How to interpret soil tests this fall

Courtesy of ISU Girl taking soil sample from field.
WAIT FOR RAIN: Soil test results may be difficult to interpret when soil samples are pulled during drought conditions.
Be cautious when interpreting fall soil test results following drought.

Sampling soil this fall following the dry conditions this past summer, and in some places in Iowa where dryness is continuing into fall, may result in lower-than-expected soil test results for phosphorus, potassium and pH.

“This is especially the case if soil samples are collected before any significant rainfall,” says Antonio Mallarino, Iowa State University Extension soil fertility agronomist. “Farmers and crop consultants should interpret those soil test results with caution.”

His advice is to wait until you get enough rain to wet the soil at least to the depth of probing (6 inches for most farmers). Sampling should be delayed for about a week after the rainfall. He offers the following observations and recommendations.

P, K removal with harvest

Estimates of P and K removal are used to decide fertilizer application rates to maintain soil test P and K levels within the “optimum” soil test interpretation category. Prolonged drought can reduce crop grain yield, and consequently, the amount of P and K removed with harvest, so the planned removal-based rates may be reduced accordingly. However, a large yield reduction is not likely if below- normal rainfall was only from late August, so in these fields the planned removal-based rate should not be reduced.

Removal-based rates in fields with low grain harvest recovery from badly lodged corn due to the August derecho or recent windstorms should not be reduced because P and K in unharvested grain will become available for next year’s crop. Although sampling harvested plant parts for nutrient analysis is an option, an easier and effective approach to estimate P and K concentrations per unit of yield is to use information provided in ISU Extension publication PM 1688, A general guide for crop nutrient and limestone recommendations in Iowa.

Concentration values in that publication are adjusted from a dry matter basis so they can be directly multiplied by the yield at the standard moisture concentration. For example, values from PM 1688 for corn grain at 15% moisture are 0.32 pound P2O5 per bushel and 0.22 pound K2O per bushel and for soybean grain at 13% moisture the values are 0.72 pound P2O5 per bushel and 1.2 pounds K2O per bushel. Remember, the yield level variation is by far much more important at determining nutrient removal than variation in P or K concentrations.

P, K recycling to soil

Although low rainfall since a crop reached physiological maturity late in the growing season may not affect yield and the amount P and K removed with harvest, it can greatly reduce the amount of plant P and K recycled to the soil. Normal rainfall leaches plant nutrients into the soil after plants mature (from standing vegetative plant parts) and from crop residue after harvest.

Potassium recycling occurs earlier and faster than for P because the K in plant tissue is soluble in water and plant P is mostly organic. Research has shown that with good yields and normal rainfall, soybeans and corn recycle about 80 and 30 pounds K2O per acre of K to the soil, respectively, between physiological maturity and grain harvest. An additional 30 and 15 pounds K2O per acre are recycled, respectively, from harvest until early December.

The recycled K is fully available to the next year crop. For P, the amounts of soluble P recycled are much lower, on average being only 10 and 7 pounds P2O5 per acre for corn or soybeans, respectively, for the entire period from physiological maturity until early December.

Therefore, below-normal rainfall from the time of physiological plant maturity until the time of soil sampling in the fall will result in much less K recycling to the soil than normal, and consequently lower soil test K levels than with normal fall rainfall. A small soil test P reduction is possible but less likely.

Effects on soil testing

With a prolonged drought, low crop yields, and low P and K removal, postharvest soil test P and K levels tend to be higher than expected. However, if below-normal rainfall was only from late August, this effect will be small. Conversely, dry soil after crop physiological maturity slows down the normal reactions between soil nutrient pools, which often results in lower soil test P and K levels.

Plants are like pumps taking up P and K from available soil pools, but as nutrient uptake decreases late in the season, normal rainfall and moist soil allow for a replenishment of the available nutrient pools (measured by soil tests) from the less available pools. Dry soil limits soil test rebound, and often affects soil test K more than soil test P. However, the greater impact at reducing soil test K levels often comes from lack of rainfall reducing K leaching from plants to the soil.

Very dry soil conditions may result in lower soil pH values (more acidic in neutral to acidic soils). Differences ranging from 0.1 to 0.4 pH units lower are common with very dry conditions from late August until soil sampling in the fall. This is because small concentrations of soluble salts normally present in the soil solution aren’t leached down to deeper layers by rainfall, which results in higher hydrogen ion concentration and greater acidity (lower pH) in the topsoil. On the other hand, the dry soil effect on buffer pH, which is used to estimate lime requirement, is not large or consistent.

Therefore, the main issue with pH measurements with dry soil is taking into consideration that the pH value may under-estimate pH and the decision if lime should be applied or not — but will not affect very much the amount of lime to apply.

Another possible problem of sampling during dry soil conditions is that it may increase sampling error because it is more difficult to control the sampling depth and accomplish proper soil core collection. This may be especially serious in no-till and pastures, due to large nutrient stratification with depth; but stratification is also present with chisel-plow and disk tillage. When the top inch of soil is very dry and powdery, it is very easy to lose this soil portion from the core, which will affect the soil test result significantly.

Drought conditions

Mallarino offers these suggestions for soil sampling and test result interpretation during drought conditions:

P and K removal. Consider yield and estimates of P and K removal with harvest during the last two years to decide maintenance fertilization rates for the optimum soil test category.

Soil sampling. Delay soil sampling until meaningful rainfall occurs because it will result in a better sample and more reliable soil test results, mainly for K and pH. It is not possible to say how much rainfall is helpful, but we believe it should be sufficient to wet the soil throughout the sampling depth (usually 6 inches) and sampling should be delayed for at least a week after rainfall.

If you still have to take soil samples during dry conditions:

Sample depth. Be careful with sampling depth control and that you collect the complete soil core.

Potassium. Soil K test results may be lower than they would be with normal conditions due to less recycling to the soil and less replenishment of the soluble or easily exchangeable soil K pools.

Phosphorus. Soil P test results probably will be affected little by the recycling issue.

Soil pH. Soil pH test result may be lower than in normal conditions, which may encourage you to apply lime when it is not needed yet. However, buffer pH, which is used to determine the amount of lime to apply, will not be affected much.

For additional information about P, K and pH-lime management, visit the ISU Extension Soil Fertility website.

Source: ISU, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.


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