Acid Soil Problems and Strategies

Jr. Stevens, Koske, Thomas J.  |  4/15/2005 12:56:40 AM

Chlorosis in St. Augustinegrass.

This chart shows the influence of soil pH on nutrient availability.

High Magnesium lime; Dolomite.

The soil's pH (acidity level, soil reaction) is a basic foundation of soil fertility and productivity. It is 'step one' in plant production because it regulates the use and efficiency of so many of the soil elements essential to plant health and growth.
 
Much fertilizer is wasted because it is applied to soils that are too low or too high in pH for efficient nutrient uptake. It also influences nutrient toxicities and microbial activity. Let's look at various relationships dealing with the fertility of strongly acid (low pH) soils.

When soil pH is near 5 or less, cation exchange sites are mostly occupied by hydrogen and aluminum ions. Manganese becomes much more soluble and toxicity can occur. I am not usually comfortable with soil pH below 5.5. Acid problems can create deficiencies in the soil supply of available nitrogen, phosphorus, potassium, magnesium, calcium, sulfur and molybdenum. Deficiencies of available manganese, boron, copper and zinc begin to kick in at about pH 5. Plant toxicities of aluminum,manganese and iron also will occur from enhanced availability at these low pH levels and affect root growth and plant development.

Extremes in soil pH affect microbial activity detrimentally, but more so from acidity. Particularly affected in acid soils are those nitrogen mineralization systems involved in organic matter ammonification to NH3 and the nitrification of NH3 to NO3-. Total bacteria and actinomycete fungi decline steadily at soil pH 5.5 and below. They may be replaced by other kinds of fungi.


Field
Observations

1) Acid soil complex - acid soil is a major soil chemical constraint on crops in our humid climates (see a pH versus nutrient availability table). It produces a combination of nutrient deficiencies, elemental toxicities and high soil strength (tight soil, compaction, poor tilth). It is particularly a problem on non expanding clay soils (common in the humid South) such as our Kaolinites, iron/aluminum oxides and Allophanes. It is a major limitation to crop rooting, and maintaining a functional root mass during  stress is critical. It is normally a problem throughout the entire soil profile rather than a soil surface problem and usually increases with depth. Alleviating soil chemical constraints in the root zone is time consuming and requires extra management, especially proper fertilization practices and liming. Ag lime reactions to raise soil pH require several months to develop fully, so instant relief will not be found.

Very acid soils may show a lower cation exchange capacity (CEC) that will improve with liming as exchanged Al ions make room for the basic ions that constitute CEC measurement.

2) Excessive Induced Acidity (by over application of sulfur or acidic nitrogen carriers) - primarily a problem in the soil surface (0.5 - 1 inch zone) of near neutral soils. This condition exposes turf crown and rhizomes to aluminum and manganese toxicity and inhibits root formation from the crown and stem nodes. Root tip injury from acid soil decreases water and nutrient uptake (a  5 to 8 lb. /1,000 ft.2 surface application of lime should correct this problem).

You might request a determination of lime response. A soil test reads only the active acidity, but soil also has residual acidity (buffering); some have more than others. The soil test's lime requirement answers how much pH change will occur on my soil for a given amount of lime. This is essential information to proper liming of a soil and maintaining adequate calcium fertility.

Acidifying fertilizers include:

  • Ammonia (anhydrous)
  • Ammonium sulfate
  • Urea, SCU, MeU, IBDU and UF
  • Diammonium phosphate (DAP)
  • Monoammonium phosphate (MAP)
  • Ammonium nitrate

3) Acidic Thatch in Turf - conditions favoring thatch development in sod:

  • High use of acid nitrogen fertilizer applications
  • High sulfur applications
  • High rainfall regions where the thatch and soil basic elements are leached
  • Sulfur deposition from the atmosphere and acid rain
  • Irrigation water that is acid

Symptoms of acidic thatch conditions:

  • Poor rooting into the thatch layer.
  • Non rooting of stolons, rhizomes or crown regions growing in the thatch layer.
  • Slow response to granular nitrogen fertilizer applications (especially ammonia sources of nitrogen).
  • Slow decomposition of organic matter, especially in grasses (perennial ryegrass, tall fescue, etc.), that normally does not cause thatch buildup problems.
  • Iron chlorosis (especially on cool-season grasses).


Cures for Acidic Turf & Liming Strategies

Deep mixing of required lime before planting and establishment is a good idea if feasible. You may try periodic topdressings of 5 - 8 lbs. lime/1,000 ft.2 when cool, to maintain thatch pH above a 5.5 level. A combination surface application of lime and subsurface lime injection or a topdressing of the turf with pelleted lime plus core aerification and dragging will increase soil pH.

Use acidifying nitrogen fertilizers with lime to increase calcium solubility and calcium movement into the lower root zone (if soil calcium has been critically low). Apply soluble calcium materials such as gypsum or calcium nitrate, bone meal or rock phosphate to increase exchangeable calcium in the rootzone without acidulation. These treatments enhance downward movement of calcium into the rootzone. They do not raise soil pH like lime does. Some soil physical improvements may occur because of the added calcium (improves soil tilth). Except for adding calcium, these treatments do not change availability of other plant nutrients. Calcium decreases the exchangeable aluminum on the soil exchange thus fighting aluminum toxicity to roots.      

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