Tuesday, March 17, 2015

International Year of Soils: Modifying soil to improve crop productivity

The essential link between productive soil and humans has been clear since the beginning of agriculture. However, our food crops come from only a sliver of the world’s land area (12%).

There is room for limited expansion of crop production in some countries, but much of the earth is covered by urban settlements, forests, and environmentally protected areas that are not appropriate for agricultural expansion. Proper stewardship of our current agricultural land is vital to long-term food sustainability.
Improving soils is an ancient practice

The earliest recorded agriculture describes attempts to improve soil properties and crop productivity through application of manures, ash, minerals, and other amendments. Our understanding of the scientific principles underlying plant growth has greatly improved, but the fundamental effort to eliminate soil constraints to food production remains the same after thousands of years.

Soil physical properties have a major impact on root growth and development. Compacted soils have reduced water-holding capacity and can form a brick-like barrier that roots cannot penetrate.
Soil crusts prevent rainfall from entering the soil and crusty soils are prone to excessive water runoff. Coarse-textured and low-organic matter soils generally retain less plant-available water, and crops growing in these soils may be more susceptible to drought stress.
Compacted soil reduces wheat root growth (R)www.agric.wa.gov.au
Soil degradation causes pollution

Soil chemical properties often are a key factor in determining crop productivity. There are very few soils in the world that contain all of the essential plant nutrients in the proper concentration. The modern fertilizer industry helps farmers to identify and eliminate any limiting nutritional factors. Addition of plant nutrients prevents the depletion and degradation of agricultural land that occurs when crops are repeatedly harvested without replacing the nutrients back into the soil.
Soil degradation impairs productivity

Many soil chemical properties can limit plant growth if left unmanaged. Soil acidity is one of the largest global constraints to plant growth. Although soil acidity is relatively simple to remedy, it remains untreated in vast areas of the world. Other soil chemical issues that hinder plant growth include excessive salinity, and pollution from poor municipal and industrial waste management.
Soil acidity stunts root growth

 The importance of biological activity in supporting crop productivity is too often overlooked. Soil microorganisms are responsible for regulating the availability of many of the essential plant nutrients. Nitrogen fixation by bacteria living within the roots of some plants provides vital support to important cropping systems and rotations. The contribution of mycorrhizal fungi to root health is clear, but not fully understood. Similarly, the intricate exchange of chemical signals between plant roots and soil microbes plays an important role in supporting plant growth.
Nodules on soybean roots host N-fixers

 We are fortunate to live in an age when we understand these soil factors that limit plant growth and have the ability to manage them. Nutrient limitations can be eliminated through proper fertilization. Soil acidity or alkalinity is easily modified through use of appropriate amendments. Converting to no-tillage practices, or sub-soil tillage can often help improve soil physical properties. Conservation of soil organic matter and crop rotation may improve soil biological activity.

Soil stewardship is fundamental to modern agriculture.  Every farming decision should be one that maintains or improves essential soil resources.

This article originally appeared as a contribution to the IPNI series: Plant Nutrition Today

Thursday, March 12, 2015

Nitrophosphate - making phosphate fertilizer using nitric acid

The production and application of nitrophosphate fertilizers is largely regional, its use centered where this technology is advantageous. The process uses nitric acid instead of sulfuric acid for treating phosphate rock and does not result in gypsum byproducts.
21-7-14 Nitrophosphate
The majority of commercial P fertilizer is made by reacting raw phosphate rock with sulfuric or phosphoric acid. The sulfuric acid method of producing P fertilizer results in large amounts of calcium sulfate (gypsum) by-product that incurs additional disposal costs. 

Nitrophosphate differs because it involves reacting phosphate rock with nitric acid. Nitric acid is made by oxidizing ammonia with air at high temperatures. A primary advantage of this method is that little or no S inputs are required. With the nitrophosphate process, excess Ca from the phosphate rock is converted to valuable calcium nitrate fertilizer instead of gypsum. The nitrophosphate method was first developed in Norway and much of the global production still occurs in Europe.

The general reaction is: Phosphate rock + Nitric acidgPhosphoric acid + Calcium nitrate + Hydrofluoric acid. The resulting phosphoric acid is often mixed with other nutrients to form compound fertilizers containing several nutrients in a single pellet. The co-generated calcium nitrate or calcium ammonium nitrate is sold separately.

 Chemical Properties
The chemical composition will vary depending on the combinations of nutrients used to make the final granule. Popular grades of fertilizer made with the nitrophosphate method include:
N & P:     ranging from 20-20-0 to 25-25-0, 28-14-0, 20-30-0
N-P-K:     ranging from 15-15-15 to 17-17-17, 21-7-14, 10-20-20, 15-20-15, 12-24-12
Agricultural Use
Nitrophosphate fertilizers can have a wide range in nutrient composition depending on their intended use. It is important to select the proper composition for each specific crop and soil requirement. Nitrophosphate fertilizer is sold in granular form to be used for direct application to soil. It is commonly spread on the soil surface, mixed within the rootzone, or applied as a concentrated band beneath the soil surface prior to planting.

Management Practices
Nitrophosphate fertilizer contains varying amounts of ammonium nitrate, which attracts moisture. To prevent clumping or caking, nitrophosphate fertilizers are generally packed in water-tight bags and protected from moisture before delivery to the farmer.

This fertilizer fact sheet originally appeared in the series of IPNI Nutrient Source Specifics.  Additional information on fertilizer materials can be found at the IPNI website.

Saturday, February 7, 2015

Another look at liming acid soils

It is estimated that soil in over 30% of the world’s cropland is acidic and would benefit from liming and soil improvement. Most soils have a natural tendency to become acidic over time through natural and managed factors. Farmers too often fail to monitor soil acidity, despite its widespread nature.
Global variation in soil pH. Red = acidic soil. Yellow = neutral soil. Blue = alkaline soil. Black = no data.

Aluminum toxicity stunts bean roots
Several natural factors contribute to the development of soil acidity. The geologic material that weathers into soil has a large influence on soil pH. Acid soils occur more frequently in high rainfall areas where leaching removes cations such as calcium and magnesium from the root zone. Poor plant nutrition is frequently a significant problem in acid soils due to the lack of adequate calcium. Phosphorus availability also becomes limited as the soil pH drops. Soil acidity also limits nitrogen fixation in many legume crops. However, aluminum toxicity is usually the largest constraint to plant growth in acid soils.

Calcium uptake near root tips is hindered in acid soils by high aluminum

 Nitrogen fertilizer can also be a contributor to the development of soil acidity. When urea or ammonium-based fertilizers are converted to nitrate by soil bacteria, hydrogen ions (acidity) are naturally released. Any nitrogen source containing ammonium (including manures, composts, or cover crops) will contribute to the gradual process of acidification.

There are many examples to show where decades of repeated nitrogen fertilizer use has led to a gradual decline in soil pH. This gradual soil acidification can occur even in regions where acidity problems are not common. For example, this natural process is often noted in areas where nitrogen fertilizer is repeatedly applied to the same place in the soil for many years, such as surrounding a drip irrigation emitter in a permanent crop. Fortunately, measuring soil pH is one of the easiest analyses to perform in the laboratory.

The addition of ground limestone to agricultural soils neutralizes acidity and reduces the presence of soluble aluminum, which is toxic to plant roots. Adding limestone to acidic soil will also enhance the solubility of phosphate, which becomes more available for plant uptake as the pH approaches neutral. Finally, limestone will provide a valuable source of calcium, which is frequently lacking in acidic soils.
Adding lime is an old ag practice

Limestone requires acidity to rapidly dissolve in soil. In regions where the soil pH is greater than 6.5, limestone dissolves very slowly or not at all. Areas with naturally occurring limestone are classified as having calcareous soils. If there is a need to supply large amounts of supplemental calcium in non-acidic soils, gypsum (calcium sulfate) is commonly used. Although gypsum does not rapidly dissolve in soil, it supplies more soluble calcium than limestone in neutral and alkaline soils.
IPNI recently released a publication entitled: Soil Acidity Evaluation & Management, which provides an overview of issues related to acidity. More information can be found at the IPNI website:  http://info.ipni.net/IPNI-3353

Thursday, January 8, 2015

Step Up as a Source of Information!

We live in an age of information overload, with an avalanche of information arriving each day. It can become a struggle to decide what information to accept and listen to, or judge which new ideas can be disregarded.

Getting reliable agronomic information is a challenge for everyone. We are all looking for in­novations that will help improve efficiency and profitability. Plant nutrition products are evaluated for safety and for concerns arising during manufacturing and shipping, but there are no labels that tell you if they will work in your individual situation.

 Several recent surveys of farmers from across the U.S. confirm the fact that crop advisers are the most frequently consulted source of agronomic information. Although the specific questions vary across regions and crops, farmers consistently look to their trusted adviser to help them sift through the information to get to the truth.

Given this critical role, it is essential to maintain that trust by staying current with the latest developments in agronomic science. This can be done through activities such as reading the latest trade journals and magazines, attending educational seminars, and asking probing questions. Practicing successful agronomy and horticulture requires using all the resources available and then using your expe­rience to sort out what will work locally. For example, do you know how to implement the 4R’s of Nutrient Stewardship in each field where you work? Can you clearly explain the cropping decisions you recom­mend if asked by a member of the general public?

Many new alternative fertilizer products have been introduced in the past decades. Some of these new products are based on sound science and their performance has been carefully evaluated in various scenarios. There are other products that have not been tested in a scientifically credible way, and lack results that are explainable and reproducible. Instead, many of these products simply rely on en­dorsements and testimonials as a substitute for good science and statistical analysis.

Economic and environmental pressures on farmers seem to increase every year. Crop advis­ers have the unique opportunity to directly influence the success of farmers by providing the best possible information. The relationship of trust between farmer and adviser is reinforced each time accurate and useful information is transferred.

Certified Crop Advisers are tested to demonstrate proficiency in the areas of nutrient manage­ment, soil and water management, pest management, and crop management. Additionally, they are required to take 40 hours of continuing education every two years to keep current with the latest agronom­ic developments.

Whether you are a Certified Crop Adviser or any other type of farm adviser, remember that you are viewed as a trusted source of information in your community. Now you need to maintain your reputation by staying current in providing accurate and reliable agronomic information.  

Thursday, September 4, 2014

Fertilizer Is Not a Dirty Word

High crop yields often come under scrutiny because of the need for fertilizers and the perception of their potential environmental impacts. Newspaper articles, letters, and advertisements from well-intended, but poorly informed, citizens seem to perpetuate old myths and clichés about modern fertilization practices.

  The fact is, maintaining food production for the growing world population requires the use of new technology and the intensification of management to grow more food on the existing cropland...and fertilizer is essential for accomplishing this.

Sometimes I get tired of hearing about the negative fertilizer issues that are associated with our abundant, affordable, and nutritious food supply...a truly amazing supply of healthy food that is clearly unprecedented in the history of the world! 

Misapplication and misuse of agricultural fertilizers have undoubtedly occurred and their impact on the environment needs to be minimized. But to fairly judge the use of fertilizers, the risks of their use should be compared with their benefits for food production.

I have had people tell me that raising yields with commercial fertilizer is somehow immoral and dangerous for our soils... that strictly organic or specialty products will meet the demand of global food production. You probably know about the “stink test”... that is, when something smells fishy there is usually a reason why! Many of these ideas and claims just don’t pass stink the test.

The time has come for all of us dispel myths about fertilizers and nutrients, and to convey a correct message to a world which is becoming increasingly urbanized and removed from what agricultural production is all about... providing healthy food.

How Does Fertilizer Contribute to the 
Food Supply?
2.5 tons: What a family eats in a year ~1970's

A survey of U.S. crop production estimated that average corn yields would decline by 40% without N fertilizer. Even greater declines would occur if regular additions of P and K were also halted. Numerous long-term studies have also demonstrated the contributions of fertilizer to sustaining crop yields. For example, long-term studies in Oklahoma show a 40% wheat yield decline would occur without regular N and P additions. A long-term study in Missouri found that 57% of the grain yield was attributable to fertilizer and lime additions. Similarly, long-term trials from Kansas show that 60% of the corn yield was attributable to fertilizer N and P.

Few people appreciate that corn yields have continued to increase in the Corn Belt of the U.S. without a similar increase in N fertilization. In fact, N use efficiency has increased at least 35% in the past 25 years (where less N fertilizer is now required to produce a bushel of grain). Remarkably, more corn is being harvested without increasing N fertilizer application rates. Some of this improvement has also come from modern genetics and improved agronomic management.

Saturday, August 23, 2014

A Closer Look at Phosphorus Uptake by Plants

ALMOST ALL PLANT PROCESSES require phosphorus (P) to operate. Phosphorus is essential for life sustaining reactions including energy transfer, activation of proteins, and regulation of metabolic processes. While the primary source of P for plants is inorganic phosphate, there are very few soils that naturally contain a sufficient P supply to allow unrestricted plant growth.

Compared with the other major nutrients, P is the least mobile and available to plants in most soil conditions and therefore it is commonly a major limiting factor for plant growth. Even in well-fertilized soils, the P concentration rarely exceeds a few hundred parts per billion (ppb) and is commonly less than 50 ppb in the soil solution.

Up to half of the soil P is commonly found in organic forms, derived from materials such as plant residues and soil organisms. However, organic P must be mineralized to inorganic phosphate (the form found in most fertilizers) before it can be taken up and used by plants for growth.

Because of its strong reactions with soil components, P is principally supplied to plant roots by diffusion rather than mass flow. Phosphorus uptake occurs primarily at the young root tip, into the epidermal cells with root hairs, and into cells in the outer layer of the root cortex.
The P depletion zone in soil

The young root tips, continually expanding into fresh soil, are exposed to P concentrations found in the bulk soil solution (Figure 1). While P is rapidly taken up along the root surface, a P depletion zone of 0.2 to 1.0 mm (about the thickness of a dime) develops surrounding the root. Root hairs help expand the surface area available for P absorption. Mycorrhizal fungi, growing in association with root cells and extending up to several centimeters into the soil, can also transfer P to the root. Various crops have been shown to have different abilities to extract P from the soil and to make these beneficial associations with mycorrhizal fungi that extend their effective root system.

Rhizosphere is the soil zone (within 2 mm of the root) that is influenced by heightened biological activity, nutrient uptake, and chemical changes that result from root activity.

Getting P into the Plant: Apoplastic and Symplastic Transport
 The apoplasm comprises the root walls, the cortical cells and the open spaces between these tissues (Figure 2 ). This “dead space” consists of interlaced fibers that form an open latticework in roots that serves to filter the soil solution. The soil solution moves through these spaces and pores until it reaches the tough “Casparian strip” that surrounds the core (stele) of the root. A net negative charge associated with the cell wall fibers repels anions...such as phosphate and nitrate (NO3 - ) in solution and confines their transport to larger pores within the apoplasm. Movement of nutrients through the apoplasm into the root is greatest near the root tip. Mucilage, a complex mixture of organic materials excreted around the root, also carries negatively charged hydroxyl groups which can further repel the movement of anions towards the root.
Root cross section: pathways of uptake

The symplast , by contrast, is a living system within the plant that connects each growing organ. It consists of thin walled cells called parenchyma, which have small openings acting as “tunnels” where the content of one cell connects with the contents (protoplasm) of the adjacent cell. The symplast is very fragile and requires the rigid framework of the apoplast to hold it in place.

Although the soil solution P concentration may be less than 50 ppb, the concentration in plant cells is much higher…50 to 500 parts per million (ppm). For plants to boost their P concentration by over a thousand times, active transport (requiring energy) across the root membranes (the plasmalemma) is required.

The movement of P from the apoplast across root cell membranes is the critical step in the transport of nutrients into the plant, requiring an energy-driven transport mechanism to move P through the membranes into the plant root cells (phosphate transporters).  Researchers are currently working on ways to increase the uptake of P by stimulating these nutrient transport proteins found in the root.

Other changes take place in the rhizosphere to improve P uptake and recovery. Many roots exude organic acids  (primarily citrate and oxalate) into the rhizosphere, which enhance P availability. While this response has been frequently measured, the major effect may be due to the organic compounds displacing P held by the soil (ligand exchange), rather than a direct effect of the organic acids on the rhizosphere pH. Some microorganisms living in the rhizosphere have been shown to help solubilize soil P.
These mechanisms can help improve P recovery, but they all require that P be present in the soil to begin with. No amount of excreted organic acid, root-zone microorganisms, or mycorrhizal fungi can allow a plant to recover P from a soil that does not contain P to begin with.
Nitrate uptake increases pH

Plant roots can also have a considerable effect on rhizosphere pH. This pH change comes from the release of H+ or OH-  (as HCO3 - ) as plants balance their uptake of excess cations or anions. Nitrogen nutrition plays the most important part in this ion balance since it is the mineral taken up in the largest quantity by most plants and it can occur as either an anion (NO3 - ) or a cation (NH4 + ). Research suggests that greater P uptake that occurs as plants are fertilized with a source of NH4+ may be the result of rhizosphere acidification that occurs with this N source. However, depending on the buffering capacity of the soil, only little change in pH may be observed in some soils. 
Ammonium uptake decreases soil pH

There are many complex reactions occurring under our feet as roots invisibly work to meet the nutritional needs of the growing plants. Research is underway in many areas to improve the efficiency of P fertilizer use. Advances in improving root function may someday lead to better ways of meeting the nutrient demands of crops. For now, the focus remains on maintaining adequate soil fertility by using regular plant and soil testing, proper fertilization techniques, and appropriate management to keep the nutrients where they are needed for the growing plant.

This article originally appeared as a newsletter of the International Plant Nutrition Institute.  The pdf is available here.

Tuesday, July 1, 2014

Kieserite... naturally occurring magnesium sulfate

Kieserite is a naturally occurring mineral that is chemically known as magnesium sulfate monohydrate (MgSO4·H2O). It is mined from geologic marine deposits and provides a soluble source of both Mg and S for plant nutrition.
Magnesium deficient grapes


Kieserite is primarily obtained from deep underground deposits of minerals in Germany. It is present in the remnants of ancient oceans that were evaporated and are now buried beneath the earth’s surface.
Mining in Germany
These mineral resources contain a variety of valuable plant nutrients. The ore is brought to the surface where the magnesium salts are separated from potassium and sodium salts using a unique, dry electrostatic (ESTA) process.

The fine crystalline kieserite is sold for direct application to soil, or it is granulated to a larger particle size that is better suited for mechanical fertilizer spreading or for bulk blending with other fertilizers.

Chemical Properties

Chemical formula:   MgSO4·H2O
Mg content:             16% (kieserite fine); 15% (kieserite granular)
S content:                22% (kieserite fine); 20% (kieserite granular)
Solubility:                 417 g/L (20°C)
Solution pH:             9
Granular kieserite

Agricultural Use

Kieserite provides a highly concentrated form of two essential plant nutrients—Mg and S. Since kieserite applications have no major effect on soil pH, it can be supplied to all kinds of soil, irrespective of soil pH. It is commonly used prior to or during the growing season to meet the nutrient require­ment of crops. Due to its high solubility it can be used to supply both Mg and S during peak periods of crop demand. Since kieserite is an earth mineral mined from naturally occurring deposits, it is permitted as an organic nutrient source by some organic certi­fying agencies.

Kieserite itself is not used as foliar fertilizer or in fertigation systems, but it serves as raw material for the production of Epsom salt (MgSO4·7 H2O), which is totally soluble and suitable for both fertigation and foliar application.
Fine kieserite particles

 Management Practices

Many soils are low in Mg and require supplemental nutrients to support crop yield and quality. Sandy-textured soils and soils with a low pH (such as highly weathered tropical soils) are frequently characterized by a low Mg supply for plants. Under these conditions, it is a prerequisite to raise the Mg content in the soil by adequate fertilization.

Splitting Mg applications into two or more doses is recommended in areas with high precipitation in order to avoid leaching loss­es. Soils in temperate climates with higher clay content may have higher Mg contents and are often less prone to leaching losses.

Fertilizer Mg application rates vary depending on factors such as the specific crop requirement, the quantity removed during harvest, and the ability of soil minerals to release adequate Mg in a timely manner to support crop yield and quality. Kieserite application rates are typically in the range of 200 to 300 kg/ha for many crops. Additional Mg and S demands during peak growth periods demand can be met by foliar application of materials such as Epsom salt or a variety of soluble nutrient sources.
Magnesium deficient soybeans

This post originally appeared as one in a series on specific nutrient sources on the IPNI website.