Fertilizing trees and nutrient deficiencies: what three extension services say

This page reports what the named codes, regulators and makers say. It is not training and does not teach the work. Each statement below is attributed to the fact sheet it comes from, and the figures are theirs, written for their own regions. The courses that teach tree work are on the certifications guide, and the wider picture of care programs is on the plant health care primer.

Does the tree need it at all

The University of New Hampshire fact sheet says that as long as a tree or shrub looks healthy and grows at an acceptable rate, there is no need to add fertilizer, and that mature trees in a natural setting seldom need it.
The University of Maryland Extension page says healthy trees and shrubs in most landscapes do not require fertilizer, especially at mature size, and that fertilizer is not a remedy for landscape problems. It says trees and shrubs also take up nutrients applied to turf next to them.
The Clemson Extension fact sheet says mature plants in favorable soil need little or no fertilizer. It lists reasons for fertilizing: treating a deficiency found by a soil test or leaf tissue analysis, encouraging growth in young trees, maintaining older trees where fallen leaves are removed, and improving flowering and fruiting. It says to avoid fertilizing plants that show poor growth on the assumption that fertilizer will fix it.

Newly planted trees

Maryland says it is not recommended to fertilize at planting, because research it cites found that nitrogen during establishment seems to suppress root growth. New Hampshire says new trees cannot absorb nutrients until they grow an adequate root system, and that quick-release sources at planting may inhibit roots. Clemson says water, not fertilizer, is the main element for establishment, and describes slow-release nitrogen only where a soil test or tissue analysis shows a deficiency. Planting itself is covered on the planting depth page and the staking and watering page.

Timing, as each source gives it

Source	Windows it names	Cautions it names
University of Maryland Extension	Fall, between late October and early December, or late winter to early spring, between late February and early April	Late summer or early fall, because new growth would be stimulated as plants prepare for dormancy; drought-stressed plants; heavy rain forecast or frozen ground; within 10 to 15 feet of waterways
Clemson Extension	Spring to early summer, and late summer to early fall; the total split into a few smaller applications	Drought-stressed plants in summer. It calls the older worry that late-season feeding causes winter damage unfounded for conifers and most hardwoods, but says plants with several growth flushes, such as red and sugar maples, may respond, so only part of the yearly amount is given then
University of New Hampshire Extension	Spring before growth starts, or mid-summer after shoot growth stops; mid-September to mid-October can also help	High-nitrogen sources in late August to mid-September; plants under drought stress

The three sources name different windows, and Clemson says its recommendations are for South Carolina conditions.

Rates and where the fertilizer goes

Clemson says recommendations are based on nitrogen, and that the three numbers on a label are the percentages by weight of nitrogen, phosphate and potash. It says landscape trees and shrubs generally prefer a ratio of 3:1:1 or 3:1:2, and that where a soil test shows no need for phosphorus the ratio becomes 3:0:1 or 3:0:2. Maryland also names 3:1:1 as common for complete fertilizers. Clemson cites the tree care industry's soil management standard, ANSI A300 Part 2, for slowly available nitrogen at 2 to 4 pounds of nitrogen per 1,000 square feet a year, not above 6, and for fast-release nitrogen at 1 to 2 pounds, not above 4 a year. Clemson says slow or controlled-release forms are preferred because of leaching and runoff.

On place, Clemson describes the root zone as running from outside the root flare to the outermost branches, and for columnar trees or trees kept small by pollarding it describes estimating the radius as the trunk diameter in inches at 4.5 feet, times 1 or 1.5, in feet. Maryland describes surface broadcasting, liquid injection into the soil, spikes and foliar sprays, and says large shade trees are not fed through the leaves. New Hampshire gives one to three pounds of actual nitrogen per 1,000 square feet of the area fertilized, and notes that there is no lab test for nitrogen because it changes form and moves quickly in soil.

Deficiencies and look-alikes

Nitrogen: Maryland lists yellowing of older leaves. New Hampshire lists small, light-coloured leaves and little new growth on established plants.
Potassium: New Hampshire says dead tissue along leaf edges may be a symptom, but that drought or other causes can produce it too, and that potassium is generally low in its soils.
Iron: Maryland describes yellowing between the veins of new leaves, tied to a soil pH above 7.0. It says foliar or soil chelated iron corrects it for a time and that the pH must be corrected or symptoms return. The iron chlorosis page has the wider picture.
Magnesium: Maryland describes yellowing of older leaves with short new growth, mostly in very sandy soils with acid-loving plants, and says a leaf tissue test is the most accurate check.
Look-alikes: Maryland lists poor drainage, overwatering, over-mulching, planting too deep, root nematodes and root injury as other causes of chlorosis. New Hampshire lists planting too deep, drought, overwatering, disease and mechanical damage, and says fertilizer will not correct cultural problems. The mulch depth page and the root problems page cover two of these.

What the sources say about excess

Maryland says too much nitrogen produces long succulent shoots that attract sucking insects such as aphids, that strong concentrations burn roots, and that excess fertilizer can contaminate groundwater, streams, rivers and the Chesapeake Bay, that runoff can easily occur on slopes, and that groundwater pollution is more likely from fast-release products on sandy soils. New Hampshire says nitrogen leaches into groundwater, that phosphorus in runoff can cause algae blooms, and that effects of disease, insects and stress may be more severe on heavily fertilized plants. Clemson describes fertilizer burn and lush growth that is more open to cold injury, drought and pests. Local fertilizer rules, where they exist, are set by each state or province and are not covered here.