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The Role of Soil pH in Phosphorus Dynamics

Soil pH dictates various aspects of soil fertility. For phosphorus, soil pH impacts the chemical form present in the soil. As soil pH increases, the concentration of hydrogen ions decreases; likewise, the number of hydrogens associated with phosphate decreases.

Iron Fixation in Highly Acidic Soils

At soil pH levels below 3.0–4.0, the predominant form of phosphate in the soil is H₃PO₄. This form is not plant-available and has a high chemical reactivity with iron. This low pH also greatly increases the water-soluble of iron, which creates ideal conditions for the two to bond.

·        Oxidized Iron Soils (Red Clay): If the iron is in an oxidized state, the bond is very strong. This creates an insoluble mineral that has extremely low solubility and plant availability. Soil pH adjustments have little to no effect on releasing phosphorus bonded to oxidized iron.

·        Reduced Iron Soils (Grey Clay): If the iron is in a reduced state, the bond is also very strong but maintains a low level of water-solubility.

Aluminum Interactions and H₂PO₄^- Availability

As soil pH increases, the predominance of H₂PO₄^- increases, which is plant-available. The concentration of H₂PO₄^-starts at a soil pH of 3.5–4.0, peaks at a soil pH of 5.5–6.0, and ends at pH levels above 6.5–7.0. This form has limited reactivity with iron but is reactive with aluminum. Aluminum water-soluble occurs at soil pH levels below 5.0–5.5. The peaks of availability/ water-soluble between H₂PO₄^- and aluminum do not quite align. Additionally, as aluminum is included with iron, or replaces iron in the reaction, the water-soluble and plant availability of the resulting mineral increases slightly.

The "Sweet Spot" for Phosphorus Availability

Between pH 6.0–7.5, phosphorus exists as either H₂PO₄^- or HPO₄^2- _, both of which are plant-available, and the reactive partners of iron and aluminum are not water-soluble. This results in the optimal pH for plant-available phosphorus, with the lowest rate and severity of fixation due to the formation of insoluble minerals.

Alkaline Soils and Calcium Fixation

It is not until soil pH levels rise above 7.5 that HPO₄^2- is the prevalent form and excessive calcium drives the reaction of phosphorus with calcium. High soil pH is caused by the over-application of lime or naturally high calcium carbonate content in the soil. Because high soil pH and high calcium levels are strongly correlated, calcium is often confused with soil pH. The mineral formed in this environment between calcium and phosphorus closely resembles the rock phosphate mined for fertilizer production, which has low water-solubility. The amount of phosphorus participating in this reaction continues to increase as soil pH increases.  However, calcium phosphate minerals are more water-soluble than the minerals formed at a low soil pH. Like the mined mineral, the bond between calcium and phosphate can be broken by acidifying the material, leading to a significant increase in water-solubility. Acidic root exudates are effective at breaking this bond, leading to increased plant availability.

Comparison of Phosphorus Fixation by Soil Environment

How to Dial in a Nitrogen Rate for the Upcoming Corn Crop

The current economic conditions for corn producers require making wise decisions when it comes to crop inputs. One of the higher costs associated with a corn crop is nitrogen (N) fertilizer. Knowing exactly how much you need to purchase can help lock in better prices with early purchase options before the prices are likely to go up as the growing season approaches. Here is a list of things to consider when determining an appropriate N application rate.

Potential Yield – Determine a realistic yield for your operation. This is probably not the year to aim for record yields. Use the average of the last 5 to 10 years of actual yield averages, not just the average you hope for. Consult your seed company agronomist to see how different varieties have yielded in local plot trials.

Nitrogen Use Efficiency – This is the amount of N it takes to produce 1 bushel of corn. Every bushel of corn contains approximately 0.67 pounds of N. However, a corn crop takes up approximately 1.0 pounds of N for every bushel produced. If N can only be applied before planting, generally 1.2 to 1.4 pounds of N per bushel should be applied to account for the greater risk of loss. In a typical system with starter N and sidedress, aim for 1.0 to 1.1 pounds per bushel. If you have the option for late season applications (VT and later), it is possible to reduce rates to 0.7 to 0.9 pounds per bushel.

Maximum Return to Nitrogen (MRTN) – This is a model developed by multiple universities using N response data to calculate the economic optimum N rate for your situation. You begin by selecting your region then entering your expected price per bushel and your cost per pound of N. This model is accessible at https://www.cornnratecalc.org/.

Estimated Nitrogen Release (ENR) – You can use the organic matter from your routine soil tests to help reduce your N application rate. For every 1% organic matter, you can estimate that approximately 20-40 pounds of N will be mineralized or naturally released by the microbes in the soil. However, this is heavily dependent on the weather. So, it is advisable to stay on the lower end of the range.

Presidedress Soil Nitrate Test (PSNT) – Collect a soil test prior to a sidedress application to see how much nitrate has been mineralized by the organic matter. For sampling instructions and data interpretations please see our fact sheet, PSNT for Corn.

Soil Compaction Fundamentals

Soil compaction is often associated with its physical properties. It is when soil particles are pressed together and pore space is decreased. Pore space can account for fifty percent depending on soil type. This can be physically altered through natural and mechanical influences.

In the pore spaces of soil, water and air are in a constant back and forth balance. As soil solution increases due to precipitation weather events or capillary action, there is less air present in pore space. Contradicting this, the soil dries from lack of precipitation and more air is present. Water infiltration and capillary action are affected by soil type and soil compaction.

There are soil types that naturally are more resistant to compaction. The higher the sand content, usually, the less compaction occurs. Soils with more clay tend to compact more and further in depth. They have a higher water holding capacity, smaller pore space and tighter particle bonds.

Compaction can occur at various levels in the soil profile. Tillage practices can influence many compaction points, but on the soil surface it experiences multiple situations. How can some no-till fields have such a hard top layer? Heavy rain events cause lots of surface compaction. What can make this worse is a seedbed preparation tillage pass before such event. This will cause crusting of the soil surface with little pore spacing for germinated seedlings to emerge.

Each pass in the field, whether it be from machine or foot, compresses the soil limiting pore space and compacting as well. A tool to help measure these actions is a penetrometer. It is a solid probe with an indicator dial on top that is pressed into the soil. As it travels through the profile, the needle on the dial will show what the PSI is at the probe tip ranging from 0-300+. Using a ¾ inch tip, 0-200 is considered optimal, 200-300 roots are restricted and anything over 300 is very compacted.

Plow layers, or subsurface compaction, is caused by smearing of the soil and done on a routine basis. These can usually be found around 7-9” deep depending on the region and tools used. These are also mistaken for soil horizon changes. Such as Topsoil A Horizon, to Subsoil B Horizon as soil changes from higher organic matter to structureless massive soils with an anerobic environment.

To manage compaction, it starts with limiting soil surface exposure. Leaving residue or practicing minimal tillage. Not applying too much down pressure with the planter gauge wheels, proper tire inflation or the use of tracks, and not disturbing the soil when field conditions are marginal to saturated.

SoilTrak Rides off Into the Sunset

Last September, the ALGL Client Portal was introduced to provide laboratory business account holders with single sign-in access to existing lab tools, alongside new and enhanced features designed to make doing business with us more convenient. 

eSub, located within the ALGL Client Portal, allows for the online creation of soil submission forms and bag labels. This feature replaces SoilTrak with a cloud-based version that is no longer limited to a single PC. Because eSub and eDocs are integrated, data from past sampling events—including growers, farms, and fields—is readily available for new submissions. Future updates will expand these tools to include additional materials.

After May 31, 2026, ALGL will cease technical support, software updates, and new installations for SoilTrak. While current installations will remain functional, they will no longer be supported.

Plan a Budget and Stick to It

It is no secret that the current agricultural economy is not doing producers any favors as another growing season approaches and crop input decisions need to be made soon. In order to survive these challenging times, it imperative that growers have a firm understanding of their input costs and build a plan to maximize their return on investments and stick to it.

To begin, all input costs need to be identified and divided into two categories, set costs and flexible costs. Set costs are things such as land/rent payments, taxes, interest on borrowed money, equipment costs, etc. There is little to nothing that can be done to lower these costs, so any savings need to be made on other inputs.

The first decision is what crop should be grown? While most producers will decide between corn or soybeans, this may be an opportunity to explore other crops such as wheat or other small cereal grains if there is a market for them in your area. It is important though to consider all impacts of trying a new crop such as equipment compatibility, access to crop protection chemicals, and transportation.

What seed variety should be grown? Work with your local seed agronomists to determine what varieties perform well in your area. While yield is important, the highest yielding varieties may not be the most profitable because they often require additional inputs to produce the highest yield. Look for varieties with a good record of resistance to pests and diseases that are known in your area and don’t pay for traits that are not needed.

Crop protection inputs (herbicides, fungicides, insecticides, etc.) need to be planned and budgeted in advance. This is where previous years of crop scouting can really help. You can generally assume the same weeds and diseases that were a problem over the last few years will likely be again. Budget for a worst-case scenario and use crops scouting and integrated pest management strategies to determine when an application is needed to get the most benefit from the application.

Soil fertility inputs need to be determined from recent soil test data. Ideally less than two years old. Identify what nutrient or other factor is the most limiting. If your pH needs to be raised, liming should take priority over fertilizer. pH determines nutrient availability. So, if the pH is off, the efficacy of your fertilizers is jeopardized. Determine if phosphorus or potassium is more limiting. Phosphorus fertilizers are much more expensive currently. So, at a minimum try to maintain phosphorus levels rather than build them up and certainly avoid applications where soil tests are high. Potassium fertilizer is currently more affordable and potassium soil test levels can drop much more quickly than phosphorus. If the budget only allows for phosphorus or potassium, prioritize the potassium.

The two most important things a grower can do in these tight times is first, do not try to do it all on your own. Rely on advice from industry agronomists, independent consultants, and grain marketing specialists. Second, stick to your plan. Don’t make any rash decisions that could lead to a complete crop failure. Don’t look for a silver bullet, there is no single product or practice that is going to save a poorly managed crop regardless of how good the sales pitch is.

Potassium Availability

Certain plant nutrients must go through biological, chemical and physical cycles to become plant available.  The potassium cycle is much simpler.  Many mineral soils contain a surplus of potassium.  However, much of this is not in the soil solution but held tightly in parent materials like micas and feldspars.  For these to become plant accessible, they must first weather.  As they naturally break down, potassium on the outside edges goes from nonexchangeable to available forms in the soil solution for plants to utilize.

Potassium, being a macronutrient, is taken into the plant in rather large amounts.  This amount can vary from 5-10 times the amount of phosphorus.  As vegetation begins to wither and die, also known as necrosis, potassium is released from the plant back to the soil.  Unlike other residue cycling, K is readily available through this process.

There are four key locations for K in the soil.  In primary mineral structure (unavailable), nonexchangeable K in secondary minerals (slowly available), Exchangeable K on soil colloids and K soluble in water (readily available).  This makes 90-98% of all soil K to be unavailable to plants.  As mentioned above, weathering is the solvent action of carbonic, organic and inorganic acids as well as acid clays and humus.  Weathering must occur to make exchangeable K from parent materials.  Some plants with finer root structures can access this nutrient between clay layers, but many row crops cannot.  The amount of K fixed depends on the nature of the soil colloids, wetting and drying, freezing and thawing and the presence of excess lime.

There are soil types that are difficult to change soil test K values.  A few clay types are Illite (mica-type clay), vermiculite, smectite and kaolinite.  Illite (Drummer, Flanagan, Sable) has the highest K content of common clays. K sits between the layers and is slowly released and has a good long-term K supply.  It can also fix K if soils become dry or compacted. Vermiculite (Graymont, Alida, Iva) has a high CEC and can hold lots of potassium.  It also has a higher fixing capacity than Smectite. Smectite (Patton, Wabash, Toledo) has a high CEC but less inherent K.  This type is a great reservoir for soil K but not a great direct source.  Kaolinite (Bluford, Berks, Gilpin) is a 1:1 clay.  It has a low CEC, poor K retention and needs frequent K fertilization to meet crop demand. 

Potassium cycling is a dynamic equilibrium.  As soon as the plant takes in K, more is released back into the soil solution from exchangeable K.  A plant may take up more K than necessary, called luxury consumption, and this does not directly increase yield.  Potassium applications vary greatly on crop type, soil type, region and residue management.  If the soil test K levels cannot be achieved because of fixation, pH and other soil influences then applications may need to be more frequent to keep up with crop demand.

Source: Brady, N. C., & Weil, R. R. (2016). The nature and properties of soils (13th ed.). Pearson Education.

The Good, the Bad and the Ugly of Wood Ash

In tight times it is common for growers to look to waste products as a source of nutrients. The products are also commonly marketed to growers.

Wood ash is often promoted primarily as a source of carbon and potash. It is often a good source of potassium, calcium, and carbon with a notable amount of phosphorus. The calcium is often calcium oxide which is powerful liming agent. The liming properties of wood ash are usually where problems can arise if not managed correctly.

While wood ash is a good source of carbon, it comes with a high C:N ratio. The more hardwood burnt to create the ash, the higher the carbon content. C:N ratios over 30:1 can become a nitrogen sink as microbes process the material. This can lead to nitrogen deficiencies in crops if not managed properly.

One ton of wood ash can have the same neutralizing capability of ½ ton of calcium or magnesium carbonate based ag lime. The concern arises when repeated applications of wood ash are made to meet a potassium nutrient recommendation without consideration for the liming potential of the material. Especially on high pH soils.

Excessive applications of calcium or magnesium carbonate based ag lime will result in a soil pH of 8.1 to 8.2. These soil pH levels can lead to a variety of challenges. The acid neutralization reactions of carbonate products stall at this pH. Unreacted carbonates will remain in the soil to neutralize any acid additions that maybe made to the soil. When soil reaches this point, it is very difficult to impossible to lower the soil pH. This is the main reason excessive lime applications are discouraged. Oxides, like those found in wood ash, can elevate soil pH into the low 9’s before stalling.

Whenever applying word ash, test the Calcium Carbonate Equivalent (CCE) of the material to compare to calcium or magnesium carbonate based ag lime application rates, avoid excessive or repeated applications, and monitor soil pH closely after application. If you have any questions about wood ash or other byproducts reach out to your regional ALGL agronomist.

Phosphorus Rate Reductions and World Demand Growth

Management of high phosphorus fertilizer prices has been common topic in the fall of 2024 and 2025. The elevated phosphate prices began in 2021, the depressed grain markets in 2024 and 2025 that drove the widespread cost cutting. A common approach has been to cut rates. More information on how to effectively reduce applications rates while limiting negative impacts can be found in a previous ALGL blog post “The Seven Most Expensive Words in Agriculture – Fertilizer Prices”.

Many rate reduction strategies implemented in the past two falls have been under the assumption that the elevated phosphorus prices will relax relatively quickly. Most fertilizer price spikes do not last very long and impact roughly two growing seasons. If this assumption is part of your phosphorus plan, a modification of the plan may be in order.

The drivers of this phosphate price elevation are a bit different than past price events that usually are a result of short-term supply interruptions or seasonal demand spikes. World demand for MAP is growing significantly as South American and Asia are looking to increase domestic grain production which has led to a significant/large increase in world phosphate demand over the past 4 years. The Asian demand increase has been driven primarily by China. China has significantly reduced phosphate exports to focus on domestic demand, thus lowering the world phosphate supply. It is foretasted that world demand should level off in 2026 with no indications of decline.

There are new sources coming online, however market forecasts are starting to project that it will be 2028 or 2029 before the demand and supply can align allowing phosphate prices to relax.

While many of us initially assumed that the high phosphate prices would necessitate reduced application volumes for 1-2 growing season based on application timing, high phosphate prices may be reality though the 2028/2029 growing season. Can your current plan support significant phosphate reduction until 2028 or 2029 without negative future impacts?

The base defense against negative impacts of altering your fertility management is routine and consistent soil sampling to monitor soil fertility changes along with tissue testing to monitor crop access to fertility. Reach out to your ALGL Regional Agronomist for help with soil fertility questions.

Potassium Fertilizer and Nitrogen Use Efficiency

When managing soil nutrient sufficiency ranges, most are attempting to keep each nutrient above the critical and within the adequate range.  This, of course, depends on the soil, region, crops being grown etc.  Sufficiency ranges have been developed over time through field calibration research, linking crop yield/response to soil test levels.  From these crop responses, different soil test ranges have been made insufficient, sufficient or excessive.  This has been the standard to determine a crop’s overall needs, but what has been studied further is the relationships between different nutrients.

There are many cause and effect relationships between soil nutrients.  Too much calcium, for example, can affect the phosphorus availability in certain soils.  Soil pH is the determining factor for nutrient availability etc.  A connection between potassium and nitrogen is not often discussed, and perhaps because there is present-day data supporting this.  Unlike calcium and phosphorus availability, potassium and nitrogen are more about aiding uptake.

Potassium (K) improves nitrogen use efficiency in a few ways.  The first is potassium contributes to root elongation.  Better root growth promotes contact with nitrates and ammonium in the soil for plant uptake.  Nitrogen is primarily brought into the plant through a process called mass flow.  Mass flow is regulated by plant transpiration. This is the movement of dissolved nutrients.  As the plant transpires, more soil solution is being brought into the plant through the root system.  Transpiration can have a large effect on nutrient uptake.  It requires soil moisture to regulate plant temperature, and water/nutrient transport.  Temperature, humidity, light and wind all play a part in the rate of transpiration.

Potassium is the primary ion controlling transpiration and plant nitrogen mobility.  Low K can cause nitrogen to accumulate in older tissues instead of moving to newer growth.  It controls the stomata movement on the leaf surface.  Stomata are the pores regulating water and gas exchange.  When stomata open, potassium ions move into the guard cells and vice versa when they close.  Without proper plant K levels, which can be greatly affected by soil moisture and clay type, the mass flow of nutrients such as nitrogen are inhibited. 

The fact is that growing organisms have a balance amongst themselves.  Yes, insufficient nutrient content can raise a plant but keeping them in a constant balance is what increases the efficiency of others for maximum yield potential.  Constantly maintaining a sufficient range of nutrients is the first step and the next is to learn the cause and effect of nutrient levels. 

Continued Challenges and Solutions in 2025

Editorial by Agronomist Jamie Bultemeier

Looking back on the past few years, a common trend is the challenges that arise within a growing season. One unique aspect to the challenges of 2025 has been that many of the challenges are continuations from 2024 while other challenges will linger beyond 2025.

Much of our service area was experiencing low level drought in 2024 that did not fully resolve through the winter leading into 2025. While rainfall may have been similar in 2025 as 2024, the lack of subsurface moisture added to the moisture stress in 2025. Dry weather can reduce nutrient loss and leaching. Dry weather also reduces nutrient uptake by plants. Low tissue test levels of potassium were a common sight at the lab though the summer months.

Our position on soil sampling in abnormally dry soil has been to first ensure that you are able to sample to correct depth. Incorrect sampling depth can bias data regardless of the chemistry impact of dry soil on lab methods. Dry soil normally leads to shallow samples with an increase in soil test data values. If correct depth can be achieved, chemical impact of the soil test results from the lab appear when a prolonged drought reaches the late D2 to early D3 status. There were indications of samples exhibiting significant soil pH and potassium level reductions in areas of extended D2 to D3 drought. Fortunately, this was again limited to a small portion of our service area again this year and the severity of the drought did not build until the later part of the fall soil sampling season.  One concern for 2026 will be the continuation of the drought. Near term precipitation forecasts indicate that drought conditions will persist in the coming winter.

The dry fall weather drove a near record harvest pace again this fall that tied 2024. Most sampling seasons have a weather event or multiple events that slows soil sampling in a significant portion of the ALGL service area. This reduces inbound soil sample volume overall and provides periods of time to catch up. There was no such event during the fall of 2024 or 2025. This in combination with record fall soil sample volumes led to increased inbound soil samples above our daily capacity without an opportunity to catch up. Our dedicated staff worked repeated Saturdays, which led to a record number of samples processed in one month, almost 10% more samples in a single month than the previous record.

2025 also marks the second year of high phosphorus fertilizer prices and elevated nitrogen prices. Many conversions between lab agronomists and clients have focused on effective ways to reduce phosphorus and nitrogen rates. Trends in the fertilizer supply indicate that nitrogen prices will remain elevated into 2026 and high phosphorus prices could endure until 2028. Some of the short-term reduction strategies for phosphorus may need to be rethought as it is likely that phosphorus prices to remain high longer than many have anticipated.