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FAQs: Fertilizers & Amendments

Topics listed pertain to what fertilizers/nutrients are needed to improve plant growth, how to apply them and what soil amendments will benefit plant growth

How do you apply micronutrients?
In recent times, we have become more interested the entire spectrum of nutrients that plants need to grow, including the secondary and micronutrients. Sulfur, iron, manganese, zinc, copper and boron are absorbed mostly by the root system in very small amounts compared to the primary nutrients, nitrogen, phosphorus, and potassium. When one or more micronutrients becomes deficient either because of the low supply or because some condition like the soil pH is too high or low, then this deficiency will limit plant growth. The only way to tell if a micronutrient is in low supply is through a soil test. If your soil test indicates that there is a single micronutrient deficiency, then you have several choices. You can either find a supply of the specific micronutrient, look for a micronutrient fertilizer containing many micronutrients in a package or, buy a complete fertilizer containing micronutrients.

Specific micronutrient fertilizers would be; iron sulfate, manganese sulfate, zinc sulfate, copper sulfate or fixed copper and boric acid or borax. With the exception of iron, the rest are not easy to find at a common garden center. Boron is a product of some detergents and this can be applied to satisfy this deficiency. Copper sulfate can be found in the pest control section since copper has fungicidal activity important in disease control. Some garden centers or nurseries might stock a complete micronutrient fertilizer, but this is not common since few customers request it. The next best supply of these micronutrients is in a complete fertilizer. You will need to read the bag to see that the fertilizer contains micronutrients. Be aware that the application of a complete fertilizer is aimed at applying nitrogen which if over applied, may cause plant damage.

Can gypsum be used to improve poor soils?
Gypsum is chemically known as calcium sulfate and this has been used on soils for a very long time to provide both plant nutrients and to improve the structure of certain problem soils. As a nutrient supply, gypsum is a good source of calcium and is frequently used instead of limestone when the pH of the soil is in a good range for plant growth and no further correction is needed. Unlike lime, gypsum will not affect the soil pH. It is also a good source of sulfur and can be used instead of elemental sulfur for correcting soil deficiencies.

As a soil conditioner, gypsum has been used to improve soil structure. But its effectiveness increases when applied to soils which have become poor due to excessive salt build up. These soils are called sodic in nature and can be found more frequently in the southwest U.S. from Texas to California. Here annual rainfall is low and irrigation water, high in sodium, is used to grow plants. Evaporation from the soil surface causes a concentration of the sodium in the upper 6 inches of the top soil and this affects the soil structure. These soils become black in color and slick. The remedy has been to add gypsum which react in such a way that the calcium replaces the sodium and this combines with the sulfur to be leached away when fresh water is applied. The calcium left behind will assist in improving the soil structure and in stimulating clod formation which adds pore space for water and air exchange. This is the most valuable use of gypsum as a conditioner. Gypsum should not be prescribed for improving soil structure unless soil tests show that the soil is high in sodium. High soluble salt soils caused by over fertilization or surface evaporation should be treated with water alone to flush nutrients out of the root zone. Gypsum will not help these soils nor will it improve clay soils more than a simple application of organic matter.

What are soil conditioners and amendements?
Conditioners and amendments are considered the same when it comes to adding them to soils. Both are used for improving soil texture, structure, a soil property like pH, or nutrient holding capacity. Conditioners are usually organic materials spread over the top of the soil and tilled or turned under to the depth of the root system -- about 6 inches. In contrast, most fertilizers are not considered amendments since they do not contribute to the soil's tilth or improve the physical properties of the soil. Likewise, mulches are not amendments either, since they are added to the top of the soil as a barrier to moisture loss, to help in controlling weeds and protect the plant from damaging low temperatures. Although many mulches also eventually work into the soil and improve it's tilth, their purpose is related to the surface application and not to their incorporation.

The types of conditioners commonly added include leaf mold, compost, sawdust, wood chips, chopped leaves, hay or straw, peat moss, fruit peels and animal manure. All of these add organic matter to the soil and this provides the benefits of improving water holding capacity, adding some nutrients, creating more air space for root development, loosening clay and compacted soils. Amendments and conditioners are best added in the fall of the year preceding the planting season. This is important because these materials typically need to be mixed when the soil is at the right moisture level and allowed to settle, voiding large air spaces. It also allows the organic matter to begin to decompose so that the particle size of the organic matter is smaller and nutrients can be released. If amendments are added in the spring, use them sparingly and choose the type of material that will be easily incorporated as well as quick to break down.

Some amendments may cause a nitrogen deficiency because of their concentration in carbon-rich materials. Sawdust and wood chips are dense and carbon-rich. When added to soils, microorganisms will begin to break the carbon-rich material down and that takes an extra nitrogen supply. Their own consumption of the material requires a large pool of nitrogen to offset the growth in the microorganism population. As a result, nitrogen is depleted from the soil and plants growing in wood-amended soils turn yellow. This is a general symptom of nitrogen deficiency. If you need to mix carbon-rich materials into the soil, then apply a nitrogen fertilizer like ammonium nitrite, urea or green lawn clippings to supply the extra nitrogen required. Taking a soil test beforehand will give you the percentage of organic matter in your soil. This is the best place to start.

Incorporating too much organic matter can only be a problem if it causes the soil to not settle and remain lofty or to hold too much water and not dry out. Generally, gardeners strive to reach levels of 5 to 10% organic matter. A standard recommendation is to apply is about 2 to 3 inches over the top of the soil and incorporate. For perennials beds and more permanent plantings, amendments are not practical since they cannot be incorporated easily to a beneficial depth without disturbing the root system. Amendments and conditioners are more appropriately used for annual flower beds, vegetable gardens and anywhere that the soil is fallow for a period of time. Gypsum and greensand are the only amendments that do not provide organic matter, yet improve the soil's tilth. Their use is restricted to only certain conditions.

What is an organic fertilizer?
Organic fertilizers are derived from once living organisms that are in the process of decomposing through the action of microbial activity to give up essential nutrients for plant growth. The term "organic" does not include synthetic fertilizer forms that are formulated from non-living sources. Good examples of organic fertilizers include seaweed, fish emulsion, kelp, poultry or cow manure, bloodmeal, cottonseed meal, sewage sludge, compost tea and bone meal. The percentage of nutrients is different for each source and tend to be low in contrast to synthetic sources. Because organic fertilizers require the action of microorganisms to release nutrients, they are considered a slow release type of fertilizer. It may be months, depending upon the type of organic material, its particle size, the ground temperature, microorganism population and other factors to gain significant release of nutrients. However, some gardeners rely completely upon organic sources for their nutrients. The supply of organic fertilizers has increased considerably and while the bulk required to maintain a steady supply of nutrients is significant, the use of organic sources illustrates how sustainable the plant system can be.

What fertilizers are best?
Fertilizers are formulated to fit into certain situations. The most common type is the complete fertilizer that contains all three essential nutrients, nitrogen, phosphorus and potassium. Their relative amount present in each fertilizer bag is expressed as a percentage and written on the bag or container. A typical complete fertilizer is 5-10-5. This has 5% nitrogen, 10% phosphorus as P2O5 and 5% potassium as K2O. Therefore, in a 100 pound bag you will have 5 pounds of nitrogen, 10 pounds phosphorus (as P2O5) and 5 pounds potassium (as K2O). When you use this fertilizer blend, you are applying all three nutrients at once. If you don't need all three, then you are adding nutrients that may not be used, might be washed way or occupy exchange sites in the soil that should be used by other nutrients.

The best fertilizer to use is one that can supply the nutrient in question in an amount that is convenient to spread without supplying other nutrients that are not needed. In areas where rainfall exceeds 20 inches annually, you will need to apply a nitrogen fertilizer as part of the routine. Nitrogen, in the form of nitrate that plants take up, is soluble and will leach from the soil. This is the reason for replacing it each year and why soil tests do not bother with its measurement it in the soil. If nitrogen is the only reason for applying a complete fertilizer, then look for one that supplies only nitrogen like urea or ammonium nitrate.

Complete fertilizers are contrasted against incomplete fertilizers. These supply one or two nutrients. An example of this is triple superphosphate with a formula of 0-46-0. The only nutrient in this fertilizer is phosphorus in the amount of 46%. This fertilizer is used only when phosphorus is deficient in the soil. It would not be used as a blanket application like a complete fertilizer. Incomplete fertilizers require soil testing information in order to use them when needed and soil testing is the only way to understand how much to apply for the type of plant in question. Other fertilizer choices include those designed for starting plants. These are called "Starter Fertilizers" and contain about 2 to 3 times the amount of phosphorus than nitrogen when compared to routine fertilizers. "Bulb Boosters" are similar to starter fertilizers and likewise, are aimed at stimulating root development. "Slow Release" fertilizers contrast against soluble fertilizers. These are commonly coated or formulated in such a way that the nutrients are released over a period of weeks to months. There are especially valuable when you are looking for season-long supply of nutrients and when rainfall or irrigation supplies abundant water.

Organic fertilizers contain nutrients that are associated with organic matter. Manure, bone meal, cottonseed meal and kelp are examples of organic fertilizers. Nutrients are released when the organic matter is decomposed by microorganisms in the soil. Therefore, the release is considered to be slower than with soluble fertilizers. The slower release allows for more efficient uptake by plants since it occurs over a broader range of time. When you choose a fertilizer, consider what nutrients need to be supplied, what amounts you need to purchase and how often you will need to repeat the process. Also, think about the cost of the nutrient on the basis of the percentage that it exists in the bag as well as how quickly the nutrient should be available for uptake. Lastly, apply only what is needed on the basis of a soil test.

When should you add lime?
Lime is an amendment we use to adjust the pH of the soil to make it more alkaline. Lime comes from limestone rock which varies in composition. Lime typically contains calcium, carbon, oxygen and may contain magnesium in different proportions. When water is added to lime, it begins a chemical process where the breakdown products are alkaline. These grab onto hydrogen that comes from exchange sites on the soil particles and makes them water. During the process, there is enough hydrogen from the soil that becomes neutralized through the formation of water. The balance of the reaction favors the alkaline side and as a result, the pH goes up past 7.0. It will stay there only as long as there is a constant supply of lime.

Since it takes months for limestone to completely breakdown in the soil, the amount applied will determine how long the soil is affected by the alkaline reaction. There are other factors that affect the liming reaction including the soil type, degree of incorporation into the soil and rainfall. The only way to know when and how much to apply is by getting the soil pH measured with a meter. These devices can provide a measure of acidity and alkalinity. The ability to get a good measurement depends upon the technique and what is mixed with the soil to register the reaction on the meter. Since there is considerable variability in doing this and experience is a key factor in how the results are interpreted, consider using a professional soil laboratory that uses precision equipment and has the experience to know whether amendments are necessary for the plant you wish to grow.

Why fertilize plants?
It is a fact that plants need nutrients and these are stored in the soil with the exception of carbon, hydrogen and oxygen which comes from the air. In nature, essential nutrients are cycled from one living organism to another. There is no one there to fertilize. Plant growth is usually slower, seasonally variable and dependent upon the weather. In residential yards and recreational parks, we generally have higher expectations for plants. We would like the growth to be more rapid, plants to be more healthy, productive and low maintenance. This active growth must come at a price and that is with the application of fertilizers and setting the conditions for optimal growth -- including some modification of the soil with amendments.

From seed, plants have some nutrients stored to give them a start, but soon the root system must develop and serve as the pipeline for taking up nutrients. If the plant cannot find enough of one nutrient, then its entire system is affected. Therefore, all it takes is one nutrient deficiency to limit plant growth. How severely a plant is affected by an inability to find the soil nutrients depends upon the condition of the soil (pH, texture, structure, water holding capacity, pore space) and the availability or supply of the nutrients. It also depends upon the plant and how fast it grows or what demands are present for leaf bud and flower formation, fruit development or replacement of leaves and stems.

Lawns need more fertilizer than shade trees because we are constantly mowing and removing the clippings. Returning the clipping to the lawn will partially offset this need for fertilizer, but to keep the lawn healthy, tolerant of diseases and insect attack and actively growing, additional fertilizers are commonly applied. Just how much to apply and when to apply them are essential questions to prevent over fertilizing and impacting the environment through nutrient runoff as well as causing the nutrients to be out of balance with each other. This is important because when one nutrient is present is excessive amounts, that can affect the availability of other nutrients. It is not a question
of enough fertilizer then, it is the over supply and the interaction of nutrients and competition for the uptake by plant roots. We are shifting from using a "blanket" application of complete fertilizer containing nitrogen, phosphorus and potassium to applying only what is needed or deficient. To do otherwise is to waste resources, pollute the ground water supply and defeat the purpose of growing healthy plants by developing a healthy soil.