Tuesday, February 26, 2013

Calcium carbonate (limestone): What it does

Handful of ground limestone

Calcium carbonate, the chief component of limestone, is a widely used amendment to neutralize soil acidity and to supply calcium (Ca) for plant nutrition.  The term lime can refer to several products, but for agricultural use it generally refers to ground limestone.

Limestone is a common sedimentary rock found in widespread geologic deposits. It has been used throughout much of recorded history as a building material, a cementing agent, and in agriculture to improve acid soils. An agricultural liming material (ag lime) is broadly defined as any substance containing Ca or magnesium (Mg) and capable of neutralizing acidity. Many materials can be classified as ag lime.

Spreading limestone
Ag lime is extracted from quarries or mines and usually requires mechanical crushing. The fineness of the ag lime is important in determining how quickly it reacts with soil acidity. Limestone of a smaller particle size reacts quickly since there is more exposed surface area for chemical reaction. Larger particles are slower to react, but provide a sustained, longer-term source of acid neutralization. A measurement of particle size is typically reported on the product label.

Other materials in the ag lime, such as clay, will reduce its purity and diminish the acid-neutralizing capacity. Ag lime effectiveness is rated based on its comparison with pure calcium carbonate (CaCO3), a value that is expressed as the percent calcium carbonate equivalent (CCE). Ag lime is more soluble in acid soils than in neutral or alkaline soils. The presence of CaCO3  in soil is detected by the effervescence or fizz’ when a drop of strong acid is applied.

Chemical Properties
Limestone/Calcite – calcium carbonate [CaCO3]   
Mostly insoluble in water, but solubility increases in acid conditions (maximum of 40% Ca).
Dolomite – calcium magnesium carbonate [Ca.Mg(CO3)2
Mostly insoluble in water, but solubility increases in acid conditions (between 2 to 13% Mg)
Hydrated/Slaked lime – calcium hydroxide [Ca(OH)2]  
Relatively insoluble in water; forms a solution of pH >12
Burned lime/Quick lime – calcium oxide [CaO
Reacts with water to form hydrated lime

Agricultural Use
The primary use of ag lime is to raise the pH of acid soils and reduce the concentration of aluminum  (Al) in soil solution. Poor crop growth in acid soils is largely due to soluble Al, whicis toxic to the root system of many plants.  Lime will reduce soluble Al by two reactions:

1) CaCO3 + H2O --> Ca2+ + 2OH- + CO2    
2) Al3+ [soluble] + 3OH- --> Al(OH)3 [insoluble]

Additions of ag lime also supply valuable Ca (and possibly Mg) for plant nutrition. Some secondary benefits of neutralizing soil acidity with ag lime include:
Increased phosphorus (P) availability
Improved nitrogen (N) fixation by legumes
Enhanced N mineralization and nitrification
Better water use, nutrient recovery, and plant performance with a healthier root system
Aluminum toxicity eliminated by adding limestone
Management Practices
The quantity of ag lime needed to bring a soil to a desirable pH range can be easily determined in the laboratory. Ag lime is most commonly spread uniformly on the soil and then mixed through the root zone. Neutralizing soil acidity is not a one-time process, but must be repeated periodically depending on the soil and environmental conditions. Typical application rates are measured in tons per acre.
Limestone spreader

Non Agricultural Uses
Limestone is one of the most widely utilized of all earth materials. In addition to its use in building and construction, lime­stone is used in diverse applications such as air pollution control, treatment systems for drinking water and waste water, soil stabilization, medicines, antacids, and cosmetics.

A pdf version of this post is available at the IPNI website here

Thursday, February 14, 2013

Will Fertigation Work for You?

Drip-irrigated almonds ready to be gathered
The application of plant nutrients through drip and sprinkler irrigation systems has increased dramatically during the past 15 years.  As careful water management becomes more important and the need for high yields of superior-quality crops increases, more growers are exploring the advantages of combining water and fertilizer application.  Fertigation offers a boost in water and nutrient efficiency by simultaneously placing moisture and nutrients directly in rootzone where they are most needed- frequently resulting in the need for less fertilizer and water! Other potential advantages from fertigation include savings in labor and energy costs, flexibility in nutrient timing to meet the actual crop nutrient requirement throughout the growing season, and the ability to add nutrients that may be otherwise difficult to apply.  Here are a few key factors to remember when considering a fertigation program.

Almonds with microsprinklers
Making a decision to fertigate begins with having a suitable irrigation system and an adequate supply of water.  Fertigating with flood or basin-type irrigation does not require high quality water- but when using drip irrigation, care must be taken to never plug the small emitters.  While many fertigation techniques can be effective, they have very different requirements and levels of management. 
Addition of fertilizers to irrigation water needs to be carefully done to avoid precipitation with the water or other added chemicals when mixed together.  For example, high levels of calcium in the water can react with some fertilizers to coat pipes, sprinklers, and emitters.   

Urea ammonium nitrate is often used with fertigation
A wide variety of fertilizers are available to supply all of the essential plant nutrients with the water.  Your fertilizer dealer can assist you with choosing any combination of primary nutrients or micronutrients to meet the crop requirements.  There are numerous sources of nitrogen, phosphorus, potassium that can be matched for a specific irrigation system and purpose.  Regular plant tissue testing will let you know the crop need at that particular growth stage as a guide for fertigation.
It is essential that nutrients used for fertigation completely dissolve and flow cleanly through the irrigation system.  Consultation with your fertilizer dealer will make sure the plant nutrients needed are compatible with your irrigation water, your irrigation system design, and your crop production goals.  When mixing chemicals for the first time, conduct a compatibility test in a jar before injecting materials in irrigation system. 

Fertigation is essential with plasticulture
In almost all of the studies conducted, there have been significant yield and quality benefits resulting from fertigation. When done properly, environmental benefits associated with of precision fertilization have been documented too.  Maintaining a stress-free rootzone allows the crop to develop to its full yield potential.  Researchers conclude that when water and nutrients are simultaneously supplied to the plant in the right quantity and form, superior crops result.  

Microsprinklers can improve nutrient use efficiency and water use efficiency
If you are currently irrigating, you should carefully consider using your present system to add fertilizer too.  However, fertigation will only be profitable if it is managed properly.  For example, if the irrigation water is not distributed uniformly in the field, then the nutrients will not be spread evenly either.  Fertigation can boost yields, crop quality, and profitability when done properly.  Consult with a Certified Crop Advisor when making your plans for the coming growing season.

Monday, February 11, 2013

Meeting the Phosphorus Requirement on Organic Farms

Using organic nutrient sources?
Phosphorus management can be difficult in organic production since approved sources are limited and the consequences of under- or over-fertilization can be significant. Since P is an essential element for plant growth involved in many critical plant metabolic functions, sustainable agricultural production depends on an adequate P supply.

This article was published in Better Crops by Nathan Nelson and myself several years ago.  The original pdf version of the article is available here.

Nutrient management in organic production systems focuses on maintaining agricultural productivity with inputs of on-farm or minimally processed materials. Nutrient inputs for organic production are typically focused on carbon-based nutrient sources (e.g., crop residue, compost, manure) and nonprocessed mineral sources (e.g., rock phosphate, lime, and gypsum).

Manure should be captured and returned to the field
In most agricultural systems…both organic and conventional… complete nutrient cycling does not occur. The nutrient reservoir in the soil shrinks when crops are removed from the field at harvest. This nutrient export creates a P deficit, necessitating regular P additions to replace the harvested P. Several studies investigating whole-farm P budgets have found nutrient P deficits in many organic farms and illustrate the need for nutrient additions. Because P is an essential nutrient for plant growth, all sustainable systems should at a minimum seek to replace the P removed in harvested crops in order to avoid declines in yield and quality. Although organic agriculture seeks to minimize off-farm inputs, it is essential that producers replace P removed in harvested crops.
Nutrient management is complex in mixed livestock/crop farming.

A brief review of the most commonly used P sources for organic production is presented here. More information and an extensive list of references are available at the website:

Soil Organic Matter
Phosphorus must be soluble for plant uptake
Soil organic matter can be an important source of P for crops. Some studies have shown that soil organic matter increases on organically managed farms, while other long-term studies do not show such a buildup. These differences largely depend on management practices such as tillage intensity, heavy manure additions, return of crop residues, the extent of cover cropping, and climatic factors. Soil organic matter serves as a reservoir of plant nutrients, but may also improve the soil physical conditions and root environment.

Soil organic matter contains a variety of organic P compounds, such as inositol phosphate, nucleic acid, and phospholipid. These compounds must be first converted to inorganic phosphate by soil enzymes before being used for plant growth. These phosphatase enzymes are produced by soil microorganisms, mycorrhizal fungi, or excreted by the plant root. Some organic P compounds are stable for many years in the soil, while others are converted to inorganic P within a few days or weeks.

Cover Crops
Cover crops can recycle phosphorus
Cover crops are frequently grown in rotation with cash crops for a variety of beneficial purposes. The advantage of cover crops for P nutrition involves the accumulation of soil P by the cover crop. This P is subsequently released when the cover crop is killed. Numerous studies have shown that some cover crops can provide a P nutritional benefit for the next crop compared to crops grown without a preceding cover crop. This is attributed to the ability of some species to draw down soil P concentrations below what some cash crops can and also to their extensive root system. This P drawdown may also be the result of root exudates and the efficient P uptake by the cover crop roots. Some cover crops can be excellent hosts for mycorrhizal fungi, which may allow a greater exploitation of the soil P reserves.

There are considerable differences in the ability of various cover crops to provide additional P for the subsequent crop. Research has generally shown a greater P benefit from legume cover crops than from grass cover crops, but the effects of cover crops on P nutrition can be highly variable. In many cases, supplemental P is still required after the cover crop to eliminate P deficiency. In some circumstances, P uptake by the cash crop following the cover crop is actually reduced due to low residual soil P caused by uptake by the cover crop and poorly synchronized P release.
Cover crops do not add phosphorus to depleted soils

Cover crops offer some P nutritional benefits in some circumstances. The variable results (positive and negative responses) are due to the complicated species, microbial, and environmental interactions that are not easy to predict. However, it must be remembered that cover crops do not provide any new P to the soil, but only allow the existing soil P reserve to be used more efficiently. With removal of P from the field in harvested products, the nutrient supply must be ultimately replaced with an additional supply to maintain sustainability.

Mycorrhizal Fungi
Enhanced P uptake is frequently cited as a primary benefit of mycorrhizal fungi colonization. In this symbiotic relationship, the plant root provides the energy (carbohydrate) for the fungi in exchange for improved nutrient uptake and other plant root benefits. Almost all crop plants form this relationship with mycorrhizal fungi, which is present in the root zone of most soils. 
Sketch of mycorrhizal assocation
Many organic growers encourage the associations of mycorrhizal fungi with crop roots through the use of cover crops and rotations. However, frequent tillage commonly used for weed control causes a disruption of the soil fungal network and may reduce its effectiveness for providing nutrients to the plant.

The value of mycorrhizal fungi for supplying P for crops is most apparent in low-P soils. In most cases, plants growing in soils with medium to high concentrations of P have less mycorrhizal association than plants in low-P conditions. Therefore, the value of mycorrhizal fungi is greatest in soils without an adequate supply of P. Similar to cover crops, mycorrhizal fungi do not provide any additional P to the soil, but can allow better utilization of the existing soil resource. Commercial sources of mycorrhizal fungi are available and may be used in specialized

Rock Phosphate
Rock phosphate
Rock phosphate (apatite) is a general term used to describe a variety of globally distributed P-rich minerals. Of the two main types (sedimentary or igneous), sedimentary rock deposits are the source of over 80% of the total world production of phosphate rock. Depending on its geologic origin, rock phosphate has widely varying mineralogy, texture, and chemical properties. Some rock P is found in hard-rock deposits, while other rock P is found as soft colloidal (soil-like) material. This great variation in properties and the accompanying elements present in the rock (such as carbonate and fluoride) has a large effect on its value as a source of plant nutrient. This range in properties makes some rock P sources excellent nutrient sources and other sources quite unsuitable. Unfortunately, the information on P availability from a specific rock source is not generally available to the consumer.

The general reaction of rock P dissolution added to soils to a plant available form is:

Equation 1: Ca5(PO4)3F + 6H+ 5Ca2+ + 3H2PO4– + F–

Note the importance of acidity (H+) and low Ca2+ in this reaction.

It is difficult to make universally applicable recommendations for rock P application because so many factors affect its dissolution and plant availability. However, the key factors to consider include:

• Soil pH is important in the dissolution of the rock P (Equation 1). Rock P is much more soluble in acidic soils (soil pH <5.5). In neutral pH to alkaline soils, rock P typically provides little benefit for plant nutrition, except under special conditions.

• Particle size influences the dissolution of rock P by controlling the surface area available for reaction. However, fine grinding a low-reactivity phosphate rock will not significantly increase P availability due to its insoluble mineralogical structure. Conversely, it may not be necessary to finely grind highly reactive rocks used for direct application to the soil. Many rock P sources are commonly ground to <100 mesh (0.15 mm) to improve reactivity, but such finely ground material may be difficult to handle and to spread uniformly.

• Low soil Ca concentrations and high soil cation exchange capacity favor rock P dissolution since Ca is one of the reaction products resulting from dissolution. Soil conditions that limit Ca availability (soil acidity, high leaching, or the presence of organic compounds that complex exchangeable Ca) also tend to favor rock P dissolution and the release of P for the plant.

• Other cultural practices that may improve P availability from rock P include broadcast applications to maximize soil dissolution reactions, and using management that promotes root colonization by mycorrhizal fungi. Application of rock P should be made several weeks or months prior to the anticipated need for plant nutrients. Although lime applications are important for reducing harmful effects associated with soil acidity, lime additions tend to reduce the value of rock P as a nutrient source.

Manure and Composts
Composted dairy manure
These materials are generally excellent sources of P for plants. Even though these materials are considered as organic products, over 75% of the total P they contain is present as inorganic compounds. It is commonly recommended that the P in manure and compost be considered as 70% available for soils with low soil-test P, but 100% available for soils testing adequate or high for P.

The ratio of nutrients in composts and manures does not closely match that required by plants nor in the harvested products. When manure and compost are used as a primary N source for crops, P is typically overapplied by 3 to 5 times compared with the crop removal rate. Long-term use of manures and compost as the primary N source leads to an accumulation of P in the soil that can become an environmental concern for surface water quality.

Bone Meal
Bone meal, prepared by grinding animal bones, is one of the earliest P sources used in agriculture. Most commercially available bone meal is “steamed” to remove any raw animal tissue. The primary P mineral in bone material is “calcium-deficient hydroxyapatite” [Ca10–x(HPO4)x(PO4)6–x (OH)2–x (0 < x < 1)], which is more soluble than rock phosphate, but much less soluble than conventional P fertilizers. Calcium-deficient hydroxyapatite present in bone meal dissolves:

Equation 2: Ca9.5(HPO4)0.5(PO4)5.5(OH)1.5 + 13H+ 9.5Ca2+ + 6H2PO4– + 1.5H2O

Similar to rock P, bone meal is most effective in acidic soils and when the particle size is small. When used properly, it can be an effective P source. One of the first commercial P fertilizers was produced by reacting animal bones with sulfuric acid to enhance the solubility of P.

Concerns have been raised regarding bovine spongiform encephalopathy (BSE) in cattle and the residual effect of bone meal as a fertilizer. There are no restrictions on the use of bone meal and most commercial bone meal products have been heat treated, so the potential for prion transmission is small.

Guano as a nutrient source
Guano is most commonly used as a source of N for plants, but some guano materials are also relatively enriched in P. Guano is mined from aged deposits of bird or bat excrement in low rainfall environments. The drying and aging process changes the chemistry of the P compared with fresh manure. Struvite (magnesium ammonium phosphate) can be a major P mineral found in guano, dissolving slowly in soil. The limited supply and high cost of guano generally restricts its use to small-scale applications.

There are several options available for meeting the P requirement for organic production. Growers are encouraged to first consider locally available materials to meet this need. Many of the allowed materials are fairly low in nutrient content, therefore transportation costs may be a concern since relatively large quantities of amendment may be needed to meet the crop demand. Regular soil and tissue testing should be conducted by all growers to avoid depletion of soil nutrients and to prevent inadvertent nutrient accumulation, regardless of production philosophy and management techniques. BC

Dr. Nathan Nelson is with the Agronomy Department, Kansas State University; e-mail: nonelson@ksu.edu. Dr.  Rob Mikkelsen is IPNI Western Region Director; e-mail: rmikkelsen@ipni.net.