Agronomy Update: Achieving Adequate Soil Fertility

ENVIRONMENTAL CROP STRESS AND THE PRIMARY AND SECONDARY NUTRIENTS

Adequate soil fertility is a goal that we should all try to achieve.  Adequate fertility is most important in years that are experiencing environmental crop stress. Growing seasons that are similar to what many growers experienced in 2020 such as too hot, too dry, too cool and too wet play havoc on a crop. Crops survive those environmental extremes best when fertility is adequate and balanced. Most soils should be soil tested at least every three to five years; however, soils with a C.E.C. (Cation Exchange Capacity) of less than eight require special attention and should be soil tested at least every two to three years.

Good fertility levels of Nitrogen (N), Potassium (K), Calcium (Ca), Magnesium (Mg) and Sulfur (S) are important for all soils but are essential on the usually stressed light textured low CEC soils.  Ideal soil levels of these nutrients on soils with a CEC of less than eight greatly enhances root growth, adds to stalk quality, reduces the impact of disease, improves the tolerance to drought and improves yield. This past growing season, I saw multiple situations where better fertility would have improved the performance of alfalfa, corn and soybeans. Inadequate fertility levels greatly magnify the effect of drought conditions.

Soils with CEC’s of less than eight are unique, since they have less ability to hold certain nutrients that all crops require for proper growth and development. N, K are primary nutrients. Ca, Mg and S are secondary nutrients essential for healthy crops that lower CEC soils cannot hold and are prone to leaching. Proper fertility management for lower CEC soils requires spoon feeding N, K, Ca, Mg or S annually or semi-annually based on soil test results, yield goals and the crop grown.

On all soils it is important to have adequate percent base saturation levels of Ca, Mg and K. On soils with a CEC of eight or less, it may require annual or semi-annual applications of these nutrients to maintain proper percentages.

Ideal % Base Saturation for balanced fertility:

• Ca – 60-75%
• Mg – 10- 15%          
• K – 2-5%

Too much or too little of the percent base saturation of the above nutrients may cause other nutrients to be unavailable and cause crop performance issues.

Most soils with a CEC of less than eight have low organic matter content. N and S are major nutrient components of organic matter and are usually at lower levels on low CEC soils.  In addition, N and S are subject to leaching on the lower CEC soils. To maintain adequate N and S levels, it will require annual applications based on yield goals.

Experience shows that adequate fertility on all soils is most important when there are growing seasons with environmental crop stress. All soils cannot be treated equally!      

PLANT NUTRITION AND SOIL FERTILITY

Sometimes it’s good to rediscover some of the reasons why we do what we do in the field. The following definitions and topics are important to keep in mind as you develop and execute on your fertility plans this fall.

Important Soil Properties for Fertility

Soil pH: soil pH is king, influencing all other nutrients and biology in the soil. pH measures the acidity or alkalinity of soil. Acidic soils have a higher concentration of H⁺ ions; identified with lower pH, such as 5 or 6. Alkaline soils have a lower concentration of H⁺ ions and more OH^- ions. Alkalinity is identified by a pH above 7 (7 being neutral). The pH scale is logarithmic, so a pH of 5 has a concentration of H⁺ that is 10x higher than a pH of 6. The target pH for most crops is near 6.0, with legumes being closer to 6.8-7.0.

Cation Exchange Capacity (CEC): Cations are positively charged ions in the soil. Exchangeable cations are those which easily move between soil particles and plant roots (both of which are negatively charged). Roots release CO₂, which reacts with water to release a H⁺ cation into the soil. This H⁺ can bump another cation from a soil particle to be taken up by a plant. Ammonium (NH₄⁺), potassium (K⁺), calcium (Ca⁺⁺) and magnesium (Mg⁺⁺) are almost exclusively used by plants in this way. CEC is the measure of cations that a soil can hold. Finer soil particles such as clays and organic matter have more negatively charged binding sites and therefore higher CECs than coarse soils.

Base Saturation: The amount of the total CEC that is occupied by base cations such as K⁺, Ca⁺⁺ and Mg⁺⁺. Base saturation can be measured for each cation and should be balanced appropriately.

Essential Elements for Plant Growth

Structural Nutrients: Carbon, Hydrogen and Oxygen from air and water and make up 95% of the plant.

Primary Nutrients:

• The Nitrogen cycle in the environment is very complex and warrants a lot of our attention as agronomists and producers. It is worth studying in-depth, but that is for another time. Nitrogen is available to plants as nitrate (NO₃^-) and ammonium (NH₄⁺). It is a primary component of proteins in the plant, but is also important for enzymes and chlorophyl.
• Phosphorous is very immobile in the soil due to its ability to quickly react with iron, aluminum and manganese in acidic soils and calcium in alkaline soils. With good soil pH management, this immobile phosphorous acts as a ‘bank’ to constantly replenish plant-available phosphorous. Plants use phosphorous for cell division and transportation of energy, making it a key nutrient for growth, flowering, grain fill and water uptake.
• Potassium, an exchangeable cation, is needed for charge balance in plants. It is important for many metabolic processed and nutrient transfers. Potassium is key for improved disease resistance and stalk strength in crops.
• Secondary Nutrients:
  Calcium is another exchangeable cation, important for cell structures and growth. Most soils have a large supply of calcium, so deficiencies are rare for most geographies of the Midwest.
• Magnesium is an exchangeable cation that serves as an enzyme activator and is a component of chlorophyl. Deficiencies are most common on sandy soils (low CEC) that have high amounts of potassium and ammonium inputs.
• Sulfur plays a key role in converting nitrate into organic nitrogen within the plant, hence the overall yellowing of sulfur deficient plants. Sulfur deficient crops also tend to accumulate nitrate for this reason. As high protein crops, legumes have an especially high need for sulfur.

Micronutrients:
Micronutrients can be managed on an as-needed basis after we see symptoms or build a history of known issues. A regular micronutrient program is not generally recommended. Focus on any corrections to soil pH as the first strategy.

• Boron is perhaps most important from a management standpoint. Needed for cell division and reproduction, boron deficiency can have a big impact relative to the amount that is needed.
• Zinc has multiple metabolic functions in the plant. Like boron, it too has a large impact relative to the quantity needed. Watch for zinc deficiency on sandy soils that have a pH higher than 6.5.
• Manganese is an important enzyme activator for photosynthesis. Its availability becomes so high to plants in acidic soils that it’s a big reason why we lime alfalfa crops to a pH of near 7.0.
• Iron deficiency is very rare with the exception of non-field crops and on very alkaline soils. Iron is important for the production of chlorophyl.
• Sometimes we hear about Molybdenum applications to legumes because of its importance to nitrogen fixation, but again we see this generally solved by proper liming. Rates are so low that only foliar applications are practical.
• Copper, Nickel and Chlorine are also essential nutrients for plant growth, but generally don’t need management other than to watch for toxicity.

Corteva Technology Use Agreements

All growers with orders for any Corteva Agriscience brand seed product, regardless of crop or trait (including non-GM products) need to have a signed Corteva Technology Use Agreement in place by September 1. Growers should sign the Corteva Technology Use Agreement electronically at www.agcelerate.com. Signing electronically is preferable, however, paper copies are available at www.traitstewardship.com.

Brian Weller Western Region 507.456.3034	Dan Ritter Central Region 219.863.0583	Branden Furseth Northern Region 608.513.4265	Rod King Eastern Region 574.596.6721	Terry Jones Eastern Region 419.630.3115

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