Quality: Potassium Management is Critical for Horticultural Crops

Quality, What is it?
Potassium is frequently referred to as the “quality” nu­trient for plants. Quality has many characteristics and the most important aspects of quality will depend on the spe­cific crop. For example, with citrus, it may be the thickness of the peel and Vitamin C concentration, for apples, sug­ar concentrations, while for tomatoes, the development of uniformly red fruit rich with lycopene. The specific quality parameters for each crop will vary and should be well un­derstood to maximize crop nutritional practices and market profitability (Kumar et al., 2006).

While many “quality” benefits are generally understood, it can be difficult to define and quantify the exact benefits of K (Lester et al., 2010a). Most notably, the lack of quali­ty is frequently observed when the plant K supply becomes
 deficient. An inadequate K supply becomes especially im­portant for horticultural crops where the visual appearance of the fruit and leaves is critical for marketing. Although the total yield may be reduced with insufficient K, it is possible that the entire crop may be unsalable due to poor quality and visual appeal.

The growth and longevity of cut flowers and ornamen­tals can also be diminished by a lack of adequate K. Ship­ping, handling, and freshness are particularly important for ornamental horticulture.

Consumer Preference
Consumers have a strong preference for fresh fruits and vegetables with appealing appearance and texture. Quality and freshness of fruits and vegetables are often cited as the primary characteristics for making purchase decisions.

Potassium plays a critical role in many of the metabolic processes that enhance the quality, nutrition, flavor, appear­ance, and longevity of fresh food crops. These beneficial im­provements clearly are desirable for farmers and will add to the marketability of crops.

 Vitamin C
Application of K to the soil or plant foliage has been shown to increase the concentration of Vitamin C in a va­riety of fruit crops. While citrus is the most frequently cited example, increased Vitamin C has been reported in crops such as cucurbits, cauliflower, onion, banana, guava, and papaya (Imas, 2013). Muskmelon also had higher concen­trations of Vitamin C as a result of foliar K sprays (Lester et al., 2010b).

Nitrate Assimilation and Protein Synthesis
Potassium plays an important role in converting nitrate into amino acids and proteins. An insufficient supply of K may result in both lower nitrate uptake from the soil and slower nitrate assimilation into amino acids and proteins. Potassium deficiency can result in accumulation of low mo­lecular weight sugars and carbohydrates, along with solu­ble-N compounds in the plant.

Nitrate accumulation in K-deficient plants can be a con­cern where limits have been established (such the Europe­an Union nitrate limit for leafy vegetables). When nitrate is rapidly converted to protein, the concern for healthier food is satisfied.

Appearance of Fruits and Vegetables
An adequate K supply has been linked to improved vi­sual appearance of many horticultural crops. For example, banana is a crop that frequently responds favorably to K fertilization. Sufficient K improves banana fruit weight and number of fruits in each bunch, increases soluble solids, sug­ars, and starch. Low K results in thin and brittle bunches with a shorter shelf life. A lack of K has been linked with premature color development and harder, dry fruit sacs in citrus. Potassium-deficient grapes are less firm and have less juice.

An adequate supply of K increased mar­ketability traits of muskmelon fruit (maturity, yield, firmness, and sugars) and quality param­eters (ascorbic acid and β-carotene) (Lester et al., 2010a). The yield, quality, and shelf-life of tomatoes are improved with an adequate K supply. A lack of sufficient K results in uneven ripening, yellow shoulder fruit, and irregularly shaped fruit with poor internal quality (Hartz et al., 2005).

Extending Shelf-life and Reducing Food Waste
Potassium has been shown to have a bene­ficial impact on properties that improve shelf life, storage, and shipping of many fruits and vegetables. Some of this occurs as an adequate K supply generally increases the firmness and strength of skins, allowing greater resistance to damage during trans­port and storage. Extending the longevity of freshness pro­vides immediate benefits to both the farmers and the con­sumers.

The positive impact of K on fruit storage has been re­ported on many crops, including bananas (shelf-life), citrus (decreased post-harvest mold and rot), potatoes (storage longevity), carrots (crispness), pineapple (greater vitamin C leading to reduced browning and rot), figs, and apples.

Disease and Insects
Plants that are deficient in K are likely to be more sus­ceptible to infection and insect damage than when sufficient K is present. In a significant literature review, Perrenoud (1990) examined 2,449 scientific citations and concluded that the use of K reduced the incidence of fungal diseases by 70%, bacterial infection by 69%, insects and mites by 63%, viruses by 41%, and nematodes by 33%. Reducing these pathogens and insects had a large benefit of allowing higher yields to be achieved.

A review by Wang et al. (2013) presented an excellent summary of how optimal K nutrition imparts significant plant resistance to both biotic and abiotic stresses. They re­viewed the important role of K in protecting plants against diseases, pests, drought, salinity, cold and frost, and water­logging.

Consumers are sensitive to the use of plant protection chemicals in production of horticultural crops. This sensi­tivity partially accounts for the growth of the organic farm­ing sector (Mikkelsen, 2007). Whenever possible, providing adequate K should be used as a first line of protecting plant health. Decreased damage to harvested fruits and vegeta­bles from pathogens and stresses will also result in a more attractive, marketable, and hence profitable crop.

Nutrient Composition
Fruits and vegetables are the most important sources of dietary K in the human diet. However, a trend for a decline in the mineral concentration of many foods has been sug­gested for over 75 years (Davis, 2009). A decline of 5% to 40% or more in minerals, vitamins, and proteins has been measured in many foods, especially vegetables. The cause for this decline may be due to dilution, changes through plant breeding, and changes in farming cultural practices. Recent reviews indicate that the decline in nutrient concen­tration of fruits and grain may not be as severe as earlier claimed (Marles, 2017).

Whatever the cause of this dilution, clearly there is a need to reexamine how the K concentration of food can be enhanced to better meet the dietary and health needs of consumers.

Functional Foods
“Functional food” is a term used to describe foods that provide health benefits in addition to the regular vitamins and minerals contained in common foods. Including them in a human diet is often considered to promote health be­yond a more typical diet. Lycopene found in tomatoes, alli­cin present in garlic, and resveratrol in grapes are examples of nutraceutical compounds in functional foods that may provide health benefits.

The concentrations of all these functional food com­pounds listed above have been shown to increase in the pres­ence of an adequate or abundant K supply to plants. The direct metabolic link between K and these functional food compounds is not always clear, but the trends are consistent.

Human Health
Animals and humans have an absolute requirement for K for proper growth and health. Potassium is involved in many essential functions in nerves, biochemical reactions, muscle function, heart health, and water balance. However, almost all human diets are quite low in K compared with the recommendations for health (Weaver, 2013). For exam­ple, in the United States the average daily K consumption is only 55% of the recommended dietary intake.

A diet rich in fruits and vegetables is one of the best ways to increase K intake, with potatoes being one of the highest sources of dietary K. Increasing the K concentra­tion of the harvested portion of fruits, vegetables, and other plant-based products would make an important contribu­tion to improving human health.

Conclusions
Potassium is essential for sustaining both the yield and the quality of many horticultural crops. Enhanced quality is frequently observed in many vegetables and fruits from an abundant supply of K. This quality can be observed in different ways for each species, but includes parameters such as size, appearance, longevity of storage, sugar and acidity, soluble solids, and nutritional benefits. Damage from dis­ease, insects, and environmental stresses are frequently re­duced when adequate K is present. All these considerations combine to underline the importance of maintaining an adequate supply of K for the production of high quality horticultural crops.

References
Davis, D.R. 2009. HortScience 44:15-19.
Hartz, T.K. et al. 2005. HortScience 40:1862-1867.
Imas, P. 2013. Potassium - The quality element in crop production. Intern. Potash Institute: Horgen, Switzerland.
Kumar, A.R. et al. 2006. Agric. Rev. 4:284-291.
Lester, G.E. et al. 2010a. Better Crops 94(1):18-21.
Lester, G.E. et al. 2010b. Plant Soil 335:117-131.
Marles, R.J. 2017. J. Food Comp. Anal. 56:93-103.
Mikkelsen, R.L. 2007. HortTechnology 17:455-460.
Perrenoud, S. 1990. Potassium and Plant Health, 2nd ed., International Potash Institute: Bern, Switzerland.
Wang, M.Q. et al. 2013. Int. J. Mol. Sci. 14:7370-7390.
Weaver, C.M. 2013. Adv. Nutrition 4:368S-377S.

This article originally appeared in IPNI's magazine: Better Crops in 2018.  It is linked here
potassium potash health vegetable fruit quality consumer nitrate shelf waste functional human animal quinic sulphoraphine indole carotenoid echinacoside polysaccharide lignans allicin flavonoids ginsenoside serveratrol quercirtin anthocyanidins lycopene carotenoid beta glucans saporins terpenoids phytic robert mikkelsen rob plant nutrition

Nanofertilizer and Nanotechnology: A quick look

The word “Nano” means one-billionth, so nanotechnology refers to materials that are measured in a billionth of a meter (nm). A nanometer is so small that the width of a human hair is 80,000 nanometers. The field of nanotechnology has resulted from advances in chemistry, physics, pharmaceuticals, engineering, and biology. The size of a nanomaterial is typically about 1 to 100 nanometers. They can be naturally occurring or engineered. Due to their extremely minute size, they have many unique properties that are now being explored for new opportunities in agriculture.

There are naturally occur- ring nanoparticles that have been previously proposed for agricultural use, such as zeolite minerals. However, engineered nanomaterials can now be synthesized with a range of desired chemical and physical properties to meet various applications.

Nanofertilizers are being studied as a way to increase nutrient efficiency and improve plant nutrition, compared with traditional fertilizers. A nanofertilizer is any product that is made with nanoparticles or uses nanotechnology to improve nutrient efficiency.
Another promising application of nanotechnology is the encapsulation of beneficial microorganisms that can improve plant root health. These could include various bacteria or fungi that enhance the availability of nitrogen, phosphorus, and potassium in the root zone. The development of nanobiosensors to react with specific root exudates is also being explored.

Three classes of nanofertilizers have been proposed:
1. nanoscale fertilizer (nanoparticles which contain nutrients),
2. nanoscale additives (traditional fertilizers with nanoscale additives), and
3. nanoscale coating (traditional fertilizers coated or loaded with nanoparticles)
Nanomaterial coatings (such as a nanomembrane) may slow the release of nutrients or a porous nanofertilizer may include a network of channels that retard nutrient solubility. The use of nanotechnology for fertilizers is still in its infancy but is already adopted for medical and engineering applications.

Another promising application of nanotechnology is the encapsulation of beneficial microorganisms that can improve plant root health. These could include various bacteria or fungi that enhance the availability of nitrogen, phosphorus, and potassium in the root zone. The development of nanobiosensors to react with specific root exudates is also being explored.

Examples of potential nanofertilizer designs  (adapted from Manjunatha et al., 2016)

• Slow release: the nanocapsule slowly releases nutrients over a specified period of time.
• Quick release: the nanoparticle shell breaks upon contact with a surface (such as striking a leaf).
• Specific release: the shell breaks open when it encounters a specific chemical or enzyme.
• Moisture release: the nanoparticle degrades and re- leases nutrients in the presence of water.
• Heat release: the nanoparticle releases nutrients when the temperature exceeds a set point.
• pH release: the nanoparticle only degrades in specified acid or alkaline conditions.
• Ultrasound release: the nanoparticle is ruptured by an external ultrasound frequency.
• Magnetic release: a magnetic nanoparticle ruptures when exposed to a magnetic field. 

Many of these nanotechnologies are still in the early development stage for both medical and agricultural uses. However, the next time you hear about nanofertilizers, you will have a better idea of where this field is headed.

Additional Reading

• Calabi-Floody, M. et al. 2017. Adv. Agron. 147:119-157.

• Fraceto, L.F. et al. 2016. Front. Environ Sci. 22 March 2016. https://doi.org/10.3389/

fenvs.2016.00020

• Manjunatha, S.B. et al. 2016. J. Farm Sci. 29:1-13.

• Tapan, A. et al. 2016. J. Plant Nutri. 39:99-115, https://doi.org/doi.org/10.1080/019041

67.2015.1044012

• Wang, P. et al. 2016. Trends Plant Sci. 21(8) https://doi.org/10.1016/j.tplants.2016.04.005

The original article can be viewed here:


Fertilizer pollution phosphorus technology mikkelsen soil fertility chemistry advance new nutrition

Swapping Organic and Inorganic Fertilizers

Rob Mikkelsen, IPNI

The bickering over the superiority of one source of plant nutrients over another gets tiresome. There are excellent arguments about why organic nutrient sources make valuable contributions to plant and soil health. However, remember that the inorganic fertilizer industry first developed to satisfy farmer’s irreplaceable need for affordable plant nutrients. Yes, both sources of nutrients play essential roles for food production sustainability.

The fundamentals of 4R Nutrient Stewardship remind us to always use the Right Source of nutrient regardless of their origin. Applying 4R principles will always assist in achieving the desired goals of each unique situation.

Predicting N release from mineral fertilizers is relatively simple (see diagram provided). The N release from organic fertilizers depends on the proportion of rapid and extended-release materials. The environmental conditions and field management practices influence the behavior of all nutrient sources applied.

One difference between many organic and inorganic fertilizers is the presence of organic carbon. Addition of organic matter is almost always beneficial for soil health. However, there are at least two ways of adding organic matter to soil: 1) harvest crops from one field,  feed them to animals and humans, and then return the manure to another field, or 2) grow plant-based organic matter directly on the field  (such as cover crops or by returning crop plant residue). Both approaches are effective in increasing organic carbon inputs to the soil.

Predicting nitrogen availability for plants is challenging from organic fertilizers

The decision of using organic or inorganic nutrient sources is often based on the availability of local resources, the economics of hauling and application, and the need to supply balanced crop nutrition. Here are a few considerations to keep in mind:

Potassium (K):  The fertilizer equivalence of K in most organic nutrient sources is quite similar to inorganic sources. Since K is not a structural component of plant cells, it remains soluble in animal manure, urine, and crop residues. The nutrient value of K in animal manures is generally equivalent to soluble K fertilizers.

Phosphate (P):  Phosphorus availability from organic materials for plant nutrition is extremely variable. In animal manure, 45 to 70% of the P is present as inorganic phosphate, the form found in most fertilizers. Most reports indicate that there is no difference in crop growth between P supplied by animal manures and composts or fertilizer P (Prasad, 2009, Zhang, 2002). Phosphorus availability in manure and compost will often range from 60 to 100% of the inorganic P fertilizer equivalent. Conditions controlling mineralization and the presence of additional organic matter can also play a role in P availability.

Some commercially available organic fertilizers have been shown to be unsatisfactory at supplying the immediate P needs of plants. However, with a multi-year perspective, even these slowly available P sources may eventually supply P for crop growth if applied in large quantities.

Nitrogen (N):  Predicting the fertilizer replacement value of N in organic materials is the most challenging of the primary nutrients. The availability of N from an organic material is partially controlled by its chemical and physical characteristics. The N-release rate from organic materials is also impacted by factors such as the site (e.g., soil, climate), soil fertility (existing C and N, turnover rate), crop type (length of growing season, rooting patterns), and multi-year field management practices (placement, tillage).

 

Some organic materials, such as urine, are equivalent to a solution of urea and N will rapidly become available for plant uptake. Other materials, such as aged beef-lot compost will be stable for several years and only begin to release N after many years.

Many organic fertilizers provide both short-term and long-term N release, which consequently requires considerable skill and knowledge to accurately predict the nutrient value. To aid in this prediction, Gutser et al. (2005) developed a chart for comparing the N fertilizer equivalence for a variety of organic materials during the first year of application.

 

Gutser et al (2005) made a chart comparing nitrogen availability from organic materials compared with soluble fertilizer

The selection of any nutrient source should be made so that it simultaneously accomplishes the 4R goals of economic sustainability, environmental protection, and societal acceptance. Whether a farmer primarily uses organic nutrient sources, inorganic fertilizers, or a combination of the two, they must all be managed properly. Let’s not argue about it.

References and Further Reading:

• Gutser, R. et al. 2005. J. Plant Nutr. Soil Sci 168:439-446.
• Mikkelsen, R. and T.K. Hartz. 2008. Better Crops 92(4) 16-19.
• Mikkelsen, R. 2008. Better Crops. 92(2) 26-29.
• Nelson, N.O. and R. Mikkelsen. 2008. Better Crops 92(1) 12-14
• Prasad, M. 2009. A Literature Review  on the Availability of Phosphate from Compost in Relation to the Nitrate Regulations SI 378 of 2006. EPA,  Wexford, Ireland.
• Zhang, H. et al. 2002. Managing  Phosphorus from Animal Manure. Univ. Arkansas. http://animalwaste.okstate.edu/bmps-regulations/ bmps-regulations

Enhance Soil Health with Fertilizer?

We can agree that healthy soil is essential for sustainable and productive agriculture. While we have a general understanding of what soil health means, it can be difficult to define and even more difficult to agree on the best was to measure it. A definition certainly includes aspects of physical, chemical, and biological properties of soil. 

Some proponents envision an undisturbed prairie or forest soil as providing the archetype of an ideal healthy soil. However, in real life, agriculture by its very nature is a disruptive human activity that we engage in to meet our existential need for farm products. Just harvesting a crop will subtly change soil properties.

We can agree that maintaining soil in its top possible condition will require careful stewardship, conservation, and greater appreciation of its unreplaceable value.

Chemical Properties

Soil pH is probably one of the most important attributes of a healthy soil. Soil acidity is especially important for determining both the microbial population and the distribution of microbial community structure. For farmers, soil acidity presents a pernicious attack on crop yields that is generally best addressed by application of lime. Applications of ammonium and urea fertilizer, when not buffered, gradually lower soil pH. When soil pH drops below 5, microbial biomass generally decreases.

However, when pH is maintained near neutral, the input of N fertilizer does not seem to have longterm negative effects on microbial biomass in annual cropping systems.

Harmful effects of acidity on plant roots: Soil Quality.org.au

Physical Properties

Soil compaction is damaging for health, and is often exacerbated by the traffic of heavy equipment across the field at an inappropriate time.

Certainly, a soil in good physical condition allows better root growth and recovery of nutrients and water. An interesting study from long-term research showed that root access into a soil is an important part of improving P availability to crops1. The importance of soil microaggregates is becoming more appreciated2. Root channels also make an important contribution to water infiltration that improve soil properties and reduce runoff.

Biological Properties

Many short-term studies have been conducted to measure the effect of fertilization practices on soil biology. Most of this work shows that nitrogen (N) fertilization has little impact on microbial communities3 apart from any acidity that may be produced during nitrification. However, long-term studies are needed to provide a full understanding.

A recent literature review concluded that long-term N fertilization of agricultural soil results in increased microbial content, most likely due to associated greater input of organic carbon (C) resulting from higher crop productivity. The measured increases in soil microbial biomass carbon (Cmic) in fertilized soils under annual crops contrasts with some observations in natural ecosystems, where N inputs may decrease Cmic4.

Another recent report from long-term research reported that applications of organic manure

(which is more diverse in nutrient content and organic content than fertilizer) resulted in strong

enhancement in soil microbial biomass and diversity5. The use of inorganic fertilizers alone resulted in a slight increase in microbial biomass, but strongly enhanced the activity of specific soil enzymes. They concluded that a combination of manures and inorganic fertilizers may be the most beneficial for microbial health. However, the authors do not suggest where the large quantities of manure could be obtained or where the nutrients in the manure likely originated.

An unanswered question is how much biological activity is optimal for soil health. Farming practices that support soil health provide benefits but may also come at a price (such as yield penalties). For example, tillage generally has a negative impact on earthworms, but may be beneficial for improving the crop root zone, incorporating nutrients deep in the profile, and reducing the susceptibility of some nutrients to be lost through runoff.

Too frequently it is parroted that any fertilizer inputs are automatically detrimental for soil health. It’s just the opposite. When properly managed, appropriate addition of fertilizer stimulates plant growth and results in greater biomass returned to the soil, a healthy plant canopy that quickly covers and protects the soil, and an extensive root system that provides a habitat for beneficial organisms. 

Many of these relationships have received insufficient attention. Instead the discussion is too often dominated by polarizing debates over the merits of organic or inorganic nutrient sources. There is no doubt that the proper use of nutrients has a great benefit on soil health and helps us to sustain agricultural production. Let’s use whatever nutrient resources are available to carefully protect and enhance our valuable soil resources.

References

1. Johnston, A.E. and C.J. Dawson. 2010. Proc. International Fert. Soc. 675:1-40.

2. Totsche, K.U. et al. 2018. J. Plant Nutr. Soil Sci. 181:104- 136.

3. Yu, H. et al., 2016. PLoS ONE 11(3):e0151622. doi:10.1371/journal.pone.0151622

4. Geisseler, D. and K.M. Scow. 2014. Soil Biol. Biochem. 75:54-63.

5. Francioli, D. et al. 2016. Front. Microbiol. 7:1446. doi:10.3389/fmicb.2016.01446

 This article originally was written as a "Plant Nutrition Today" newsletter by Rob Mikkelsen HERE

Ugly Pictures Help to Grow Beautiful Potatoes

Potatoes are one of the most important crops in the human diet. They are grown worldwide, with China being the largest potato-producing country, followed by India and Russia. There are hundreds of potato varieties, representing a wide range of colors, shapes, sizes, flavors, and cooking properties.

Potatoes have a fairly shallow fibrous root system. Compared to other crop species, they are less able to utilize moisture and nutrients from deeper zones in the soil. Additionally, potatoes have relatively low root length density (about one-fourth that of wheat) and also have relatively few of the root hairs that are critical for the uptake of many plant nutrients.

Tindall, Pitchay, & Mikkelsen (L to R)

IPNI recently teamed with Dr. Dharma Pitchay at Tennessee State University and Dr. Terry Tindall of Simplot to develop an e-Book of potato nutrient deficiency symptoms that will assist farmers to recognize and diagnose emerging nutrition problems.

Nutrient deficiencies do not immediately result in visible symptoms. Plant growth and metabolism is usually hindered for some time before visual symptoms occur. This period of so-called “hidden hunger” occurs with low levels of chronic nutrient deficiency and is far more common than visible deficiency symptoms. By the time obvious visual symptoms first appear, the plant can no longer properly function and there is already a loss of yield and quality. 

Nutrient deficiencies too often result in permanent loss of potato yield and quality. Tuber formation and development is often disrupted, with the full impact not observed until harvest, or during storage, or when cooking.

Other stresses cause abnormal symptoms to appear on potato leaves that may not be directly related to nutrient deficiency. In these situations, there may be an adequate nutrient supply in the soil, but unfavorable conditions restrict uptake by the roots. Additionally, plant disease, insect damage, herbicide impacts, and excessive salinity are examples of non-nutrient factors that cause leaf disorders and stunting.

Good nutrition is essential for supporting potato plant health and provides defense against plant disease and stress. Nutrient deficiency results in secondary plant damage that is not readily visible. A variety of nutrient-induced disease resistance mechanisms have been reported for potatoes. For example, balanced NPK applications will induce disease resistance. Similarly, sulfur and magnesium fertilization enhances resistance to potato scab bacterial infection. Potassium deficiency increases susceptibility to various diseases and insect pests.

This new guide to potato nutrient deficiencies is being provided as a component of the IPNI mission to develop and promote scientific information about the responsible management of plant nutrition for the benefit of the human family. With good nutrient planning, we will not see these ugly nutrient-deficient potatoes in farmer fields, but only beautiful and tasty tubers! 

For more details, please visit:

http://info.ipni.net/ebooks

Soil Nutrient Mining... Good or Bad?

I will frequently be asked when talking about issues of plant nutrient management, “so, is that good or bad?” Experience tells me that most complex issues do not have a clear distinction between good and bad, but require a little more exploring on how to make the difference clear.

Rob Mikkelsen in cranberry field

Nutrient mining is an agronomic concept that gets discussed as either good or bad, but the real answer lies somewhere in between depending on the circumstances and the specific nutrient. The discussion here mainly refers to the more immobile nutrients, such as phosphorus (P) and potassium (K), but some of the same concepts apply to the more mobile nutrients.

Nutrient balances are measured by the difference between nutrient additions and removals. As nutrients are removed more quickly than they are replaced, a negative balance results in nutrient depletion (a.k.a. mining). When more nutrients are added to the soil than are removed, a positive balance results in accumulation or buildup.

The Bad
Nutrient mining and depletion of low-fertility soils exhausts the crop-producing potential of the soil, harms soil health, and degrades the valuable natural resource. On soils that are already low in crop nutrients, further depletion results in lost economic opportunities too. Continual nutrient depletion is a major soil-degrading practice that persists in many parts of the world.

Nutrients are not really “lost” from soil, but they are harvested and transported off of the field, eroded, leached beneath the root zone, or sometimes burned as crop residues. At some point, it is necessary to replenish the nutrients to avoid excessive soil mining and severe nutrient depletion. 

Recycling on-farm residues (e.g., crop residues, compost, green manures, animal manures) will help return nutrients to the production fields. But these organic additions will not eliminate soil nutrient mining if they are produced on the same farm where they are used.

On fields where all of the crop residue is removed, the extent of soil nutrient mining is accelerated, compared with fields where the residue remains on the soil and the nutrients are recycled. If the residues are removed for animal feed, returning the manure to the field would slow the process of nutrient mining to some degree.

The Good
When a soil testing program is carefully followed, there may be fields that have an adequate nutrient concentration for healthy plant growth and fertilization can be temporarily halted. 

There are two major philosophies concerning interpretation of soil test results. There is no simple answer of which approach is superior, since there are many factors that determine what is the appropriate route for you.

Great potato crop in Idaho

Nutrient Sufficiency:
Apply only the minimum amount of nutrient required to maximize profits in the year of application.

Build and Maintenance:
Build the concentration of nutrients to a non-limiting range and then apply sufficient nutrient to maintain that desired concentration.

The Build and Maintenance approach allows farmers to take a break in fertilizer application if economic circumstances change. An investment has previously been made to boost the soil nutrient concentrations and soil mining can be allowed to occur for a period of time without devastating results.
Many fields that receive repeated applications of animal manure eventually accumulate high concentrations of P in the soil. When manure is applied to meet the nitrogen requirement of crops, the amount of added P far exceeds crop uptake and removal, resulting in soil P build up. This accumulated P, often termed “legacy phosphorus” can be viewed as a valuable resource, but may also become a source of pollution to nearby water bodies through runoff and erosion. In this case, P application may need to cease and soil mining adopted to lower the P concentration and envir
   
The key to managing soil nutrient mining is to understand the balance between inputs and outputs. Where available, a comprehensive soil testing program should be used to maintain nutrient concentrations above their critical value. When nutrient concentrations are less than recommended, a phase of nutrient build up is needed to avoid loss of crop yield and quality. At soil test concentrations far greater than recommended, fertilizer applications less than crop nutrient removal may be appropriate.

Soil testing services are not available in many parts of the world. IPNI has developed easy-to-use  software (Nutrient Expert) that allows farmers to make fertilizer recommendations in the absence of local soil testing information.

So, is soil nutrient mining good or bad? It can have devastating effects, leading to soil degradation, or it can have significant economic and environmental benefits. Begin by understanding the nutrient budget for each field and then adopt specific practices appropriate for your conditions
onmental risk.

Agriculture has advanced a long ways!

Nitrification Inhibitors

Some compounds added to nitrogen (N) fertilizers can reduce the rate at which ammonium is converted to nitrate. Under appropriate conditions, this can help reduce N losses through denitrification and leaching. Nitrification in Soil Nitrification is a natural process in soils that converts ammonium to nitrite and then to nitrate. The soil bacteria Nitrosomonas spp. extract energy from ammonium by converting it to nitrite. A second group of bacteria, Nitrobacter spp. then convert nitrite to nitrate. Both types of bacteria are common in soil and it is the first reaction that limits the overall rate of nitrate production. 

  

With moderate temperature and soil water content nitrification occurs on most soils within a few days or weeks after application of ammonium sources. The ammonium can come from urine, manures, composts, decomposing crop residues, or fertilizer containing ammonium or urea.

Nitrate is the dominant form of plant available N in soil and unless taken up by roots, it can be transferred to either water or the atmosphere. Nitrate can leach below the root zone with the potential to be transferred to surface or sub-surface waters. Under waterlogged conditions, nitrate can be denitrified to form nitrous oxides and dinitrogen by other soil bacteria. While dinitrogen is the most common gas in the atmosphere and inert, nitrous oxide is a powerful greenhouse gas. Some nitrous oxide can also be produced during nitrite decomposition during nitrification. 

Nitrification is rapid in warm (>25^o C) soils, and it largely ceases below 5^o C. It occurs most rapidly where the soils are well aerated and near field capacity, and decreases with higher or lower moisture contents. In saturated soils nitrification nearly stops because of the lack of oxygen. 

The nitrogen cycle involves many biological processes

Regulating Nitrification

The rate of nitrification can be controlled by preserving N as ammonium, which can be held on soil colloids rather than leached. Nitrification inhibitors are compounds that delay nitrate production by depressing the activity of Nitrosomonas bacteria.

There are at least eight compounds recognized commercially as nitrification inhibitors although the most commonly used and best understood are 2-chloro-6-(trichloromethyl)-pyridine (Nitrapyrin), dicycandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP). These compounds suppress microbial activity for a few days to weeks depending on soil moisture and soil type although there are differences between them in the way they are deployed. In general, nitrification inhibitors are more effective in sandy soils, or soil low in organic matter and exposed to low temperatures. 

Management Practices 

Nitrapyrin can be injected directly into the soil with anhydrous ammonia or coated onto solid N fertilizers or mixed with manures. Because nitrapyrin is volatile it needs to be incorporated into the soil. Nitrapyrin is usually broken down within 30 days in warm soils. 

DMPP is usually supplied pre-blended with fertilizers. It is considered a highly specific nitrification inhibitor active for 25 to 70 days, but its effect is reduced at higher temperatures. It is relatively immobile in the soil. 

DCD can be coated on solid fertilizers, and is also used where manures are surface applied, and can be used post-grazing to reduce nitrate leaching from urine patches. The inhibitory effect of DCD usually lasts between 25 to 55 days, and it is readily leached, lowering its effectiveness.

A study in New Zealand showed that DCD applied to grazed pastures reduced nitrate leaching from urine patches by 59% and nitrous oxide emissions by 82%, as well as increasing herbage production by 30%. 

Because there are many factors that control both the rate of nitrification and the activity of the inhibitor, there is considerable variability in the reduction in the amount of nitrate leached, the reduction in nitrous oxide produced, economic return and the potential benefits associated with their use. So, the yield benefits occur when sufficient N is preserved to provide additional growth.

Feel free to contact me for additional information

This overview of nitrification inhibitors originally appeared as part of the IPNI series "Nutrient Source Specifics".  They can be viewed at  https://www.ipni.net/specifics-en

Phosphate Rock

Phosphorus (P) additions are needed in most areas of the world to improve soil fertility and crop production. Direct application of unprocessed phosphate rock (PR) to soil may provide a valuable source of plant nutrients in specific conditions, but there are several factors and limitations to consider. 

Rock phosphate production and final product

  

Production Phosphate rock is obtained from geologic deposits located around the world. Apatite, a calcium phosphate mineral, is the primary constituent of PR. It is primarily extracted from sedimentary marine deposits, with a small amount obtained from igneous sources. Most PR is recovered through surface mining, although some is extracted from underground mines. 

North Carolina, USA phosphate mining

The ore is first screened and some of the impurities removed near the mine site. Most PR is used to produce soluble phosphate (P) fertilizers, but some is used for direct application to soil. While PR can be a valuable source of P for plants, it is not always appropriate for direct application. Its suitability depends partly on naturally occurring mineral impurities, such as clay, carbonate, iron, and aluminum (Al). The effectiveness of PR for direct application is estimated in the laboratory by dissolving rock in a solution containing a dilute acid to simulate soil conditions. Sources classified as “highly reactive” are the most suitable for direct soil application.

 

Florida, USA phosphate mining

Direct use of PR avoids the extra processing associated with converting apatite to a soluble form. The minimal processing may result in a lower-cost nutrient source and make it acceptable for organic crop production systems. 

Agricultural Use When a water-soluble P fertilizer is added to soil, it quickly dissolves and reacts to form low solubility compounds. When PR is added to soil, it slowly dissolves to gradually release nutrients, but the rate of dissolution may be too slow to support healthy plant growth in some soils. To optimize the effectiveness of PR, these factors should be considered:

 •Soil pH: PR requires acid soil conditions to be an effective nutrient source. Use of PR is not usually recommended when the soil pH exceeds 5.5. Adding lime to raise soil pH and decrease Al toxicity may slow PR dissolution. 

• Soil P-fixing capacity: The dissolution of PR increases with a greater P-fixing capacity of soil (such as high clay content). 

• Soil properties: Low calcium and high organic matter in the soil tend to speed PR dissolution. 

• Placement: Broadcasting PR and incorporation with tillage speeds the reaction with the soil. 

• Species: Some plant species can better utilize PR due to their excretion of organic acids from the roots into the surrounding soil. 

• Timing: The time required for the dissolution of PR necessitates its application in advance of the plant demand. 

 

Global P Resources Map

Management Practices Not all sources of unprocessed PR are suitable for direct application to soil. Additionally, many soils are not suitable for PR use. The total P content of a material is not a good predictor of the potential reactivity in the soil. For example, many igneous PR sources are high in total P, but are of low reactivity and provide minimal plant nutrition because they dissolve so slowly. However, mycorrhizal fungi may aid in the acquisition of P from low-solubility materials in some environments. Over 90% of PR is converted into soluble P fertilizer through reaction with acid. This is similar to the chemical reaction that PR undergoes when it reacts with soil acidity. The agronomic and economic effectiveness of PR can be equivalent to water-soluble P fertilizers in some circumstances, but the specific conditions should be considered when making this choice.

Rob Mikkelsen examines phosphorus fertilizer response of wheat