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How to Change soil pH

By Edited Nov 13, 2013 0 0

So you want to change your soil pH but you haven’t the foggiest how to do it.  Well your in luck I have had people asking me recently just how does one change your pH.  Well I'm hear to tell you its not to complicated and isn’t even that expensive if done correctly.  When you changing your pH you can only go one of two ways.   Either you are making your soil more acidic or you are making it more basic a.k.a alkaline.  Changing soil pH is done for a couple of reasons, the main one being that a person or persons want to be be able to grow a certain type of plants in that that location.  Most plant prefer soil that are in the 6.5-7.5 range in pH.  It is here that soil nutrients are the most available for plant uptake.  But not all soils are in this “sweet spot” of soil pH some need to be altered in order to provide maximum productivity 

 The first step you must take is to determine what direction you are going.  This can be done through a few simple at home tests that I explain more about in another article.  If you haven't tested your soils pH I would suggest going to that article and read up on a few techniques on how to do that.  If you know where you are at and you know where you are going then all it takes is some simple math to do the rest.

 

 Lowering the pH

 There are many materials or chemical that can be used to alter pH.  If you wanted to make your soil more acidic you would use substances like,

  • Sphagnum Peat Moss

  • Elemental Sulfur

  • Aluminum sulfate

  • Iron sulfate

  • Acidifying Nitrogen (ammonium nitrate)

  • Organic mulches

 These substances all vary in cost and in how well and fast they work.  Sphagnum peat moss for example is very desirable for small garden plots and can change the pH of a soil just by working in 1-2 inches before planting.  But it can be expensive so large scale applications are cost prohibitive.  A option for those wanting to apply to large areas would be the elemental sulfur.    Below is a table showing how much sulfur you would need to apply in pounds/10 square feet of loam soil.  These figures need to be altered to suit the soil.  In sandy soils you should reduce the rate by 1/3 and in clays you should increase is by 1/2.

 

Pounds of Sulfer to Lower Soil pH

  Desired pH 6.5 6.0 5.5 5.0 4.5
Present pH  
8.0   .3 .4 .5 .6 .7
7.5   .2 .3 .4 .5 .6
7.0   .1 .2 .3 .4 .5
6.5     .1 .2 .3 .4
6.0       .1 .2 .3
             

 How fast this sulfur will react is dependent on several factors including the fineness of the sulfur particles, how wet the soil is, the presence of soil bacteria and soil temperature. 

 For more immediate results you can used Aluminum sulfate which will begin to acidify the soil upon contact. The application ratio are similar except it must be applied at about 5 times the rate of the sulfur. 

 

Pounds of Aluminum Sulfate to Lower soil pH
  Desired pH          
Present pH   6.5 6.0 5.5 5.0 4.5
8.0   1.8 2.4 3.3 4.2 4.8
7.5   1.2 2.1 2.7 3.6 4.2
7.0   .6 1.2 2.1 3.0 3.6
6.5     .6 1.5 2.4 2.7
6.0       .6 1.5 2.1

 Take care not to over apply with either substance, no more than 2-3 lbs/ 100 sqft for the sulfur and 5 lbs/100 sqft for the aluminum sulfate  and to avoid plant injury wash plants off if they were covered in these substances as they could cause leaf burn.

 Some fertilizers such as urea and ammonium nitrate can also be used to lower a soils pH.  Just make sure to check the label on the fertilizer before application.  It should be able to tell you how much fertilizer to apply as well as how acidic the fertilizer is.  If you have any question about either application rate please contact either the manufacture of the fertilizer, your local university extension agent or a crop consultant.

 

Raising the pH

 If instead you want to raise the soil pH and make it more alkaline you have several options.  The most common of these is the use of limestone.  Limestone contains large amounts of calcium carbonate which when present in the soil make it more alkaline.  For the purposes of agriculture, limestone comes in two types, 

  • pulverized agriculture lime

  • hydrated lime

 The agricultural lime is composed of calcium carbonate and magnesium carbonate which is neutralize acidity in the soil.  The most favored of the two is typically dolomite lime since it is slow acting and contains magnesium which is a crucial plant nutrient that can sometimes be limiting.  The following table shows the amount of agricultural lime that needs to be applied to raise soil pH to 6.5.

 

Pounds of Calcitic or Dolomitic Lime Needed to
Raise Soil pH to 6.5 (Rate= lbs/100 sqaure feet)
Soil pH Sand Loam Clay
6.0 2 lbs 3.5 lbs 5 lbs
5.5 4.5 lbs 7.5 lbs 10 lbs
5.0 6.5 lbs 10 lbs 15 lbs
4.5 8 lbs 15 lbs 20 lbs
4.0 10 lbs 17.5 23 lbs

Hydrated Lime is the strongest form of lime available and is very fast acting.  Its use however should be discouraged except in heavy clay soils due to its strong nature. If over applied, which is easy to do it can burn plants roots and then you have dead plants rather than just struggling ones.

 To change the pH of the soil is not something that can usually be accomplished overnight.  It will take repeated applications over several years to cause any lasting impact and even then without outside inputs the inherent properties of the soil may eventually win out against any changes we make.  However when it is successfully done the results such as better plant growth and yields are worth the work and cost of changing the pH.

 This article is another one in my series on soil and soil sciences feel free to check out what else I have written and make comments or ask questions about what you find there. 

 

JPLarson

 

 

 

Sources: 

http://www.ipm.iastate.edu/ipm/hortnews/1994/4-6-1994/ph.html

 http://www.clemson.edu/extension/hgic/plants/other/soils/hgic1650.html

 http://www.sustland.umn.edu/implement/soil_ph.html

 http://ohioline.osu.edu/agf-fact/0507.html

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