Understanding what's in the fertilizer bag or bottle is the key to being successful in the landscape maintenance business today. Getting no plant response from a fertilizer application is an expensive problem. To the uninformed user fertilizer is fertilizer is fertilizer. However, nothing could be farther from the truth. Unfortunately, the fertilizer manufacturing industry has helped to create some of this confusion by introducing a dizzying array of product formulations. If we focus on the fertilizers normally used in landscape maintenance we can, however, make some sense of it all.
There are two major classifications of fertilizer. The first is synthetic (chemical) fertilizer and the second is organic fertilizer. These classifications refer to the parent source of the end product.
For landscape uses we will focus on what the nutrient combinations are in a fertilizer product. Then we must focus on what combinations of nutrients are agronomically appropriate. When shopping for fertilizer, the most common product information is what is known as the NPK analysis. This series of three numbers is easily identified as the Guaranteed Analysis required by the state Department of Agriculture. It is to be prominently displayed on the product label.
Each number represents the percentage by weight that each nutrient is of the whole product. For example, if a label on a 50-pound bag states that the analysis is 16-6-8, then nitrogen (N) is 16% of 50 pounds which equals eight pounds of actual N. Phosphorus (P) is 6% of 50 pounds which is three pounds of actual P and potassium is 8% of 50 pounds, which is four pounds actual K. The nutrients total only 15 pounds out of the total 50. The balance of the weight of the bag of fertilizer is the carrier, which is the material on to which the fertilizer is sprayed, or impregnated, to permit it to be broadcast easier. The carrier can be designed to either break down quickly to release the nutrients rapidly or to break down slowly to retard the release of nutrients into the soil.
Each manufacturer is required to list the "guaranteed" amount of nitrogen, phosphorus, and potassium in the product. The amount of each nutrient can actually be more than is listed on the label, which is permissible. However, it should never be less than the label shows.
The next decision that needs to be made is in what form the nutrients should be in order to get the most for your money. Nitrogen usually is delivered in several forms, the first of which is ammoniacal. The second is called the nitrate form and the third is usually urea. The first two forms of N are very fast acting, and yield a rapid response (usually greening occurs in one or two days) from plants. The urea form is slower because urea must undergo nitrification by soil micro-organisms which convert it to the ammoniacal form then quickly into the nitrate form. Surface applied, pure urea is volatile and can rapidly turn into a gas and be lost into the air. From an economic point of view this is not a good choice unless there is a volatilization inhibitor added to the product.
Phosphorus sources tend to be one or another form of phosphoric acid that is very stable in soil. Once it?s applied it stays in the soil because it does not volatize nor leach through the soil as does nitrogen and potassium. Potassium is usually either a form of potassium sulfate or potassium chloride (muriate of potash). Both are equally effective sources.
In addition to NPK, many fertilizers contain other nutrient elements. Some of these elements are deliberately added to give extra value to a product. Most are listed because they are beneficial nutrient contaminants that just happen to be in the parent source mix. An important thing to understand about these elements is the difference between plant macronutrients and micronutrients. The macronutrients include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). The micronutrients include iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), molybdenum (Mo), boron (B), and chloride (Cl).
Macronutrients are needed by plants in amounts many, many times greater than are micronutrients. However, it is not unusual to see landscapers applying "micro-nutrient packages," thinking them to be beneficial when it is probable that they are accumulating to levels in the soil and in plant tissues that could inhibit the uptake of other beneficial nutrients. Therefore, as a rule of thumb, apply micronutrient packages once a year to most landscape plant material, usually in the spring.
How Do Plants Absorb Nutrients?
There are four main ways that plants absorb nutrients. It is too simplistic to say that plant roots absorb nutrients. Understanding the ways plants absorb water-borne nutrients can help you understand what type of fertilizer to buy and apply.
1. Diffusion is the way most plants absorb fertilizer immediately after it is applied to the soil surface and irrigated. Application instructions on a bag of granular fertilizer almost always recommend watering after application to dissolve the product. Obviously, in order to be moved into the soil in a form the plant can use, the granules must be dissolved in water. This dissolution creates a very high concentration of the nutrients in a small volume of soil. As the high concentration of nutrients moves toward the root mass it grabs lesser concentrations of other nutrients, usually micronutrients, and carries them with the main stream toward the roothairs.
2. Mass Flow is also related to a fertilizer application but usually occurs a few days after application. Subsequent irrigation continues to dissolve the fertilizer granules at a more steady rate creating a consistent flow of nutrients into the root zone. Most plant benefit is derived from mass flow feeding, provided the nutrients are the ones that are deficient.
3. Root Interception is the process by which most plants grow. As root growth increases, plant roots grow past nutrient cations that have become adsorbed (not absorbed) onto soil colloid cation exchange sites. With the release of humic acid exudates, the plant roots can remove the cations from the sites and transfer them through the soil water solution into the plant.
4. Foliar absorption of nutrients is a popular method of fertilization. To be done properly, it does require some expertise along with agronomically appropriate fertilizer materials. The results from foliar feeding can be quite dramatic if the deficient nutrient is accurately identified and then applied. Most plants are capable of responding to foliar feeding if the leaf cuticle is not too thick.
Understanding how plants absorb nutrients and how fertilizers work, can take the mystery out of how to make plants look better and healthier.
Using the same fertilizer product year in and year out is not wise. Serious soil and plant problems could and do occur with this practice. The best policy is to invest a little money in an annual soil test and quarterly plant tissue tests. Tissue tests done by Near Infrared Reflectance Scanning are inexpensive at about $20 per sample and very accurate. They literally take the guesswork out of deciding which fertilizer to apply. Growing good quality plant material today is not rocket science, but it is science.