Grain Test Weight Factors, P and K after a Big Crop  01/04/17 4:08:46 PM

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Low Test Weights in Corn--Does Test Weight Matter??   ABSOLUTELY—Weight is weight and bushels are weight generated.  So what factors influence test weight?  So here are some observations we have made.


High Grain Moisture

Test weight increases by 1 pound per bushel for every 4 points of moisture drydown. For example, corn harvested at 23.5% moisture with 54 pound test weight is likely to have 56 pound test weight when dried to 15.5%. However, if corn was not able to supply kernels with enough energy (starch/carbohydrates)  to fill, test weight may not increase as corn dries.

Inability to Supply Kernels with Sufficient Energy

Conditions from R2 through black layer (R6) can greatly impact test weight. Once corn is past R2, kernel abortion is unlikely and the number of kernels are fairly set. After black layer, the movement of moisture, nutrients, and energy between kernels and the plant is minimal. If yield potential is higher than what the environment or plant can support (i.e. fertility or soil moisture) from R2 through R6, something has to give. This can result in lower test weights because the corn plant cannot supply kernels with enough energy to fill due to lack of production, damage to plumbing, or premature death.

Corn Plant Cannot Produce Enough Energy. Plants produce energy through photosynthesis (Ps). Drought, leaf diseases, or cloudy or rainy weather can reduce Ps, thereby reducing energy production. Drought stress limits water uptake as well the ability to photosynthesize efficiently. Leaf diseases destroy leaf tissue needed for Ps. Cloudy or rainy weather limits sunshine, thereby reducing Ps.

Damage to Plumbing. The term “plumbing” refers to the xylem andphloem in the plant. The plumbing is needed to move moisture, nutrients,and energy, to sustain the plant and grain. If the plumbing is damaged due to insect injury, stalk rots, stalk lodging or root lodging, it can hinder the ability of the plant to produce and/or send enough energy to developing kernels, resulting in low test weight.

Premature Plant Death. Frost or severe stalk rot infection are common causes of premature plant death (prior to black layer). Often premature death results in the corn cob being mushy and bending easily with your hands. Premature death causes slow drydown. Even when grain from plants that have died prematurely is dried, the water can be removed, but the starch within each kernel does not shrink as it normally would. That results in larger, softer, less dense kernels, and low test weights.

What Happened in 2016?

The majority of corn last year maintained a “higher” average test weight.  Years were low test weight corn occurs had conditions where corn was harvested between 20 to 28% moisture and droughty conditions existed after pollination. This drought stress limited energy production in the plant. It also encouraged cannibalization. Limited energy and damaged plumbing from cannibalization restricted the plant from supplying kernels with enough energy and resulted in low test weights. Earlier relative maturity corn can maintain better test weight than later maturing corn, which is likely due to the corn being at different stages of development when drought stress was most severe.

Source: J. Beuerlein, “Bushels, test weights, and calculations”

The Ohio State University Extension. Fact

Sheet # AGF 503-00

0503.html (Accessed 10-22-2008)

Importance of P and K in Corn after a record crop is removed.


We have had this discussion a number of ways and a number of times before.  With decreased commodity prices, some growers are considering minimizing fertilizer applications, especially with

phosphorus (P), potassium (K), and lime.  Remember the record crop we just harvested?  You also harvested record amounts of fertility along with that crop! 


This has been a “residual” question during the past few years.  While skipping or reducing applications of P and K might not be as detrimental as reducing nitrogen (N), it can still impact yield potential and stress tolerance.



Uptake of P increases rapidly after about the V6 growth stage, approximately 4 to 6 weeks after corn planting. Uptake continues until near maturity. Plants that are phosphorus deficient typically have a purple or dark green color because leaf expansion is retarded more than chlorophyll and chloroplast formation

Plants grow slowly, stalks are thin and shortened, and maturity is usually delayed. Other factors that may cause purple corn include the following: cold temperatures, wet soils, compaction, and root injury. Soil testing can reveal if P is deficient. Unlike nitrogen, P has a relatively short range of movement in the soil and is therefore considered an “immobile nutrient.” P requires moist soil for effective root uptake. As a result, dry soil conditions can negatively affect soil uptake by corn roots.


Role of P. Skipping or reducing fertilizer applications for one year may have minimal impact on yield potential in many cases, due to the “banked” levels of P. However, having less available P can impact the plant in other ways, especially under stressful conditions. Basically, P is the main nutrient responsible for energy production in the plant. Without enough P to properly support energy production, corn cannot grow, produce, or handle stress as it should. It sounds simple, but it is absolutely critical to a successful crop.

Management. Soil fertility level for P is greatly impacted by the inherent availability in the soil and crop removal. Each bushel of corn harvested per acre removes approximately 0.4 pounds per acre of P2O5. Each bushel of soybeans harvested per acre removes approximately 0.85 pounds per acre of P2O5.

Soil tests are recommended every 4 years or less. If there is concern about fertility, especially due to very high yields in 2016, take soil tests to aid with fertility decisions. Base fertilizer applications on the values received from the soil test. Always consider residual fertility from previous crops when determining application amounts.



Uptake of K. Potassium uptake increases rapidly after about the V6 growth stage, approximately 4 to 6 weeks after corn planting. Therefore, when K demand becomes large and there is not enough available K, plant deficiency symptoms become visible Uptake of potassium is completed soon after silking (R1 stage). Potassium deficiency is characterized by yellow and brown margins beginning at the leaf tips and can often be confused with nitrogen deficiency. Like nitrogen, potassium is mobile in the plant so lower leaves are affected first. Potassium deficiency is favored by low soil test K, compaction, and conservation tillage where no subsurface band of K is used. Dry soil conditions can also negatively affect soil

uptake by corn roots.

Role of K. Potassium is one of 12 nutrient elements required for normal corn growth and development. Specifically, K has been linked to improved stalk strength. When corn takes up sufficient K, stalk drydown is slowed after maturity. Thus, the risk of lodging after maturity may be reduced if K levels are adequate for

crop production.

Management. Similar to P, soil fertility level for K is greatly impacted by the inherent availability in the soil andcrop removal. Each bushel of corn harvested per acre removes approximately 0.29 pounds per acre of K2O, and each bushel of soybeans harvested per acre removes approximately 1.45 pounds per acre of K2O. Crops cut for silage or baled for stover remove significantly more, approximately 8 pounds per acre of K2O, because the majority of the above-ground tissue is harvested. As previously suggested for P, basing fertilizer applications on soil tests and residual fertility is also recommended for K.

In Summary, as yield potential increases, it is critical to manage soil fertility. While reducing fertilizer inputs may decrease your costs, it may not improve your bottom line. Please carefully consider the risks of reducing fertilizer inputs on crop production



Corn & Soybean Field Guide. 2007 Edition. Purdue

Pest Management Program. Purdue University.

S. Ritchie, et al. How a Corn Plant Develops. Iowa

State Univ. Special Report No. 48. 1997.

5th Edition Soil

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