Potassium Fertilizers on Nitrogen Rate by Drip Irrigation Trial

Roland D. Meyer
Extension Soils Specialist, UC Davis

 Growers that have applied higher rates of nitrogen to obtain high almond yields often observe a decline in potassium concentrations in the leaf tissue samples. This may lead to reduced yields if adequate potassium is not supplied to the trees. Leaf potassium concentrations below the desired level of 1.00% in the range of 0.80 to 0.90% have been observed in the study site. The experimental site has a history of receiving ten treatments; for the past 6 years, 5 nitrogen rates--0, 4, 8, 16 and 32 oz/tree each at two water levels--0.6 and 1.0 ET_c being applied with nitrogen rates having varied over the previous years. The ten treatments have been applied to 3 varieties; Butte, Carmel and NonPareil. The trees were planted in 1981 with the fertilizer and drip irrigation treatments initiated in 1982. Urea has been the nitrogen source used throughout the study. The urea fertilizer has acidified the soil under and to the side of the emitter which has dramatically increased the uptake of manganese by the trees and resulted in manganese concentrations in leaf tissue of up to 700 ppm in late season samples. The application of potassium sulfate as a potassium source beginning in 1989 has significantly increased the leaf manganese concentration an average of 150 ppm more than where no potassium sulfate has been applied. The application of mono-potassium phosphate as a potassium source for the almonds and the phosphate to immobilize some of the manganese in the soil and reduce the tree uptake of manganese is being evaluated.

Objectives

(1) To evaluate the influence that two potassium fertilizer sources: mono-potassium phosphate and potassium sulfate, have on meat yields and leaf nutrient concentrations of potassium, phosphorus, sulfur, nitrogen, zinc and manganese, and (2) To evaluate the effect of the phosphate ion from mono-potassium phosphate fertilizer might have to immobilize some of the manganese in the soil and reduce the negative impact of manganese upon almond yields where potassium sulfate has been applied and where it has not been applied.

Results

Almond meat yields have shown a progressive trend from 1995 through 1998 for an increase from the application of mono-potassium phosphate which was significant in 1997 (Table 1). There was a trend for a yield increase with potassium sulfate in 1997 (1576 versus 1777 meat lbs/A), but a much larger increase in 1997 (1777 versus 2304 meat lbs/A) where mono-potassium phosphate was applied following previous years’ applications of potassium sulfate (Table 1). The consistent trend across the four years with significant differences in 1997 makes for a firm conclusion that the mono-potassium phosphate is impacting tree growth and yield. Almond yields in 1995 for the three varieties were very similar, 1708 lbs/A for Butte, 1612 lbs/A for Carmel and 1659 meat lbs/A for NonPareil while in 1996 they were 2016 lbs/A for Butte, 1190 lbs/A for Carmel and 2418 meat lbs/A for NonPareil. In 1997 almond meat yields were 1940 lbs/A for Butte, 1640 lbs/A for Carmel and 2249 meat lbs/A for NonPareil and in 1998 they were 2077 lbs/A for Butte, 1345 lbs/A for Carmel and 2318 meat lbs/A for NonPareil. The decline in yields of the Carmel variety has been caused by bud failure.

The influence of nitrogen rate and water level upon almond yield during the four years 1995-1998 is given in Table 2. The intermediate nitrogen rates of 8 and 16 oz N/tree have resulted in the highest yields at the 0.6 ET_c water level while the 16 and 32 oz N/tree nitrogen rates have resulted in the highest almond yields at the 1.0 ET_c water level. Only in 1998 were the highest yields recorded at the nitrogen rate of 4 and 8 oz/tree.

 The leaf analyses for manganese concentrations are given in Table 3 for the three sample dates in 1995 – 1998. It can be noted that the control where no potassium fertilizer was applied, consistently had the lowest manganese concentration in the leaves while the highest manganese concentration often occurred with the MKP applied on the previously applied potassium sulfate treatment. Given the yield increases that have resulted from the MKP treatments, it would be assumed that lower manganese might have occurred in these treatments. Apparently the application of MKP makes it easier for the almond trees to deal with the higher manganese and perhaps aluminum concentrations.

Plans are to continue this experiment for several more years, primarily to evaluate the yield response to a potassium fertilizer where the possible negative effect of high leaf manganese levels is reduced. The yield trends indicated for the four years 1995 - 1998 are particularly promising. Our plan is to reapply the two potassium fertilizers in 1999 at the same rate as in 1996, two applications of 0.5 lb K₂O/tree/application. Leaf samples will be taken the first week of April, July and October as in 1995-1998. Meat yield data will be collected in 1999 as well.

Table 1

Almond meat yields in 1995, 1996, 1997 and 1998 as influenced by potassium fertilizer source.

Table 2

Almond meat yields for the four years 1995-1998 as influenced by nitrogen rate and drip irrigation level.

Table 3

Almond leaf analysis for manganese taken on three dates each year during 1995-1998 as influenced by potassium fertilizer source.

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Copyright © 1999 Colusa County Cooperative Extension, Univ. of California
Last modified: September 01, 2000