Fertilizing frost-damaged fruit crops
Should you adjust your fertilizer program to account for few fruit? Many Michigan fruit crop levels will be low this year due to frost injury and poor pollination. Consider this information if you haven’t fertilized or applied only a portion of fertilizer.
A good estimate of the nutrient demand associated with fruiting is the nutrient content of fruit at maturity, calculated from reported nutrient concentrations and typical fruit yields. This is the same as nutrient removal. Nitrogen (N) and potassium (K) are present in relatively high levels in most fruits, so these are two nutrients most affected by cropping levels. Typical nutrient removal levels for Michigan fruit crops are summarized in the table below. The N content of the fruit ranges from 8 lbs. per acre (blueberries, cherries) to as high as 50 lbs. per acre (15 ton per acre peach crop). The K contents range from 8 to 80 lbs. per acre. A good rule of thumb for grapes is that 5 lbs. K are contained in each ton of fruit.
Nitrogen and potassium removed from fruit
plantings in harvested fruit (lb per acre).1
|
Crop |
N |
K |
|
Apples |
18-20 |
30-80 |
|
Blueberries |
8-12 |
8-12 |
|
Cherries |
8 |
16 |
|
Grapes |
28 |
30-40 |
|
Peaches |
50 |
80 |
1Estimated from reported nutrient concentrations and typical Michigan yields.
Using these figures to adjust fertilizer rates is not straight-forward because plants obtain only part of their nutrient needs from current-season fertilizer (the rest from soil and tissue reserves). Generally, though, if the fruit of apples or grapes is lost to frost, N rates can be reduced by 50 percent (on lighter, sandier soils) to 100 percent (heavier, fertile soils) of typical applications. If the entire crop of cherries, peaches or blueberries were lost, N rates can safely be reduced be a third on sandier soils, to as much as a half on heavier soils. Reduce rates proportionately in the case of partial crop failures.
The effect of crop loss on K requirements is difficult to estimate. Fruit are strong sinks for K, so K demand is clearly reduced when no crop is produced. Frost-damaged plantings on heavier soils likely will not benefit from K additions this year. Plantings on sandy soils with a low K reserve or where tissue analysis has indicated a need for K may benefit from K, but will require lower rates, perhaps half of the typical application. Applications of K could be discontinued this year where K levels in the soil are moderate to high, and an annual maintenance application of K is typically applied.
Dr. Hanson's work is funded in part by MSU's AgBioResearch.
