Now that harvest is underway for many, we are gaining insights into how our 2024 crop will perform. As we go through harvest, it may be worth the time to keep a notebook in the cab of the combine and, if nothing else, record a few thoughts on the field performance of any items that worked well or areas for improvement.

Phosphorus and Potassium Applications

One item I know that may be on the minds of many growers is low grain prices and high input prices, such as fertilizer. It may be a natural reaction that, in times of high fertilizer prices, to cut back for the 2025 growing season. This may be an appropriate item to consider for some fields, but it may come with some risk for the following year’s crop, depending on current soil test levels and field conditions next year. If soil test levels currently exceed 25-30 ppm for Phosphorus (P) and 150-175 ppm for Potassium (K), especially for soils classified as medium to heavy-textured soils, it may pose a low risk for yield loss in 2025. However, this reward of investing in soil fertility and building soil test levels does not come without some risk if soil fertility rates are reduced for the 2025 growing season. The main concern is root growth and if it becomes restricted for any reason next year, it may translate into nutrient deficiencies even though soil fertility may be adequate. Many of the nutrient deficiencies I witnessed this year, not including nitrogen, generally fall into the following two categories:

1. Restricted Root Growth—Generally, some compaction was generated early in the season that prevented normal root growth. Examples may include soil compaction generated from either tillage or planting equipment, resulting in sub-surface compaction layers and sidewall compaction in many cases. Evidence of restricted root growth may be visible as “Tomahawk Roots,” pancaking of plant roots or horizontal root growth.
2. Unrealized need for key nutrients such as sulfur (S), boron (B), and zinc (Zn). Until recent years, soil organic matter and managing soil pH to a range of 6.2 – 6.7 have been key to providing crops with these key nutrients. However, with the acceleration of modern hybrids and varieties, along with clean air acts, soils can’t keep up with these demands, and the need to supply these nutrients may be needed.

Photos provided by Eric Beckett, Illini FS

Field conditions that promote restricted root growth should be avoided. Plant roots generally take up nutrients through at least one mechanism, but often more than one process such as:

1. Mass Flow – Movement of dissolved nutrients into the plant as it takes up water for transpiration. Most all nutrients are taken up in this manner except phosphorus.
2. Diffusion – Movement of nutrients to the root interception in response to a concentration gradient from high concentration to low concentration to form an equilibrium. This is key for P, K, S, Fe (Iron), and Zn.
3. Root Interception – Root growth must occur to intercept soil colloids that contain nutrients for absorption. This is a key pathway for calcium and magnesium but minor for other nutrients taken up in this pathway. Calcium, Magnesium, Iron, Manganese, and Zinc may all be taken up through root interception.

Photos provided by Eric Beckett, Illini FS

In conclusion, anything that restricts root growth or movement of roots and the flow of water throughout the soil will also prevent the movement or interception of key plant nutrients. A key example I witnessed many times this year, from in-season nutrient testing to field scouting, was potassium. Every time that I witnessed potassium foliar deficiency symptoms this year, soil test levels were adequate. Restricted root growth of some sort was the root cause of potassium not making its way into the plant. If soil fertility is reduced in 2024 and we have another wet spring, real nutrient deficiencies may be observed later into the season as plant roots cannot uptake adequate amounts of potassium and other essential plant nutrients.

Stay tuned for part two where I discuss weed control and considerations for 2025.