September Forage Soil Testing: When Low Magnesium Changes the Potash Decision
September is a good time to look beyond potassium when reviewing the fertility of hayfields and pastures. After a summer of repeated cutting, grazing, heat, and uneven rainfall, many forage fields have removed a substantial amount of potassium from the soil. That naturally puts potash near the top of the fall fertility discussion, but potassium should not be evaluated by itself when soil magnesium is marginal or when sulfur supply may also be limiting.
This matters because forage production places unusual pressure on the soil nutrient budget. Grain systems leave large amounts of stalks and leaves behind, allowing much of the potassium in vegetative tissue to cycle back into the soil. Hay harvest removes those stems and leaves from the field repeatedly, which means potassium, magnesium, sulfur, and other nutrients leave with every cutting. A productive forage field can therefore develop multiple nutrient shortages at the same time, particularly when fertilizer replacement has focused primarily on nitrogen or when soil testing has not been updated for several years.
The solution is not to assume that every forage field needs a fertilizer containing more nutrients. A three-nutrient fertilizer only becomes useful when the field actually needs the three nutrients it supplies. September soil testing, crop-removal history, stand condition, soil texture, pH, manure history, and forage use should determine whether potassium, magnesium, and sulfur need to be managed together or whether a simpler fertility program would be more economical.
For fields where all three nutrients deserve attention, a potassium-magnesium-sulfur source can simplify the program. Where magnesium is already adequate, however, adding more Mg simply because it comes in the bag may provide little value. The same reasoning applies to sulfur. The goal is not to apply the fertilizer with the longest nutrient list; it is to match the fertilizer analysis to the limitations that are actually present in the field.
Repeated Hay Harvest Can Change More Than the Potassium Number
Hay removal is often discussed primarily as a potassium issue because forage crops can remove large quantities of K, but the same harvest also exports magnesium and sulfur. When that cycle is repeated several times during a productive year, the nutrient balance of the field can change faster than it would under a system where most plant residue is returned to the soil.
This is especially important in high-yielding alfalfa, bermudagrass, tall fescue hay, orchardgrass, and mixed-forage systems. A field that produced several strong cuttings may have supported excellent summer growth while drawing heavily from existing soil reserves. The crop can look productive right up until those reserves approach a level where another season begins exposing a deficiency.
September provides a useful opportunity to evaluate that drawdown because much of the season’s tonnage is already known. Farmers can compare actual harvest with previous soil-test results and decide whether the nutrient program has been keeping pace. A field that has produced more forage than expected for several years may deserve closer attention even if obvious deficiency symptoms were not widespread during the summer.
Crop removal should still be treated as context rather than as the fertilizer recommendation itself. The amount of nutrient leaving in hay helps explain why soil-test levels may be falling, but the soil test and regional recommendations determine whether fertilizer is likely to produce an economic response.
Potassium and Magnesium Need to Be Considered Together on Some Forage Soils
Potassium and magnesium are both positively charged plant nutrients, and very high potassium availability can interfere with magnesium uptake in certain situations. That interaction becomes particularly important when the soil is already marginal or deficient in Mg rather than when both nutrients are present at adequate levels.
Penn State’s forage fertility guidance notes that high soil potassium can depress magnesium uptake by forage grasses. This may not always create an obvious yield penalty, but it can influence forage mineral composition and becomes especially important when pasture is the primary feed source for cattle.
The practical lesson is not that potassium fertilizer should be avoided. A low-K forage field still needs potassium, and failing to correct a genuine K deficiency can reduce yield, persistence, and stress tolerance. The better lesson is that a field testing low in magnesium deserves a different potassium discussion from a field with a strong Mg reserve.
A farmer who sees low soil-test K and low Mg should therefore resist the temptation to address only the potassium number because potash has historically been the larger forage expense. Correcting one deficiency while leaving another unresolved may still limit crop performance, and applying very large amounts of K to an Mg-deficient soil can make the nutrient balance more difficult for the plant.
High Potassium Should Not Be Managed by Chasing a Perfect Soil Ratio
The relationship between potassium, magnesium, and calcium sometimes leads growers toward complicated base-saturation ratios or claims that every soil must maintain one ideal balance among the three cations. That is not necessary for most crop fertility decisions.
The stronger approach is to use calibrated soil-test sufficiency ranges and crop recommendations. If potassium tests low, correct potassium. If magnesium also tests low, address magnesium. If both are adequate, there is little reason to purchase additional fertilizer simply to chase a theoretical ratio.
Missouri Extension does recognize that excessive potassium can suppress magnesium uptake, particularly on low-CEC or coarse-textured soils, but that concern is strongest where the system is already vulnerable to Mg deficiency. The existence of nutrient competition does not mean every field needs to be managed toward a precise mathematical K-to-Mg ratio.
That distinction protects farmers from replacing one oversimplification with another. Applying potassium automatically because hay was harvested is too simplistic, but so is withholding needed potassium because someone claims the soil has the “wrong ratio.” The field should be managed from actual soil-test categories, crop response research, and the production goals of the forage system.
Magnesium Deficiency Has Its Own Field Pattern
Magnesium is part of the chlorophyll molecule, so deficiency commonly appears as loss of green color between leaf veins. Because magnesium is mobile within the plant, the crop can move Mg from older tissue toward younger growth when supply becomes inadequate, which means symptoms typically become visible first on older leaves.
Mississippi State describes magnesium deficiency in corn as interveinal striping or yellowing that is more pronounced on older leaves, while sulfur deficiency tends to appear on younger tissue. The same nutrient-mobility principle is useful when diagnosing many forage and crop species.
Visual symptoms are useful clues, but they should not be treated as a fertilizer prescription by themselves. Low pH, poor roots, saturated soil, drought, disease, or a damaged stand can also affect nutrient uptake and leaf color. A field showing suspected magnesium deficiency should be compared with a healthy area, and soil testing should be used to determine whether the root zone is actually low in Mg.
The pH result is particularly important because it can change which magnesium source makes the most sense. A field that is both acidic and low in Mg may need an entirely different correction strategy from a field that has an appropriate pH but inadequate magnesium.
If the Soil Needs Lime and Magnesium, Dolomitic Limestone May Be the Better First Tool
A multi-nutrient fertilizer is not automatically the best way to correct low magnesium. When the soil also requires lime, a magnesium-containing liming material can sometimes correct acidity and Mg deficiency in the same application.
Penn State’s forage fertility guidance specifically notes that magnesium recommendations may be addressed through a magnesium fertilizer or through an appropriate magnesium-containing limestone when lime is needed. UConn Extension makes a similar distinction for vegetable soils, recommending magnesium-containing limestone when pH correction is required and a fertilizer Mg source when no lime is needed.
This is an important September decision because fall is a good time to correct soil acidity. If the soil test shows low pH and low magnesium, applying a K-Mg-S fertilizer while ignoring the lime requirement may leave the larger root-zone problem unresolved. Dolomitic or another properly analyzed magnesium limestone may provide a more logical foundation, with potassium and sulfur managed separately according to their actual requirements.
If pH is already where it should be, applying lime simply to obtain magnesium could create the opposite problem by raising pH unnecessarily. Under those conditions, a fertilizer source containing Mg becomes much easier to justify.
The fertilizer choice should therefore follow both the magnesium number and the lime recommendation rather than treating low Mg as an isolated issue.
Sulfur Risk Is Often Highest on Sandy, Low-Organic-Matter Ground
Sulfur behaves differently from potassium and magnesium because plants absorb most S as sulfate, a negatively charged ion that can move with soil water. Coarse-textured soils hold sulfate less strongly than heavier soils, which means sulfur deficiency risk increases where soils are sandy, organic matter is low, rainfall is substantial, and little manure has been applied.
Mississippi State identifies sandy, low-CEC, low-organic-matter soils as common settings for sulfur deficiency and notes that heavy crop removal and reduced atmospheric sulfur deposition have increased the importance of S management in some production systems. Penn State similarly identifies coarse-textured soils with low organic matter and little recent manure as higher-risk environments for sulfur deficiency.
This risk-based approach is important because sulfur soil testing is not equally reliable in every region. Organic matter mineralization can supply meaningful quantities of sulfur during warm weather, while sulfate may accumulate deeper in the profile beyond the depth of a routine surface soil sample. A shallow test can therefore miss sulfur that deeper roots may eventually reach.
For a sandy forage field with very little organic matter and no manure history, sulfur deserves much more attention than it does on a deep, high-organic-matter soil that regularly receives livestock manure. The fertilizer program should reflect that difference rather than applying sulfur uniformly across every field.
Sulfur Deficiency and Nitrogen Deficiency Can Be Confused
Sulfur and nitrogen deficiency can both make a crop appear pale, but the location of the symptoms differs because the two nutrients move differently within the plant. Nitrogen is relatively mobile, so deficiency normally appears first on older leaves as the plant redistributes N toward new growth. Sulfur is less mobile, which means young leaves tend to remain pale when S supply becomes inadequate.
Mississippi State describes sulfur-deficient plants as small, light green, and slow growing, with symptoms often becoming stronger on younger tissue. That pattern can help distinguish sulfur shortage from a simple nitrogen deficiency, although cool soil, poor rooting, and waterlogging can still complicate the diagnosis.
A farmer evaluating a pale forage field in September should therefore consider more than the possibility of additional nitrogen. If the stand has received adequate N but newer leaves remain unusually light on coarse, low-organic-matter soil, sulfur becomes a more plausible concern. Paired tissue samples from healthy and affected areas can strengthen the diagnosis where university interpretation guidelines exist for the crop.
The reason for making this distinction is economic as well as agronomic. Additional nitrogen will not correct a sulfur deficiency, and additional sulfur will not correct an N shortage. A combined product is useful only when its nutrient package corresponds with what the crop actually lacks.
Manure History Can Completely Change the Sulfur and Potassium Discussion
Livestock farms often have substantial nutrient sources that are not represented by the commercial fertilizer invoice. Manure can supply potassium, sulfur, nitrogen, phosphorus, magnesium, and other nutrients, and repeated applications can significantly alter the soil fertility profile.
Penn State identifies recent manure history as one reason sulfur deficiency becomes less likely, while high manure rates can also build soil-test potassium. This means a forage field near the livestock facility may have a very different K-Mg-S requirement from a hayfield located several miles away that exports forage every year and rarely receives manure.
The nutrient cycle becomes particularly important when hay is fed in a different location from where it was produced. Potassium and other nutrients leave the hayfield in bales, pass through livestock, and may then accumulate around winter feeding areas or on fields that receive the manure. The farm as a whole may recycle nutrients, but the individual hayfield can still be mined.
September soil testing can identify whether that redistribution has created a shortage. Commercial fertilizer should be used to correct the remaining need after realistic manure credits are considered rather than duplicating nutrients already supplied.
Pro-Mag Trio Fits When Potassium, Magnesium, and Sulfur Needs Actually Overlap
When soil testing and field history indicate that potassium and magnesium both require attention and the production system also has a credible sulfur need, Supply Solutions 0-0-22 Pro-Mag Trio provides potassium, magnesium, and sulfur together without adding nitrogen or phosphorus. Supply Solutions currently lists Pro-Mag Trio as a potassium-magnesium-sulfur fertilizer intended for situations where those nutrients are useful together.
The reason to use Pro-Mag Trio in a September fertility program is that the field has more than a potassium problem. A forage soil that tests low in K and Mg, has acceptable pH, and carries meaningful sulfur-deficiency risk is a much stronger fit than a field needing only potash. The combined nutrient analysis can simplify handling and allow one material to contribute to several confirmed fertility needs.
The appropriate timing depends on the soil and the next period of crop demand. Potassium and magnesium can fit a fall fertility program on many suitable soils, particularly after forage harvest when removal is known and field access is good. Sulfate sulfur is more mobile, however, so coarse-textured soils with substantial leaching potential may benefit from timing the product closer to active crop uptake rather than applying it many months before the forage needs the sulfur.
The problem Pro-Mag Trio solves is a combined K-Mg-S fertility shortage. It does not correct excessive acidity, replace lime, supply nitrogen or phosphorus, repair compaction, improve drainage by itself, or solve a nutrient-uptake problem created primarily by drought.
A Multi-Nutrient Product Still Has to Match the Required Nutrient Ratios
One of the limitations of any multi-nutrient fertilizer is that the nutrients come in a fixed proportion. The soil, however, may not need them in that same proportion.
Suppose a hayfield requires a large potassium correction but only a modest amount of magnesium. Applying enough of a K-Mg-S product to satisfy the entire potassium requirement could supply considerably more Mg or sulfur than is economically necessary. In that case, Pro-Mag Trio could provide part of the K requirement while another potassium source supplies the remainder.
The opposite situation can occur as well. A field may test adequately in potassium but show a meaningful magnesium shortage. Using a potassium-containing product strictly to correct Mg would add K that the soil does not need, making a more focused magnesium source the better choice.
This is why the fertilizer analysis should be compared with the complete soil-test recommendation before the rate is chosen. A combined product can simplify an application when the nutrient requirements overlap reasonably well, but simplicity should not come at the expense of applying unnecessary nutrients.
High Potassium Forage Creates a Livestock Nutrition Consideration
The interaction between potassium and magnesium has significance beyond plant growth in pasture systems. High potassium concentrations in cool-season grasses can reduce magnesium uptake by the plant and can also influence magnesium availability to grazing livestock. In susceptible cattle, this can contribute to grass tetany risk when dietary magnesium is inadequate.
Penn State identifies this concern particularly in heavily fertilized cool-season grass pastures and notes that livestock mineral management may still be required even when soil fertility is handled carefully. The issue tends to receive the most attention during periods of lush spring growth, when cool soil and rapidly growing grass can create conditions that favor high K relative to Mg.
This does not mean potassium fertilizer should be withheld from a genuinely deficient pasture. Low soil K can reduce forage production and weaken the stand, so creating a potassium deficiency is not a sound livestock strategy. The better approach is to avoid unnecessary K buildup, correct genuine magnesium shortages, and manage animal mineral intake with guidance from livestock nutrition professionals or veterinarians where grass tetany risk is significant.
September soil testing helps prepare for that spring risk months before cattle encounter the lush forage. If soil K has become excessively high while Mg remains marginal, the farmer has time to reconsider the fertilizer program instead of automatically spreading more potash before winter.
Soil Texture Should Influence Fall Timing
A fall application that fits a silt loam may not be the best program for deep sand. Potassium and magnesium are positively charged and can be retained on soil exchange sites, but coarse soils with low cation-exchange capacity have fewer sites available to hold those nutrients. Sulfate is even more mobile because it carries a negative charge and is less strongly retained in many surface soils.
This creates an important timing question for September. On medium- and fine-textured soils with adequate nutrient-holding capacity, a fall K-Mg application may fit well where regional recommendations support it. On very sandy soil, applying a combined K-Mg-S fertilizer far ahead of spring demand can increase the amount of time those nutrients are exposed to leaching or movement below the most active root zone.
Wisconsin Extension identifies sandy forage soils as particularly vulnerable to sulfur loss because low organic matter and leaching combine with the substantial nutrient demand of forage crops. Rutgers makes the same point, explaining that sulfate is particularly mobile on sandy, low-organic-matter soils.
The product can still fit those soils, but the application may belong closer to active crop growth. September planning should determine both what nutrient is needed and when the soil is most likely to hold it where roots can use it.
Fall Soil Testing Should Use Consistent Sampling Zones
Forage fields often contain more nutrient variability than their uniform canopy suggests. Manure distribution, feeding locations, water sources, gateways, slopes, eroded knolls, different soil textures, and historical fertilizer patterns can all produce distinct fertility zones.
A single composite sample that mixes heavily manured ground with nutrient-depleted hay areas may produce an average that represents neither one accurately. The result can lead to fertilizer being applied where it is unnecessary while failing to supply enough where the field has actually been depleted.
September provides an opportunity to sample those zones separately, particularly after a difficult growing season has made differences more visible. Areas that consistently produce less hay, show unusual leaf color, or respond differently to drought should be evaluated against healthier portions of the field rather than disappearing into a whole-field average.
This becomes particularly important with magnesium because soil texture and pH can change its availability substantially across short distances. The same field may contain a sandy ridge with low Mg and S risk while a heavier lower area remains adequately supplied.
Soil Testing and Tissue Testing Answer Different Questions
A soil test evaluates the nutrient supply environment, while tissue analysis shows what the plant has actually taken up. Neither should automatically replace the other.
If soil magnesium tests low and forage tissue also shows low Mg, the diagnosis becomes stronger. If soil Mg appears adequate while the plant remains deficient, root health, potassium competition, pH, moisture, and sampling conditions deserve further investigation. A tissue value should not automatically lead to fertilizer without understanding why the plant failed to obtain the nutrient.
Sulfur deserves particular caution because routine surface soil testing does not always predict crop response reliably in humid regions. Penn State notes that sulfate deeper in the profile and variable organic matter mineralization make sulfur soil tests only marginally useful for many field crops. In those situations, soil characteristics, manure history, crop symptoms, and properly interpreted tissue samples can become more informative than a single shallow sulfur number.
September records should bring those pieces together. A multi-nutrient fertilizer makes the most sense when several lines of evidence point toward the same shortage rather than when one laboratory report simply contains several numbers labeled “low.”
Do Not Ask Pro-Mag Trio to Replace a pH Correction
One of the most important boundaries in this fertility decision is soil acidity. Magnesium deficiency can occur on acidic soils, but supplying magnesium fertilizer without correcting a serious lime requirement may leave root growth and overall nutrient availability compromised.
If the soil test shows low pH and low Mg, a magnesium-containing limestone may be the better foundation because it addresses acidity while contributing Mg. Potassium and sulfur can then be supplied separately according to their needs. If the pH is already appropriate, a K-Mg-S fertilizer becomes much easier to justify because lime is no longer the missing piece.
This order protects farmers from paying for secondary nutrients while leaving the more fundamental soil problem unresolved. A product containing potassium, magnesium, and sulfur can be valuable when those nutrients are limiting, but it cannot neutralize soil acidity simply because magnesium appears on the label.
The broader lesson applies across the entire fertility program: choose the input that corrects the actual limitation rather than the input that happens to contain the greatest number of useful nutrients.
September Is a Good Time to Decide Whether the Field Needs One Product or Several
Fall fertility planning does not have to end with a single fertilizer source. In many cases, using two products provides a better match than asking one product to supply the entire requirement.
A field with a large potassium need and a smaller magnesium-and-sulfur need may receive part of its K through a K-Mg-S material and the remaining potassium from another source. A field needing K and Mg but little sulfur may justify a different combination. Another field may need dolomitic lime first because acidity and magnesium are linked, with potash handled separately.
The economics should be calculated from pounds of nutrients actually needed rather than from the number of nutrients listed on each bag. A three-nutrient fertilizer can save handling and application costs when all three nutrients are useful, but those savings disappear if the rate forces the grower to purchase nutrients that have little probability of producing a response.
September provides enough time to make those comparisons before spring urgency begins. Once the soil test is available, fertilizer sources can be priced and matched against the nutrient requirement rather than selected by habit.
A Good K-Mg-S Program Begins With the Field, Not the Fertilizer Label
The strongest September forage fertility plan treats potassium, magnesium, and sulfur as related nutrients without assuming they always need to be applied together. Potassium deserves close attention after heavy hay removal because large amounts can leave the field with every cutting, while magnesium deserves additional scrutiny where soil tests are low, pH has declined, or high K availability could further restrict Mg uptake. Sulfur becomes more important on coarse-textured, low-organic-matter soils with little manure history, especially where repeated forage harvest removes nutrients faster than the soil can naturally replace them.
Those conditions create a legitimate role for a multi-nutrient fertilizer, but they also define its limits. If soil pH is low and magnesium is deficient, a properly selected magnesium-containing lime may be more valuable than a fertilizer Mg source. If potassium is the only nutrient that tests low, a more focused potash product may provide the better economic fit. If sulfur risk is low because the field has strong organic matter and recent manure history, adding sulfur simply because it comes with the potassium may provide little additional return.
Where the field truly needs potassium, magnesium, and sulfur together and pH does not require a different correction strategy, Supply Solutions 0-0-22 Pro-Mag Trio provides those nutrients without adding nitrogen or phosphorus. The reason to use it is the combined nutrient requirement, the timing should reflect soil texture and crop demand, and the problem it solves is an overlapping K-Mg-S shortage rather than generalized forage stress.
September is valuable because the season’s forage removal is largely known and there is still time to turn that information into a better fertility plan. Farmers who compare soil tests with actual hay production, separate low-pH problems from true fertilizer needs, account for manure, watch the interaction between potassium and magnesium, and recognize where sulfur loss risk is highest can make a much more precise decision than simply replacing whatever nutrient received the most attention during the growing season.
Supply Solutions can help growers determine whether Pro-Mag Trio fits a field where potassium, magnesium, and sulfur all need attention, but the strongest recommendation begins with the soil and the crop rather than the product. When the nutrient needs genuinely overlap, a combined source can simplify the program. When they do not, using separate or more targeted amendments is often the better agronomic and economic choice.
