N-P-K Fertilizer Requirements
and Recommendations in Pumpkins and Related Crops
(J.M. Swiader)
1. Nitrogen fertilization recommendations
Due to the various physio-chemical
transformations and mobility of N in most soils, chemical tests for
plant-available soil N can be highly inaccurate and misleading. Subsequently, fertility recommendations for
N in pumpkins are not based on soil N tests, but rather are based primarily on
crop yield potential, with adjustments made for soil organic matter
content. In Table 1, N fertilizer
recommendations are for a crop yield potential of 15-25 tons/acre, a relatively
high but achievable yield level.
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Table 1. Nitrogen fertilizer
recommendations in pumpkins (and related crops).z |
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soil organic matter content (%) |
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<
2 |
2-9.9 |
10-20 |
>20 |
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(lbs N/acre) |
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100-120 |
80 |
60 |
30 |
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zfor a yield goal of 15-25 tons/acre |
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On sandy soils, and other soils with less
than 2% organic matter, it is common practice to split the N application, with
1/2 the N applied preplant, and 1/2 sidedressed when vines run.
If the previous crop was soybean, a
credit of 30-40 lbs N/acre can be taken.
If the prior crop was a legume vegetable, such as green bean or peas, a
credit of 20 lbs N/acre can be taken.
However, in both cases, no credit should taken if pumpkins are to be
grown on sandy soils. For each ton of
solid dairy or cattle manure applied, N recommendations can be reduced 3-4
lbs/acre. If liquid sources of dairy or
cattle manure are used, subtract 10 lbs N/acre for each 1000 gallons/acre of
material applied. However, in no case
should amounts of manure in excess of N fertilizer requirements be applied.
2. Phosphorus (P) and potassium (K) fertilization
recommendations
P and K soil test levels. As regards P and K
fertility requirements, pumpkins and winter squashes are classified as
demand-level ‘5’ crops, which means they have a relatively high demand for P
and K. Unlike N, however, P and K
fertilizer recommendations are based on the levels of plant-available nutrient
in the soil. Optimum soil test levels
of exchangeable P and K for pumpkins are shown in Table 2 (next page).
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Table 2. Optimum soil test levels for P and K in pumpkins and related crops. |
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soil type |
soil
P1 |
soil
K |
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(lbs/acre) |
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loam, silt, clay |
60-75 |
250-350 |
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sands |
60-75 |
200-300 |
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P and K fertilizer recommendations.
When P and K soil tests are in the optimum range, P and K fertilizer
recommendations are set to a rate approximately equal to the amount removed in
the harvested part (pumpkin fruit) of the crop. This is known as the ‘maintenance’ level and is roughly equal to
10 lbs P/acre and 105 lbs K/acre. Based
on these totals, and converting P and K to their respective oxide equivalents
(and increasing the P requirement 50% to account for fixation by soil
particles), the fertilizer maintenance requirement (FMR*) for P and K in
pumpkins for a yield goal of 20-25 tons fruit/acre calculates out to approximately
125 lbs K2O/acre and 50 lbs P2O5/acre.
When soil test results are below the
optimum range. Additional P and K is
added in with the maintenance P and K level.
Conversely, when soil test results are above the optimum range, P and K
fertilizer recommendations are reduced to approximately 1/4 to 1/2 the
maintenance levels. These calculations
are factored in to the P and K fertilizer recommendations in Tables 3 and 4,
respectively. The recommendations in
each table are based on a fruit-yield potential of 20-25 tons/acre, for plant
population densities of 1,850-4,500 plants/acre. Note; soil test results are expressed in lbs/acre, while
fertilizer recommendations are given as P2O5 and K2O
equivalents.
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Table 3. Phosphorus fertilizer requirements (P2O5)
in pumpkins for various levels of soil P fertility |
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expected
yield level |
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P1 soil test |
maximum |
95% |
90% |
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(lbs P/ac) |
(lbs P2O5/ac) |
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100 |
0 |
- |
- |
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90 |
30 |
- |
- |
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80 |
60 |
0 |
- |
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70 |
90 |
20 |
- |
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60 |
120 |
50 |
0 |
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50 |
150 |
80 |
30 |
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40 |
180 |
110 |
60 |
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30 |
210 |
140 |
90 |
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20 |
240 |
170 |
120 |
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10 |
270 |
200 |
150 |
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P ‘maintenance’ level (~
50 lbs P2O5/acre) |
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Table 4.
Potassium fertilizer requirements (K2O) in pumpkins for various levels of soil K
fertility |
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expected
yield level |
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K soil test |
maximum |
95% |
90% |
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(lbs K/ac) |
(lbs K2O/ac) |
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400 |
60 |
- |
- |
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300 |
150 |
60 |
- |
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280 |
160 |
70 |
10 |
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260 |
180 |
90 |
30 |
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240 |
200 |
110 |
40 |
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220 |
210 |
120 |
60 |
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200 |
230 |
140 |
80 |
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180 |
250 |
160 |
90 |
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160 |
260 |
170 |
110 |
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140 |
280 |
190 |
130 |
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120 |
300 |
210 |
140 |
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100 |
310 |
220 |
160 |
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80 |
330 |
240 |
180 |
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60 |
350 |
260 |
190 |
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40 |
360 |
270 |
210 |
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K ‘maintenance’ level (~
125 lbs K2O/acre) |
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