Here's some more guidance if you have managed to narrow the likely causes of decline to one of the following;
Deep planting or build-up of soil level
For optimum growth, the root flare (first roots on the trunk) should be just below soil level. Planting too deeply may cause establishment problems soon after planting, but it could also become a problem later on. Over time the soil level can build up around the base having similar effect.
Solution: With a spade or trowel, carefully remove excess soil from around the base to expose the root flare. Take care to avoid damaging larger roots and the roots of trees and shrubs that are prone to suckering (root damage can stimulate or aggravate suckering). Unless the roots are very close to the surface, also reduce the soil level in the main rooting area under the canopy. Cover the area with mulch such as composted bark or wood chippings, but keeping the root collar free from mulch.
With high value trees and shrubs, consider hiring in a professional with air-tools that use high pressure air to remove excess soil and mulch from around the trunk and main rooting area with minimal damage to the roots. At the same time bio stimulants may be added to the soil. See 'Alternative treatments' above.
All soil can become compacted, but clay and silty soils are the most vulnerable. Compaction causes poor gas exchange in the soil meaning the roots quite literally asphyxiate, leading to root death and poor top growth. The soil structure can be damaged by heavy foot traffic or machinery, especially when the soil is wet, causing compaction. Compaction can lead to problems with poor drainage and waterlogging.
Solution: Avoid walking on or working wet soils. Divert paths away from the base of ailing specimens. Remove all vegetation including turf from the main rooting area under the canopy. This eliminates the need to enter the area for regular mowing or weeding, as well as competition from other plants. Cover the area with an organic mulch and replenish as needed. The microbiological activity can help to gradually improve the soil. Consider applying general fertiliser to the rooting area in spring to boost growth. Using compressed air tools (air-spade, Terravent and Terralift) can help to reduce compaction in the main rooting area. See 'Alternative treatments' above.
When soil is saturated its air pockets are filled with water instead of air and roots can ‘suffocate’ and start rotting. Wet conditions also provide favourable conditions for certain soil borne diseases.
Solution: It is difficult to improve drainage around established plants but in areas prone to waterlogging consider installing drainage. Where practical consider moving smaller trees and shrubs to a better suited position. To reduce further soil structure damage avoid compaction.
Though well-established trees and shrubs can generally cope with a degree of drought stress, extremely dry conditions can result in significant leaf loss or premature leaf fall. Drought stress may contribute to or speed up decline, if the tree or shrub is suffering other problems already.
Solution: Apply mulch when soil is moist to reduce evaporation. In spring apply a general fertiliser to boost growth. However, avoid overfeeding that encourages soft, lush growth that is more prone to drought damage.
Lack of nutrients
Poor growth of established ornamental trees and shrubs is seldom caused by lack of nutrients in the soil. The problem often lies with poor uptake of nutrients due to the growing conditions such as drought, compaction or waterlogging or is caused by a root disease problem. However, they may suffer from specific nutrient deficiencies if soil conditions are unsuitable.
Established trees and shrubs do not generally require regular fertiliser, but fruiting trees and shrubs and shrubs with a long flowering season such as roses benefit from annual feeding.
Solution: Aim to identify the underlying problem and address it. Consider soil analysis. Application of a general fertiliser as directed by the manufacturer can boost growth of stressed plants. Preferably apply fertiliser early in the growing season. Where possible, remove any vegetation from under the canopy before feeding. In the long term is it worth investing in organic mulches as these will gradually improve the soil’s texture, reducing leaching nutrients and evaporation from the soil leading to better nutrient availability.