Boosting Human Health with Fertilizer

In discussions about using fertilizers and various plant nutrients, the entire purpose of using these materials is too frequently overlooked. All of the effort that goes into

Artwork by Roy Doremus

acquiring, transporting, applying, and managing these valuable resources is for the primary goal of growing healthy and abundant crops for humans and animals.

Removing crop products from the field extracts nutrients from the field. The harvested products may be used for things such as blue jeans, biodiesel, animal feed, or eaten directly by people. It becomes necessary to replenish the soil as the nutrient reservoir becomes gradually depleted. 

Planting in Africa

Some of the goals of productive agriculture include:

Food security—refers to having access to adequate food, without fear of hunger or starvation. Commercial fertilizers are estimated to support over half of the current global food production and clearly have a vital role in meeting the goal of food security for everyone.

Nutrition security—means having access to adequate food components for human nutritional needs. Many of the

Vegetable market in Malaysia

healthful components of food are boosted by the application of fertilizer (such as protein, carbohydrates, oil, vitamins, and minerals). This security involves access to nutritional food and making wise dietary choices.

Micronutrient nutrition—is increasingly important in many parts of the world as human diets in many less-developed countries have shifted towards greater consumption of “staple” cereal crops (such as corn, wheat and rice). The yield of many micronutrient-rich crops (such as various beans, fruits and vegetables) has not benefited as much from the Green Revolution, and these foods now comprise a smaller proportion of the diet of the world’s poor­est people.

Floating market in Thailand

A variety of practices are being used to boost the nutritional value of crop plants. This includes improved agronomic practices to achieve “biofortification” with minerals such as zinc (Zn), selenium (Se), and iodine (I). Genetic approaches are effective in boosting the concentration of iron (Fe) and vitamins in plants.

Some recent scientific papers indicate that there may have been a decline in the nutrient concentration of some vegetables during the last 50 to 100 years. This is likely due to the well known “dilution effect”, where higher-yielding and larger plants may take up the same quantity of nutrients from the soil. This results in an apparent dilution of the mineral concentration in the harvested crop.

To a large extent, the supply of soluble plant nutrients in the soil determines the composition of the plant. Crops grown in nutrient-deficient soils will likely have low yield and may have poor nutritional qualities for humans and animals. Adding the appropriate plant nutrients to soil supports high yields, and also sustains the essential nutritional food properties. Without the proper nutrients, plants cannot possibly provide an adequate and nutritional food source for people.

IPNI has recently published a comprehensive scientific review of this topic, which can be downloaded for free at: http://info.ipni.net/FCIHH.

The Roots of Nutrient Uptake

We clearly see that plant growth suffers when there are low concentrations of nutrients in the soil, when the nutrients are not sufficiently soluble, or if nutrients do not move to the roots. But in our quest to grow abundant and healthy crops, it is easy to overlook all of the complex chemical and biological activity occurring around the plant roots that make nutrients available for uptake.

Model of root growth

The availability of plant nutrients for roots is controlled by factors such as soil properties, root characteristics, and interactions with surrounding microorganisms. Traditional soil testing techniques measure the availability of nutrients in the general soil, but this may differ from the nutrient concentration in the immediate vicinity of the root (the rhizosphere). Nutrients with restricted mobility in the soil (such as P, K, zinc, iron, manganese, and copper), may be in adequate supply in the bulk soil, but their concentration may be low near the root if the movement in the soil is too slow to replenish the nutrients entering the root.

Calcium deficiency results in damaged root tips

Focusing on P as an example, supplying this nutrient to the root includes several complicated mechanisms. This involves excretion of organic acids, increased root hair formation, and enzyme release.

• Release of Organic Acids: When soil P supplies are low, many plants excrete a wide range of organic compounds to increase the availability of relatively insoluble compounds, such as some calcium phosphate minerals. The organic acids have a role in dissolving nutrients (due to pH), complexing soil cations, and providing an excellent growth substrate for soil microorganisms. Common organic exudates include substances such as malate, citrate, acetate, and oxalate which can lead to root-zone modification. Most soils have populations of microorganisms that are capable of dissolving P-containing minerals, so addition of an organic substrate may encourage their growth in low-P conditions. Mycorrhizal fungi also form complex relationships with most plant species, where the fungi provide various benefits for the plant, including improved nutrition, in exchange for carbohydrate for fungal maintenance and growth.

Rhizosphere soil on wheat roots

• Changes in Root Structure: Plants growing in a low-P soil tend to direct more of their photosynthate energy to root development and often have more fine roots with a small diameter, resulting in a larger surface area. A large root surface area allows plants to access more of the soil and scavenge any soluble phosphate that may be present.
• Enzyme Release: In low-P conditions, plants generally increase the production of enzymes that enhance the rate of P release from soil organic matter. The phosphatase enzymes are not effective in mineralizing phytate, the dominant form of organic P in many soils. Phytase, the enzyme responsible for phytate hydrolysis, is primarily released by microorganisms, which indirectly improves the P availability for nearby roots.

Root hairs increase surface area

These root modifications occur as a result of low soil P availability, requiring plants to devote additional energy to the roots and away from above-ground growth. The excretion of organic compounds from roots can consume as much as half of all the carbon allocated to the root system, although this number is highly variable. The energy costs of mycorrhizal associations with various plant species ranges from 4 to 20% of the daily net photosynthesis. Plant geneticists are looking for ways to make plant roots more efficient at recovering nutrients from the soil. While we wait for improved plant genetics, there are many practical things that can be done to get the maximum benefit from added nutrients. Begin by placing nutrients in the soil in the proper form and in the correct place so plant roots can support abundant yields of high-quality products. 

Cross section of rice roots (notice the aerenchyma, the air channel that allows gas exchange between shoot and root)

Sodium nitrate... a naturally occuring source of nitrate fertilizer

Sodium nitrate was one of the first commercially available inorganic nitrogen (N) fertilizers. It was very important in plant nutrition before the discovery of ammonia synthesis by the Haber-Bosch process in the early 1900’s. Sodium nitrate is a naturally occurring mined product, and as such is used to provide a portion of N nutrition in some organic cropping systems.

Production

Raw caliche ore containing valuable nutrients

Sodium nitrate ore is mined from surface deposits in the Atacama Desert of northern Chile. The ore body occurs within the top two meters in a zone nearly 500 miles (800 km) long and 10 miles (16 km) wide. Sodium nitrate accumulates in this remote region due to very low rainfall and unique geologic conditions.

The nitrate ore, called caliche, is crushed and washed with hot water to dissolve the sodium nitrate. The solution is then filtered and chilled to recover the final product. It is ultimately sold as crystalline or prilled products.

Hauling caliche ore for cleaning

Small deposits of sodium nitrate are reported in other countries, but the Republic of Chile is the only commercial source of this product, so it is frequently referred to as Chilean nitrate. 

Chemical Properties

Chemical formula:            NaNO₃

Nitrogen content:             16% (present as nitrate)

Sodium (Na) content:       26%

Water Solubility:               880 g/L (20^o C)

Agricultural Use

Sodium nitrate provides an immediately available source of N nutrition to plants since it is highly soluble. It has been used as a source of N nutrition since the mid 19th century and has a distinguished history as a valuable fertilizer material. It has been a preferred source of plant nutrition for many crops, notably for tobacco, which is typically fertilized with a nitrate form of fertilizer.

Prilled sodium nitrate fertilizer ready for use

Sodium nitrate is approved by the U.S. National Organic Program for use as a supplemental source of N nutrition. The permitted use recognizes that mineralization of carbon-based organic N sources is not always rapid enough to meet the N demand of the growing crop. This deficit between N release and plant demand can be overcome with appropriate applications of sodium nitrate. Organic farmers are urged to check with their local certifying agency to determine the appropriate use of sodium nitrate.

Management Practices

Appropriate management is needed to achieve maximum advantage of any fertilizer, including sodium nitrate. Since nitrate is highly mobile in soils, careful consideration of placement, timing, and rate will minimize undesirable losses. Sodium nitrate can be broadcast onto the soil surface or applied in a concentrated band on top, or beneath the soil surface. This source of N is not susceptible to volatile losses, so it can provide added flexibility compared to ammonium and urea-containing N fertilizers.

Concern is sometimes expressed over sodium (Na) in the fertilizer. Excessive Na in soils can have damaging effects on soil structure, but this risk is minimal at typical application rates of sodium nitrate. When used in organic production, Na inputs are quite low. For example, application of 30 lb N would supply only 50 lb Na to the soil. Sodium is held less strongly on soil cation exchange

sites than other common cations, so it can be leached during typical rainfall or irrigation events.

Sodium nitrate ore is a naturally occurring product. Therefore, it may contain traces of various elements and compounds such as iodate, borate, perchlorate, magnesium, chloride, and sulfate.

Non Agricultural Uses

Sodium nitrate is a strong oxidizer and is used in a variety of industrial and food processes. For example, it is commonly added to charcoal briquettes to make them easier to light, and is used for making glass and in wastewater treatment. It is used as a food additive in meats and poultry (not to be confused with sodium nitrite which is used as a preservative in deli meats).

Sodium nitrate is combined with other nitrate materials to store heat from solar thermal projects. Solar thermal plants store energy in molten nitrate salts instead of storage in electrical batteries.
[photos supplied courtesy of SQM.  More information at sqm.com]
A pdf version of this information can be found here.