February 26, 2014
The following is an article that is conjunction with a presentation that I made last week at the Canadian International Turf Conference in Vancouver. It will appear later this year in the Canadian Groundskeeper Magazine.
Here is the link to the article as it appeared in the July 2014 issue of the Canadian Groundskeeper:
FERTILITY MANAGEMENT
FOR SAND-BASED SYSTEMS
Nick Christians
Iowa State University
While management of plants on sandy soils is relatively rare
in agriculture, it is very common in the turf industry. Sports fields andgolf course greens, where
all of the soil has been excavated from the site and replaced with a high sand
media, are the norm rather than the exception.
Many turf managers are faced with the task of growing grass on soil in
some parts of their operation and on sand on others. There are some important differences between
soil and sand-based systems and a different approach, particularly in the
management of fertility programs, are required for the two areas.
One of the main differences will be cation exchange capacity
(CEC). CEC is defined as the ability of
the soil to exchange cations. The
cations are a series of positively (+) charged elements in the soil. The primary cations in the soil are hydrogen
(H+), calcium (CA++), magnesium (Mg++),
potassium (K+), sodium (Na), and ammonium (NH4+). Each of these exists in the soil with a
positive electrical charge. The fine
soil particles and the organic component of the soil generally have a
predominately negative (-) charge. These
charges act like magnets in the soil and the positive charges are attracted to
the negative charges and are held in place for “exchange” with the root system
(Figure 1). This is a key part of soil
fertility.
The measurement of CEC on your soil test form is usually
expressed in millequivalents (meq)/100 grams (g) of soil. Clay loam soils are relatively high, with a
CEC of 20 to 30 meq/100 g, whereas sands are usually very low, often in the
range of 1 to 4 meq/100 g. Including
organic matter, like peat, in the sand-based media can raise the CEC, but it
will still be much lower than soil. This
difference in CEC between soil and sand has a major impact on the soil’s
ability to provide cations to the plant and it changes how the fertility
program for the area is conducted.
The pH is another factor that has a big impact on the
fertilizer program. The H in pH stands
for hydrogen and pH measurement is basically a measurement of the hydrogen in
the soil. The p stands for “potential”. While H is not an element that is generally
considered in the development of a fertilizer program, H does have some very
important ramifications for the plant.
The pH scale ranges from 0 to 14 (Fig. 2). Everything below 7 is acidic and everything
above 7 is alkaline. Seven is the
neutral point. The pH scale is an
inverse scale and the lower the number, the more H is in the system and on the
CEC sites. The scale is also
logarithmic and changes by a factor of 10 with each increase of one number on
the scale. To go up from 5 to 6 is a
factor of 10 and to go from 5 to 7 is 100.
Figure 2. The pH
scale and the cations that predominate in various pH ranges.
The soil pH has a major effect on the availability of essential elements (Fig. 3) and is a very important piece of information in determining the nutritional needs of the plant. Iron (Fe), for instance, is usually readily available in the acidic range, but limited in high pH soils. Iron chlorosis (the lack of chlorophyll), a yellowing of the plant is common on turf grown on high pH soils. Whereas chlorosis on low pH soils, particularly those with low CEC’s, is often due to a magnesium (Mg) deficiency. The wider the line in figure 3, the more available the element to the plant. While calcium (Ca), potassium (K), and Mg are readily available in high pH soils, they may be limited in acidic soils (pH below 7).
Sands can vary greatly in pH. Silica based sands can have a low pH, whereas
calcareous sands will have a pH of 8.2.
The best starting point in developing a fertility program for a
sand-based system is to get a good soil test.
Because of their low CEC’s, sands may also be deficient in some of the
key cations for plant growth. Grass
grown on an acidic sand may experience deficiencies of Ca, Mg, and K, whereas
sands grown on high pH sands may have problems with Fe, or some of the other
micronutrients. In addition to the pH, a
good soil test will show which elements are there in sufficient quantities and
which are lacking.
Figure 3. The effect
of pH on nutrient availability to the plant.
There are 17 elements currently considered to be essential
for plant growth (Table 1). These are
generally further subdivided into the macronutrients and micronutrients. These terms do not reflect on the importance
of the element, all are essential. The terms refer to the amount needed by the
plant. Macronutrients like nitrogen (N),
phosphorus (P) and K are found in the plant in 1000 parts per million (ppm) or
more, whereas the micronutrients are found in the plant in 100 ppm or less. By this definition, Ca, Mg, and sulfur (S)
are also macronutrients, although they are often thought of in the turf
industry as micronutrients.
Nitrogen is generally the most important element for
sand-based systems. Sandy soils require
more irrigation. They are also low in
organic matter, which is an important source of N in soils. Nitrogen can easily leach from the rootzone. Controlled release N sources can help govern
the release of N into the soil solution.
Availability of N can also be governed by “spoon-feeding”, the process
of applying small amounts of N on a regular basis. This will usually involve the application of
0.1 to 0.2 lb N/1000 ft2 on a 10 to 14 day interval. Rates this low will usually require liquid
applications with a soluble N source such as urea to provide uniform
application. There may also be a need
for periodic applications of granular N sources if the area becomes
chorotic. The plant is very responsive
to N applications. Initial deficiency
symptoms include chorosis and slow growth.
A noticeable greenup and increase in growth can take place in as little
as 24 to 48 hours after application to a N deficient turf.
Phosphorus can be highly variable in sand-based
systems. It depends on the parent
material of the sand. Silica sands are
generally low in P and applications of this element will be necessary. Some sands have rock phosphate as a part of
their parent material and the grass could be grown on it for years without
additional P. The deficiency symptoms of
P are a dark green to purple discoloration of the tissue. There are good soil tests to measure the
availability of P and soil tests should be conducted before developing a
fertility program. There are other
factors such as cold weather that can cause some types of grass to turn
purple. The best way to determine if
that discoloration is a P deficiency is to apply some P to a test area and see
if the condition goes away. If the discoloration
continues after P application, the problem is not due to a P deficiency.
Potassium is generally a limiting factor in low CEC, sandy
media. Soil tests can provide some
information, but the current tests were developed for soil-based systems and
they may be misleading. This element is
important in stress tolerance, including wear, heat, and cold. Visual deficiency symptoms are generally not
present unless the deficiency is extreme.
Light application of 0.25 to 0.5 lb K2O/1000 ft2
two or three times during the season is usually appropriate. Avoid the temptation of using too much
K. Potassium is a cation and its overuse
can result in saturation of the cation exchange sites at the expense of other
essential element (Fig. 4). This should
be avoided and light applications are usually satisfactory.
The need for Ca, Mg, S and the micronutrients will usually
be dependent on soil pH. Calcium and Mg
will often be deficient in low pH ranges, particularly on low CEC sandy
media. This can easily be solved with
the application of lime, or Ca + Mg products like dolomitic limestone. Iron deficiency, which expresses itself as
chorosis, will be most prevalent on high pH soils, whereas chlorosis in low pH
soils will usually be due to Mg deficiency.
Other micronutrient deficiencies may also occur on more inert sands and
the spray application of a micronutrient “package” that contains small amounts
of several essential elements may be appropriate if the turf fails to respond
after the application of Mg or Fe.
For a more in depth analysis of soil chemistry and fertility
programs for turf, see “Fundamentals of Turfgrass Management” by Nick
Christians. It’s ISBN number is
978-0-470-58731-7 and on the web at http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470587318.html. For those using it as a text book, there are
quizzes and other instructor information available from the publisher, Wiley
Publications.
More information is also available from the author at:
Nick Christians, Ph.D.
University Professor
Iowa State University
Department of Horticulture
133 Horticulture Bldg.
Ames, IA 50011
515-294-0036
e-mail: nchris@iastate.edu
turf blog: http://www.iaturf.blogspot.com/
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