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5.3 Acidification 135

Table 5.5 Critical levels of manganese for some plants (Upjohn et al. 2005)
Manganese tolerance Critical leaf Mn
category Plants level mg kg −1
Highly sensitive Alfalfa, pigeon pea, barrel and bar medics 200–400
Sensitive White clover, strawberry clover, chickpea, canola 400–700
Tolerant Subclover, cotton, cowpea, soybean, wheat, barley, 700–1,000
triticale, oats
Highly tolerant Rice, sugar cane, tobacco, sunflower, most pasture >1,000
grasses, oats, triticale, tiga, currency, cereal rye

sorghum (Sorghum bicolor) (Mgema and Clark 1995), and wheat (Triticum aestivum
L.) (Taylor et al. 1991). Aluminum sensitivity of some plants is shown in Table 5.4.
Critical levels of Mn in some plants are shown in Table 5.5.

5.3.2.3 Plant Growth

A slightly acidic soil (a pH around 6.5) is the optimum requirement of most terrestrial
plants. At this pH soil microorganisms are most active, and plant nutrients are readily
available. At extreme alkaline and acid conditions, this delicate balance is disturbed,
and plant nutrients that were in adequate supply can become either deficient or toxic
to plant growth. Some essential nutrients such as phosphorous, calcium, magnesium,
and molybdenum become unavailable if the soil pH becomes too acid. Acid condi-
tions will result in a lowering of plant production in farming systems leading to
reduced profitability and an increased reliance on fertilizers to sustain any form of
productive agriculture. Plants have pH preferences. A list of plants showing their
suitable pH ranges is cited from Jett (2005) below:
pH range Suitable crops
4.5–5.5 Azalea, Camellia, Rhododendron
5.5–6.0 Blackberry, chicory, gooseberry, magnolia, pineapple, potato, oats, rosemary
5.5–6.5 Apple, avocado, barley, cranberry, capsicum, melon, turnip, strawberry, wheat
6.0–6.5 Broad bean, bean, carrots, lemon, lentil, olives, parsley, pear, pumpkin, raspberry,
soybean, squash, watermelon, white clover
6.0–7.5 Asparagus, broad bean, broccoli, cabbage, cauliflower, celery, cherry,
chrysanthemum, cucumber, dahlia, French bean, garlic, grape, horse radish,
lettuce, mulberry, mustard, onions, peach, pear, peas, peppers, plum, radish,
spinach, sweet corn, tomato, turnips
6.5–7.5 Alfalfa, asparagus, avocado, barley, sugar beet, beetroot, grape vine, mushroom,
spinach

5.3.2.4 Microorganisms

Soil acidity limits Rhizobium survival and persistence. Elevated levels of aluminum
are toxic to the growth of Rhizobia affecting nodule initiation and the nitrogen