Forests and woodlands are full of trees that take care of themselves (no one goes around fertilizing them!). In urban and suburban areas throughout Connecticut, however, trees often need some help from us – usually in the form of fertilization and/or soil amendment.
Because soils in urbanized areas have a range of problems that can hinder tree health. In this article, we cover common problems with urban soil, how those issues affect your trees, and what to do about it.
Problems With Soil In Connecticut
Some soils are well-draining, have all the nutrients a tree needs, and generally support plants without any intervention or addition. But, frequently, soil in urban areas and suburban developments is less than ideal and needs to be improved in order to support healthy trees and plants.
The most common soil problems for urban and suburban trees in Connecticut are:
- Low nutrient levels
All of these can keep trees from reaching their full size and spread, can make them more vulnerable to insect pests and tree diseases, and can even kill them.
Compacted soil usually gets that way from three things: heavy vehicles and loads atop it, mechanical compaction for construction, and rainfall.
All these processes compress soil molecules together, which prevents enough air and water from penetrating the soil. It also hinders tree root penetration into the soil, which affects a tree’s water and nutrient uptake, as well as the development of a root system that can support and stabilize a large tree.
Soil acidity is measured by a soil’s pH rating. The pH scale is from 0 to 14, with 7 being the ideal neutral for soil. Most native soils range from 3 to 10 on the pH scale; the lower the pH, the more acidic the soil, while high pH soils are alkaline.
Connecticut soils are naturally acidic and fall at or below the pH range most beneficial to plants and trees (5.5-7.5 pH). Our native plants adapted long ago to acidic soils, and plants like Connecticut’s state flower, the mountain laurel (Kalmia latifolia), actually prefer acidic over alkaline soil. However, many common landscape trees planted throughout our area grow better in more neutral soil; some will even struggle in typical Connecticut soil.
Low Nutrient Levels
Poor soil is known by its low nutrient levels. If your property has only native plants (which have evolved to grow in poor soil), they will likely survive without additional nutrients. However, many other plants and trees that are commonly used for landscaping cannot thrive in those conditions – they need more fertile soil.
Low nutrient levels can be the result of leaching from rainwater, over-cultivation of crops, and lack of organic matter. Subsoil, or soil from deeper layers, is naturally low in nutrients because eroded minerals and organic matter have not reached it.
How To Tell If You Have Poor Soil
You can tell that your trees and shrubs are lacking nutrients in several ways:
- Leaves are yellow, brown, stunted, or sparse
- The ends of stems die back
- Flowering and fruiting are reduced
- Plants don’t grow to their full size
Specific nutrient deficiencies present specific characteristics. For example, iron deficiency causes leaf tissues to turn yellow while leaf veins stay green. Magnesium deficiency produces yellow leaf edges.
Since some symptoms of nutrient deficiency look like water stress, many people increase irrigation in an attempt to “fix” the problem. Unfortunately, this doesn’t address the underlying issue; as a result, nutrient deficiency symptoms will continue or even worsen. In fact, a struggling tree or shrub may struggle even more with overwatering.
How to Improve Your Soil (& Fertilize Your Trees)
Fortunately, soil amendments can improve both a soil’s pH and its nutrient levels. The best long-term amendment is compost, which improves the overall quality of the soil by increasing the amount of organic matter.
But when plants need more than compost can provide, or need it more quickly, synthetic or organic fertilizer can be applied to boost fertility.
There are several options for fertilizing your trees. The option you choose depends on the existing condition of your soil, the species and health of trees growing on your property, and your personal preferences for organic vs synthetic options.
The best thing for your soil fertility is to amend it by adding compost.
Compost makes humic acid, a kind of carbon that’s rich in nutrients. It bonds to soil particles and improves soil in several ways, such as:
- Increased soil fertility
- Increased soil texture
- Increased beneficial soil fungi
- Increased water-holding capacity
Nutrient molecules from humic acid attach themselves to other soil molecules, making nutrients available to plant roots for the long-term. As you replenish compost atop the soil, humate nutrients continue to bond with soil molecules, slowly improving the soil over time.
Slow-Release Granular Fertilizer
You can add slow-release fertilizer directly into your soil, where it breaks down slowly to prevent a spike in nutrients that would otherwise get quickly washed away. These are similar to the little colored beads of fertilizer you find in potting soil mixes.
You can also add granular fertilizer to the soil surface around your trees and shrubs. Since you don’t want to disturb their roots, gently scratch the soil surface and add fertilizer, then cover with a layer of mulch.
Water well after applying fertilizer to help dissolve the nutrients into the soil and make them available to tree roots. Rain and irrigation will continue to dissolve the fertilizer over time.
You can find granular fertilizers in both organic (carbon-based) and synthetic (chemical) versions. Both will provide nutrients needed by your trees.
Deep Root Fertilization
One of the best solutions to provide nutrients for trees is deep root fertilization. This method injects a specific fertilizer mix designed for your needs down into the soil around your tree, where established roots can take it up for use. It also helps aerate the soil so that oxygen and water can move through it.
The targeted nature of this application means only the nutrients that are lacking are injected, and injections are done only where it is most beneficial to a tree’s roots. Deep root fertilization is best done by tree-care professionals as it is an intensive process and requires training and equipment designed specifically for that purpose.
When To Fertilize Trees In Connecticut
Whatever type of fertilization method you use, be sure to apply fertilizer at the best times for your trees and shrubs.
Here’s when to fertilize:
- In late winter, when trees and shrubs are emerging from dormancy and using up their stored energy reserves
- During the spring and summer growing seasons, when plants need more energy for flower, fruit, and leaf production, and pests and diseases are preying on struggling plants
Do NOT fertilize trees in fall, when trees and plants are preparing for dormancy. If you stimulate new growth in the fall, plants will use their stored energy reserves to make leaves and stems that will quickly die at the first frost.
When you fertilize your garden, don’t assume that if some is good, more must be better. It’s not. In fact, over-fertilization can cause a whole set of new problems for your trees.
Nitrogen and phosphorus are two of the main nutrients needed by trees and shrubs for green leaves, flowers, and growth. A tree’s root systems takes up these nutrients as the tree requires them.
When fertilizer is over-applied, the excess nitrogen and phosphorous are wasted and washed away by rain, either as surface runoff or by percolating through the soil. Eventually, these concentrated amounts of nutrients reach bodies of water, such as ponds, streams, lakes, and even Long Island Sound, where they cause algae blooms and reduce the oxygen level in the water. This kills fish and other aquatic life. Fertilizer runoff also enters groundwater and can sicken people and domestic animals who drink it.
To address the problems with fertilizer runoff, Connecticut legislation prohibits the application of lawn fertilizers containing phosphorus on established lawns unless a soil test, done within the past two years, shows that phosphorus is deficient and phosphorus needs to be applied.
The same principles may be applied to general garden fertilization, including trees and shrubs, which is why we recommend a soil test before applying fertilizer.
Want To Test Your Soil?
Worried your soil is low in nutrients? Concerned that you might have over-fertilized?
Connecticut has publicly-available resources for soil testing that will tell you whether your soil needs more – or fewer – nutrients, as well as which nutrients you need.
If you’d like to run a nutrient test on your property’s soil, check out this information from U. Conn’s soil nutrient analysis laboratory. It tells you how to take a soil sample and submit it for analysis. You can also find soil testing information on Connecticut’s agricultural station’s website.
If you really dig soil, U. Conn has a soil database full of information.
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