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

Good Nutrition: Key to Plant Health

Getting crops off to a good start is critical for achieving high yields. During this early stage of growth, seedlings are especially vulnerable to many environmental and biological stresses. Protecting plants from stress  and disease begins with providing balanced nutrition from planting through harvest. The critical link between plant nutrition and disease resistance has become apparent
as the frontiers of plant health are better understood. A few of these examples are explained here:

How potassium protects plants; Wang, M. et al. 2013.

Potassium
Potassium plays an essential role in many well-recognized metabolic processes for plants. Potassium’s contribution to sustaining high yielding crops with top quality is well understood. However, the role of potassium in plant stress resistance is less known and appreciated. Potassium is unique among the essential mineral nutrients in its role for plant survival against environmental stress, pests, and diseases.

Supplying adequate potassium to crops through proper fertilization is a simple way to lower the requirement for pest-control treatments that may be costly, time-consuming, and troublesome. The frequently observed benefits of potassium on plant health were reviewed by Wang et al. (2013), which summarizes many recent scientific studies.

Potassium deficiency symptoms on squash (normal on left, severe deficiency on right. Photo by D. Pitchay

When there is a lack of sufficient potassium in plants, low molecular weight compounds begin to accumulate. This build-up of soluble nitrogen-containing compounds (such as amino acids and asparagine) and sugars (such as sucrose) makes a particularly favorable environment for numerous pathogens and insects. For example, aphids are severely nitrogen limited, making potassium-stressed plants an attractive host as an abundant nitrogen source. The presence of sufficient potassium also promotes the production of defensive compounds (such as phenols) which are an important component in plant pest resistance.

An adequate potassium concentration within the plant decreases the internal competition with various pests and pathogens for resources. This results in more resources available for hardening cell walls and tissues to better resist penetration of pathogens and insect pests, and to repair any damaged tissue. Air-borne pathogens are more rapidly shut out from stomatal invasion when adequate potassium is present.
Phosphate
The link between adequate phosphate and plant health is also well known, but perhaps less understood than the association with potassium. Phosphorus is involved in the synthesis of many organic molecules and complex metabolic functions within plants. Crop growth and yields will be significantly reduced when phosphorus is deficient in soil or when plant roots cannot access it.

Pythium root rot on melon

A shortage of phosphorus frequently leads to more disease for many crops. Some of the protective response occurs because healthy plants with sufficient phosphorus have vigorous root growth which allows them to outgrow and escape disease. More specifically, an adequate phosphorus supply has been linked with decreased incidence of Pythium root rot for wheat, leaf blight for rice, numerous tobacco diseases, blight in soybean, and many other diseases. Foliar application of phosphorus-containing sprays is reported to induce protection against powdery mildew.

Chloride

Severe wheat chloride deficiency (L) and moderate (R)

The important role of chloride as a nutrient is often overlooked, especially in regions where soil salinity is a  concern. However in many areas, the addition of chloride results in increased plant vigor and disease resistance. The occurrence and severity of a number of plant diseases have been documented to be reduced following the application of chloride. This includes take-all, stripe rust, and Septoria in wheat, and stalk rot in corn. Promoting plant health clearly includes a solid foundation in proper nutrition. Strong and vigorous crops are able to produce abundant yields of high quality, while better resisting diseases and pests.

Rob Mikkelsen, IPNI, discusses plant health

References
Wang, M. et al. 2013. Internat. J. Molec. Sci. 14:7370-7390.
Available at: http://www.mdpi.com/1422-0067/14/4/7370

Avoid Growing Pains for your Crop... The "Right Time" for 4R Nutrient Management

Preteen children sometimes wake up at night complaining of sore legs and an uncomfortable ache that is slow to go away. These growing pains will be different for every child, ranging from no pain to a lot of discomfort.

While you won’t hear crops complain during their growth spurts, they still need the right supply of nutrients to match their demands for growth and development.

Most grain or fruiting crops have distinct peak periods of nutrient uptake that correspond to their growth pattern. Crops such as grasses and forage have a more consistent pattern of nutrient uptake through the growing season.

One of the fundamental principles of 4R Nutrient Stewardship is making certain that the proper nutrients are in the root zone at the time plants need them. This “Right Time” approach needs to be adjusted for different nutrients and individual crops.

Rob Mikkelsen

In general, it is best to apply nutrients as close to the time of uptake as practical. Nutrients such as P, K, Ca, and Mg are slow to move in the soil and can be applied several months ahead of when the plants will need them. For nutrients that readily move in the soil (such as nitrate and sulfate), it is best to apply them as close to the time for uptake as possible. This reduces the potential for nutrient loss from leaching or denitrification.

When the nutrient supply in the soil cannot match the plant’s demand, crop growth slows and a portion of the final yield is lost each day. Nutrient shortages often cause plant stress and yield loss far earlier than when deficiency symptoms become visible, a deficit known as “hidden hunger.”

It is important to understand when the peak demand periods occur for the specific crops you work with. This allows the “Right Time” component of the 4R’s to support crop yields to their full potential. A few examples of various crops are shown here to highlight some of the differences in total seasonal accumulation or daily nutrient uptake rate.

Maize: Nitrogen and K uptake follow a traditional S-shaped pattern during the growing season, with two-thirds of the total nutrient requirement acquired by the silking stage (VT/R1). However more than half of the total P uptake occurs after the VT/R1 stage, indicating the need for a continual supply of P from the soil throughout the season until crop maturity.

 N and P accumulation in maize (Better Crops, 2013).

Accumulation patterns of soybeans
Better Crops, 2013

 Soybean: For most of the crop nutrients, uptake continues during the entire growing season. The exception is K, where nearly 75% of the total K is taken up before the late vegetative and early reproductive growth stage. The season-long accumulation of most nutrients is a reminder of the importance of maintaining an adequate nutrient supply all the way to maturity.

Daily nutrient accumulation rates of potatoes,
Hermiston, Oregon (Better Crops, 2008)

Potatoes: Approximately two-thirds of the total N is accumulated in the first few months following planting. The rate of plant P uptake generally peaks during the middle of the growing season, with a daily demand of between 0.4 and 0.9 lb P/A/day depending on the variety and location. Potassium uptake reached a peak of 14 lb K/A/day during the midseason of this study from Oregon.

Sugar beets: High yielding sugar beets produce very large amount of biomass. During the growing season, the total nutrient accumulation in Idaho was approximately 240 lb N/A, 60 lb P/A, and 470 lb K/A. In addition to large amount of nutrients taken up, there were distinct peak periods where the daily N accumulation exceeded 4 lb N/A and the daily K accumulation reached 10 lb K/A.

Accumulation patterns of N, P,
and K in sugar beets
(Mriganka et al. 2018)

Link to the pdf version of this article is here: