Friday, 18 September 2015

Changing Soil pH

Changing Soil pH to Match Plant Needs 

If your spinach is spindly and your tomatoes are troubled, changing soil pH may help. “pH” refers to potential hydrogen, or the hydrogen ion concentration of soil. pH is a measure of soil acidity.  

The pH scale ranges from 1 to 14. It’s a logarithmic scale, like the Richter scale to measure earthquakes. A soil with a pH of 6 is ten times more acidic than a soil with a pH of 7.  Neutral pH is 7.0. A soil with a pH lower than 7 is an acidic soil. A soil with pH higher than 7 is an alkaline soil. Soil acidity determines the availability of mineral nutrients for your vegetables. In alkaline soils, phosphorous, iron, and zinc are limited. In acidic soil, calcium and magnesium are less available to plants. 

Lowering Soil pH  |  Buffering Soil pH  |  Raising Soil pH 
Soil pH varies by up to half a point over the year. Soil pH tends to be higher (more alkaline) when the soil is cool, and lower (more acidic) in summer, when increased bacterial activity in warmer weather has an acidifying effect on soil. Factor this in when changing soil pH. Garden soil pH is usually neutral to slightly acidic, pH 6.5-7.0. This also happens to be the ideal soil pH for vegetables. 

However, if you want to really tweak performance: 

Green, leafy vegetables (like spinach and lettuce), cruciferous vegetables (like broccoli, cauliflower, cabbage, and kale), and other Winter Vegetables prefer a more alkaline soil, pH 7.0-7.2. 

Fruiting plants, like nightshades (tomatoes, peppers, eggplants) and cucurbits (cucumbers, melons, squash) prefer a more acidic soil, pH 6.0-6.8. See  Summer Vegetables for more information on the preferences of these summer favorites 

Adding organic matter is an indirect method of soil pH adjustment. Organic matter “buffers” soil, especially sandy soil.  The higher the organic matter content of a soil, the more lime it takes to raise the soil pH 1 point, and the more sulfur it takes to lower the soil pH 1 point. 

Plants grown in soil with a lot of organic matter have healthier roots. They’re able to extract enough nutrients from the soil even when the pH isn’t optimal.  In a healthy soil with adequate organic matter, changing soil pH may not be necessary, because plants continue to grow at pH levels that would stunt growth in leaner soils.  

When you increase soil organic matter, you’re not really changing soil pH, you’re increasing your plants’ tolerance for acidic or alkaline conditions. For information on increasing soil organic matter, see how to improving Garden Soil. 

Lowering Soil pH 

Why would you want to lower soil pH?  

If soil pH testing indicates your soil is greater than 7.0, you have an alkaline soil, and changing soil pH may be called for, depending on what you’re growing.  As soil acidity increases, minerals like phosphorous, iron, and zinc become more available. 

In alkaline soils, these minerals—especially iron and zinc—are bound up and less available. Fruiting plants like tomatoes, peppers, squash, and melons need these minerals to flower and set fruit. Soil acidification also makes life more difficult for many weeds. Weeds are early successional plants that evolved in thin, alkaline soils. In acidic soils, many weeds are weaker competitors.  

Ways to Lower Soil pH (Make Soil Acidic) 

Soil pH can be lowered by half a point—from 7.0 to 6.5, for example—by increasing soil nitrogen. Adding compost, manure, or organic soil amendments like alfalfa meal to the soil can help drop pH over time by increasing bacterial populations.  

There's a myth that coffee grounds (2-0-0) are a quick fix for lowering soil pH. Most of the organic acids in coffee are water-soluble, and flush out into the brew. Coffee grounds have a pH around 6.8, close to neutral, so they won't do much to lower pH. They do add a little nitrogen, so they can help reduce pH over time, just like manure or compost.  

If you need to drop soil pH more quickly, try watering your plants with leftover (cold) coffee, diluted 50-50 with water. This works especially well for houseplants and container vegetables. To lower soil pH by larger amounts (more than half a point), use Elemental Sulfur, sometimes called “Flowers of Sulfur”.   

When using sulfur for changing soil pH, be aware that the acidifying effect depends on soil bacteria (thiobacillius), which oxidize the sulfur and release dilute sulfuric acid into the soil over a period of weeks to months.   

Because the acidifying effect of sulfur depends on soil bacteria: 

The sulfur must be dispersed through the soil to be in contact with these bacteria. Make sure you mix the sulfur thoroughly into the soil. Otherwise, there will be strongly acidic areas around blobs of sulfur, and no effect elsewhere in the soil.  

Sulfur only works during the summer, when the soil is warm and bacterial activity is at its highest. 

Sulfur is not a quick-fix for changing soil pH. After application there is a delay of several weeks to several months before soil bacteria break down the sulfur to acidify the soil. 

Elemental sulfur is acceptable as an organic soil amendment for changing soil pH under National Organic Program (NOP) guidelines. 

When using elemental sulphur for changing soil pH, it's best to divide the amount to be applied to achieve the desired drop into 2 or 3 applications over the entire season, instead of a single application. Applications should be 6-8 weeks apart. 

NOTE: Application amounts in the table below apply to loam soil. 2.4 lbs of elemental sulphur (per 100 square feet) will drop loam soil pH by 1 point. 

  • For Clay Soil, INCREASE amounts by half (50%). 
  • For Sandy Soil DECREASE amounts by one-third (33%). 

 Present Soil pH 
 To pH 6.5 
 To pH 6.0 
 To pH 5.5 
 To pH 5.0 
 To pH 4.5 
3.6 lbs 
4.8 lbs 
6.0 lbs 
7.2 lbs 
8.4 lbs 
2.4 lbs 
3.6 lbs 
4.8 lbs 
6.0 lbs 
7.2 lbs 
1.2 lbs 
2.4 lbs 
3.6 lbs 
4.8 lbs 
6.0 lbs 
1.2 lbs 
2.4 lbs 
3.6 lbs 
4.8 lbs 
1.2 lbs 
2.4 lbs 
3.6 lbs 
Pounds of Elemental Sulfur Needed for Reducing Soil pH  
(100 square feet of Soil 6” Deep, LOAM soil) 

NOTE : 1.2 lbs is around 543 gms. 

Raising Soil pH 

Why would you want to raise soil pH? 

If you’re growing fall or cool-season vegetables, these green, leafy vegetables perform better in soils with a slightly higher pH, between 6.8 and 7.5. 

Ways to Make Soil More Alkaline (Reduce Acidity) 

Dolomite Lime, (calcium magnesium carbonate), is the most common soil amendment for raising soil pH (reducing acidity). It’s used by both organic and conventional farmers, but should Not be used in soils with adequate or excess magnesium.  

Plants need magnesium in small amounts, and excess magnesium stunts and kills vegetables. Where a soil test indicates adequate or high magnesium levels, use an alternate calcium source for changing soil pH.  

The following table provides application rates according to soil textural type:  

 Soil Texture Type 
 Raise pH 1 pt (4.5 to 5.5) 
 Raise pH 2 pts (5.5 to 6.5) 
Sandy & Loamy Sand 
2.3 lbs 
2.75 lbs 
Sandy Loam 
3.6 lbs 
5.9 lbs 
5.5 lbs 
7.8 lbs 
Silt Loam 
6.9 lbs 
9.1 lbs 
Clay Loam 
8.7 lbs 
10.5 lbs 
17.4 lbs 
19.7 lbs 
Pounds of LIME Needed for Changing soil pH  
(100 square feet of Soil 7” Deep) 

NOTE : 1.2 lbs is around 543 gms. 

You can see the buffering effect of soil organic matter in the table above. As organic matter increases with each soil textural class, the amount of lime needed for changing soil pH increases significantly.  

The same thing happens when you’re lowering soil pH with sulfur. The heavier your soil, the more sulfur it takes to drop the pH 1 point.  

Clay and loam soils give you more wiggle room when changing soil pH. Sandy soils, not so much.  

Where dolomite lime may create magnesium toxicity, use any of the following alternative calcium sources instead:  

Ground Oyster Shell (1-2 lbs/ 100 sq ft) 
Dried, Crushed Eggshells (1-0.4-0), a kitchen byproduct, are a great source of calcium and a good method of changing soil pH to reduce acidity. (1-2 lbs/ 100 sq ft) 
Hardwood Ashes Short-term soil pH adjustment. (Use up to 1.5 lbs/ 100 sq ft.) 
Calcite (high-calcium lime). Use up to 1.5 lbs/ 100 sq ft. 

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