Subsoil Nutrients
When you buy a product and receive a rebate coupon for your
next purchase, it's like throwing money away to discard the
coupon if you know you'll buy the product again. The same is
true when it comes to the rebate of extra plant nutrients often
available in subsoil clay for crops grown on Coastal Plain soils.
POTASSIUM AND MAGNESIUM
Oppositely charged particles are attracted to each other the
same way that positive and negative poles of magnets are
attracted. Even though potassium and magnesium are
positively charged ions and held by negatively charged sites
on clay and organic matter, they may still leach down through
the soil with time. This is especially true for Coastal Plain
soils, which have small amounts of clay and organic matter
in the sandy surface depth. The potential for these nutrients
to move through the surface depth is also indicated by the
low cation exchange capacity (CEC) values on the soil test
reports. The CEC is a quantitative measure of the soil's
capacity to retain positively charged nutrients (cations)
such as potassium, calcium, and magnesium.
The following table illustrates the effect of time and rainfall on
the movement of surface-applied potassium in a Coastal Plain
soil (Marlboro loamy sand). The soil has 7 percent clay in the
surface depth and 40 percent clay starting at the 1-foot depth.
| Weeks after application |
Accumulative rainfall (inches) | Potassium accumulation (fallowed soil) |
| 5 | 1.3 | Most of the potassium is near the surface |
| 17 | 10.3 | Potassium accumulation at 3 inches (135 lbs/acre) and at 12 inches (115 lbs/acre) |
| 46 | 42.0 | Most of the potassium accumulation is at 12 inches |
| 76 | 69.4 | Only 10 lbs/acre potassium left near the surface. The rest is at 12-inch depth |
| Boswell, F.C. and O.E. Anderson. 1968. "Potassium movement in fallowed soils," Agron. J. 60:688-691. | ||
The following data shows the soil-test results for
phosphorus, potassium, magnesium, and sulfur in a field
which has several soil types and had been managed under
conventional tillage for at least 10 years prior to sampling.
| Soil type | Depth to clay (inches) | Soil pH | Phosphorus | Potassium | Magnesium | Sulfur |
| (lbs/acre & rating*) | ||||||
| Norfolk --plow layer --subsoil |
8 | 6.5 5.9 |
71 H- 0 VL | 106 M 263 VH | 151 H 333 H |
6 I 112 S |
| Goldsboro --plow layer --subsoil |
12 | 6.4 4.6 |
63 H- 2 VL | 116 M 128 M | 215 H 136 H |
9 I 97 S |
| Dunbar --plow layer --subsoil |
15 | 6.5 4.5 |
66 H- 0 VL | 75 M- 99 M | 199 H 135 H |
9 I 124 S |
| Noboco --plow layer --subsoil |
14 | 6.6 5.1 |
37 M- 0 VL | 115 M 215 H+ | 184 H 249 H |
8 I 112 S |
| * H = high, VH = very high, M = medium, VL = very low, I = insufficient, S = sufficient | ||||||
| Samples collected Sept. 8, 1995, at the Coastal Plain Soil, Water and Plant Research Center by Bob Lippert, Extension Soil Fertility Specialist, Clemson University and John Sadler, Soil Scientist, USDA-Agricultural Research Service. | ||||||
The crop rotations included corn, wheat, sorghum, and
soybeans. Each year's fertilizer applications were based on
Clemson University's
recommendations. Notice how the phosphorus accumulates
in the plow layer but the potassium, magnesium, and sulfur
accumulate in the subsoil clay.
For the plants to utilize subsoil nutrients efficiently,
subsoiling may be necessary to ensure that a hardpan will
not deter access of the roots to the subsoil. The subsoil clay
should also be no greater than 15 to 20 inches from the
soil surface and have a pH value higher than 5.0.
SUBSOIL SAMPLING AND TESTING
Some states have developed a subsoil sampling and testing program so farmers can adjust for these nutrients with their fertilizer applications. The following are the guidelines developed by Clemson University for subsoil Potassium, Magnesium and Sulfur.
To sample the subsoil, take samples from the top 4 inches
of the clay layer at several locations in the field. Both
plow layer and subsoil samples can be taken from the
same core locations. Mix the clay cores in a clean
container and send the sample to the
Agricultural Service Laboratory. The fee is $5.00 per
sample. The lab report will give the pH; CEC; percent
base saturation; amounts of phosphorus, potassium,
calcium, magnesium, zinc, manganese, copper and boron;
plus comments regarding fertilizer application reduction.
A test for sulfur requires an additional $5.00 fee.
If the test results show high or very high levels of
potassium in the subsoil, the recommendation for
potassium from the plow layer soil test can be reduced by
as much as one-half. If a subsoil test indicates a high level
of magnesium, then any recommendations for magnesium
indicated by the plow layer soil test can be ignored.
SULFUR
Just as magnet poles with the same charge repel each other,
the negatively charged sulfate ion will be repelled by the
negatively charged clay and organic matter exchange sites
in the soil. Subsequently, the sulfur will easily move down
through the sandy soil layer with rainfall and accumulate
in the subsoil clay. A plow layer test for sulfur, therefore,
will give misleading results. The test for sulfur in plow
depth samples will likely show insufficient levels in sulfur
even though there is a sufficient amount in the subsoil clay
(see previous table). The subsoil test is the best analysis to
determine if sulfur is needed. The sample collected for the
subsoil potassium and magnesium test can also be used for
the subsoil sulfur test. If the subsoil contains 40 pounds per
acre or more sulfur, then there will be sufficient sulfur for
that year's crop.
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