Rosie posted a great link about nutritional deficiency: http://www.enst.umd.edu/people/Weil/Symptoms/Iron.cfm
Here's a link from Australia that shows what's deficient in alkaline soil, and what's deficient in acidic soil: http://www.agric.wa.gov.au/objtwr/imported_assets/content/pw/gard/nutritional_gn_formatted.pdf
Acid soil is more common in England, so I'll post the info. for acid soil by Wikipedia:
"Problems in in acid soils: aluminium (Al), hydrogen(H), and/or manganese(Mn) toxicity, and nutrient deficiencies of calcium (Ca) and magnesium(Mg). Aluminium toxicity is the most widespread problem in acid soils ..."
My soil in Chicagoland is rock-hard alkaline clay. I bought a soil-test kit from HomeDepo - it showed false reading, neutral. I bought a second kit from Lowe's, it asked for one teaspoon of soil ... not accurate! EarthCo. soil-testing company tested 1 cup of my soil to be alkaline, at pH 7.7, super-high in magnesium.
Even a $200 pH meter needs to be re calibrated. The fish-tank litmus paper sold at Walmart for $5 is used to test your water, and not your soil. Red cabbage is accurate, since it allows time for soil elements to dissolve.
Shopping list: 50 cents red cabbage and $1 distilled water. Rain water is acidic (pH 5.6) versus my tap water at pH 8. That's why you need DISTILLED WATER to boil red cabbage, resulting in a VIOLET solution. When I boiled red cabbage in my pH 8 water, I get a BLUE solution... shortcut to see if your water is alkaline or neutral.
pH of indicator samples: pine bark (4.5), tomatoes (4 to 4.5), potatoe (5.4 to 5.9), whole wheat bread (pH 5.5), frozen cooked peas (6.4 to 6.7), cooked brocolli (6.3 to 6.5), cow's milk (6.4 to 6.8 pH), lettuce (5.7 to 6), cooked oatmeal (6.2 to 6.6), cooked rice (6 to 6.7), dry tea (pH 7.2), and cooked corn (above 7.3).
I use tiny plastic cups and place indicator-samples in separate cups: 1 teaspoon vinegar (pH 2.2), 1 teas. baking soda (pH 8.3), 1 tablespoon of MiracleGro potting soil green bag (pH 6.5), 1 tablespoon of peatmoss (pH 4), 1 tablespoon used coffee grounds (neutral pH), 1 tablespoon of MG Moisture Control potting soil (neutral pH).
Soil samples from different places in your garden placed in PRE-LABELED cups.
If you test 10 samples, chop a quarter of a cabbage head. Boil in 4 cups of DISTILLED WATER for 10 minutes, discard the solids. Pour hot juice over samples, and WAIT FOR AT LEAST 30 MINUTES.
If your soil is neutral, the juice will be clear like coffee ground or Organic potting soil (both are buffers, takes time to clear up). Most acid is fuchsia pink like vinegar. Dark pink is pH 4 (peat moss, pine bark, and tomatoes). Medium pink is pH 5.5 (leaves, potatoe, and bread). Slight pink is pH 6.5 (MiracleGro green bag, milk, oatmeal, and rice).
Light blue is slightly alkaline (decomposed grass or tea), darker blue is pH 7.5 (cooked corn), blue with green tinge is my 7.7 pH soil, more green is baking soda (pH 8.3), and bright green is wood ash at pH above 10.
You can also use fresh leaves - but this varies. See http://www.asecular.com/forests/phleaves.htm
Most acidic is Eastern Redbud (pH 4.3), Virgninia pine (pH 4.4), sugar maple (4.5), black maple (5.4), black walnut (pH 4.6), white oak (4.6), black oak (5 to 5.5), white ash (5.8 to 6.1), American Beech (5.8 to 6.9), flowering dogwood (5.5 to 6), Slipper elm (7 to 7.9), Hackberry at pH 8. If the leaves are fully-decomposed over a year, they become neutral to alkaline, according to U. of Illinois Extension. Same with decomposed grass, slightly alkaline.
Coffee ground is pinkish at first, then it clears up after 1/2 hour. Coffee ground is a buffer, neutral pH. MiracleGro ORGANIC potting soil (brown bag) also tested neutral, best for seedlings in pepper plants experiment.
Some cities put lime in their water, raising the pH of tap water, so pipes won't get corroded. MiracleGro indoor potting soil (green bag) has a pH of 6.5. After months of watering with my tap water at pH 8, I tested the pots again and the red-cabbage juice turned blue, slightly alkaline.
Here's a great link that shows color changes and the corresponding pH:http://www.erowid.org/archive/rhodium/chemistry/equipment/pictures/ph-cabbage.jpg
Here's another link that shows color changes of red cabbage juice: http://www.abc.net.au/science/surfingscientist/pdf/lesson_plan16.pdf
Problems with high pH: Iron, manganese, copper, zinc, potassium, and especially phosphorus deficiency ... I'll include the link to Cornell University on how to identify boron deficiency, which can happen at neutral pH. http://www.greenhouse.cornell.edu/crops/factsheets/boron_deficiency.pdf