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Fertilizing Trees

by James B. “Jim” Kea
Area Extension Forestry Agent – now retired
Thursday, February 9th, 2006

Fertilizing trees generally increases growth. Researchers are beginning to learn exactly what occurs when trees grow and why growth, as we know it, can be bad for trees.

Fertilized trees show remarkable increases in size, fruit production, and leaf or needle size. Several experiments are currently underway to see just how fast trees can be made to grow with as much water, fertilizer, light, carbon dioxide and oxygen as they can use. We know that there is a point where to much of any of these essential needs can kill. Salts in fertilizer can severely restrict water uptake in established plants or the already stressed roots of newly transplanted plants. Nutrients in high concentrations can also kill plants or inhibit growth. A sudden increase in sunlight intensity can cook leaves. And of course sudden changes in soil moisture can also be fatal.

To much growth on fruit trees can cause long, spindly limbs that snap under the weight of fruit or in wind storms. Fast growing wood in forest trees or yard trees is also structurally weaker than “slow growth”. Researchers are just now understanding other implications of fast growth.

Scientist believe that carbon becomes a limiting resource as trees grow faster. As carbon is separated from oxygen in carbon dioxide and converted to sugar, it must be allocated to elongation (growth) of buds, to cell wall thickening, to photosynthesis, to food stored as carbohydrates in fruit, buds, stems or roots, to toxic substances used by plants to fight off insects and diseases, to form callus tissue over wounds, to root growth and to increase the thickness of stem wood.

Photosynthesis has priority over most allocations, but its needs are quickly and efficiently met. Fruit production can be next on the list if conditions favor pollination, fruit set and expansion. Elongation of buds will be the most demanding of carbon reserves and the least efficient. These three can actually severely deplete food reserves in the plant even when all other resources are plentiful. This “hogging” of carbon will cause spindly stems, low resistance to insects and diseases and poor root development.

Stress severe enough to limit photosynthesis can cause trees to allocate food reserves and any food left after photosynthesis to go into fruit production. Pines under severe stress often produce enormous quantities of cones. This reaction is often the final gasp in the tree’s life. It’s almost as if the tree sacrifices its last bit of energy to its offspring.

Moderate stress actually triggers a more efficient and balanced allocation of carbon. Moderation in all things (including stress) seems to benefit trees as well as people.

Revised 2/16/2006.

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