Learning about nutrient availability from tile lines

As wet weather continues this season, the positions of tile lines are abundantly clear. Crops over those lines are darker, larger and more robust than crops between the lines. This variation illustrates differences between the presence and bioavailablity of nutrients.

A soil test measures the concentration of available nutrients per unit of soil. Many (arguably, too many) years ago, the relationship between crop yield and nutrient concentrations were quantified; these relationships continue to be the basis of modern recommendations.

However, adequate nutrient test levels do not guarantee availability throughout the season. Compaction and dry weather can limit root growth, nutrient solubility or even movement within the plant.

This year we can see how excessive moisture is depriving plants of the oxygen and energy they need to concentrate nutrients in roots. With the exception of well-drained sections over tile lines, plants are pale, stunted and starving.

Presumably these fields received fertilizer at either a straight rate or a variable rate that matched fertility with production goals. Either practice should have ensured enough nutrition for plants across the field. But while nutrients are present in adequate amounts, they are not bioavailable to plants lacking in oxygen and energy.

When soil-based nutrients fail due to excessive or limited moisture, foliar feeding bypasses the soil so that nutrients directly enter the plant. While these few pounds or ounces of nutrition don’t replace the hundreds of pounds of soil-based nutrients consumed over a growing season, they buy time until weather returns to more normal patterns.

While wet fields may not be accessible with foliar fertilizers, this is a great time to reflect on the relative advantages of foliar- and soil-based nutrition.

The role of boron in plants

Boron is thought to play important roles in cell wall synthesis and sugar transport. As such, it affects cell wall strength and resistance to disease, as well as the efficiency of nutrient movement between sources (such as leaves) and sinks (either roots or grain). The latter has important implications for maintaining sufficient sugar movement into embryos to avoid abortion during grain fill. It may also explain why boron availability is associated with nodule formation in legumes.

Like potassium and manganese, boron is severely affected by drought conditions. Dry soils fail to support mass flow, whereby boron flows with moving soil water to the roots. Secondly, boron is not plant mobile meaning the plant cant move the nutrient from a leaf of higher concentration to new growth that may have a limited supply.

The dependency of boron on xylem transport may also explain why shortages are often more pronounced in flower and seeds than leaves and roots. Because reproductive structures are more poorly supported by vascular tissues and have lower transpiration rates, supply of boron may be more limited.

Foliar applications of boron are the most reliable form of application as the nutrient is used in very small quantities. With most soils receiving a variable rate P and K application it is very hard to adequately include Boron as a soil broadcast partner.

Because of its critical role in cell wall and cell membrane integrity and function, boron availability is also associated with other plant functions. This includes the establishment of nodules in legume roots, which requires the successful construction of cell walls and membranes around nodules following Rhizobium infection.

Cell wall and membrane integrity also affects uptake and retention of nutrients. Healthy membranes allow nutrient uptake and retain nutrients once absorbed. Adequate boron is associated with improved soil phosphorus uptake. Conversely, boron deficiency is associated with potassium leaching from excessively wetted leaves.

These characteristics, combined with the general challenges of efficiently delivering micronutrients to plants through the soil, shows how effective foliar boron can be, especially when used in combination with manganese and potassium.

The importance of foliar nutrition in soybeans

Foliar fertilization of soybeans is a critical step in achieving greater yields. In recent years, we have burnt the proverbial candle at both ends when it comes to crop micronutrients. On one end, yields have increased and with each additional kernel or bean removed from the field, we have taken away more nutrients.

On the other end of the candle, we have not returned micronutrients to the soil as quickly as in past years. That’s right, we applied micronutrients for decades, long before they became a hot issue. The micronutrients were in the manure we spread, in the rain that fell downwind from factories and even in the N-P-K fertilizer that we used to apply.

Manure is now applied to fewer acres, and the Clean Air Act has resulted in cleaner air technology that has also removed trace elements like zinc, a critical micronutrient for root development and stalk extension. And finally, in formulating more concentrated N-P-K fertilizers, we have removed the micronutrient by-products in those fertilizers.

Another concern in recent years has been that glyphosate may alter the chemistry of micronutrients inside plants and the availability of micronutrients in the soil. However, this is not true as empirical research has shown no significant differences in manganese uptake in soybeans that are glyphosate tolerant. Rather, the specific variety of beans will show different manganese uptake from soil.

Therefore, we need to add micronutrients to our soybean program – especially manganese and boron. Broadcasting these nutrients in dry form, however, makes them more difficult for plants to intercept. Banding these nutrients in the root zone improves their availability, however, in-row starters are less common in soybean production.

The most efficient and common way to feed micronutrients to plants is through the leaves. Foliar application of micronutrients to soybeans allows us to bypass chemical reactions in the soil that may reduce micronutrient availability. It also allows nutrients to enter the plant during dry conditions, when soil moisture may be too low to move micronutrients toward plants.

Dry soil conditions not only threaten micronutrient availability to plants, but nitrogen, potassium and phosphorus availability as well. Potassium deficiencies are exacerbated by low soil moisture. Add to this a general reduction of K soil test levels across the region and we now have need for supplemental K along with critical applications of Mn and Boron.

Balanced nutrition is also important in a foliar program. Single-nutrient approaches can be dangerous. Too much of a single nutrient may block the availability of other nutrients to plant cells. In addition, supplying only one nutrient will provide a short-term solution until the next most-limited nutrient slows plant growth. Remember, micronutrients are required in very small amounts, so highly-concentrated single nutrient products that look like a bargain may supply that nutrient far in excess of what the plant requires.

Among the micronutrients recommended for soybeans are manganese, which is important in chlorophyll development and enzyme activation, and boron, which promotes reproductive growth and stem branching in soybeans. Plant growth regulators can also be added to the foliar mixture to more precisely shape soybean development.

Some foliar ingredients may trigger the plant’s immune system. This trigger gives the plant the advantage of manufacturing defense proteins before the attack of fungi and bacteria, thereby reducing the time required to contain an infection.

Finally, soybean foliar products should include a basic sugar such as sucrose, which has been shown to reduce leaf burn in soybeans. It is also thought to act as a humectant, raising moisture levels at the leaf surface so nutrients stay dissolved for uptake.

Foliar nutrition is an important step in taking good soybean management programs to a higher level.

PCT | Sunrise offers three foliars developed in-house: Soy Foliar BAM, Soybean Foliar LITE and Soybean Foliar EXTRA, along with a late-season foliar, PCT Soy Finisher.

The other livestock on your farm

In a healthy soil a small sample the size of the tip of your little finger or about 1 gram of soil contains about 10 billion microbes. That is not a misprint 10 billion. Soil microorganisms can be used for the good of a plant like breaking down residue and returning nutrients to your crop but can also be present in quantities that can overwhelm a plant like pythium and fusarium root disease.

As farmers and agronomists, we are often preoccupied with managing the physical and chemical aspects of crops: soil texture, chemical composition of soil, compaction, seedbed preparation, residue management and chemicals that directly suppress weeds, pest insects and disease. We often emphasize practices that directly manipulate these properties and pay less attention to cultivating microorganisms that surround our crop and dramatically impact its growth.

Science and industry understandably recognize the relative ease of perfecting inputs that directly affect crops and pests, in contrast to complex microbial systems that respond to a myriad of different factors. However, we know that microorganisms above and below ground have profound effects on plant growth and vice versa.

The practice of introducing and nurturing beneficial microbes – plant-growth promoting rhizobacteria (PGPRs) – has increased, especially where regulations or availability limits fertilizer use or where diseases are resistant to conventional fungicides. Some farmers even refer to these microbes as their livestock as these organisms account for thousands of pounds of living biomass per acre. In the Corn Belt, we are seeing more microorganism-based products, including our in-house Progressive Crop Technology (PCT) products.

Microorganisms:
•    dissolve nutrients, making them chemically available to plants;
•    release hormones to stimulate root growth;
•    produce chelates that improve micronutrient availability;
•    physically deliver nutrients to plants;
•    provide additional sites on roots’ nitrogen-fixing nodules;
•    reduce plant stress;
•    compete with and produce antibiotics that suppress plant pathogens;
•    and stimulate the plant’s own immune system.

PGPRs make phosphorus and micronutrients more available for plant use by directly producing acids or stimulating plants to produce acids to dissolve phosphorus. PGPRs also produce siderophores, a kind of chelate that wraps around nutrients and protects them from both soil tie-up and use by pathogens.

Another benefit of PGPRs is the production of hormones that influence plant growth. Many produce IAA, a kind of auxin that encourages lateral root development. The result is a better-branched root system with more root hairs. In legume crops inoculated with nitrogen-fixing bacteria rhizobia, these additional root hairs provide more sites for nodule development. Even more nodules can be produced if the seed is inoculated with both rhizobia and a secondary bacteria that increases root hair development.

Plants inoculated with beneficial soil bacteria have shown reduced responses to heat stress. In this way, beneficial bacteria may reduce plant stress similar to strobulurin fungicides such as Headline and Stratego.

PGPRs also can reduce disease in at least four ways:
1. Crowding – Crowding root zones and leaf surfaces with good bacteria, they reduce sites where pathogens establish.
2. Competition – Beneficial bacteria steal nutrients from pathogens.
3. Antibiotics – Many beneficial bacteria produce their own antibiotics to fight off pathogens.
4. Immune Stimulation – Beneficial bacteria stimulate the plant’s own immune system to defend against disease.

At PCT | Sunrise, we have included beneficial microorganisms in our starter and foliar fertilizers for years. We comb through dozens of peer-reviewed university studies from around the world to identify which species are best to include in our products. Organisms are selected for biological benefits, compatibility with other ingredients and shelf-life in our products. While our exact species are trade secrets, we select spore-forming bacteria that are able to quickly enter dormancy under adverse conditions and germinate as conditions improve.

Beneficial bacteria/PGPR technology is here to stay. In the face of increasing regulations and pest resistance, they represent a win-win technology for growers who seek to improve yield and conserve the environment. Emerging technologies to identify and engineer these organisms will continue their advance in agriculture.