Field assessment of soil quality as affected by compost and fertilizer
application in a broccoli ®eld (San Benito County, California)
S. Stamatiadis
a,*, M. Werner
b, M. Buchanan
baGoulandris Natural History Museum, Ecology-Biotechnology Laboratory, 13 Levidou Street, Ki®ssia, 14562, Greece bUniversity of California±Santa Cruz, Center for Agroecology and Sustainable Food Systems, Santa Cruz, CA 95064, USA
Received 6 January 1999; accepted 8 February 1999
Abstract
Selected in-®eld physical, chemical and biological indicators were measured for the rapid assessment of soil quality changes in a Sorrento silty clay loam as a result of compost and ammonium nitrate application to a broccoli ®eld (San Benito County, CA). Plots were laid out in a randomized complete block design with four replications of 0, 22 and 44 Mg haÿ1
compost treatments which were split to include fertilizer (165 kg N haÿ1
) and no-fertilizer subplots. Soil samples were taken on 11 and 24 October 1995 during the active growth phase of the crop, and soil quality evaluation was compared to crop nutrient content and yield which were determined at harvest in November.
Surface application of ammonium nitrate initially stimulated soil nitri®cation and acidi®cation processes in the top 7.6 cm as evidenced by an 80-fold increase in nitrate-N and accumulation of nitrite, a 1.5-unit increase in electrical conductivity (EC) and a 1.4-unit decrease in pH. Following irrigation, this pattern was reversed by nitrate leaching and root uptake, although nitri®cation and acidi®cation effects remained detectable at both sampling depths (0±7.6 and 0±20 cm). Nitri®cation was positively correlated to soil respiration and negatively correlated to soil water content. The estimated nitrate-N levels of fertilizer-containing plots in the top 20 cm were two times higher than those reported in the literature as minimal levels for optimal corn growth and indicated a high risk for groundwater contamination in this irrigated ®eld by taking into account the rapid water in®ltration and low soil buffering capacity.
The detected short-term bene®cial effects of compost application were the stabilization of pH and the decrease of water in®ltration rate. Stabilization of pH prevented acidi®cation effects due to fertilizer application at both sampling depths. The high soil EC of plots receiving 44 Mg haÿ1of compost at the 0±20 cm depth probably resulted from a high compost salt
content, other than nitrates, and warns against repeated use of high EC composts that may result in N depletion, reduced nutrient cycling and impaired crop growth.
The relatively small differences among treatments in crop yield, head number and weight, leaf and petiole nutrients indicated that there was suf®cient residual fertility to grow a crop in the absence of any amendments. However, the relevance of selected soil quality indicators to plant productivity and health was evidenced by the strong correlation of soil nitrate-N with leaf-N and head weight, despite the adequate-to-excessive amount of soil nitrate-N in most treatments.#1999 Elsevier Science B.V. All rights reserved.
Keywords:Nitri®cation; Acidi®cation; Crop yield
*Corresponding author. Tel.: +30-1-8087345; fax: +30-1-8080674.
1. Introduction
The use of synthetic ammonium fertilizers is known to cause a rapid shift of soil chemical properties which are initiated by microbial nitri®cation. This shift may result in soil acidi®cation. The magnitude of the effect of these processes has been directly measured in soil extracts by a rise of nitrate and electrical conductivity (EC) and a decline of pH (Patriquin et al., 1993; Smith and Doran, 1996). However, the slower nitri®cation of N and the high cation exchange capacity of organic amendments stabilize soil chemical properties by increasing soil buffering capacity and slowly releasing essential nutrients for more sustainable plant growth. In order to fully assess the effects of such manage-ment practices in the soil environmanage-ment, we need to integrate chemical with physical and biological attri-butes that can serve as appropriate indicators of soil functions. In this study, a basic set of soil quality indicators (bulk density, in®ltration rate, water con-tent, water holding capacity, pH, EC, NO3-N, soil
respiration) were measured in a broccoli ®eld sub-jected to compost and inorganic fertilizer inputs. These indicators were selected in order to demonstrate the utility of in-®eld measurements for the quick detection of soil quality changes under organic and chemical management practices. They comply with the proposed selection criteria of soil quality indica-tors in that they are sensitive to variations in manage-ment, they de®ne major ecological processes in soil and they re¯ect conditions as they actually exist in the ®eld under a given management system (Doran et al., 1996). The techniques employed were relatively inex-pensive, simple, and could be conducted on site by many users such as farmers, researchers, consultants, extension agents and resource conservationists (Lie-big et al., 1996; Sarrantonio et al., 1996). Spacial and temporal changes of soil attributes were evaluated and compared to plant nutrient content and yield in order to assess effects on both environmental quality and plant productivity.
2. Materials and methods
2.1. Site description
The experimental site was a commercial broccoli ®eld located near Hollister, San Benito County. The
soil type is Sorrento silty clay loam, described as a Mollisol. The climate is Mediterranean, with summer drought, and cool, rainy winters. The site had been in a lettuce/broccoli rotation (lettuce in the summer, broc-coli in the fall) for at least the previous 3 years, using conventional soil fertility and pest management prac-tices. No organic matter inputs were made other than crop residues. Water was provided during the cropping season through sprinkler irrigation. The ®eld was laser-leveled every 6 or 7 years.
At the time of the experiment, the `Sundance' tillage system was in use. Permanent beds (45 cm wide, 102 cm between two consecutive bed centers) are maintained by a front and rear-mounted shank, shovel and disk system, which can incorporate prior residue while rebuilding the bed in one pass. The pre-existing crop is ®rst mowed, the shanks rip the center of the bed to 25±30 cm depth, followed by discs which incorporate residues while moving soil back into a rough triangular bed. Furrows remain undis-turbed, thus wheel-traf®c is controlled and permanent. The beds are cultivated by dragging a 46 cm shank through the middle of the bed. Two passes with a Lilliston cultivator were made to further incorporate prior lettuce residues and compost prior to shaping the raised beds and transplanting. Transplanting was on 10 August and broccoli was harvested on 10, 14, and 17 November 1995.
2.2. Experimental design and sampling
A longitudinal section of the broccoli ®eld, six rows or 612 cm wide, was divided into four blocks and three ®elds of equal area in each block were randomly assigned to receive three rates of compost (0, 22 and 44 Mg haÿ1
) in a complete randomized block design. Green wastes (30%), cow manure (20%), spoiled hay (15%), clay soil (5%) and various crop processing residues were reported to be the initial sources of compost. The presence of soil was indicated by the high ash content of compost (Table 1). Each ®eld was partitioned into two subplots that received a random assignment of the two levels of fertilizer, 0 or 165 kg (NH4NO3)-N haÿ1 sprinkled
com-post application. Each subplot was 768 cm long and 612 cm (six rows) wide and the total number of subplots in the experiment was 24. Broccoli is trans-planted in double rows (30 cm apart) on each bed with plants spaced approximately 20 cm apart within each row.
Soil quality sampling was conducted on 11 and 25 October 1995, just prior to the start of budding, with at least one irrigation event occurring within this time interval. Top soil samples (0±7.6 cm depth) were taken from the center row of each treatment plot. These samples were analyzed with a ®eld soil quality kit (Sarrantonio et al., 1996; Liebig et al., 1996), established by Dr. J.W. Doran (USDA±ARS, Lincoln, NE), for gravimetric water content, bulk density, electrical conductivity and soil pH of 1 : 1 soil±water mixture using individual calibrated Hanna pocket meters and nitrate-N estimated from ®ltered extract of 1 : 1 soil±water mixture using AquaChek test strips (Whatman). Soil EC and nitrate-N values were adjusted for deviation from a 1 : 1 soil±water mixture.
On-site water in®ltration rate was measured within PVC in®ltration rings which were inserted to a 7.6 cm soil depth. Soil respiration (pre-irrigation and post-irrigation) was measured from the head-space of covered in®ltration rings for 30 min using 0.1% CO2 gas sampling tubes (National Draeger).
Post-irrigation samples of surface soil (0±7.6 cm) from within the irrigation rings were analyzed for soil bulk density and water holding capacity (WHC). Further details of the ®eld soil quality procedures used are given by Sarrantonio et al. (1996). Post-irrigation samples could not be
re-trieved for the ®rst sample date due to an unexpected pesticide application. Respiration and in®ltration measurements were made on Control, Fertilizer, 22, and 44 Mg haÿ1
plots only. Additional composite cores (0±20 cm depth, 10 cores per sample) were collected from all plots on 25 October 1995 and analyzed for gravimetric water content, NO3-N, EC,
and pH.
Crop harvesting was performed sequentially on 10, 14, and 17 November 1995. Only market grade broc-coli was cut from the inner 3.1 m length of a single bed. The market for bunching requires a tight ¯oret of no more than about 10 cm in diameter, cut with a stalk of approximately 15 cm length. All harvested heads for each plot were counted and weighed. Petiole and leaf samples were collected from 10 plants per plot at ®rst harvest on 10 November. Plant tissue analysis was performed by the University of California (Depart-ment of Agriculture and Natural Resources Labora-tory, Davis). After oven drying and grinding, petiole samples were extracted and analyzed for NO3-N (by
colorimetry) and K (by inductively coupled plasma, ICP), and leaf samples for total N (dry combustion by an elemental analyzer), P, K, Ca, Mg, Zn and Fe (by ICP).
2.2.1. Statistical analysis
Analysis of variance using a complete rando-mized block design was performed on data obtained with the soil quality kit on the ®rst sampling date (11 October) and for in®ltration, WHC, BD and soil respiration of the second sampling date (24 October). Split-plot ANOVA was performed on soil pre-in®ltration data for both depths on 24 October where the variability due to subplots (fertilizer by block and fertilizer by compost by block interactions) was used as the error term to test fertilizer effects. The effect of compost was evaluated separately by an
F-test using the compost by block interaction as the error term. All plant data was similarly analyzed based on a split-plot randomized complete block design. In all cases, Bonferonni multi-comparison of means was used at the level ofP< 0.05. Correlation coef®cients were computed by standard analytical procedures at the level ofP< 0.05. All the employed procedures are reported in the statistical analysis system (SAS Institute, 1985).
Table 1
3. Results and discussion
3.1. General soil properties
The control treatment of this experiment had an acceptable soil bulk density and water holding capa-city, but very rapid water in®ltration rate, low nitrate levels, high pH and background electrical conductivity in the top 7.6 cm (Tables 2 and 3). High pH > 8 are usually due to the presence of Ca and Na salts (Smith and Doran, 1996) while bicarbonate is a signi®cant counter ion for these basic cations in alkaline soils of low nitrate and high EC values (Patriquin et al., 1993). High soil pH can restrict nitri®cation, result in sig-ni®cant loss of N through ammonia volatilization and lead to nitrite accumulation (Smith and Doran, 1996). Soil samples to a 0±20 cm depth had higher nitrate levels and pH < 8 (Table 4) indicating better condi-tions in the rooting zone for plant growth. Effects of fertilizer or compost application were detected in all measured soil properties of the top soil except for soil bulk density and water holding capacity.
3.2. Fertilizer effects on soil quality
Fertilizer application caused changes of soil che-mical properties, water content and respiration. Nitrate-N and EC were increased and pH was reduced in the top 7.6 cm at both sampling times (Tables 2 and 3). Nitrate levels were extremely high, 80 times higher
than that of the control, and nitrite accumulation was detected at the ®rst sampling date 3 weeks after fertilizer application (Table 2). The high nitrate levels caused a rise in EC by 1.5 units above background levels and resulted in a signi®cant correlation between these two variables (Table 4). Soil acidi®cation was produced by microbial nitri®cation during formation of nitrate from the ammoniacal fertilizer, while the decrease in pH by 1.4 units indicated a low soil buffering capacity (Patriquin et al., 1993; Smith and Doran, 1996; Table 2). Two weeks later and after irrigation, however, nitrate and associated EC levels dropped considerably in the top 7.6 cm of the fertilizer treatment and soil pH increased by 1.1 units (Table 3) although acidi®cation effects were still pronounced (Fig. 1). These rapid chemical changes are attributed to nitrate leaching from the soil surface after irrigation which may explain the higher nitrate-N concentration in all fertilizer-containing plots within the 0±20 cm depth (Fig. 1). The absence of signi®cant correlations between NO3-N and pH at this depth (Table 4)
sug-gests that nitri®cation was more pronounced in the surface soil and was followed by nitrate leaching to greater depths.
As illustrated in Fig. 2, estimated nitrate-N levels of fertilizer-containing plots were on average two times higher than those needed by crops such as corn1during
Table 2
Changes of top soil properties caused by fertilizer and compost application on 11 October 1995 (means for 0±7.6 cm depth,n3) Soil quality indicator Control Compost Fertilizer
22 Kg haÿ1 44 Kg haÿ1
Pre-irrigation
Soil bulk density (g/cm3) 1.09 1.09 1.16 1.04
Electric conductivity (dS/m) 0.52 0.50 0.57 2.06
Soil pH 8.1a 8.1a 8.3a 6.7b
Soil NO3-N (kg NO3-N/ha) 2.4b 9.9b 2.7b 186.0a
Soil NO2-N (kg NO2-N/ha) 0 0.04 0 1.65
Respiration (kg CO2-C/ha/d) 60%
water-filled pore space, 258C
[63.2ab] 17.1b 31.5b [84.9a]
Infiltration time
First inch (min) 0.07 0.07 0.09 0.07
Second inch (min) 0.57 0.40 0.82 0.57
Data in square brackets ([ ]):n2.
Same letters or absence of letters within rows indicates no significant difference between means. Means within rows followed by different letters are significantly different atP< 0.05.
120±25 mg N kgÿ1in the top 30 cm of soil or 53 kg N haÿ1in
the top 20 cm, kg haÿ121
the early growing season when crop demand is great-est (Bundy and Meisinger, 1994), although broccoli needs differ, at least seasonally, as N uptake peaks in the ®nal third of the seasons between the start of budding and the formation of heads (Magni®co et al.,
1979; Stivers et al., 1993). Given these high levels of available N, the very rapid in®ltration rates and low buffering capacity of this soil, great potential of leaching and ground water contamination is possible with this management practice. Even greater rates of
Table 3
Comparison of mean soil properties of inorganic fertilizer and organic compost treatments on 24 October 1995 (n3 for 0±20 cm depth and post-irrigation data,n4 for remainder)
Sample depth Soil quality indicator No fertilizer Fertilizer added 0 Mg haÿ1
(control)
22 Mg haÿ1 44 Mg haÿ1 0 Mg haÿ1 22 Mg haÿ1 44 Mg haÿ1
Pre-irrigation
0±20 cm Soil water content (g/g, %) 17.3a 17.6a 17.0a 13.7b 14.7b 15.0b
Electric conductivity (dS/m) 0.55b 0.74ab 1.18a 0.60b 0.69b 0.87ab
Soil pH 7.9 7.7 7.8 7.5 7.7 7.8
Soil NO3-N (mg/g) 11.4c 19.5bc 12.3c 39.6ab 40.9ab 55.5a
0±7.6 cm Soil bulk density (g/cm3) 1.15 1.13 1.12 1.15 1.19 1.08
EC1:1(dS/m) 0.43b 0.51ab 0.45ab 0.45ab 0.63a 0.52ab
Soil pH 8.5a 8.4a 8.6a 7.8b 8.4a 8.3ab
Soil NO3-N (kg NO3-N/ha) 4.3 6.7 3.1 20.7 27.9 11.8
Infiltration time
First inch (min) 0.06b 0.06b 0.12a 0.06b ± ±
Second inch (min) 0.39ab 0.34b 0.88a 0.35b ± ±
Post-irrigation
Water holding capacity (g/g) 0.30 0.29 0.30 0.28 ± ± Soil bulk density (g/cm3) 1.18 1.15 1.14 1.19 ± ±
Respiration (kg CO2-C/ha/d) 60%
water-filled pore space, 258C
8.8b 14.4ab 14.0ab 18.1a ± ±
Means within rows followed by different letters are significantly different atP< 0.05. Same letters or absence of letters within rows indicates no significant difference between means.
Table 4
Correlation coefficients of soil NO3-N*with EC, pH and respiration
Sampling time Sampling depth DATA subset Correlation coefficient of NO3-N with
EC1:1 pH CO2-C
11 October 1995 0±7.6 cm All (n12) 0:99 ÿ0:66 0:75 a
Controlfertilizer (n6) 0:99 ÿ0.58 0.76 b
Compost (n6) 0.09 ÿ0:90 ÿ0.50
24 October 1995 0±7.6 cm All (n24) 0:85 ÿ0:62 0:67 c
Controlfertilizer (n8) 0:80 ÿ0:91 0.79 d
Compost (n8) 0:91 ÿ0:87 0.00 d
Compostfertilizer (n8) 0:93 ÿ0.53 ±
24 October 1995 0±20 cm All (n18) 0.22 ÿ0.37 ± Controlfertilizer (n6) 0.58 ÿ0.74 ± Compost (n6) 0.50 ÿ0.49 ± Compostfertilizer (n6) 0:83 ÿ0.36 ± *ppm for 0±20 cm depth and kg haÿ1for 0±7.6 cm depth
an: 10,bn: 4,c:n12,d:n6
fertilization (175 to 350 kg N haÿ1
) than those applied in this experiment are reported to be typically used by growers for lettuce, celery, broccoli and cauli¯ower in the western US and groundwater contamination by levels of nitrate-N above the public health drinking-water standard is a growing problem in such agricul-tural areas (Stivers et al., 1993).
The higher soil respiration in the fertilizer-contain-ing plots (Table 3) was associated with greater nitri-®cation rates (Table 4). Gravimetric water content was signi®cantly lower in all fertilizer plots, with or without compost, at 0±20 cm depth (Table 3) as a
likely result of greater plant growth (vegetative mate-rial) and greater extraction of soil water. Such a relationship is suggested by a signi®cant negative correlation between water content and nitrate-N in plots that did not contain any compost (r ÿ0.92,
n6).
3.3. Compost effects on soil quality
Compost application decreased water in®ltration rate, increased EC at 0±20 cm depth and increased soil buffering capacity as indicated by the unchanged soil pH even after fertilizer application (Table 3, Fig. 3). Increased soil buffering capacity is one of the bene®ts of building up soil organic matter through compost application. Ulrich (1987) found no acidi®-cation effects and no relationship of pH to NO3, but
higher cation exchange capacity and base saturation, in soil amended with compost as compared to soil without compost. Our data show signi®cant correla-tions between pH and NO3-N in compost-treated plots
(Table 4), but in such narrow ranges that acidi®cation effects were undetectable. Despite the initially high nitrate content of compost (Table 1), nitrate levels of compost soil were low and similar to the control indicating net loss through leaching, root uptake and possibly immobilization and denitri®cation pro-cesses. The addition of fertilizer to compost in com-bined treatments did not bring about any changes in soil properties other than an expected increase in nitrate-N (Table 3).
The positive relationship of soil EC to compost application for the 0±20 cm depth (Fig. 3) shows an excessive content of salts, other than nitrates, in this compost. Soil EC values doubled, relative to the control, in plots amended with 44 Mg haÿ1
compost to levels characterized as slightly saline that may adversely affect the growth of salt-sensitive crops (Smith and Doran, 1996). The rise in EC warns against continued application of high EC composts that may lead to soil salinization and result in N depletion, reduced nutrient cycling and impaired crop growth. Leaching of soluble salts contained in compost explains the low soil EC of the top 7.6 cm (Fig. 3) and illustrates the need to sample at greater depth in order to detect such adverse management effects especially in soils of low buffering and cation exchange capacity.
Fig. 2. The relationship between soil nitrate-N (top 20 cm on 25 October), leaf N and head weight of broccoli harvested in November 1995.
Table 5
Broccoli yield and nutrients at harvest in response to compost and nitrogen fertilizer applications Soil quality indicator No fertilizer Fertilizer added
0 Mg haÿ1
(control)
22 Mg haÿ1 44 Mg haÿ1 0 Mg haÿ1 22 Mg haÿ1 44 Mg haÿ1
Crop yield
No. of heads (heads plotÿ1) 17.0 17.3 16.3 14.5 17.5 14.8 Head weight (g headÿ1) 222 236 236 240 240 245 Head weight (Mg haÿ1) 13.9 14.6 13.6 12.6 15.2 12.9
Petiole nutrients
NO3-N (ppm) 872 985 970 960 915 1002
K (ppm) 2150 2075 2175 2200 2150 2275
Leaf nutrients
N (%) 5.27 5.34 5.33 5.52 5.50 5.55
P (%) 0.62 0.64 0.67 0.66 0.66 0.63
K (%) 2.54 2.53 2.77 2.84 2.81 2.77
Ca (%) 2.10 2.32 2.19 1.93 2.22 2.16
Mg (%) 0.68 0.67 0.60 0.61 0.66 0.62
Zn (ppm) 28.3 29.7 30.3 32.7 30.7 31.3
Fe (ppm) 70.0 84.3 83.0 87.3 84.3 85.6
3.4. Crop growth and its relation to soil quality
Differences between treatments in crop yield and number of heads were insigni®cant and relatively small, the biggest difference being observed between 22 Mg haÿ1
compost and fertilizer plots (the former being 20% higher than the latter, Table 5). Even smaller differences were found in head weight, leaf nutrients (N, P, K, Ca, Mg, Zn, Fe) and petiole nutrients (NO3, K) and indicated that there was
suf®-cient residual fertility to grow a crop in the absence of any amendments.
Although the values of these plant variables ¯uc-tuated within narrow ranges, a few signi®cant
correla-tions emerged. The number of broccoli heads was a good predictor of crop yield (r0.97,n6) with the chemically fertilized plants producing the smaller number of heads which may be seen as a negative plant response, direct or indirect, to excessive soil N levels. Although, a similarly low number of heads in 44 Mg haÿ1
compost plots may have been caused by high soil EC (Fig. 3), the greatest number of heads produced by 22 Mg haÿ1
plants, without or with fer-tilizer added (Table 5), suggests that compost applica-tion at reasonable rates was associated with highest yields. However, crop yield or number of heads was not signi®cantly correlated to any measured soil attri-butes so that compost nutrients, other than nitrates, might be related to the higher yield of 22 Mg haÿ1
plots. Moreover, plant elemental analysis did not show any major differences between compost and no-com-post plants, or between plants of 22 and 44 Mg haÿ1
compost (Table 5).
Broccoli head weight and leaf N content were signi®cantly correlated to soil nitrate-N (Fig. 2), thus, illustrating the relevance of this indicator in soil quality assessment from plant productivity and health perspectives. Leaf N content was directly proportional to soil nitrate-N in the top 20 cm at the second sampling time (r0.97,n6, Fig. 2) or in the top 7.6 cm at the ®rst sampling time (r0.96, n4). Patriquin et al. (1993) also obtained a signi®cant correlation between leaf and soil nitrates in organi-cally fertilized lettuce. The lack of relationship of crop yield with any of these variables appears to have resulted from adequate-to-excessive levels of soil nitrates beyond crop needs in most treatments and from other compost-related factors that were unrelated to N content.
Acknowledgements
This project was supported by the Fulbright Foun-dation in collaboration with the University of Cali-fornia±Santa Cruz/Center for Agroecology and Sustainable Food Systems (USA) and the Goulandris Natural History Museum/Gaia Environmental Research and Education Center±Ki®ssia (Greece). Thanks are extended to Dr. John Doran for supplying the USDA-ARS soil quality test kit and for advice on soil quality assessment methods.
References
Bundy, L.G., Meisinger, J.J., 1994. Nitrogen availability indices. In: Weaver, R.W. et al. (Eds.), Methods of Soil Analysis, Part 2. SSSA Book Series 5. Soil Sci. Soc. Am., Madison, WI, pp. 951±984.
Doran, J.W., Sarrantonio, M. Liebig, M.A., 1996. Soil health and sustainability. In: Sparks, D.L. (Ed.), Advances in Agronomy, vol. 56. Academic Press, San Diego, CA, pp. 1±54.
Liebig, M.A., Doran, J.W., Gardner, J.C., 1996. Evaluation of a field test kit for measuring selected soil quality indicators. Agronomy J. 88, 683±686.
Magnifico, V., Lattanzio, V., Sarli, G., 1979. Growth and nutrient removal by broccoli. J. Am. Soc. Hort. Sci. 104, 201±203. Patriquin, D.G., Blaikie, H., Patriquin, M.J., Yang, C., 1993.
On-farm measurement of pH, electrical conductivity and nitrate in soil extracts for monitoring coupling and decoupling of nutrient cycles. Biological Agric. Hortic. 9, 231±272.
Sarrantonio, M., Doran, J.W., Liebig, M.A. Halvorson, J.J., 1996.
On-farm assessment of soil quality and health. In: Doran, J.W., Jones, A.J (Eds.), Methods for Assessing Soil Quality. Soil Sci. Soc. Am. Spec. Publication #49 SSSA, Madison, WI, pp. 83± 105.
SAS Institute, 1985. SAS User's Guide: Statistics, Version 5 edn. Cary, NC, USA.
Smith, J., Doran, J.W., 1996. Measurement and use of pH and electrical conductivity for soil quality analysis. In: Doran, J.W., Jones, A.J (Eds.), Methods for Assessing Soil Quality. Soil Sci. Soc. Am. Spec. Publication #49. SSSA, Madison, WI, pp. 169± 185.
Stivers, L.J., Jackson, L.E., Pettygrove, G.S., 1993. Use of nitrogen by lettuce, celery, broccoli, and cauliflower: a literature review. California Department of Food and Agriculture, Fertilizer Research and Education Program, Sacramento, CA, 79 pp. Ulrich, B., 1987. Stability, elasticity and resilience of terrestrial
