Correct light, moisture and nutrients make trees thrive. When soil lacks the right nutrition, fertilizers are applied in quick- and slow-release varieties. Products sold at garden centers often contain a combination of the two. Seeking fertilizers with the right amount of slow-release ingredients can benefit you and the environment.
Make sure you are purchasing the right slow-release fertilizer for your trees by having a local extension agency due a soil test. The results of the test include the specific nutrients your soil is lacking, but they also contain the schedule and rate for application.
Apply slow-release fertilizer to trees in the early spring before they start growing. Make a second application in summer, no later than the middle of July. Applying slow-release fertilizer later promotes growth while trees should be dormant, and it is wasted on deciduous trees which will soon drop their leaves.
Natural Slow-Release Fertilizers for Trees
Manure is an inexpensive (and often free) source of nitrogen. Trees like it because it is non-burning and feeds them for a long time. Manure decays and changes existing phosphorus to a form that is easier for trees to absorb. This phosphorus stays in the soil for years and travels to deep sections of the root system unlike synthetic phosphates.
The problem with manure is that its content is unreliable. You might find weed seeds and high levels of salt. Weeds are a nuisance, but salt can accumulate in the soil over time and reach toxic levels. Gypsum is a soil amendment used to help remove salt from soil. Heavy irrigation or consistent rain also flush salt from the ground.
Other natural slow-release fertilizers used on trees include blood meal, corn-gluten, fish meal and cottonseed meal. Natural sources of nitrogen like this are more expensive than their synthetic counterparts.
Manufactured Slow-Release Fertilizer for Trees
Bags of tree fertilizer sold in garden centers state information about their nitrogen sources. Low-release nitrogen comes from urea. Urea is water-insoluble nitrogen, and labels abbreviate it as WIN. Buy fertilizer that contains a minimum of 30 percent of this type. Water-insoluble nitrogen feeds over long periods and is less likely to get into the water table.
The correct ratio for trees is 16-4-8 or 12-4-8. Using complete fertilizers like 10-10-10 will not hurt your tree, but it is unnecessary.
Always follow the fertilizer rates listed on the label. Leave a 3-foot margin around the trunk and apply fertilizer evenly out to and just beyond the edge of the canopy. This is called the drip line and the soil here contains hair-like roots that absorb nutrients.
Even too much slow-release fertilizer is not good for trees. Too much growth makes trees vulnerable to severe weather and more attractive to insects. It also creates more maintenance if the tree outgrows its location.
Do not use fertilizers that contain herbicides around trees. Trees can absorb them and be damaged. Signs of this can take a season or two to show themselves. Trees exposed to systemic herbicide take longer to leaf out, and the leaves are small and distorted.