Nitrogen Source - Soil Acidification Remediation Trial
Roland D. Meyer
Extension Soil Specialist, UC Davis
Drip irrigation involves having a relative small volume of soil being used as the
reservoir for water and nutrient uptake by the tree. Urea and other ammonium based
nitrogen sources acidify the soil rather rapidly, particularly under the emitter. This
usually increases the plant uptake of manganese and aluminum, perhaps to excess
concentrations. Calcium nitrate has several advantages, supplying the nitrogen in the
nitrate form results in little or no soil acidification and the addition of calcium helps
to neutralize any acidity. Acid fertilizers like the urea-sulfuric "N-phuric"
material that are utilized to clean out the drip lines offer even greater acidification
potential than urea alone. Once the soil has been acidified, particularly to the depth of
several feet, remediation of the zone of low pH becomes rather difficult.
Objectives
(1) To evaluate the effect of different nitrogen fertilizer sources on
almond meat yields, tree growth, and leaf nutrient concentrations of nitrogen, potassium,
calcium, magnesium, zinc, manganese and boron, and (2) To evaluate the effect of different
soil acidification remediation treatments on almond meat yields, tree growth, and leaf
nutrient concentrations of nitrogen, potassium, calcium, magnesium, zinc, manganese and
boron.
Results
The list of treatments and yield data for 1995, 1996, 1997 and 1998 are
given in Table 1. Meat yields for 1995 showed no consistent differences due to fertilizer
or soil amendment treatments. Meat yields for 1996, 1997 and 1998 were not significantly
different, indicating that the acidification of the soil by different nitrogen sources and
the subsequent soil amendment treatments are not having a major impact upon meat yields.
In most years and sample dates leaf nitrogen concentrations are lower
for the calcium nitrate, urea-calcium nitrate alternated (which receives calcium nitrate
in the odd years, 1995, 1997) and UN 32 treatments than the other nitrogen fertilizer
sources. Apparently the nitrate form of nitrogen fertilizer has been denitrified in the
saturated drip zone resulting in less uptake by the trees. Leaf manganese concentrations
were lower on all sample dates if calcium nitrate was the nitrogen source applied. Even if
calcium nitrate was applied in alternate years with urea the leaf manganese concentrations
were often lower than urea-N sources. The concentrations recorded for the N-phuric
treatment of nearly 700 ppm would be considered to be high but still somewhat below the
expected toxic range of 1000 ppm or more. No changes have been observed from the soil
amendment treatments. Soil samples were taken in early December 1998 and will be analyzed
to evaluate the changes in soil pH and other characteristics from the drip emitter areas
where the different nitrogen fertilizer sources and soil amendment treatments have been
applied.
Plans are to continue this experiment for several more years to measure
both the short term (1-2 year) and long term (4-5 year) effects of nitrogen fertilizer
source and soil pH remediation treatments upon almond yields, leaf nitrogen, manganese and
other nutrient concentrations as well as soil characteristics such as pH, exchangeable
calcium, potassium, sodium, manganese and aluminum.
Lime and gypsum treatments (#4, 5 and 7) were applied on April 12, 1995
by making the drip emitter basins slightly deeper and placing the lime or gypsum into the
basin. The lime application for treatment #6 was mixed with a small volume of soil by
placing a 10" diameter PVC cylinder (18" height) over the basin hole and adding
lime as an auger (8" diameter) went down to a depth of 18". When the auger was
lifted out of the hole, the lime-soil mixture fell back into the hole and was packed down
to form a basin again.
A total of one pound of nitrogen per tree (202 lbs N/A) from the
different sources was split into thirds and applied the first week in April, May and July
of 1996, 1997 and 1998. The NaK(NO3)2 + gypsum treatment was applied
as a 1:1:1 Na:K:Ca ratio beginning with the second application on June 15, 1995.
Table 1.
Almond meat yields in 1995, 1996, 1997 and 1998 for the nitrogen source
trial at the Nickels Soil Laboratory near Arbuckle, California.
