ABOUT PLANT PH
 

Many of our plant species can only thrive at a narrow soil pH, ranging from slightly acidic to neutral; or at slightly alkaline pH.

Soil pH is one of the most important measures of soil fertility. pH is necessary for maintaining plant health and maximizing yield.

PH is a measure of how acid or alkaline a substance or solution is. The pH measurement scale starts at (very acid) at 0.0 to 14.0 (very alkaline); with pH
7.0 being neutral.

Vinegar is an example of an acidic substance and baking soda is an example of an alkaline substance. Pure water is neutral at 7.0

The importance of soil pH is that it directly affects the amount and kind of nutrients that your plant receive. Most of the nutrients that plants need for strong growth are only available only at around pH 6.5 - 7.0.

Why You Should Know Your Soil pH?

Essential minerals and nutrients that plants require for healthy growth is dictated by soil pH. At very high or very low pH, some important minerals remain  bound to the soil, becoming soluble only when the soil pH changes.

For example, nitrate nitrogen is soluble at pH greater  than 5.5 only. At pH below 5.5, nitrogen is bound to the soil matrix. Phosphorus that plants require for growth is relatively immobile  and insoluble in soil.
It changes to a soluble form only at about pH 6.5. Above soil pH 6.5, phosphorus, iron, manganese, copper and zinc become insoluble.

Beneficial soil microorganisms that we want to encourage to flourish in our garden soil desire a pH range that is slightly acid or alkaline.

Plants that grow in soil either above or below their preferred pH range are affected either by an abundance of  nutrients that they cannot use or/and a depletion of essential nutrients. These plants will therefore suffer from nutrient deficiency.

This results in loss of plant vigor leading to decline, susceptibility to insect and disease attack; and  eventual decline and death.

What Happens When Soil Is Too Acidic?

When soil is too acidic, plants cannot utilize nitrogen, phosphorus, potassium (NPK) and other micronutrients. Iron, manganese and  aluminum become more soluble at pH of 4-5, and become toxic to plants; while other essential elements such as calcium and magnesium, become tied-up in the soil.

When soil pH gets more acid, toxic metals such as lead and arsenic become soluble, resulting in their uptake and accumulation in plant tissues.

When edible plants are grown in these soils, these toxic compounds can accumulate in plant tissues up to a dangerously high level and endanger our health.

Plants are able to absorb only very small quantities of lead at pH of 6.6 or higher pH. Arsenic is least available at pH 7.0. Therefore, adjust your soil pH to 7.0 whenever lead and arsenic are present together.

       What Happens When Soil Is Too
                      Alkaline?
    At 6.5 and above, phosphorus and most
      micronutrients get less soluble and therefore
      become less available to your plants. As soil
      pH rises to 7.4 to 7.8 or higher, other important
      plant micronutrients such as iron, manganese, boron,
      copper and zinc become  immobilized.
        
            Soil pH And Soil Microorganisms


     The severity of several plant diseases caused
       by many soil pathogen is directly affected by
       soil pH. Good examples are the soybean cyst     
       nematode and soybean brown stem rot.
       The soybean brown stem rot disease gets
       more severe as soil pH decreases. On the
       other hand, the soybean cyst nematode
       infestation is worse when soil is alkaline.
       Manipulating soil at a certain pH is currently
       being used in the management of these
       soybean diseases.

  The potato scab disease, is another plant
  disease affected by soil pH. The disease
  becomes more severe as the pH increases.
  Control or suppression of this disease
  organism is achieved  by adjusting soil pH
  to 5.2 or lower.

Soil bacteria that are useful in the decay of
soil organic matter flourish and gets over
active at high pH over 7.5. This is bad
because when this happens, they use up
organic matter at a very high rate. This in
turn creates a problem because the C/N
ratio balance in the soil is upset. The end
result is soil nutrient depletion.

    Why Soil gets more Acidic with Time

      The pH of your soil  is dynamic, never constant
      or static. It can change from season to season     
      and from year to year. The change is due to
      many factors such as what is grown there,
      soil amendments and amount of compost
      applied. Also, with time, soil reaction naturally    
      becomes more acidic due to leaching away
      of calcium, magnesium, potassium, and
      sodium by rain water.

     Decaying organic matter from manure and 
     normal respiration activity occurring in plant
     roots produce carbon dioxide that result  in
     the production of carbonic acid.

Strong acids in soil are produced by
excessive application of ammonium and
sulfur fertilizers. Strong acids destroy the
nutrient content, microbial content, and
texture belonging to good soil.

When "sweet" soil deteriorates and turns
poor and acidic, the biggest culprit is over
fertilization.

The current human mentality is that "more
is better". Excessive nitrogen fertilizer is
currently being dumped into crop land, fields,
home gardens, and turf with the expectation
that their plants will grow bigger, greener
and faster. Unused nitrogen from these
fields ends up polluting our streams, rivers
and drinking water. The balance of the
nitrogen not used up by plants are washed
away to be oxidized later into strong acids
that destroy our soil.

           Soil Remediation

To fix your acidic soil problem, apply calcium
limestone, dolomitic limestone, and wood ash.
To make your alkaline soil more acidic, use
elemental sulfur, iron sulfate (fast acting), organic sphagnum peat, ammonium sulfate or
urea as nitrogen source.

When designing your garden, it is wise to group together plants with similar soil pH demands. A comprehensive soil test is a good investment. Not only will you know the composition and quality of
your soil, you will also receive good recommendations on how to adjust the pH of your
soil for the plants you want to grow there. Most importantly, you will also get advice on the amount and type of fertilizer to apply - to eliminate the problem of fertilizer dumping.

pH testing kits are readily available from your garden center to test the soil pH yourself. For more comprehensive tests, it is best to send your soil sample to your county extension office soil-testing
lab. Privately owned soil testing labs can also do the same tests for you.
 
If your garden area is situated in a previous
manufacturing site or an old fruit orchard, it may be wise to include checks for heavy metal accumulation such as lead and arsenic. Prior to 1988, lead arsenate was approved for control of fruit tree pests.

If your land has a history of intensive farming, check also for presence of persistent pesticides.

A comprehensive soil test involves testing for:

     -  soil pH

     - nutrient levels of nitrogen, phosphorus
       and potassium

     - test for the presence of toxic chemicals,
       persistent pesticides, PCBs and heavy
       metals ( mercury lead, arsenic ).

     - soil texture, composition (amounts of sand,  
       clay, silt and organic matter )

     - recommendations of how to fix your soil
       problem

  Publication cited :

  Soil pH Influences Soybean Disease Potential.
  http://www.planthealth.info/yields2/grau2.pdf

        Realated Article: Plant pH Chart    

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