the harvest of wheat. In one of the experiment (Fig. 2A), to each of the three treat- ments, equal amounts of N (120 kg N/ha) were applied to wheat and maize. When no P and K were applied, much NO3--N remained unutilized in the profile. The amount of unutilized NO3--N in PI3K2s treatment was moderate, but when P and K were applied at the rate of 26.2 kg P/ha and 24.9 kg K/ha,

Soi[ N O ~ - N ( p p m ) 0 ~.8 3.s , ~.4 , z2, , 9.0, ,~° .e .

0 I i I I 0 1.8 , 3;6 ,

!,

~ K

Fig. 2. Profile distribution of nitrate after wheat in April 1974 in various treatments of t w o long term fertility experiments. A. Experiment started in 1968--1969 with wheat-- maize rotation; e, N ~20PoK0; ×, N120PI~K:5; A, N120p26K:5. B. Experiment started in 1971--1972 with maize--wheat--eowpeas rotation; e, 150% NPK; X, 100% NPK; ~, 50% NPK; o, no NPK. (Adapted from Singh and Sekhon, 1976c).

215

there was little NO3--N in the profile down to 2 m depth, for leaching below the root zone. In the second experiment, the supply of N, P and K in the three t reatments was increased or decreased in a proport ionate manner from the recommended dose. This resulted in nitrate distribution patterns (Fig. 2B) similar to those of the control treatment, where no N was applied.

Singh and Sekhon (1976b) emphasized the role of proper coordination of split application of fertilizer N with water management. Data in Table I indicate that as the amount of irrigation was increased while its frequency decreased, more NO3--N was leached to deeper soil layers. With lighter and more frequent irrigation schedules, NO3--N from only the third or fourth splits of fertilizer N remained unutilized in the root zone. These results pointed out the need to delay large nitrogen applications until the growing crop can utilize it and to avoid irrigation when a large amount of NO3--N is present in the root zone.

It is generally held that NO3--N is not affected by the soil and behaves as though it was a tracer of water. This is particularly true of softs which are high in water content , medium to low in cation exchange a capacity, low in iron and aluminium oxides and have a pH near neutral (Thomas, 1970). Jurinak and Griffin (1972) have reported the possibility of nitrate retention by alkaline soils having free calcium carbonate. Singh and Sekhon (1978) have shown in column studies with disturbed softs that retardation in NO3--N leaching through calcareous softs is a function of the surface area of calcium carbonate.

Unsaturated z o n e

It has generally been assumed that nitrate in the unsaturated zone is stable because there is insufficient supply of oxidizable carbon for the denitrifiers to utilize (Viets and Hageman, 1971). Stewart et al. {1967) found a low populat ion of bacteria in this zone. The experiments of Foerster (1973) in a sandy soil confirmed that even with high application rates of energy-rich

TABLE I

Amount of NO 3--N expressed as percentage of total applied N, present in 180 cm soil profiles of different treatments under wheat (adapted from Singh and Sekhon, 1976b)

Irrigation Fertilizer I N application rate ~ (cm/irrigation) Single In 2 In 3 In 4

dose splits splits splits

5.5 14.5 17.9 34.1 47.4 7.5 29.5 21.3 32.1 32.0 9.5 39.4 44.3 53.5 88.0

A total of 150 kg N/ha and 40 cm irrigation was applied to each treatment.

216

farmyard manure, denitrffication at a depth of 6 m was not high enough to reduce, in a 3-year study period, the nitrate content of soil water below 22.6 ppm NO3--N. This is in agreement with results of Kimble et al. (1972) who found that energy for anaerobic microbial activity is limited in soil samples from layer below I m.

Pratt et al. (1972) and Adriano et al. (1972a) have computed under field conditions a 'not accounted for' (NAF) loss of nitrogen in balance sheets of several profiles to depths of 15 m or more. The NAF includes losses by denitrification, chemical decomposition of NO3, volatilization of NH3 and experimental errors. The NAF in the studied profiles ranged from --73% to 68% of the nitrogen input. The positive values indicate denitrification while negative ones suggest immobilization. It was found that in 15 out of 19 profiles, NAF was in the range of 0 to 60%, thus indicating a net loss via denitrification. This range in NAF is in good agreement with similar data of lysimeter experiments of MacVicar et al. (1950), Allison (1955) and Owens (1960).

Investigations on nitrate concentration in cores have usually found large variations with depth (Stout and Burau, 1967; Stewart et al., 1967; Adriano et al., 1972a,b). In studies carried out at University of Calfornia Agricultural Experiment Station (Rible and Pratt, 1977), much variation was found in NO3--N concentration in the soil water of the unsaturated zone, not only among sites, but also among samples taken from the same site. Statistical treatment of the soil nitrate concentrations revealed that the individual sites were so variable that seventeen 15.25 m cores per site would be required to give a desired reliability to only 75% of the site average reported. Nitrate content expressed on the basis of dry weight of soil is expected to differ markedly because soil texture and water content vary widely.

Pratt and his associates (Pratt et al. 1972; Adriano et al., 1972a,b) investigated in detail the fate of NO3--N in the unsaturated zone. They ob- tained data on the concentration of NO3--N in the unsaturated zone in rela- tion to agricultural practices by collecting soil samples to a depth of 15.25 m at several irrigated farm locations in coastal Southern and Central California, and in the San Joaquin valley. Data on previous fertilization, yields and quantity and quality of irrigation were collected at 40 sites to enable the cal- culation of transit times and the amounts of NO3--N that had moved past the root zone at those sites. The summary of the results presented in Table II indicates that the wide range in percolation volumes contributed appre- ciably to the highly variable amounts of NO3--N moving past the root zone annually. The average values for the four cropping systems indicated differences based on both location and crop type. The deciduous tree crop group, for example, is markedly lower in average NO3--N concentration and the amount of NO3--N percolating past the root zone than the coastal citrus group, where average amounts of applied N and irrigation water were higher. Average transit times for surface to 15.25 m depth for all sites was 10 years, the cropping history period chosen for the project. Three main factors were

TABLE II

Summary of the data on NOs--N in the unsaturated zone by site averages and ranges (adapted from Rible and Pratt, 1977)

Site Number NO s--N in soil water of sites of unsaturated zone

(ppm) Average Range

NOs--N percolating Percolation volume Transit time from root zone (ha-cm/year) (year/15.25 m) (kg/ha/year) Average Range Average Range Average Range

Coastal sites Citrus 14 67 9--220 Field vegetables 6 31 1 6 - - 3 6

Inland sites Deciduous 7 18 10--44 Field vegetables 13 29 9--51

All sites 40 40 9--220

159 59--353 13 2--28 9 4--95 174 73--320 24 9--38 8 4--22

59 28--118 15 6 - - 2 4 9 7--21 83 28--191 12 5--23 12 7--30

119 28--353 14 2--38 10 4--95

t ~

218

stated to govern NO3--N concentrat ion in the unsaturated zone: the difference between N inputs and crop removal; drainage volume; and soil profile characteristics that apparently control denitrification.

S a t u r a t e d z o n e

All voids are filled with water in this zone and the rate and direction of water movement depends on the hydraulic and transmissive characters of the basin sediments dnd on the hydraulic gradient. Rates of lateral movement may vary from a few decimetres a year to several kilometres a month (Biggar and Corey, 1969). Mean residence t ime of groundwater may vary from 30 days (Carlson, 1964) in a humid region (New Jersey) to 186 years (Duke, 1967) in semi-arid regions (Ogallala aquifer in Colorado). Thus pollution and depollut ion of the New Jersey basin can occur 2265 times faster than in the Ogallala formations. According to Ayers and Branson {1973), hydrological responses are in general damped in space and time. Consequently the presence and movement of NO3--N is associated with slow movement of water and the high nitrate concentrat ion persisting in groundwater of some areas must be related directly to a N source in the overlying soft.

The range of NO3--N content of groundwaters is frequently very broad, even within a limited geographical area, such as the one in Northeastern Colorado from where Viets (1971 a) has reported the NO3--N concentrations (ppm) of water tables as follows: water tables underlying virgin grasslands, 0.1--19; those underlying fallowed wheat, 5--9.5; those underlying irrigated alfalfa, 1--44; and those underlying cattle feedlots, 0--41. Nitrate concentra- tion of well water fluctuates seasonally and often erratically. Even adjacent wells will differ when sampled at the same time, because of the differences in depth and position of the aquifers. Investigations of Ayers and Branson (1973) indicate that high NO3--N concentrations occur in wells perforated near the top of the saturated zone and that lower concentrations occur near the b o t t o m of the unsaturated zone.

The concentrat ion of NO3--N in the groundwater can change as a result of dilution and biochemical and chemical reactions. Feth (1966) discussed data on fluctuations in well water NO3--N concentrat ion in terms of dilution phenomena. Viets and Hageman (1971) consider that much microbial denitrification cannot occur in the water table because about 0.9 to 1.3 g of biodegradable carbon is needed to denitrify a gram of NO3--N. But according to Steenvoorden (1976), condit ions for denitrification in the groundwater of agricultural lands are quite favourable. It is not necessary that the ground- water should be entirely anaerobic, but oxygen demand must exceed oxygen availability. For example, Oosterom and Steenvoorden (1974) observed that at about 2 m below the groundwater table, the average oxygen content is only some tenths of a milligram O2/1. The amount of organic matter measured as KMnO4 consumption in the upper 2 m of the groundwater is in the range of 10 to 15 mg O2/1 for sandy areas and 50 to 150 mg O2/1 for

219

eutropic peat areas. These observations are supported by the work of Stewart et al. (1967), who obtained much higher counts of total bacteria in the capillary fringe above the water table than in the unsaturated zone. Besides biochemical denitrification, in which bacteria use NO3--N as a hydrogen acceptor instead of oxygen, chemical reactions between nitrate and soil const i tuents such as ferrous ions (Chao and Kroontje, 1966):

NO3- + 3H ÷ + 2Fe 2+ -* HNO2 + 2Fe s+ + H20 HNO: + H + + Fe 2+-* NO (g) + Fe 3+ + H20

can also be responsible for denitrification. Denitrification, however, does not mean that permanent groundwater pol-

lution with nitrate cannot occur. The work of Steenvoorden (1976) itself indicates that lack of an adequate energy source can completely prevent denitrification. He observed that in East Gelderland (The Netherlands) most of the NO3--N-containing groundwater samples were found in the soil classes with the lowest organic matter content.

EFFICIENCY OF FERTILIZER NITROGEN, MAXIMUM YIELD AND POTENTIAL NITRATE POLLUTION

Percent recovery of fertilizer N by a crop is known as the fertilizer use efficiency. It varies with the N source and the rate at which it is applied, the nature of the chemical and biochemical reactions between soil and fertilizer, the timing and placement of fertilizer, the type of crop and its N require- ment, the adequacy of o ther nutrients and a host of soil, climatic and management factors. In fact, much is known about the factors that influence efficiency of fertilizer use, but the manner in which they operate is not always clear (Stanford et al., 1970). The range encountered in percentage recovery of applied nitrogen by crops is very wide, whether considered on a local, regional or natural scale. Under favourable conditions, 80% or more of the fertilizer N may be recovered by the crop, but under most situations efficiencies of 50% or less are common (Allison, 1955, 1966). Low efficiency of fertilizer is caused by loss of N from the root zone by leaching and denitrification. According to Stewart et al. (1968) the fertilizer efficien- cy factor deserves careful at tention if agriculturists are to produce maximum crop yields and prevent pollution of natural waters with plant nutrients.

In 1971, Viets (1971b), while discussing the relationship between fertilizer use efficiency and pollution, remarked:

"Efficiency of fertilizer means different things to different people, depending on the time and circumstances. When fertilizers are relatively expensive and the market price for the product low, one wants the maximum utilization of the nutrient by the crop. The economic or marginal return of further increments of fertilizer is not suffici- ent to justify decreasing efficiency in the use of fertilizers. Such conditions often exist in many parts of developing countries as they did in now developed ones 30 years ago."

220

These facts emerge from a consideration of the response curve in which each successive increment of a fertilizer produces a lower increase in yield. The yield--input curve (response curve) flattens near the maximum yield and the optimum rate of fertilization cannot be closely defined (Stanford et al. 1965; Stanford, 1966). In most cases, the point of greatest economic return to applied N is somewhere below the point of maximum yield and it is doubtful whether the few last increments can be justified. Although neither of these points can be predicted accurately, most farmers in developed nations think in terms of maximum yield rather than the point of greatest economic return'. This is because fertilizer N is relatively cheap and marginal returns obtained near the flat portion of the response curve turn out to be economic. This practice, however, results in a very low fertilizer N use effi- ciency and a large fraction of the applied N remains unutilized in the soil and creats a potential threat to nitrate pollution of natural waters. These facts are verified from the investigations carried out by Broadbent and Rauschkolb (1977) using isotopically labelled nitrogen ('SN). Fig. 3 shows the results obtained on an extremely N-deficient soil. There were large yield increases upto 224.2 kg N/ha. Addition of nitrogen beyond this point resulted in very little additional N taken up by the crop and increased the amount of N that could be potentially leached.

Fertilizer N is applied to ensure that crop yields are not limited by lack of nitrogen. However, relatively few field experiments give information regarding minimum amounts of N that must be absorbed for various attain- able yields to be obtained (Stanford et al., 1965; Stanford, 1966). As a means of minimizing over-application of fertilizer N, Stanford and his associates have described an approach to estimate the amount of fertilizer N needed to produce maximum yield. They computed the fertilizer need from knowledge

1 2 . 5 5 I I ' [ ' I 5 6 0 . 5 ~ .

10.04 / ~ - 44e,4 b o

Y . c

7 . 5 3 3 3 6 , 3

5 . 0 2 / / 2 2 4 . 2 "7

. N / / /

N 2 . 5 1 ~ _ _ . . ~ ° - ~ 112.1 - - -

t / . /

0 . . / L _ ~ - - - P " / t I i I i 0 0 112.1 2 2 4 . 2 3 3 6 . 3 4 4 8 . 4 5 6 0 . 5

Fer t i l i ze r appl ied (kg N /ha )

Fig. 3. Relationship between maize yield, amount of applied nitrogen fertilizer and nitrogen recovered in grain or remaining as leachable N in soil. yield; leachable N in soil; - - - - f e r t i l i z e r N in grain. (Adapted from Broadbent and Rausch- kolb, 1977).

221

of N uptake by the above ground portions of the crop, potential yield, fertilizer N use efficiency and the capability of the soil to supply N. Parr (1973) extended the idea of Stanford and coworkers and gave a rational approach to fertilizer N recommendations. He assumed that mineralizable and residual N are utilized at the same efficiency, and this approach seems quite practical. Singh et al. (1978) have described another approach to deter- mine opt imum rates of fertilizer N at which yields are least affected while unused N, which is a potential pollution threat, is reduced to a permissible level. They have used functions relating yield and N uptake with applied fertilizer N, to define application rates (i) at which greatest economic returns are obtained, and (ii) at which the amount of applied N in excess of the environmentally permissible amount of N in the soil profile is absorbed by the crop. To compute the latter quant i ty the permissible level of unutilized N must be known. The opt imum rate is the smaller of the two above defined fertilizer N rates. The approach seems to be applicable only over small areas, since the equations describing yield and N uptake as function of applied N are very much affected by factors like soil, climate and management practices.

The basic philosophy of these approaches is to apply fertilizer N at such rates that use efficiency is high, so that the amount of unutilized N is reduced to environmentally acceptable levels. Fortunately practices that result in the greatest N uptake efficiency provide both environmental protection and high crop yields. Therefore, increasing the efficiency of fertilizer N should be among agriculturists' highest research priorities. A breakthrough in this direction will allow a significant reduction in fertilizer N application rates without lowering the yield potential.

REFERENCES

Adriano, D.C. Takatori, F.H., Pratt, P.F. and Lorenz, O.A., 1972a. Soil nitrogen balance in selected row-crop sites in Southern California. J. Environ. Qual., 1 : 279-282.

Adriano, D.C., Pratt, P.F. and Takatori, F.H., 1972b. Nitrate in unsaturated zone of an alluvial soil in relation to fertilizer nitrogen rate and irrigation level. J. Environ. Qual., 1 : 418--422.

Aldrich, S.R., 1970. Influence of cropping patterns, soil management and fertilizers on nitrate. In: Nitrate and water supply: Source and control, 12th Sanitary Engg. Conf. Proc. Univ. Illinois, Urbana-Champaign, pp. 153--176.

Allison, F.E., 1955. The enigma of soil nitrogen balance sheets. Adv. Agron., 7 : 213--250.

Allison, F.E. 1965. Evaluation of incoming and outcoming processes that affect soil nitrogen. In: W.V. Bartholomew and F.E. Clark (Editors), Soil Nitrogen. Agronomy, 10 : 573--606.

Allison, F.E., Roller, E.M. and Adams, J.E., 1969. Soil fertility studies in lysimeters con- taining Lakeland sand. U.S. Dept. Agric., Tech. Bull. No. 1199, 62 pp.

Amberger, A., 1972. Belastung und Entlastung der Oberflachengewa~er durch die Land- wirtschaft. Landw. Forsch. Sonderh., 27/1: 13--24.

Anonymous, 1971. Management of nutrients in agricultural land for improved water quality. Environmental Protection Agency Rep. No. 10320 DPB 08/71, 151 pp.

222

Anonymous, 1972. Effects of intensive fertilizer use on the human environment. FAO Soils Bull. No. 16, 360 pp.

Anonymous, 1973. Impact of fertilizers and agricultural waste products on the quality of waters. OECD Rep., Paris, 72 pp.

Anonymous, 1974a. The pollution of waters by agriculture and forestry. UN Econ. Social Coucil, Econ. Comm. Eur., Comm. Water Problems, Water Sem., 124 pp.

Anonymous, 1974b. Fertilizer application rates and nitrate concentration in Illinois Institute for Environmental Quality, IIEQ Document No. 74-38, 85 pp.

Ayers, R.S. and Branson, R.L., 1973. Nitrates in the upper Santa Ana river basin in relation to groundwater pollution. Univ. Calif. Agric. Expt. Sta. Bull. No. 861.

Bartholomew, W.V. and Clark, F.E. (Editors), 1965. Soil Nitrogen. Agronomy, 10, Am. Soc Agron., Madison, WiSconsin, 615 pp.

Biggar, J.W. and Corey, R.B., 1969. Agricultural drainage and eutrophication. In: Eutro- phication: Cases, consequences and correction. Nat. Acad. Sci., Washington, D.C., pp. 4O4--445.

Biggar, J.W. and Nielsen, D.R., 1967. Miscible displacement and leaching phenomena. In: R.M. Hagan et al. (Editors), Irrigation of Agricultural Lands. Agronomy, 11 : 254--274.

Boyce, J.S., Muir, J., Edwards, A.P., Seim, E.C. and Olsen, R.A., 1976. Geological nitrogen in Pleistocene loess of Nebraska. J. Environ. Qual., 5: 93-96.

Bremner, J.M., 1965. Organic nitrogen in soils. In: W.V. Bartholomew and F.E. Clark (Editors), Soil Nitrogen. Agronomy, 10: 93--149.

Broadbent, F.E. and Rauschkolb, R.S., 1977. Nitrogen fertilization and water pollution. Calif. Agric., 31(5): 24--25.

Carlson, C.W., 1964. Tritium-hydrologic research: Some results of the U.S. Geological Survey research program. Science, 143: 804--806.

Chao, T.T. and Kroontje, W., 1966. Inorganic nitrogen transformations through the oxidation and reduction of iron. Soil Sci. Soc. Am. Proc., 30: 193--196.

Commoner, B., 1968. Threats to the integrity of the nitrogen cycle: Nitrogen compounds in soil, water, atmosphere and precipitation. Proc. Ann. Meeting Am. Assoc. Advance- ment Sci., Washington, D.C. 40 pp.

Cooke, G.W., 1973. The possibilities of polluting soil by fertilizers and the way they are used. Bull. Rech. Agron. Gembloux, No. Hors Set., pp. 561--583.

Czeratzki, W., 1972. Transport yon N~hrstoffen aus der mineralischen D~ingung durch Bodenperkolation unter den Wurzelhorizont. Bet. Landw., 50: 465--476.

Czeratzki, W., 1973. Die Stickstoffauswaschung in der Landwirtschaftlichen Pflanzenpro- duktion. Landb. Forsch. V~lkenrode, 23: 1--18.

Duke, H.R., 1967. Groundwater in the high plains of eastern Colorado. Colorado Agric. Expt. Sta., General series paper No. 852, 9 pp.

Feth, J.H., 1966. Nitrogen compounds in natural water -- a review. Water Resources Res., 2: 41--58.

Foerster, P., 1973. Einfluss hoher GUllegaban and Ublicher MineraldUngung auf die Stoff- belastung (NO3, NH 4, P und SO4) im Boden- und Grundwasser in SandbSden Nord- westdeutschlands. Z. Acker-Pflanzenbau, 137: 270--286.

GKchter, R. and Furrer, O.J., 1972. Der Beitrag der Landwirtschaft zur Eutrophierung der Gew~sser in der Schweiz. Schweiz Z. Hydrol., 34: 41--93.

Gardner, W.R., 1965. Movement of nitrogen in soil. In: W.V. Bartholomew and F.E. Clark (Editors), Soil Nitrogen, Agronomy, 10: 550--572.

Gass, W.B., 1969. Recovery of residual nitrogen by corn plants from various soil depths as measured by N '5 tracer techniques. Ph.D. Thesis, Univ. of Nebraska, Lincoln.

Gilliam, J.W., Daniels, R.B. and Lutz, J.F., 1974. Nitrogen content of shallow ground- water in the North Carolina coastal plain. J. Environ. Qual., 3: 147-151.

Gruener, N. and Shuval, H.I., 1970. Health aspects of nitrates in drinking water. In: H.I. Shuval (Editor), Developments in Water Quality Research. Ann Arbor-Humphrey Sci. Publ., Ann Arbor, Michigan, pp. 89--105.

223

Hedlin, R.A., 1971. Nitrate contamination of groundwater in Neepawa-Langruth area of Manitoba Can. J. Soil Sci., 51 : 75--84.

Jung, J., 1972. Factors determining the leaching of nitrogen from soil, including some aspects of maintenance of water quality. Quai. Plant. Mater. Veg., 21: 343--366.

Jurinak, J.J. and Griffin, R.A., 1972. Nitrate ion adsorption by calcium carbonate. Soil Sci., 113: 130--135.

Keeney, D.R. and Gardner, W.R., 1970. The dynamics of nitrogen transformations in the soil. In: S.F. Singer (Editor), Global effects of environmental pollution. U.S. Dept. Interior, Washington, D.C., pp. 96--103.

Kimble, J.M., Barlett, R.J., McIntosh, J.L. and Varney, K.E., 1972. Fate of nitrate from manure and inorganic nitrogen in a clay soil cropped to continuous corn. J. Environ. Qual., 1: 413--415.

Kolenbr.ander, G.J., 1972. Does leaching of fertilizers affect the quality of groundwater at the water works? Stikstof, 15: 8--15.

MacVicar, R., Garman, W.L. and Wall, R., 1950. Studies on nitrogen fertilizer utilization using N is. Soil Sci. Soe. Am. Proc., 15: 265--268.

Miller, M.A. and Nap, W., 1971. Fertilizer use and environmental quality. Report prepared for the Advisory Board of Ontario, Canada, 40 pp.

Muir, J., Seim, E.C. and Olson, R.A., 1973. A study of factors influencing the nitrogen and phosphorus contents of Nebraska waters. J. Environ. Qual., 2: 466-470 .

Muir, J., Boyce, J.S., Seim, E.C., Mosher, P.N., Deibert, E.J. and Olson, R.A. 1976. Influence of crop management practices on nutrient movement below the root zone in Nebraska soils. J. Environ. Qual., 5: 255--259.

Nightingale, H.I., 1970. Statistical evaluation of salinity and nitrate content and trends beneath urban and agricultural areas -- Frenso, California. Groundwater, 8 : 22--28.

Nightingale, H.I., 1972. Nitrates in soil and groundwater beneath irrigated and fertilized crops. Soil Sci., 114: 300--311.

Olsen, R.J., Hensler, R.F., Attoe, O.J., Witzel, S.A. and Peterson, L.A., 1970. Fertilizer nitrogen and crop rotation in relation to movement of nitrate nitrogen though soil profiles. Soil Sci. Soc. Am. Proc., 34: 448--452.

Olson, R.A. Seim, E.C. and Muir, J., 1973. Influence of agricultural practices on water quality in Nebraska: A survey of streams groundwater and precipitation. Water Resour. Bull., 9: 301--311.

Oosterom, H.P. and Steenvoorden, J.H.A.M., 1974. Chemical and physical composition of ground and surface water in some areas. Nota 810 (in Dutch), Institute for Land and Water Management Research, Wageningen.

Owens, L.D., 1960. Nitrogen movement and transformations in soils as evaluated by a lysimeter study utilizing isotopic nitrogen. Soil Sci. Soc. Am. Proc., 24: 372--376.

Parr, J.F., 1973. Chemical and biochemical considerations for maximizing the efficiency of fertilizer nitrogen. J. Environ. Qual., 2: 75--84.

Peele, T.C. and Gillingham, J.T., 1972. Influence o f fertilization and crops on nitrate con- tent of groundwater and tile drainage effluent. Clemson Univ., Water Resour. Res. Inst., Rep. No. 33, 19 pp.

Pfaff, C., 1960. N|ihrstoffauswaschung aus dem Boden bcin Anbau yon Reben. Weinberg Keller, 7: 225--229.

Pratt, P.F., Jones, W.W. and Hunsaker, V.E., 1972. Nitrate in deep soil profiles in relation to fertilizer rates and leaching volume. J. Environ. Qual., 1: 97 -101 .

Rible, J.M. and Pratt, P.F., 1977. Nitrate-nitrogen in unsaturated zone below irrigated fields. Calif. Agric., 31(5): 28--29.

Schwille, F., 1969. Hohe Nitratgehaite in den Brunnenw~ssern der Moseltalaue zwischen, Trier und Koblenz. Gas-Waaserfach., 110: 35--44.

Seim, E.C., Mosher, P.N. and Olaon, R.A., 1972. How much pollution from fertilizers? Nebr. Agric. Exp. Stn. Q., 18(4): 20 -24 .

Singh, B. and Sekhon, G.S., 1976a. Nitrate pollution of groundwater from nitrogen fertilizers and animal wastes in the Punjab, India. Agric. Environm., 3: 57--67.

224

Singh, B. and Sekhon, G.S., 1976b. Some measures of reducing leaching loss of nitrates beyond potential rooting zone. I. Proper co-operation of nitrogen splitting with water management. Plant Soil, 44: 193--200.

Singh, B. and Sekhon, G.S., 1976c. Some measures of reducing leaching loss of nitrates beyond potential rooting zone. II. Balanced fertilization. Plant Soil, 44: 391--395.

Singh, B. and Sekhon, G.S., 1977. Some measures of reducing leaching loss of nitrates beyond potential rooting zone. III. Proper crop rotation. Plant Soil, 47: 585--591.

Singh, B. and Sekhon, G.S., 1978. Leaching of nitrate in calcareous soils as influenced by its adsorption on calcium carbonate. Geoderma, 20: 271--279.

Singh, B., Biswas, C.R. and Sekhon, G.S., 1978. A rational approach for optimizing appli- cation rates of fertilizer nitrogen to reduce nitrate pollut ion of natural waters. Agric. Environm., 4: 57--64.

Smith, G.E., 1965. Water forum, Nitrate problems in water as related to soils, plants and water. Mo. Agric. Exp. Stn, Special Rep. No. 55, pp. 42--52.

Smith, G.E., 1967. Fertil izer nutrients as contaminants in water supplies. In: N.C. Brady (Editor), Agriculture and Quality of our Environment. Am. Assoc. Adv. Sci., Washington, D.C. pp. 173--186.

Smith, W.D., Hill, R.D. and Walker, W.H., 1970. Washington county nitrate study, Phase I. unpublished report, Illinois State Water Survey, Urbana, Illinois.

Stanford, G., 1966. Nitrogen requirements of crops for maximum yield. In: M.H. McVickar et al. (Editors), Agricultural anhydrous ammonia - - technology and use. Am. Soc. Agron., Madison, Wisc., pp. 237--257.

Stanford, G., Ayers, A.S. and Doi, M., 1965. Mineralizable soil nitrogen in relation to fertilizer needs of sugarcane in Hawaii, Soil Sci., 99: 132--137.

Stanford, G., England, C.B. and Taylor, A.W., 1970. Fertilizer use and water quality. U.S. Dept. Agric., ARS 41--168, 19 pp.

Steenvoorden, J.H.A.M., 1976. Nitrogen, phosphate and biocides in groundwater as influenced by soil factors and agriculture. Insti tute for Land and Water Management Research, Wageningen, Tech. Bull. No. 97, pp. 52--69.

Stewart, B.A., Viets, F.G., Hutchinson, G.L., Kemper, W.D., Clark, F.E., Fairbourn, M.L. and Strauch, F., 1967. Distribution of nitrates and other water pollutants under fields and corrals in the middle South Platte valley of Colorado, U.S. Dept. Agric. For t Collins, Colorado, 206 pp.

Stewart, B.A., Viets, F.G. and Hutchinson, G.L., 1968. Agriculture's effect on nitrate pol- lut ion of groundwater. J. Soil Water Conserv., 23: 13--15.

Stout, P.R. and Burau, R.G., 1967. The extent and significance of fertilizer buildup in soils as revealed by vertical distr ibution of nitrogenous matter between soils and under- lying water reservoirs. In: N.C. Brady (Editor), Agriculture and the Quality of our Environment. Am. Assoc. Adv. Sci., Washington, D.C. pp. 283--310.

Sturm, G. and Bibo, F.J., 1965. Nitratgehalte in Trinkwasser unter besonderer BerUch- sichtigung der Verh~ltnisse in Rheingaukreis. Gas- Wasserfach., 106: 332--334.

Taylor, R.G. and Bigbee, P.D., 1973. Fluctuat ions in nitrate concentrations utilized as an assessment of agricultural contaminat ion to an aquifer of a semi-arid climatic region. Water Res., 7: 1155--1161.

Thomas, G.W., 1970. Soil and climatic factors which affect nutrient mobili ty. In: O.P. Engelsted (Editor), Nutrient mobil i ty in soils: Accumulat ion and losses. SSSA Special Publication Series No. 4 Soil Sci. Soc. Am., Madison, Wisc., pp. 1--20.

Thomas, G.W., 1972. The relation between soil characteristics, water movement and nitrate contaminat ion of groundwater. Univ. of Kentucky, Water Resour. Inst., Res. Rep. No. 52, 33 pp.

Tomlinson, T.E., 1971. Nutrient loss from agricultural land. Outl. Agric., 6: 272--278. Viets, F.G., 1970. Soil use and water quali ty - - a look into the future. J. Agrie. Food

Chem., 18: 789--792.

225

Viets, F.G., 1971a. Water quality in relation to farm use of fertilizer. BioScience, 21: 460--467.

Viers, F.G., 1971b. Fertilizer efficiency and pollution. Rec. Adv. Plant Nutri., 2: 345--351.

Viets, F.G. and Hageman, R.H., 1971. Factors affecting the accumulation of nitrate in soil, water and plants. U.S. Dept. Agric., Agric. Handbook No. 413 .63 pp.

Vollenweider, R.A., 1968. Scientific fundamentals of the eutrophication of lakes and flowing waters, with particular reference to nitrogen and phosphorus as factors in eutrophication. OECD Pep., Paris, 160 pp.

Wadleigh, C.H., 1968. Wastes in relation to agriculture and forestry. U.S. Dept. Agric., Washington, D.C., Misc. Publication No. 1065, 112 pp.

Walker, W.H., 1969. Illinois groundwater pollution. J. Am. Water Works Assoc., 61 : 31--40.

Walker, W.H., Peck, T.R. and Lembke, W.D., 1972. Farm groundwater nitrate pollution -- a case study. Unpublished report, Univ. of Illinois, Urbana-Champaign, Illinois.

Ward, P.C., 1970. Existing levels of nitrates in waters - - the California situation. In: Nitrate and Water Supply: Sources and control. 12th Sanitary Engg. Conf. Proc., Univ. of Illinois, Urbana-Champaign, pp. 14--26.

Warrick, A.W., Biggar, J.W. and Nielsen, D.R., 1971. Simultaneous solute and water transfer for an unsaturated soil. Water Resour. Res., 7 : 1216--1225.

Welch, L.F., Bazzaz, F.A., Harmeson, R.H., Hodges, T.K., Jones, B.A., Stevenson, F.J. and Switzer, R.L., 1970. Plant nutrients as water pollutants. Proc. First Allerton Conf., Univ. of Illinois, pp. 17--22.