Tree Fruit Soils and Nutrition











 

 

Soil pH

Soil pH is one of the most indicative measurements of the general chemical status of soil. Knowing the pH of your soil may help answer the following questions:

  1. Do I need to add lime or sulfur to my soil?

  2. What plant nutrient problems might I face at a given soil pH and how might I correct these problems?

  3. Which fertilizers should I use (i.e., some fertilizers will acidify the soil and others may raise the pH)?

  4. Can I tank-mix a particular pesticide with a particular fertilizer?

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Because soil pH is typically measured as soil-solution pH, it is  also an indicator of the proportions of basic and acidic exchangeable ions present in the soil.  This is because these ions in the soil solution are in equilibrium with the exchangeable ions (see also Cation-exchange capacity). 

 

The definition of pH is a measure of the activity of the hydrogen ions [H+] and is expressed according to the equation:

pH = -log[H+]

Because the equation uses a logarithmic scale, a one-unit decrease in pH is a ten-fold increase in hydrogen ions.  For example: A pH value of 5.5 is 10 times as acidic as a pH of 6.5 and 100 times more acidic then a pH of 7.5. 

 

A pH value of 7.0 is considered neutral because there are equal concentrations of H+ and OH-.  Values below 7.0 indicate acidity and values above 7.0 indicate alkalinity.

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Native soil pH is dependent upon soil minerals and amount of rainfall. The pH values of low rainfall areas of Central and Western Washington soils are commonly in the neutral to alkaline range.  Some soils have a pH of 7.8 to 8.3.  These alkaline values are indicative of calcareous soils (soils containing calcium carbonate).  Soils in the moderate to high rainfall areas tend to be acidic.  

 

Ideally, a range of 6.0 to 7.5 is optimal for orchards; however, excellent orchards occur on soils ranging in pH from 5.0 to 8.0.  In general, the availability of micronutrients is lower in alkaline soils.  Nutrients such as iron and zinc may not be in a form available to plants. In contrast, phosphorus may become limiting in acid soils.  Also, in acid soils, aluminum can become available. It is not a nutrient and is toxic to plants in high concentrations.  At pH 6 and higher, very little aluminum is in solution.  

 

Additionally, soil pH affects the abundance of microorganisms.  Bacteria are generally more prevalent in alkaline soils and fungi dominate in acidic soils.  This is important because microbes are responsible for the cycling of nutrients.  The most diverse and numerous populations are found in near-neutral soils.  Furthermore, soil pH influences pathogenic microbes, and growers can adjust pH to manage some plant diseases.

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Nutritional problems associated with low pH (<5.0)

  • bark measles of Delicious apple

  • manganese (Mn) toxicity 

  • calcium (Ca) and magnesium (Mg) deficiencies

  • restricted root growth or regeneration due to aluminum (Al) toxicity

  • reduced availability of P

  • reduced efficiency of N and K use and poor response to N and K fertilizers

  • bark necrosis

  • stunted growth

 

High pH values may also lead to nutritional problems.  

  • The availability of many micronutrients (Mn, Cu, Zn, and B for example) decrease as soil pH increases.

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Effect of fertilizers on soil pH

Different fertilizer materials vary in their impact on soil pH. The following generalizations may be used as a guide:

  • Ammonium (NH4+) or ammonium forming fertilizers (ex. urea) will cause a decrease in soil pH over time.

  • Nitrate (NO3-) sources carrying a basic cation should be less acid-forming then NNH4+ fertilizers.

  • The presence of Ca, Mg, K, and Na in the fertilizer will slightly increase or cause no change in soil pH.

  • Elemental sulfur, ammonium sulfate, and compounds such as iron or aluminum sulfates can reduce the soil pH

Acidic soils may be neutralized over time with the application of lime.  The amount of lime required to adjust the pH of a soil to a desired value is termed "lime requirement." This amount is dependent upon the buffering capacity of the soil and how much the pH needs to be adjusted.  The higher the amount of clay and organic matter, the higher the buffering capacity and more lime needed. There are many sources of lime available. Dolomitic lime also contains magnesium as well as calcium but is slower acting than agricultural lime (calcium carbonate).

 

Gypsum is calcium sulfate and has very little effect soil pH.  In areas where excess sodium is a problem, it can help improve soil structure. 

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Updated November 22, 2004

 

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