Low
Soil pH Continues to be a Problem
Jim
Camberato and Bruce Martin
Clemson
University
Pee Dee Research and Education Center
Adjusting
and maintaining proper soil pH is fundamental to growing high quality turfgrass
and should not be ignored.
When soil pH is low, toxic levels of aluminum, hydrogen, and manganese as
well as deficient levels of calcium and magnesium may occur (acid soil complex).
Application of dolomitic limestone to provide proper soil pH levels (5.5 to 6.5
for most grasses) eliminates aluminum, hydrogen, and manganese toxicity as well
as calcium and magnesium deficiency. In
an article entitled “Tales from the Annals of Disease” (Sept.-Oct. 1999, Carolinas Green, page 42), we discussed problems on golf courses
where acid soil complex stressed the turfgrass allowing unusual diseases, such
as bermudagrass rust, Fusarium and Ascochyta blights, and Dreschlera (Helminthosporium)
leaf spots to devastate the turf. Unfortunately, the basis of several recent trouble-shooting
excursions was once again acid soil complex.
In these cases, involving bermudagrass greens and bermudagrass fairways
overseeded with ryegrass, turfgrass growth and quality were poor, but diseases
were not involved in the turf decline.
Soil samples to monitor soil pH should be analyzed annually or semi-annually, depending on soil texture and nitrogen fertilizer use. Annual sampling in sandy soils that receive high nitrogen rates (5-10 lb N/1000 sq ft) is necessary because soil pH can change rapidly under these conditions. Greens, especially during grow-in and in the first two years, warrant even more frequent sampling. Fairways constructed in heavy soils (loamy or clayey in texture) and not heavily fertilized with nitrogen can be sampled every other year, if annual sampling is considered too costly.
Soil texture is correlated with the buffer capacity of the soil. Buffer capacity is the resistance of the soil to pH change. Sandy soils have low buffer capacity, meaning that a relatively large decrease in soil pH occurs with a relatively small addition of acid. Heavier soils have high buffer capacity and the decrease in pH is relatively small with relatively large additions of acid. This buffer capacity concept also applies to increasing soil pH with lime. Sandy soils require less lime to increase pH than clayey soils even though initial pH may be the same. All soil test laboratories measure buffer capacity and factor this into the lime recommendation.
Nitrogen fertilizer use influences soil sampling frequency because
ammonium-containing and ammonium-forming (urea) fertilizers generate acidity.
Commonly used sources can generate enough acidity to neutralize as much as 2 to
3 pounds of lime per pound of nitrogen.
Representative soil sampling is necessary to manage soil pH on a golf
course. A large amount of soil is
moved around when a golf course is constructed.
The pH of soils on a course can differ by as much as four pH units.
For example: We visited a golf course under construction along the coast
this spring. Sediments containing
seashells had pH levels around 8. Those dredged from low-lying areas had pH
levels near 4. Not only did these
sediments end up adjacent to each other on fairways, but also, in one instance,
half a green collar had high pH soil and the other half had low pH soil.
Variability in soil pH needs to be identified so that
samples of each soil can be taken separately and lime applied appropriately.
In many cases, soils that have extremely different natural pH levels,
will also have different fertility levels and will always need to be limed and
fertilized differently. Constructing
maps to identify areas of soil differing in pH and fertility level is necessary
for accurate sampling. The global positioning system technology that has become
commonplace in the last ten years is an excellent tool for detailing aerial
photographs as sampling templates. Initiate
a detailed soil sample map, before extensive areas of turf decline reveal areas
of acid soil complex.
Take good soil samples. For each representative area, obtain and combine10 to 15 soil cores in a clean plastic bucket, mix thoroughly, and remove a pint sub-sample for analysis. Discard the vegetation from the soil core prior to mixing. Recommended sampling depths are 3 to 4 inches for established turf and 6 inches prior to establishing turf.
It’s much better to apply lime frequently at low
rates than infrequently at high rates because the effects of lime move very
slowly into the soil and rapid changes in pH do not occur.
The soil should be limed to maintain pH in the acceptable range,
somewhere between 5.5 and 6.5 for most grasses in mineral soils. Lime
applications should be made when soil pH is in the lower portion of the
acceptable range or no more than 0.2 less than the lower limit. If drastically
low pH levels occur, then portions of the root zone will remain below the
acceptable pH for a long period of time, since the effects of lime move slowly
downward in the soil profile.
Lime is slow to react, especially when applied to the
turf surface. To hasten the reaction and movement of lime effects into the soil,
application of lime after core aerification is strongly recommended. Maintaining
soil pH at favorable levels is a continual process. Keep up with it every year
to avoid problems.