1. Zero-Acreage Farming
Solutions for Food
Desert Communities
Sarah Austin Chris Brianna
A.B.C.S. Engineering Firm
Clemson University Department of
Biosystems Engineering
BE 4750
2. What is Zero Acreage
farming?
Forms of food production
that are characterized by the
non-use of arable land, or
acreage1
http://www.tuvie.com/wp-content/uploads/urban-skyfarm-by-aprili-design-studio2.jpg
2
3. Recognition of Problem/Need
โ Land Usage
โ 17% of US land area used as
cropland14
โ Erosion in dryland regions15,18
http://www.sciencedirect.com/science/article/pii/S2095633915300472
3
4. Recognition of Problem/Need
โ Water Usage8
โ 40% of total water usage in North America
โ 70% of total water usage in the World
โ Groundwater Depletion from inefficient irrigation practices
4
5. Recognition of Problem/Need
โ Environmental Damage
โ Dead Zones29
โ Resource Recycling
โ Future Issues
โ Global population increase
http://www.noaa.gov/media-release/gulf-of-mexico-dead-zone-is-largest-ever-measured
5
6. Recognition of Problem/Need
โ Food Deserts
โ 23.5 million Americans lack access to a supermarket within 1 mile of home2
โ Increased rates of obesity and cardiovascular disease in urban areas35
6
http://3.bp.blogspot.com/-kK4dF5epIBw/UYwn0kUMFQI/AAAAAAAAAAU/91pWKeXWPvE
/s1600/us-obesity-chart--organisation-de-coop--ration.jpg
8. Definition of Problem/Need
โ Water Usage
โ Conventional farming uses 75%-95% more water than Z-farming1,3
โ Groundwater depletion18,26
โ Abstraction of 1000 km3
/yr
โ Water Use Efficiency
โ Recycling/monitoring within controlled environment
8
9. Definition of Problem/Need
โ Land Usage
โ Conventional farming uses 99% more land than vertical farming3
โ Erosion18
โ 10 million ha of cropland lost annually20
โ Soil loss is 10-40 times faster than rate of soil formation20
9
10. Definition of Problem/Need
โ Environmental damage
โ Distance between consumer and
producer prevents resource recycling
and increases petrofuel use1,5
โ Dead zones17, 18
โ Decrease fisheries production
โ Loss of aquatic biodiversity
https://serc.carleton.edu/microbelife/topics/deadzone/index.html
10
11. Definition of Problem/Need
โ Future Issues
โ 9.7 billion people in 205018
โ 70% increase in food production
โ Increased competition for land and water usage
โ Environmental tipping points30
11
12. Goals of Project
โ Biological
โ Select cost efficient, nutritional crops
โ Optimize climate conditions for plant growth
โ Determine total biomass yield
โ Structural
โ Construct an indoor containment system for crops
โ Maximize space, light, and water usage
โ Mechanical
โ Implement energy-efficient irrigation system utilizing pumps
โ Potentially design lighting system
12
13. Constraints
โ Skills
โ Upcoming graduates to Biosystems Engineering
โ Kinetics
โ Fluid mechanics and hydrology
โ Instrumentation
โ Structural mechanics
โ Technical skills include AutoCAD, SolidWorks, Matlab, and Excel
โ Space
โ Adequate space to house system in a Food Desert area
13
14. Constraints
โ Equipment
โ Clemson University laboratory equipment and online markets
โ Industry implementation would call for professional equipment
โ Logistical
โ Transportation of design to client
โ Conveying design information to client for operation and management
14
15. Considerations
โ Safety
โ Electronic equipment near water
โ Water as a slipping hazard
โ Toxicity of pesticides
โ Ethical
โ Food grade produce
โ Affordable and sustainable design
โ Community members
15
16. Considerations
โ Ecological
โ Reduce
โ Water use
โ Damage to arable land
โ Pesticide use
โ Ultimate Use
โ Sustainably produce nutrient rich crops
โ Conserve resources used for farming
16
17. Questions of User, Client and Designer
โ User - Consumer/Community
a. How much space will the farm consume?
b. What crops can be grown?
c. How will this design increase accessibility to produce?
โ Client - County
a. How much will the design cost?
b. How will this farm increase community health and wellbeing?
c. How much food will the farm produce?
โ Designer - A.B.C.S. Engineering Firm
a. What three crops are most beneficial for consumers?
b. What kind of water and nutrient needs do the crops require?
c. How will we obtain the water needed?
17
18. Governing Equations
โ Energy and Dimensional Analysis
โ Manningโs Equation
โ Open Channel
โ Irrigation system
โ Light Intensity
โ Potential lighting system
โ Heat Transfer
โ Fourierโs Law
18
20. Plant Growth
โ Different growth stages use differing
amounts of water and nutrients
โ 4 different stages for
evapotranspiration estimates
โ Initial
โ Canopy development
โ Mid-season
โ Maturation4
โ Evapotranspiration values estimated
from a reference crop and crop
coefficients
United States Department of Agriculture (1993). The National Engineering
Handbook, Part 623, 19-22, 69-70.
20
21. Plant Nutrients
โ 16 essential nutrients for maximum plant growth13
โ Macronutrients include O, C, H, N, K, Ca, Mg, P and S
โ Must be balanced for proper uptake
โ Dissolved oxygen concentrations must be high for optimum root
respiration
21
22. Irrigation Methods
โ Spray irrigation
โ Drip irrigation
โ Nutrient Film Technique
โ Extremely shallow water โfilmโ
โ All necessary nutrients dissolved
โ Can recycle water
Water with Nutrients
Image by Sarah Van Brunt
22
23. Evapotranspiration Rate
โ Evapotranspiration rate for
leafy greens
โ 1.88 g/plant/h for greenhouse
NFT system11
โ 21 day old plants
https://blog.1000bulbs.com/home/how-to-set-up-a-nutrient-film-technique-nft-system
23
24. Lighting Methods
โ Sun
โ Fluorescent light
โ Red and Blue โGrowโ lights
โ Strips, arrays, or light filters
http://www.ledgrowlightsforsale.ca/full-spectrum-300w-led-grow-light-for-growing-weeds.html
24
25. Lighting Methods
โ Red Blue light: University of Tennessee
โ Chl a and b have highest absorption in red and blue light spectra
โ 663-640 nm and 453-430 nm respectively
โ Beta - carotene and lutein absorb strongly in the 445-650 nm range
โ Light intensity
โ Spinach accumulated highest lutein and Beta - carotene at 335
ฮผmol/m2
s
โ Concentrations of soluble sugars increased with increased light
intensity33
25
26. Lighting Methods
Dr. Adelberg, Clemson University
26
https://www.hubbell.com/hubbellindustriallighting/en/Products/Lighting-Controls/I
ndustrial-Lighting/Enclosed-Gasketed/NutriLED/p/212399
http://ledt8bulb.com/hubbell-nutriled-led-horticultural-light-fixture.html
RelativePower
27. Growing Media
โ Expanded Clay Pebbles
โ Inert, porous pebbles made from clay
โ Contain tiny air pockets for good drainage
โ Can accumulate salts if not flushed regularly
http://greenscape-ltd.com/product/expanded-clay-pebbles/
27
28. Growing Media
โ Coconut Fiber (Coir)
โ Made from waste coconut husks from coconut industry
โ Holds water well and allows oxygenation
โ Can break apart overtime and clog pump systems
28
http://woodethic.blogspot.com/2017/03/coconut-coir-what-is-co
co-coir-and-how.html
29. Growing Media
โ Gravel
โ Relatively cheap
โ Heavy and poor water retention
โ Good aeration
โ Perlite/Vermiculite
โ Lightweight
โ Perlite - poor water retention
โ Vermiculite retains water well
โ Perlite dust can be hazardous
http://www.white-skips.co.uk/gravel_hull_14.html
https://www.epicgardening.com/perlite-vs-vermiculite/
29
30. Growing Media
โ Rockwool/Stonewool
โ Made of liquid rock spun into a
wool like fiber
โ Industry standard holds water
well
โ Does not decompose after
disposal
https://www.amazon.com/Rockwool-Grow-Cubes-1-5-Inch
es/dp/B00IA8AJMI
30
31. Growing Media
โ No media
โ Using just water so no media to hold water
โ Deep water or nutrient film technique
โ Salt accumulation in media is negated and nutrient solution easily monitored
31
https://www.citycrop.io/why-citycrop-uses-hydroponics/
32. Hydroponic Media
โ Comparison of gravel, floating, and nutrient film technique
hydroponic systems for tomatoes
โ Used as treatment for an aquaculture system
โ Gravel yielded the highest biomass gain followed by floating then
NFT12
โ The daily water usages were not significantly different for all three
systems
32
33. Growing Hydroponic Lettuce
โ Shown to use water more efficiently than conventional land based
farming at an average of 13 times less 7
โ The yearly lettuce yield increased 11 times when grown
hydroponically
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC44
83736/figure/ijerph-12-06879-f001/
https://www.ncbi.nlm.nih.gov/pmc/articles
/PMC4483736/figure/ijerph-12-06879-f00
2/
33
36. Location Selection: Union City, GA
โ Classified as food desert by
USDA
โ Population of 6,343
โ 8% of households without
vehicles and more than ยฝ
mile from supermarket
โ Poverty rate of 30.7%
36
https://www.ers.usda.gov/data-products/food-access-research-atlas/go-to-the-atlas.aspx
USDA (2017)
39. Nutrient and Water Requirements
39
Kale Spinach Romaine Lettuce
Water Requirement
[cm/week]
2.5-3.8 2.5 2.5
Nutrients
Requirement
[N-P-K]
16-4-17 16-4-17 16-16-8
Mattson et al. (2014)
Bonnie Plants (2011)
Sanders (2001)
Harrington (2017)
40. Selected A-Frame Design
โ Benefits
โ Housed indoors
โ Central lighting can be used
โ Provides vertical growing space
โ Offers space beneath frame
Image by Brianna Chavarria
40
42. Total Plant Yield
42
0.6 m
1.5 m
30.5m
2.5m
โ Horizontal Farming
โ 80 plants in 18.6m2
area
โ 4.3 plants/m2
โ Our Vertical System
โ 60 plants in 3.9m2
area
โ 15.5 plants/m2
Sustainable Seed Company (2017)
44. Growing Media Selection
โ Due to A- frame design, the media must be lightweight so that the
frame can support the channel weight
โ This rules out deep water culture and gravel
โ Porous media accumulates salts and nutrients which can influence
pH
โ Must be flushed regularly and sterilized between uses
โ NFT system chosen because of ease of nutrient concentration
monitoring and lightweight design
โ Electrical conductivity to monitor nutrient concentration
44
45. NFT System Design
โ Flow rate can range from 0.5L/min to 9L/min depending on
channel geometry32
โ Higher flow rates can allow greater oxygenation of water
โ Slopes need to range between 2.5%and 1.0%
โ Flow rate and slope adjusted to produce shallow channel depth
โ We chose a 2L/min flow rate and a 1.3% slope
โ Flow depth: 1.24 cm via Manningโs Equation
45
46. Irrigation Selection
โ Positive Displacement Pump
โ Fulfills NFT
โ Uses negative space
โ Run for 15min on then 15min off
โ Save energy and wear on pump
โ 61,259 Joules
โ Energy used to pump to all levels for 24
hours
Image by Sarah Van Brunt
46
47. Pesticide Selection
47
Insecticidal Soap Pyrethrum Neem Oil
Cost Rank
(1 being most
expensive)
3 1 2
Properties Sprayed about
once a week
Strong insecticide,
sprayed about once
a week
Contains fungicidal
and insecticidal
properties, sprayed
about once a week
Hydroponics-Simplified (2016)
52. Alternative Design Options
โ Other substrates can be used in the A-frame to created an ebb and
flow system that retains water
โ Horizontal hydroponics could also be used for a system that is at a
good working height for harvesters
โ NFT system can still be used or deep water hydroponics with a floating raft
โ Can be used to grow taller plants like tomatoes or vine plants like beans
https://www.epicgardening.com/deep-water-culture-get-started/
http://www.commercial-hydroponic-farming.com/contact-us/
52
53. Alternative Design Options
โ Rainwater collection
โ Solar implementation to
mitigate electricity costs
โ Gravity flow system
โ Pump water to roof
โ Solenoid valves release tubes
โ Operated by microcontroller
53
54. Location Alternative: Coin Laundry
โ Vacant laundromat in
Union City, GA
โ Building size: 195 m2
โ High ceilings
โ $175,000
http://www.loopnet.com/Listing/20461596/6335-Roosevelt-Hwy-Union-City-GA/
54
57. Location Alternative: Greenhouse on Plot of Land
57
https://www.zillow.com/homes/for_sale/Union-City-GA/pmf,pf_pt/land_type/2098602806_zpid/48085
_rid/globalrelevanceex_sort/33.610544,-84.472075,33.536172,-84.617644_rect/12_zm/
https://www.google.com/maps/dir/6335+Roosevelt+Hwy,+Union+City,+GA+30291/Lower+Dixie+Lake+Rd,+Union+City,+GA+30291/@33.5812545,
-84.5619578,16z/data=!3m1!4b1!4m14!4m13!1m5!1m1!1s0x88f4e6cc333ea2e7:0x302778b2a94a6c46!2m2!1d-84.5509936!2d33.5827278!1m5!1
m1!1s0x88f4e72a11590283:0xec164a5abbbf2ab8!2m2!1d-84.5643864!2d33.583549!3e2
58. Economic Analysis
58
Building Land (8 Greenhouses)
Price [m-2
] $900 $2.50
Total Price $175,000 $15,500
Grow Lights $23,500 -
Greenhouse - $67,000
A-Frame $4000 $32,000
Pump $3200 $25,500
Air Compressor $2250 $18,000
Irrigation System $3000 $24,000
59. Economic Analysis
59
Building Greenhouse
Total Biomass [kg/yr] 3,927 31,416
Capital Cost $211,000 $178,500
Annual Cost $3,950 $24,550
Total Year 1 Cost $214,950 $203,000
The greenhouse is more cost effective and produces ~8x more biomass
60. Budget - Capital Costs
60
Item Number of Items Cost / Item Total Cost
Land Plot 1 $15,500 $15,500
Greenhouse 8 $8,400 $67,200
3โ Pro-Flo Shift
Pump
8 $3,195 $25,560
10 hp Air
Compressor
8 $2,250 $18,000
64. Budget - Capital Costs
64
Total Cost / GH System $3,000
Total Cost / Land $24,000
Irrigation System
65. Budget - Yearly Operating Cost
65
Union City Utilities Electricity (Air
Compressor)
Water (irrigation)
Monthly Rate 1. 8 months $0.0476/kWh
2. 4 months $0.097/kWh
1. $9.56/1000 gal (water)
2. $3.94/1000 gal (sewer)
Monthly Use 2738 kWh/mo 1300 gal/mo
Yearly Cost / GH $2100 $210
Yearly Cost / Land $16,800 $1680
Total Utility Cost / Year: $18,480
66. Budget - Yearly Operating Cost
66
Item Number of Items / GH Cost / Item Total Cost
Fertilizer (bag) 1 $65 $65
Seeds 28,500 $0.00012 $4.00
Pesticides (bottle) 36 $18.75 $675
Total Cost / Year: $744
Total Yearly Cost / Land: ~$5950
67. Year 1 Total Cost
67
Capital Cost $178,500
Annual Cost $24,550
Total Year 1 Cost $203,000
68. Sustainability Measures
โ Economic
โ Utilizes vacant space
โ Ecological
โ Uses less water than conventional farming
โ Uses less land than conventional farming
โ Social
โ Provides nutrient-rich produce for โfood desertโ community
โ Ethical
โ Efficiently uses resources and funding
โ Grows nutritional, low cost produce
68
69. Conclusion
โ Zero acreage farming
โ 3.6 times more biomass yield
โ Design overview
โ A-frame
โ Greenhouse, Sun
โ NFT, Leafy greens, Neem oil
โ Greenhouses vs. Yuma, AZ transportation
โ Through non-profit organizations, this could be more viable
โ Could be coupled with conventional farming
69
70. User - Consumer/Community
โ How much space will the farm consume?
โ 1.6 ac or 6,475 m2
โ What crops can be grown?
โ Leafy greens
โ How will this design increase accessibility to produce?
โ Designed in close proximity to Food Desert community
70
71. Client - County
โ How much will the design cost?
โ $203,000
โ How will this farm increase community health and wellbeing?
โ Helps increase access to healthier foods
โ Educate youth on valuing personal health & nutrition
โ How much food will the farm produce?
โ 31,416 kg/year
71
72. Designer - A.B.C.S. Engineering Firm
โ What three crops are most beneficial for consumers?
โ Kale, Spinach, and Romaine Lettuce
โ What kind of water and nutrient needs do the crops require?
โ Each require about a ratio of 16-4-17 and about the same water requirement of 1
in/week
โ How will we obtain the water needed?
โ Municipal water will be used
72
86. References
1. Thomaier, S., Specht, K., Henckel, D., Dierich, A., Siebert, R., Freisinger, U. B., & Sawicka, M. (2015). Farming in and on urban buildings: Present practice and specific novelties of zero-acreage farming
(ZFarming). Renewable Agriculture and Food Systems, 30(1), 43-54. doi:http://dx.doi.org/10.1017/S1742170514000143
2. Treuhaft, S., Karpyn, A. (2010).The Grocery Gap: Who Has Access to Healthy Food and Why It Matters. PolicyLink. Retrieved from http://thefoodtrust.org/uploads/media_items/grocerygap.original.pdf
3. AeroFarms. (2004). Our Story: Environmental Impact. Retrieved from http://aerofarms.com/environmental-impact/
4. United States Department of Agriculture (1993). The National Engineering Handbook, Part 623, 19-22, 69-70.
5. Ward, April. (2015). โ2015 Crop Report Shows Large Volume of Leafy Greens Grown Under Government Inspectionโ. LGMA: California Leafy Green Products. Retrieved from
http://www.lgma.ca.gov/2016/07/2015-crop-report/
6. Coleman-Jensen A., Rabbitt M.P., Gregory C.A., Singh A. (2017). Household Food Security in the United States in 2016. Economic Research Report Number 237. U.S. Department of Agriculture,
Economic Research Service.
7. Lages Barbosa G, Almeida Gadelha FD, Kublik N, et al. (2015). Comparison of Land, Water, and Energy Requirements of Lettuce Grown Using Hydroponic vs.Conventional Agricultural Methods.
Bhamidiammarri R, Tota-Maharaj K, eds. International Journal of Environmental Research and Public Health. 2015;12(6):6879-6891. doi:10.3390/ijerph120606879.
8. Global Agriculture. (2014). Water: Facts and Figures. Aquastat. Retrieved from http://www.globalagriculture.org/fileadmin/files/weltagrarbericht/AquastatWithdrawal2014.pdf
9. . Ploeg, M. V., Rhone A. (2017). United States Department of Agriculture: Food Access Research Atlas.
https://www.ers.usda.gov/data-products/food-access-research-atlas/about-the-atlas.aspx#definitions
10. Mattison, N. S., Peters, C. Inside Grower Magazine: A Recipe for Hydroponic Success.Cornell University, 16-19.
11. S. Zolnier, G. B. Lyra, R. S. Gates (2004). Evapotranspiration Estimates for Greenhouse Lettuce Using Nutrient Film technique. Transactions of the ASAE. American Society of Agricultural Engineers.
47. 271-284.
12. Wilson A. Lennard, Brian V. Leonard (2006). A comparison of three different hydroponic sub-systems (gravel bed, floating and nutrient film technique) in an Aquaponic test system. Aquaculture
International. 14(6), 539-550. https://doi.org/10.1007/s10499-006-9053-2
13. James E. Rakocy, Michael P. Masser, Thomas M. Losordo (2006). Recirculating Aquaculture Tank Production Systems:AquaponicsโIntegrating Fish and Plant Culture. Oklahoma Cooperative
Extension Service. 454. http://factsheets.okstate.edu/documents/srac-454-recirculating-aquaculture-tank-production-systems-aquaponics-integrating-fish-and-plant-culture/
86
87. References
14. Bigelow, D. P., Borchers, A. (2017). Major Land Uses in the United States, 2012. United States Department of Agriculture, Economic Research Service.
https://www.ers.usda.gov/webdocs/publications/84880/eib-178_summary.pdf?v=42972
15. Qadir, M., Quillerou, E., Nangia, V., Murtaza, G., Singh, M., Thomas, R.J., Drechsel, P., Noble, A.D. (2014). Economics of salt-induced land degradation and restoration. Natural Resources Forum, 38(4), 282-295.
http://onlinelibrary.wiley.com/doi/10.1111/1477-8947.12054/full
16. University of California Cooperative Extension: Vegetable Research & Information Center. (2017). San Joaquin Valley Agriculture. Retrieved from http://vric.ucdavis.edu/virtual_tour/sanjoq.htm
17. Hudson, Andrew. (2013). Environment and Energy: Ocean Hypoxia - โDead Zonesโ. New York, New York: UNDP.
18. UNESCO. (2012). The United Nations World Water Development Report 4: Managing Water Uncer Uncertainty and Risk. Vo1. 1, pp. 46-184. Retrieved from
http://www.unesco.org/new/fileadmin/MULTIMEDIA/HQ/SC/pdf/WWDR4%20Volume%201-Managing%20Water%20under%20Uncertainty%20and%20Risk.pdf
19. Loudenback, T. (2017). Rich Millennials are Ditching the Golf Communities of their Parents for a New Kind of Neighborhood. Business Insider. Retrieved from
http://www.businessinsider.com/agrihoods-golf-communities-millennial-homebuyers-2017-10
20. Burgess, M., Pimentel, D. (2013). Soil Erosion Threatens Food Production. Agriculture, 3(3), 443-463. http://www.mdpi.com/2077-0472/3/3/443/htm
21. Letery, J. (2000). Soil salinity poses challenges for sustainable agriculture and wilflife. California Agriculture, 54(2), 43-48. http://calag.ucanr.edu/Archive/?article=ca.v054n02p43
22. N. Mattson, C. Peters. (2014). A Recipe for Hydroponic Success. Cornell University. Retrieved from http://www.greenhouse.cornell.edu/crops/factsheets/hydroponic-recipes.pdf
23. D. Sanders. (2001). Spinach: Horticulture Information Leaflets. NC State Extension. Retrieved from https://content.ces.ncsu.edu/spinach
24. D. Sanders. (2001). Beets: Horticulture Information Leaflets. NC State Extension. Retrieved from https://content.ces.ncsu.edu/beets
87
88. References
25. Bonnie Plants. (2011). Growing Kale. Retrieved from https://bonnieplants.com/growing/growing-kale/
26. VeggieHarvest. (2017). Kale Growing and Harvest Information. Retrieved from http://veggieharvest.com/vegetables/kale.html
27. H7OPOLO. (2015). Weight of Kale (Derived Mathematically). Wordpress. Retrieved from https://h7opolo.wordpress.com/2015/04/01/kale/
28. Sustainable Seed Company. (2017). Siberian Dwarf Kale Seeds. Retrieved from http://sustainableseedco.com/siberian-dwarf-kale.html
29. NOAA. (2017). Gulf of Mexico โdead zoneโ is the largest ever measured: June outlook foretold New Jersey-sized area of low oxygen. Retrieved from
http://www.noaa.gov/media-release/gulf-of-mexico-dead-zone-is-largest-ever-measured
30. OECD. (2012). Environmental Outlook to 2050: The Consequences of Inaction. Retrieved from http://www.oecd.org/environment/indicators-modelling-outlooks/49846090.pdf
31. FAO. (2012). Livestock and Landscapes. Retrieved from http://www.fao.org/docrep/018/ar591e/ar591e.pdf
32. Burrage, S.W. (1999). THE NUTRIENT FILM TECHNIQUE (NFT) FOR CROP PRODUCTION IN THE MEDITERRANEAN REGION. Acta Hortic. 491, 301-306
33. Colonna, E. Rouphael, Y. Barbeiri, G. De Pascale, S. (2015). Nutritional Quality of Ten Leafy Vegetables Harvested at Two Light Intensities. Retrieved from https://doi.org/10.1016/j.foodchem.2015.12.068
34. Lefsrud, M. G. Kospell, D. A. Sams, C. E. (2008). Irradiance from Distinct Wavelength Light-emitting Diodes Affect Secondary Metabolites in Kale. HortScience, 43(7). 2243-2244.
35. Sohi, I., Bell, B. A., Liu, J., Battersby, S. E., Liese, A. D. (2014). Differences in food environment perceptions and spatial attributes of food shopping between residents of low and high food access areas. Journal of
Nutrition Education and Behavior, 46(4), 241-249. Retrieved from https://doi.org/10.1016/j.jneb.2013.12.006
88
89. References
36. . Hydroponics-Simplified. (2016). Organic Pesticides. Retrieved from https://www.hydroponics-simplified.com/organic-pesticides.html
37. USDA. (2017). Economic Research Service: Go to the Atlas. United States Department of Agriculture. Retrieved from https://www.ers.usda.gov/data-products/food-access-research-atlas/go-to-the-atlas/
38. Harrington, J. (2017).Soil Needs of Romaine Lettuce. San Francisco Chronicle. Retrieved from http://homeguides.sfgate.com/soil-needs-romaine-lettuce-74033.html
39. Seasoned Advice. (2012).How much is a bunch of spinach?. StackExchange. Retrieved from https://cooking.stackexchange.com/questions/24637/how-much-is-a-bunch-of-spinach
40. MSF. (2016).Use Your Head When it Comes to Lettuce. Money Smart Family. Retrieved from https://moneysmartfamily.com/blog/head-lettuce/
89
