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Fertilization, Bio stimulants, and Weed and Feeds 

By Jerry Naiser Arborist Austin when to fertilize live oaks. How to fertilize Live Oaks. Certified Arborists Tree Doctor

When to Fertilize Trees                                                    

The quick answer: Fertilize your trees in the Spring and Fall. Any plant must be actively growing for fertilizer to be of any use. In conversations with people through the years, I have come to realize that there is a overall misunderstanding of fertilizers: what they are, what they do, why to use them, and when to use them. Fertilizers are not food. Plants make their own food through photosynthesis. To be healthy all plants require sixteen nutrients. This rarely happens because most of these nutrients are already present in soil. There are times when there is not enough of some of these nutrients or they are not available because of the high PH of our soil due to the large amounts of limestone in the soil where we live. This is where fertilizers and Bio stimulants come in.

Fertilizers contain varying amounts of macro nutrients, which are the three numbers that you see on the bag that you buy. These numbers represent (in this order) Nitrogen (N), Phosphorus (P), and Potassium (K). There are also very small amount of micro nutrients.

Fertilizers generally contain large amounts of the macro nutrients, with very little micro nutrients, while bio-stimulants contain very little macro nutrients, with a greater quantity of micro nutrients, and also beneficial fungi known as Mycorhizae. This fungi works in a symbiotic relationship with the root system, drawing in moisture and nutrients for the plant, and in return the plant feeds it carbohydrates. Of the sixteen essential nutrients, three of them (Carbon, Hydrogen, and Oxygen) are from water and air. These elements are always available. The other thirteen are absorbed by the roots. The thirteen essential mineral elements obtained from the soil are divided into three groups, based on the amount of nutrients that are needed by plants.

Micro nutrients include boron, chlorine, copper, iron, manganese, molybdenum, and zinc. Acid loving plants have trouble obtaining iron in Austin because our soil is so alkaline. The secondary nutrients; calcium, magnesium, and sulfur, are used in greater quantity and deficiencies are more common. Sulfur is often added to most quality fertilizers that are sold for acid deficient soils, as a buffering agent. At Real Green, we add sulfur to all fertilizers and bio stimulants we apply.

Nitrogen (N), Phosphorus (P), and Potassium (K) are the macro nutrients that are necessary in the largest quantity. Although these nutrients are required in greater quantity, they are of no more greater importance than the micronutrients.

People tend to focus on these macro nutrients because of the great quantity that they are used. Often times these nutrients are unavailable to the plant because of over fertilization with products that have a high salt content.

Nitrogen – Nitrogen forms new cells and is essential to total plant development. A shortage of nitrogen halts plant growth and cell production. Symptoms of nitrogen deficiency are yellowish brown along the veins and tips of leaves, stunted growth of the plant or paleness in color on older leaves. Too much nitrogen can cause tall, spindly plants that easily topple over and nitrate poisoning (look for a strong red tint to the leaves). Nitrogen is also essential to the compost pile, as it aids in the breaking down of old plant residues. Good sources of nitrogen include: blood meal, cottonseed meal, fish meal, soybean meal, rabbit manure and tea grinds. You can also grow a cover crop of legumes in the fall to raise the nitrogen level for the next springs crop.

Phosphorus – Phosphorus produces vigorous seed and root development. A shortage of phosphorus slows cell division and results in stunting of growth and late maturity of the plant.  A symptom of phosphorus deficiency is spindly plants with purple streaks in the stems. Since phosphorus moves very slowly in the soil, it is essential to have it available during early plant development. Good sources for phosphorus include: bone meal, rock phosphate and colloidal phosphate. Incorporating organic matter into the soil makes the phosphorus present more readily available to the plants.

Potassium – Also called Potash, this nutrient helps produce strong and sturdy stems. It advances root growth and helps plants resist disease and cold weather. A shortage of this nutrient causes stunting and stem weakness. Symptoms include a yellowing of leaf edges and yellowing of the leaves veins. This nutrient must be available during early plant development. Good sources of potash are: cow manure, compost, granite meal, greensand and wood ashes.

pH - pH is a measure of acidity or alkalinity on a scale of 0 to 14, with 7 being neutral. The pH of the soil affects many elements, including nutrient availability, plant nutrient uptake, and microorganism activity. Because soil pH affects many factors, it is important to maintain proper pH throughout the growing season. Considering the complexity of factors involved, the recommended soil pH for each individual variety is very important. Regular pH testing will allow you to make informed decisions if soil pH adjustment is necessary.

  • For soils that are too acidic, add lime to the soil in the fall. Some sources of lime are: Dolomitic limestone. This will raise the pH of the soil while supplying calcium and magnesium. Apply in late fall for next years crops. Hydrated lime will provide quicker results because of the smaller particle size and is especially useful for raising the soil pH quickly. Be aware that hydrated lime may cause severe injury to young plants. Apply at least 3 weeks before planting your crops. Wood ashes (not charcoal ashes) can also be added to acidic soil to correct the pH level.

  • For soils that are too alkaline, add powdered agricultural sulfur, aluminum sulfate or iron sulfate to the soil for a quick fix. For a long term solution, continually add acidic organic matter to the soil, such as peat moss, pine needles and oak tree leaves until the pH level is correct.

What does all this have to do with my trees?

Lawn fertilization is applied on the surface, and rarely reaches the root system of a tree. Furthermore, many lawn fertilizers actually contain a herbicide. These fertilizers are called a “Weed and Feed”. These herbicides are targeted towards broad leafed plants (weeds) The big problem here is that trees are also broad leafed plants, and these herbicides can cause fatalities in trees. Some trees, such as a Post Oak, will become severely ill after one application, while others, such as a healthy Live Oak, can withstand many years, of repeated  applications, before becoming ill. Another issue is timing; Pre-emergent herbicides should be applied when everything is dormant, while fertilizers are only effective when plants are growing.

Trees are targeted by many defoliating pests. These pests can completely strip a tree in a matter of days. A healthy tree has the ability to replace this damage. Without foliage a tree has no way of feeding itself. In the past few years, heavy rains, saturated soils, insects and fungal problems, have reeked havoc on our trees. This necessitates the use of all available tools that we have in our arsenal to keep the remaining trees healthy. Remember, it's easier to keep a healthy plant healthy, than it is to save a sick one.

Test your soil, add the recommended fertilizer and/or Bio stimulant to your trees, in the proper amounts.

If you don’t have the time to do it, we can do it for you. Call me (Jerry), or e-mail me: Jerry@RealGreenLawns.com

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