Agronomic Information - Fertility Management
Chapter I - Introduction
Life is fueled almost entirely by plants. Because plant products are so important to modern day man, a huge agricultural economy has developed around crop production. As crop production has increased, so, too, has the crop's need for extra plant nutrients. These nutrients are provided in the form of fertilizers. The science and business of plant fertility has developed rapidly and continues to develop.
Primary agricultural producers would probably have a hard time convincing their average urban cousin that plants play such a huge role in the technology based lifestyle of the average North American . Almost all of the energy we fuel our fast paced lifestyle with comes from the sun captured by plants. To date the only 2 usable forms of energy not derived from or dependant on the sun are nuclear energy and geothermal energy. There is strong resistance from society to the use of nuclear energy and geothermal energy seems to have limited applications. Of course, mankind strives to harness other sources of energy and no doubt soon will. Fossil fuels, however, are our major source of energy. These are derived from plant material.
Plants have the natural ability of capturing the sun's energy through the process of photosynthesis. They use the energy derived from photosynthesis to take carbon dioxide out of the atmosphere and build it into plant structures. The energy photosynthesis builds into these carbon structures fire all our fossil fuel based conveniences and businesses. It also provides all of mankind with their personal source of energy and nutrition either directly or indirectly!
As the earth's population continues to rise, the demand for agricultural plant production also rises. World population has just reached 6 billion. The latest stats from the United Nations suggest that the rate of increase in this population is slowing down but we will be at 8 billion by 2030. Of course, all of the soil fertility texts say one of the ways to meet this demand is to maximize output on the land we already have in plant production. Very difficult to sell this theory to Saskatchewan farmers who have been hearing of increased demand for 20 years but have seen a decrease in the price of their grains.
One of the dangers of trying to meet the pressing necessities of the future population growth may be the exploitation of our primary crop growing resource, the soil. If this population growth really does cause a food shortage, then there will be great pressure to produce as much as possible in the near term either for monetary gain or for survival. This will certainly be done at the expense of sustainability in some, if not many, instances. Many of Saskatchewan's top producers have seen the economic and conservation benefits of direct seeding so have made a low disturbance seeding system work in their operations. They may be at a distinct advantage because they have already developed a profitable, high producing and sustainable production system.
Canadian prairie grain farmers have honed the efficiency of their operations very sharply. Because they are price takers, they know that profit comes from producing bushels. They have maximized production with all of the tools they can find while keeping their production costs at an acceptable level. For many, one of the ways they are keeping production costs down is through low disturbance seeding. Another way they have maximized production is by supplying plants with all of the nutrients they can profitably provide. Plants do need nutrients to function. The Saskatchewan farmer transports a large percentage of his plant production out of the natural ecosystems in which it has been grown. This exporting business means that plant nutrients that would naturally be recycled for future plant production are permanently removed from the ecosystem.
Hundreds of years before the birth of Christ, there is literature that refers to manuring land for various types of agricultural production. During the first 18 centuries after the death of Christ, scientific progress in understanding plant growth was slow. In the late 1700's and early 1800's the understanding that plants take in CO2 was accepted. Various scientists began to show that other elements were necessary for plant growth. An understanding of the importance of nitrogen and how plants use it began to be developed. Tisdale et. al. state that corn yields in the U. S. began to increase with the use of hybrids in the 1940's. The addition of chemical fertilizers began in the 1950's and these further increased crop yields. Hermanson claims that fertilizer use began on the Canadian prairies in the 1950's. Effective pest management and fine tuning cultural practices also started to boost yields at about this time. Gardner et. al. state that as much as 50% of the current yield, quality and nutritional value of grain crops can be attributed to the use of commercial fertilizers.
What nutrients do plants really need to grow and produce energy, nutrition, and fiber for a growing world population? To date, plant scientists have found that of the more than 100 known chemical elements there are 16 that are essential for the growth of all of the world's major crops. There are another 3 that are essential to some plants and a few that stimulate plant growth under certain conditions. Plants also contain many elements plant scientists label as nonessential.
All of the essential chemical elements found in any particular plant come from the soil, water, and air(gaseous atmosphere) around the plant. These 3 sources are collectively referred to as the biosphere. Three of these essential elements, carbon (C) , oxygen (O) and hydrogen (H) are the most abundant elements in plants. They are taken in by the plant primarily from the surrounding air and water (H2O). Carbon which forms the skeleton of all organic molecules is taken in from the atmosphere in the form of carbon dioxide(C02). The photosynthetic process converts carbon dioxide and water into carbohydrates. From these amino acids, sugars, proteins, nucleic acid, and other organic compounds are synthesized. Oxygen is also needed by plants for respiration which provides energy through the break down of carbohydrates. Water, which makes up a large percentage of total plant weight, is also crucial for plant growth. It transports materials around the plant and is involved in many of the chemical reactions taking place in the plant. Carbon dioxide and oxygen are present in the natural atmosphere around plants and are not normally limited in supply. Photosynthesis is rarely limited directly by the supply of water but indirectly water does play a huge role in any plant's functional ability to keep photosynthetic processes going.
The remaining 13 essential nutrients are divided into 2 groups based on their concentration in plants: macronutrients and micronutrients. See Table 1.
Fertility issues in Saskatchewan focus on ensuring that the macronutrients and micronutrients listed in Table 1 do not economically limit production. There are small areas in Saskatchewan where we can irrigate to supplement limited water supply but those areas are too small to be included in a general discussion on fertility in direct seeding in Saskatchewan.
The remaining sections of this topic will, therefore, focus first on a general discussion of the function of macronutrients and micronutrients in plant processes. The next section will focus on the supply of these nutrients and plant uptake. One of the sources of plant nutrients is fertilizer. A brief discussion on the forms of fertilizer being utilized in Saskatchewan will follow. Fertility in direct seeding is really all about the application of an economical amount of fertilizer to maximize our return and protect our soil resource. The next section will explain the principles of fertilizer placement in direct seeding systems. Various options for placing fertilizer in direct seeding will be discussed in great detail.
There is a great deal of overlap between the various pillars of direct seeding. Direct seeding is not just a matter of seeding into standing stubble. If direct seeding is to be successful on any particular farm, there will be great benefit to carefully thinking through the whole crop production system.
Chapter II - Essential Nutrients - Macro and Micro
Plants are made up of chemicals and a grasp of some chemical principles of plant nutrients will be very helpful in understanding plant nutrition. Plant biochemistry can become very complicated and there are probably still many things for plant scientists to discover. However, even understanding the basic functions of the various essential plant nutrients will be a good foundation for planning a profitable fertility program in a direct seeding system.
Definition of essential
It is important to understand what factors make macro and micro-elements essential. Scientists decide if an element is essential by comparing plants growing in a medium containing the element to those growing in a medium not containing the element. Elements get the label essential only if plants fail to grow and complete their life cycle in the absence of this element. Certain visual symptoms of deficiency are usually evident for any particular nutrient deficiency. The symptoms are corrected or prevented when the nutrient is provided Scientists continue to refine procedures to determine essential elements. This implies that even though some elements are only needed in minute quantities, they can still affect plant growth significantly if they are not being taken in.
Plant Nutrient Concentrations
The quantity of each of these particular essential elements required by different plants varies greatly. Tissue testing is being promoted to diagnose crop problems even on the lower value crops we grow on the Canadian prairies. The quantities of nutrients that a plant contains are listed in general categories: i) Deficient - Concentration is low enough to limit yield. In many plants there is a range where yield may be limited without visual symptoms. This is called hidden hunger. With more severe limitations symptoms can be distinct. ii) Critical Range - This is the nutrient concentration where yield begins to be affected. As nutrients are applied to reach the critical range, the law of diminishing returns applies. Each added fertilizer increment produces progressively less yield. iii) Sufficiency - Increasing nutrient concentration will not increase yield but will still increase the concentration in the plant. iv) Excessive (toxic) - At this point, increases in concentration reduce plant growth and yield.
The concentration of some elements in some plants can be affected by the concentrations of other elements in the plant or in the soil. Plants can take up many elements that are not essential to plant growth, which can affect the up take of essential elements. This uptake depends on what the soil contains. In some cases, the uptake of one element may be impeded by the uptake of others. In other cases, one element is needed in a certain proportion to another. If one element is deficient, then even if quantities of the other element are above the critical range, the first element's deficiency effects will still be evident.
Nutrient Deficiency Evaluation
When a plant does not have enough of any one nutrient its normal processes become unbalanced. There may be a build-up of certain intermediate compounds and shortages of others. This causes the visual symptoms. Symptoms to look for include: i) Abnormalities at germination or seedling stage
ii) stunted growth
iii) specific leaf symptoms appearing at various times
iv) internal abnormalities such as clogged conductive tissue
v) delayed or abnormal maturity
vi) yield differences
vii) poor production quality such as protein, oil, or starch content
vii) storage quality
viii) extent and type of root growth.
It is important to understand that there are other plant stress factors that can cause symptoms similar to nutrient deficiencies. Two examples are disease and insect damage. Weather variations from normal can cause temporary nutrient shortages as growth and nutrient uptake rates change. The use of tissue testing should be used to validate visual observations of questionable symptoms or to test for hidden hunger.
By the time symptoms appear, the crop has suffered and yield may be unrecoverable. Symptoms are a good signal for changes to be made for the following year but the ideal is to supply all the nutrients that will bring an economic yield increase before the crop is affected. One of the best means available to date is the use of a soil test. It will provide an average recommendation for the field at a low cost.
Some scientists have grouped nutrient functions under these 4 categories: i)basic structure - Most of the elements in this category would be C, H, and O. ii) energy storage and transfer - Nitrogen (N), Sulfur (S), and phosphorus (P) iii) charge balance - Potassium (K), Calcium (C), Magnesium (Mg), iv) enzyme activation and electron balance - Iron (Fe), Manganese (Mn), Zinc (Z), Boron (B), Copper (Cu), Molybdenum (Mo), Chlorine (Cl)
Nitrogen is the nutrient element most frequently deficient in crop plants. A plant's nitrogen content can range from 2% to 6%.. Plants take in N as NO3- (nitrate) or NH4+ (ammonium). N is used in the formation of amino acids which are synthesized into proteins and nucleic acids. Proteins provide the framework for various plant structures, many of which are associated with plant biochemical reactions, such as enzymes.
Adequate supplies of N are associated with a crop that has: a dark green color, lots of photosynthetic activity, and vigorous vegetative growth. The quantity of N available affects carbohydrate utilization. Under reduced N supplies, carbohydrates are deposited in vegetative cells. If conditions favour growth and sufficient N is available, this carbohydrate is converted to protoplasm The protoplasm contains more water. The plants are more succulent and one of the detrimental results on grain crops is that they may be more susceptible to lodging. This lodging will be more pronounced with lower K supplies or in certain varieties. Also, too much N in proportion to P, K, and S can delay crop maturity. If the right proportion of nutrients is available, N has been shown to speed crop maturity.
Nitrogen deficiency symptoms are stunting and yellowing or chlorosis. Chlorosis occurs as protein N in chlorophyll is lost in the older leaves. In more severe deficiencies, the lower older leaves will turn brown and die. This death or necrosis of tissue begins at the leaf tip and progresses along the midrib. N is very mobile in the plant. When the roots are unable to absorb enough N from the soil, protein in the older plant parts is converted to soluble N and translocated to the growing points where it can be used in new tissues.
Grain protein is becoming a very important issue for some cereal grain crops. There are now a number of classes of wheat in the Canadian prairies that pay premiums based on protein. Maltsters do not want barley with high protein. Two of the main factors affecting grain protein are moisture and nitrogen availability. Growing seasons with low moisture reduce yield potentials without a corresponding reduction in N uptake. This results in higher N concentrations in the plants which produce seed with higher protein levels. In high moisture years, yields increase, and unless more N is available, the N concentration is lower so protein is also lower. Piebald or white starchy wheat kernels are usually an indication of insufficient N and possible yield and protein losses. 13% protein is ideal in durum because it produces a milled semolina with 12% protein which produces good quality pasta. Canada is known for exporting high quality bread wheats. High protein is required for good bread making qualities. There is some suggestion that in wetter years, we may be able to increase protein by top dressing some N. N taken up before the flag leaf stage may increase both yield and protein. N taken up after the boot stage mostly influences protein content.
Phosphorus is, on average, the second most deficient plant element. It is found in plants in concentrations of 0.1% - 0.4%. Plants absorb P most consistently as orthophosphate ions H2PO4- or H2P42-. These ions are found in very low concentrations in the soil solution. Certain components of the decomposition of organic matter can be taken up as soluble organic phosphates. Because the quantity of these fluctuates so much in soil depending on microbial activity, they are not important sources of P for crops. Plant roots are able to absorb P from very low concentrations in the soil and hold up to 1000 times that concentration in plant structures.
The most important function of P in plants is energy storage and transfer. P is also an important component of a wide variety of biochemicals in the plant. It is very important in seed formation and is found in large quantities in seed and fruit. This is mobilized to support the high rate of metabolism at germination. P has been shown to speed grain crop ripening. It also provides added straw strength. It improves the quality of the production of some grains and forages and increases disease resistance. It has been particularly shown to increase resistance to root rots in grain crops. It is also very important to increase winter hardiness of winter cereals, especially in unfavorable growing conditions.
Plants need significant amounts of P at very young stages of their life cycles. It has been shown that the absorption of P by young roots is much greater than that of older roots. P, like N, is mobile in plants. If a deficiency occurs, P is translocated from older tissues to the growing points. Symptoms show first in older leaves. In young cereals, a red to purple colour may be seen, especially during lower temperatures, such as at night. Foliar symptoms are not as pronounced as N deficiency because P deficiency slows general plant growth. In severe deficiencies, areas of leaf tips and stems may die.
Different crop plants contain varying concentrations of S. The crops grown in Saskatchewan with the highest concentrations of plant S are canola at 1.1% to 1.7% and alfalfa. Concentrations of S in the legume family are about 0.25% to 0.3%. This compares to concentrations in the grain family of 0.18% to 0.19%. Sulfur is absorbed mostly as the sulfate ion SO42-.
A large portion of the S in plants is found in certain amino acids which are essential components of proteins. S has a number of functions in plant growth and metabolism. It is specifically related to the formation of the oil content of certain crops. It is known, for example, to enhance the formation of oil in flax. It is thought to be involved in the hardening of plants to cold and drought.
S deficiency retards plant growth causing stunted, uniformly chlorotic (yellowing) plants. Thin, spindly stems are characteristic. This is similar to N deficiency but S is not as mobile in plants as is N so the youngest plant parts are the first affected. In canola, deficiency symptoms appear at younger stages as cupped leaves with purple or reddish discolourations. At blossoming, the flowering can be delayed and prolonged and flowers are paler. As the plant progresses to podding, there is reduced branching. Pods are shorter and swollen. Seed set is severely reduced.
S and N need to be available in certain proportions for certain crops. For example, in canola, it's recommended that S be applied at 1 part to every 6 - 7 parts of N. It's been shown that applying high levels of N to canola and barley without S in a S deficient soil may actually lower yield as compared to a non fertilized check.
Plants contain concentrations of 1% to 4% K. In fact, some of our prairie crops take up more K than N. K is taken up as a potassium ion K+. Potassium does not form any coordinated compounds. As a result, it performs many functions related to the ionic strength of various plant solutions. It plays a large role in enzyme activation. It affects water movement in plants through osmotic regulation. It provides the osmotic pull that draws water into the plant roots. A deficiency creates poor water use efficiency. K affects the rate of transpiration because it is involved in producing the turgor pressure that opens and closes the stomata. K is needed to produce the energy rich ATP energy rich phosphate molecules. It has been shown that K deficient plants move sugars much slower. Total N uptake and protein synthesis are reduced when K is deficient.
K is mobile in the plant and deficiency symptoms show up on leaf margins of grassy plants as yellowing, browning or scorching. On alfalfa, these symptoms are whitish in color. If the deficiency becomes more severe, it can move to younger leaves in fast maturing crops like wheat. Another symptom of deficiency is weaker straw and lodging. K deficiency can really reduce yield even without producing symptoms. It has also been shown that K deficient stress can increase the amount of crop damage from bacterial and fungal diseases and insect and virus infection.
Ca is taken up as C2+. It has an important function in the permeability and structure of cell walls. It is also crucial for the formation of new cells. It is immobile in plants and the growing points and young tissues are affected first by deficiencies. It has not been found to be limiting in Saskatchewan soils and will not be discussed further.
Mg is taken up as the Mg2+ ion. It is part of the chlorophyll molecule and essential for photosynthesis. It is also an activator for many plant enzymes. It is somewhat mobile in plants. Symptoms of deficiencies are interveinal chlorosis (yellowing in between the veins while the veins remain green ) in older leaves. In all but a very few cases, it has not been found to be limiting in Saskatchewan soils and will not be discussed further.
Important micro nutrients known to be deficient in some parts of the grain producing areas of Saskatchewan are Cu, B, and Mn. Recent research on Cl has indicated response to fertilization in some cases and not in others.
Cl is absorbed as Cl- ion and it does not seem to be involved in the synthesis of other more complicated structures. It interacts with K regulating such things as leaf turgor. It plays roles in photosynthesis, cell division and enzyme activity. It helps the plant resist moisture stress. It improves the use of N and assists the crop to mature. Chloride helps the plant overcome many disease pressures particularly root diseases such as take-all and common root rot; leaf diseases such as rusts and tan spot; and physiologic disorders of small grains.
Cu is taken up as Cu2+ or Cu+. It is an activator of several enzymes in plants and it is involved in cell wall formation. It is not mobile in plants. High levels of soil P can depress Cu uptake. Cereals are highly sensitive to Cu deficiencies and symptoms include yellowing, wilting, and leaf tip pigtailing on younger leaves. As the crop progresses other symptoms are excessive tillering, aborted heads, delayed maturity (ie. prolonged flowering), and poor grain filling. Cu deficiencies will show up in patches. These patches will have a drought-like appearance. When suffering from Cu deficiency, cereal crops are more prone to root rot and stem and head melanosis. (These areas look purple and then a darker brown than the rest of the field at harvest).
B is involved in cell development, and sugar and starch formation and movement in the plant. Symptoms first appear on new leaves and include stunted plants with misshapen brittle leaves. There can be confusion with sulfur deficiency in canola because they both exhibit yellowing of the youngest leaves. Symptoms in alfalfa include: rosetting, tops are yellow, poor flowering, terminal buds die, and poor seed set. B deficiencies have been observed in canola. They have also been suspected in alfalfa on sandy soils in the Gray soil zone.
One of the dangers with boron is that the difference in B concentrations between deficiency and toxicity is close. Practically this means that overlap during application to correct a deficiency could result in toxicity!
Mn is a component in enzyme systems, accelerates germination and maturity, and increases the availability of P and Ca. It is reported not to move in the plant except in oats, so symptoms appear on younger leaves. The main symptom of deficiency is yellowing between the veins. In oats, this discoloration is gray.
Fe deficiencies are rare in field crops in Saskatchewan. Fe affects chlorophyll levels, acts as an oxygen carrier, and aids in respiratory enzyme systems. It does not move in the plant so symptoms of interveinal chlorosis with distinct green veins shows up first on young leaves.
Mo deficiencies have not been reported in Saskatchewan. The first place to monitor would likely be when growing high yielding legumes on acid soils. Symptoms similar to N deficiency are yellowing and stunting. Mo is necessary for reactions changing forms of N and P in the plant. Another important function is in the fixation of N in root nodules on legumes. Mo is needed in such small amounts that seed treating would probably be the common way to correct deficiencies.
Zn is involved in many enzyme activities, metabolic reactions, and is involved in the formation of chlorophyll and carbohydrates. Deficiencies are likely to occur on soils that are calcareous, have high pH, are sandy, have high P contents, are eroded, or are poorly drained. Symptoms differ between crops. Older leaves in wheat and barley may have light blotches between the veins. Younger leaves in flax contain grayish brown spots and the internodes appear stunted.
Chapter III - Nutrient Sources
Sources of Plant Nutrients
The chemical reactions that affect plant nutrition and even the source of plant nutrients is very complex. A basic knowledge of the various sources of plant nutrients and a brief understanding of some of these chemical reactions will give producers some insights that will lead to wise fertilizer application decisions.
Soil organic matter is one of the most important sources of plant nutrients. The need to rebuild soil organic matter lost through many years of tillage has motivated many producers to make direct seeding work on their farms. Soil organic matter is made up of the remains of living organisms such as plants and microorganisms that are under going various stages of decomposition in the soil. The release of plant nutrients from soil organic matter is a microbial process called mineralization.
The soil organic matter supplies a large amount of the N plants need each growing season. Briefly consider the basics of the N cycle. The earth's atmosphere is 78% nitrogen gas (N2). However this gas must be converted to ammonium (NH4+) or nitrate (NO3-) before plants can take it up. This process has been happening naturally through several types of microbes which can fix this gas into forms they can use. Through the process of mineralization, some of this N eventually becomes available for plant nutrition. (See AB Agriculture Web Page -Soil Organic Matter)
When a producer sends a soil sample to the soil testing lab in Saskatchewan, now owned by Enviro-Test Labs, for a fertilizer application recommendation, the lab first measures the amount of nitrate N in the soil sample. The lab extrapolates the amount of N that will be plant available in the soil from the soil test by considering the depth of the soil sample. On the basis of soil moisture levels, soil organic matter percentage, soil climatic zone, amount of fallow in the past 3 years and a number of other factors, the lab has developed a method of estimating the amount of N which is going to be available to the crop throughout the growing season. The nutrients that are needed to produce a certain yield of a particular crop are known. The difference between what the soil will supply and what the crop needs must be made up with fertilizer. The current crop will take up about half of the N in the fertilizer you apply.
One of the important factors affecting the amount of N that will be released from the soil is whether the crop residue has been removed and what kind of crop residue is present. When plant residues are added to the soil, microbial activity begins to break down these residues. The plant residue contains significant quantities of carbon (C) and various concentrations of N depending on the type of residue. The microbes need a certain ratio of C:N for decomposition to continue. They will take up plant available N from the soil if the residue, such as cereal residue, does not contain a sufficient concentration of N. This process is called immobilization. Pulse residues contain higher concentrations of N so less plant available N in the soil solution is immobilized. Microbes can also take up N released from fertilizer to carry on their activities.
Recent crop production practices have tried to enhance the process of fixing N2 from the atmosphere by increasing the use of legume type crops in our rotation. Furthermore we use inoculants to increase the population of specific microbes which will fix the most N2. In 1991 it has been estimated that annual world microbial fixation of N was 1.5 to 2.25 times that used in synthetic fertilizers.(Tisdale et.al, pg111) Considering the increasing environmental pressures and cost of inputs agricultural production is facing, Saskatchewan producers need to examine their rotations to see how they can profitably increase this microbial synthesis of N. Although the lab does not measure the amount of N which will be released from the breakdown of pulse residues, it gives a credit of a certain quantity of N depending on the legume crop and the yield it produced.
An important characteristic that affects plant nutrition is that clay particles and soil organic matter have a net negative charge. Most of the elements exist in the soil solution as ions which means that they have a net charge. Cations have a positive charge and anions have a negative charge. Cations will easily become fixed to these negatively charged soil particles while many anions will not. The quantity of exchange sites is indicated by a soil's cation exchange capacity (CEC). The CEC of different soils is quite different. Soil organic matter has a high CEC. Nitrate (NO3-) is very mobile in the soil solution because it does not become bound on the CEC. Therefore N fertilizers do not need to be placed close to the seed for adequate plant up take.
Agricultural production is also extremely dependent on the synthesis of N fertilizers. N fertilizers tend to be converted to products that are not tightly bound on the CEC of soil particles so they are very mobile with the soil solution. This means that it is much easier for plants to access the N because it moves in the soil but that it can also easily be lost to plants. N fertilizers are produced by the reaction of H2 + N2 in a catalytic process including natural gas. Since the 2001 cropping season, all North American producers know that the cost of N fertilizer is closely tied to the price of natural gas. This process produces NH3 which can be used directly as anhydrous ammonia fertilizer (82-0-0) or further processed to produce urea (46-0-0), ammonium nitrate (34-0-0) and other N containing fertilizers.
When, for example, urea is applied to soil, it is converted through an enzyme reaction to NH3 and further to NH4+. In warm moist soils or soils which are conducive to crop growth, most of this conversion will happen in a few days. If this happens on the soil surface, the NH3 can become a gas (called volatilization) and is lost to crop production. Free NH3 is extremely toxic to both plants and microorganisms. It rapidly reacts in the soil in various ways but during this time of reaction it can affect germinating seeds in close proximity detrimentally. Guidelines have been established for safe concentrations of N fertilizers that can be placed in the seed row. (See section on openers for discussion on seed placing fertilizer)(See PAMI Web Site Publication index.php/fertility-management#731 Research Update - Don't Gamble With Fertilizer Rates)
Ammonium (NH4+) in the soil is less subject than NO3- to losses by soil water washing it down to depths below crop rooting zones (called leaching), or being given off as gaseous forms of N (called denitrification). Denitrification mostly occurs when soils are water logged through a microbial reaction in the absence of O2 (anaerobic conditions). The products given off are N2, and nitrous oxide(N2O). N2O is going to become extremely significant to agricultural producers because it is a very potent greenhouse gas and there is postulation that it could lead to restrictions on the use of N fertilizer in agricultural production. NH4+ is also converted to nitrate (NO3-) through various microbial processes called nitrification.
Ammonium nitrate (34-0-0), though not commonly used as a N fertilizer in Saskatchewan anymore, is not subject to NH3 volatilization like urea so is useful for surface broadcasting without incorporation on crops such as forages and winter wheat. It is quite possible that it will soon not be available because it is becomes very explosive when mixed with fuel. Ammonium sulfate (NH4)2SO4 (21-0-0-24) is another source of N but more commonly used as a source of S. There are 2 grades of 21-0-0-24 which are classified based on the type of prill. The cheaper form has very fine prills and cannot be put through an air seeder. It is used by a number of producers as a source of S when growing canola and is usually broadcast prior to or soon after seeding. It is an industrial by-product. Liquid N is called UAN (28-0-0) and is a combination of urea, ammonium and nitrate. (For a general discussion on N fertilization see SAFRR Web Page-Nitrogen Fertilization In Crop Production.)
Soil pH is a measure of the relative concentration of hydrogen ions (H+) and hydroxyl ions (OH-) in water held in the soil called the soil solution. The pH range is from 0 to 14 with 7 being neutral. A neutral soil has equal concentrations of both. An acid soil has a higher concentration of H+ ions with a pH value lower than 7. A basic soil, called an alkaline soil, has pH value higher than 7. Soil pH is important because it affects nutrient availability, solubility of toxic ions, and microbial activity.
Many prairie soils contain significant quantities of P but very little of it is plant available. P is found in plant available forms in the soil solution which is in flux with the P held in forms which can quite easily be broken down into plant available forms (labile forms). Labile forms are also held in flux with soil P which is very slowly released. As plants use up the soil solution forms the concentration gradient tends to cause reactions to release more labile P. The concentration of H2PO4- is more abundant at pH values below 7.2 and it is more rapidly taken up by plants as compared to HPO42- forms. These ions are held very tightly in soil because they attach or adsorb to clay surfaces and form bonds with aluminum (Al3+), iron (Fe3+), and calcium (Ca2+) and precipitate out. This means that they are not mobile in the soil solution and plant roots must come in very close contact to take up these ions. The maximum amount of P is available in most soils at a pH of 5.5 to 6.5. Above a pH of 7, another reaction occurs and P becomes less available. The benefit is that P is not easily lost from soil, just tied up in forms which will, in time, be available to plants.
Additions of synthetic fertilizers can be made more plant available by applying them as a band. The high concentration in the band as compared to broadcasting them will increase the time it takes for the P to be tied up. Cooler soil temperatures and higher soil moisture content such as the soil conditions we often have in spring time reduce the amount of P that plants are able to access. The best recommendations for P are to place it in the seed row or very close to the seed row. The monoammonium phosphate form of fertilizer most common in Saskatchewan is completely water soluable. About 10 to 30% of the fertilizer applied will be taken up in that year. Seed placed rates are limited because of the salt affect. Fertilizer compounds applied to the soil break down into electrically charged ions. Plants are able to take up these ions in low concentrations. If the concentrations become too high the ions create a high osmotic attraction for water which restricts plant water uptake. Seed placed fertilizer that creates these salts can restrict germination, and create stress that makes the seedling more susceptible to disease, and delayed maturity. See table for the salt index of various fertilizers(For a general discussion on P fertilization see SAFRR Web Page-Phosphorus Fertilization In Crop Production.)
Much of the K in our soils occurs in the primary minerals of our soils and is not available to plants. A much smaller portion of the K is held on clay exchange sites. This portion is slowly available. The last percentage of the K in our soils is in the soil solution and is plant available. K is held relatively tightly in clay soils but is more mobile in sandy and organic soil. In Saskatchewan, lighter texture soils such as sandy loam to loamy sand and peat soils in the north and northeast may have more severe K deficiencies. These deficiencies are often corrected by broadcasting K. General recommendations call for 0 to 15 lbs of K seed placed or side banded on most crops. This is due to the fact that K may not move fast enough to seedling roots in dry cool soils or plant rooting may be inhibited and K deficiencies may be induced. Another benefit to applying K fertilizer is that it contains Cl which may also increase yield. In those areas with more severe deficiencies producers are applying higher rates of K. (For a general discussion on K fertilization see SAFRR Web Page-Potassium and Chloride Fertilization In Crop Production.)
Sulphur is taken up by plants as SO42-. This form can be leached from soil but different soils are affected quite differently by leaching. Many canola producers apply more than adequate sulphur for the canola crop in rotation and find that the other crops in between canola crops have adequate S. Soil test results for S are not considered an adequate guideline as saline areas in the field can drastically skew soil test results. The common recommendation is to apply a ratio of about 6 - 7:1 N:S for canola. Sulphate S is mobile in soil but is not subject to volatilization like urea. It does not have to be applied close to the seed for adequate plant nutrition. For a discussion on the use of elemental S see SAFRR Web -Page Feasibility of Elemental Sulphur Fertilization on Cereals)
Crop Nutrition Changes With Direct Seeding
Crop nutrition includes how the chemical compounds plants require for growing and producing fruit or seeds are supplied and how the plants takes up these compounds. When producers first begin to direct seed, most suggest that their fertilizer application rates do not change significantly. Soil testing, with its various pros and cons, continues to be a general recommendation. If carried out properly, it certainly can be a useful guide to evaluate what is happening from year to year in any particular field. The big difficulty with soil testing is that it cannot measure the amount of nutrients that will be mineralized over the growing season. That must be estimated based on research and past averages. A number of producers indicate that after some time in a low disturbance crop production system (5 years plus) they have cut back slightly on nitrogen use and yet have continued to maintain or improve crop yields and quality. Of course, every producer's production system is different, depending in part, on the soil climatic zone they are in so this benefit does not necessarily apply to every producer.
In rolling topography fertilizer use becomes more efficient with direct seeding because crop production becomes more even. One of the effects of direct seeding and leaving stubble standing is more crop residue on the soil surface. This increases water infiltration and reduces run-off. Upper slopes which traditionally produced very little crop because a large portion of rainfall and snow melt ran into the depressions, now have more soil moisture. This means that fertilizer applied on the knolls and upper slopes also enhances crop production.
A number of expert recommendations suggest that fertilizer levels should be increased when producers first switch to direct seeding. ( See AB Ag Web site Soil Fertility Implication When Converting To Direct Seeding 1996)They suggest that because there is a big change from incorporating residues into the soil to maintaining a thatch layer of residue on the soil surface, nutrient cycling will change. More nutrients will be tied up in the residue for a number of years before the cycle reaches an equilibrium where the thatch is being broken down and releasing nutrients as fast as the residue is added. Most producers recently switching to LDS do not seem to be following this recommendation. One of the ways to insure that larger proportions of applied fertilizer is available to the crop is to band it below the thatch layer.
One of the studies indicating more crop nutrients are available from the soil in a direct seeding system was conducted in the Brown soil zone. (Campbell et.al., 1989) They compared tilled fallow-wheat with zero-tilled continuous wheat for 6 years. Of course in the Brown Soil Zone moisture for continuous crop production can be extremely limiting. They were also testing increasing soil moisture with cereal strips to trap more snow. In the continuous wheat they also used proper fertilizer management as compared to the conventional management system which has traditionally received little fertilizer. After 6 years, they determined that the direct seeded system had more potentially available nitrogen in the top 7.5 cm. In the 7.5 to 15 cm soil depth the amount of available N ranged from moderate increases to no changes.
Another study looked at 4 different sites (Lethbridge, Scott, Melfort, and Watrous) that had zero-till and tilled comparisons carried out for varying time periods. (Carter et.al., 1982) These researchers observed that after 16 years of zero-till at Lethbridge, there were slight but significant increases in plant available forms of P and K in top 4 cm of the soil. All other plant available nutrient contents were the same. They also measured the amount of organic matter tied up in microbial biomass. This represents a significant quantity of "labile" N which could potentially be available for plant growth throughout the growing season. They found that for the longer term comparisons (4 years and greater) the N mineralization potentials in the no-till trials were increased by 71 - 132% in the top 1 or 2 inches. At deeper soil levels of 2 to 4 inches the results were reversed with 35 to 110% more N mineralization potential in the tilled comparisons.
Selles reported from 11 year zero-till conventional till comparisons at Swift Current that the amount of labile P increased in the top 6 cm of soil under zero-tillage by 28 lb/ac. (Selles et. al., 1993). Research has also been conducted to indicate that some nutrients can be washed out of this surface thatch with rainfall. A project carried out in northwestern Alberta compared the K released from various residues under conventional and zero till. (Lupwayi et. al. 2002). They concluded that with all of the residues almost all of the K was released within 4 weeks of the residue application. See Figure 2.
Another soil nutrient change with direct seeding can be the effect of placing nutrients which don't move readily in the soil in narrow bands. If producers switch to banding nutrients on wider row spacing, immobile nutrients such as P and K may be found in higher concentrations where the bands have been placed. This can be particularly important when soil testing because samples taken in the band zone will contain significantly higher levels of these nutrients compared to samples taken outside of the band. After a number of years of applying these nutrients in bands they will be randomly located in the field and the effect will be minimized.