As explained in Lecture 21, plants need more than just N, P and K. They need at least ten other nutrients. These other ten nutrients are briefly described in this lecture, as are various concepts related to soil fertility and plant nutrition.
1. Secondary nutrients. The secondary nutrients are calcium, magnesium, and sulfur.
|Calcium is the most abundant of soil nutrients, and is environmentally benign. It is used by plants in large amounts, and is take up in the form of Ca2+. It is an exchangeable cation, and typically dominates the cation exchange sites on soil.|
|Magnesium is less abundant than calcium and is used by plants in smaller quantities. The chemistry of magnesium is very similar to that of calcium. It is a divalent cation, used by plants as Mg2+.|
|Sulfur, like nitrogen, exists in many forms and participates in many reactions. It is involved in various redox reactions, both as an electron donor (as is the case with elemental sulfur or organic sulfur forms), and as an electron acceptor (as is the case with sulfate). Sulfur is used by plants as the oxidized form, sulfate, SO42-.|
2. Micronutrients. Seven micronutrients are required by plants. These nutrients and the chemical form used by plants is given below:
|Copper: Cu2+, Cu+|
|Iron: Fe2+, Fe 3+|
In addition to the seven known micronutrients, other micronutrients may be required in such small quantities that the need has not yet been detected. Also, some elements are called "beneficials" because they improve plant performance, even though they may not be essential, or may be essential for only a few plants. For example, some grasses grow better with silicon than without; and clovers grow best with some cobalt in the soil, because the nitrogen-fixing bacterial than inhabit the roots of clovers need cobalt even though the clover itself does not.
3. Plant nutrient contents. Typical nutrient contents in plant shoots are given in the table below.
|N 1.5 - 2%|
|Ca 0.5 - 1.0%|
|S 0.1 - 0.2%|
|Cl 50 - 100 ppm|
|Fe 50 - 100 ppm|
|Mn 50 - 100 ppm|
|B 20 - 25 ppm|
|Zn 20 - 25 ppm|
|Cu 6 ppm|
|Mo 0.1 ppm|
4. Acquisition by plants. Nutrients come into contact with plant roots by three mechanisms. They are mass flow, diffusion, and interception. Mass flow refers to the process that delivers nutrients to roots in the water that flows toward the root during transpiration. Enough calcium is delivered to roots by mass flow, so that no other mechanism is necessary. Diffusion is flow down a concentration gradient. Phosphorus is delivered in only minute amounts by mass flow, but is primarily delivered to roots through diffusion as plants keep the P concentration near zero at the root surface. Interception is the process in which the root intercepts nutrients by growing into the region where the nutrients are found.
Once the nutrient is in contact with the root, it can enter passively or actively. Passive uptake is typical of nutrients delivered primarily by mass flow. They simply enter the plants along with the water. The plant actively acquires some nutrients; in other words, the plant expends energy to actively pull nutrients into the root cells. Phosphorus and potassium are acquired actively.
5. Diagnostics. The fertility of soil can be evaluated by soil testing, plant tissue testing, or by the observation of visual symptoms. Soil testing is based on the idea that one can extract nutrients from the soil, measure the nutrients in the extract, and thereby predict how well the soil and supply nutrients to plants. This approach works well on annual crops for which soil testing is done prior to planting and fertilizing. Tissue testing is done on existing plants, and is therefore more useful for perennial plants than for annual crops that will only be in the field for a short time. The grower can perform simple tests, called "quick tests", or can collect samples to be sent to a lab that will perform thought tissue analysis after digesting and extracting all tissues. The grower can also learn to recognize visual symptoms, but symptoms are often misleading.
6. Fertilizers. To the extent possible, growers do use, and should continue to use, organic waste materials such as manure to replace nutrients lost from the soil. Generally, these organic amendments are found to be inadequate for optimal yield, so commercial fertilizers are widely used. Most fertilizer N comes from the Haber process as described in the previous lecture. Fertilizer P comes from phosphate mines where rock phosphate is treated with acid to produce calcium or ammonium orthophosphates, and other materials. Potassium comes from underground salt mines, in situ mines, brines, and open-pit excavations.
7. Fertilizer grade. Fertilizers have a "grade" or "guarantee" that tells the user how much N, P and K are in the material. The standard notation is three numbers such as 18-46-0. This means 18% nitrogen, 46% phosphate equivalents, and 0% potash (as potassium is called in the fertilizer business). The nitrogen numbers are pure, actual percentages. The phosphate number is the percentage of P2O5 that would be present in the fertilizer if the phosphorus were P2O5 (which it is not). This notation is admittedly confusing and deceptive, but persists partly because it reports much larger P amounts than are actually present in the material. Actually percent P in the fertilizer is the middle grade number (46 in the above example) multiplied by 0.44. Similarly, potassium values are the percentage of K2O that would be present in the fertilizer if the K in the fertilizer were K2O (which it is not).
8. Fertilizer forms. Fertilizers can be in many forms. Many fertilizers are sold as blends of various dry fertilizers, mixed at the plant. Usually dry fertilizers are granulated for easy application in the field. One fertilizer, anhydrous ammonia, is applied as a gas. It is actually a liquid in the pressurized tank, but becomes a gas as it exits the tank. Liquid fertilizers have gained considerably in popularity. These are typically aqueous mixtures of various fertilizer ingredients. Liquid forms are easily and accurately applied. Micronutrients are often applied as chelates. This prevents them from reverting to unavailable forms in the soil. These micronutrients are often applied to the leaves of plants rather than to the soil.
9. Fertilizer application. Fertilizers can be applied before planting, either by broadcasting fertilizer over the field, or by injecting a gas or liquid in rows. Fertilizers are often applied at the time of planting, either by placing a row of fertilizer to the side and below the row of seed (called banding), or by adding fertilizer directly to the seed hopper (called pop-up). Application of fertilizer to plant leaves directly is called foliar feeding. Adding fertilizer to irrigation water is gaining in popularity and is called fertigation. Adding fertilizer to an existing crop such as a pasture is called topdressing.
Students are encouraged to look up the following vocabulary words in the textbook glossary or elsewhere and to browse the following web sites.
A web site for practical evaluation of deficiency symptoms;
a few pictures are provided.
An academic approach to nutrient deficiency symptoms.
A downloadable program to calculate fertilizer requirements
from New Mexico State University.
USDA economics and statistics library, maintained by
Cornell University. Data includes fertilizer statistics.
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