Plum trees bear a stone fruit popular for its sweet flavour. Fertilising these trees properly requires close observation. When plum trees are under-fertilised, they do not grow well. When over-fertilised, they grow very well, but produce too many leaves and no fruit. The key to fertiliser is a delicate balance.
Pay Attention to Growth
The amount of fertiliser to apply to a plum tree depends on the growth the tree had the prior year. For a plum tree, growth of less than 8 inches per year results in a reduction of fruit. Judge the growth from one year to the next by measuring the ring left when the tip of a shoot grows over the course of a year.
If your plum tree had less than 8 inches of growth in the previous year, it requires fertilisation with nitrogen. Use a maximum of 56.7gr of nitrogen per inch of trunk diameter, depending on how much less than 8 inches your tree grew. If the growth was close to 8 inches, you can use less fertiliser; use the maximum amount if the growth was 7 inches or less. For trees that had healthy growth in the previous year, apply 4/5 ounce of nitrogen per inch of trunk diameter. Too much nitrogen on a plum tree will result in lots of dense leaves and little fruiting.
Apply nitrogen to plum trees by broadcasting it evenly over the soil. The nitrogen should not be applied directly to the tree's trunk, but in a circle around the roots. Do not extend the circle outside the reach of the tree's branches. Water the soil to help the nitrogen to soak in. Plum trees should be fertilised in the early spring, after the last frost date.
An organic mulch, such as sawdust, bark or wood chips around the trunk of a tree, will help the tree retain moisture and protect the roots against unexpected frosts or too much sun. The mulch will decompose and enrich the soil as well. Apply a fresh layer of mulch each early spring after fertilising your tree with nitrogen. Mulch will also help deter weeds, which may leach the nutrients away from your trees.