Nutrient management after a failed corn crop

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Where the corn crop failed in 2011, farmers are asking questions on the best ways to handle their nutrient management programs for 2012. In most cases, the vast majority of the fertilizer that was applied to unharvested, failed corn should still be there in 2012 – either in the soil or in the crop residue. Farmers will need to do some soil testing to know for sure the nutrient status of fields with failed corn. Farmers will also want to have some idea of the amount of nutrients present in the corn residue remaining, and how quickly those nutrients will become available to crops.

There are a number of potential sources of nutrients other than applied fertilizers that could contribute to a 2012 wheat, corn, sorghum, or soybean crop. These include:

  • Nitrate, sulfate, and chloride in the soil profile
  • Phosphorus, potassium, and zinc in the surface soil
  • Nutrients in crop residues

The first category consists of mobile nutrients, and the second category consists of immobile nutrients. The difference is important. Mobile nutrients are able to dissolve in soil water and can move through the soil in water, while immobile nutrients generally stay where applied. Of the 14 essential mineral elements, the common mobile nutrients we apply as fertilizer are N, S, and Cl, and the common immobile nutrients we apply as fertilizer are P, K, and Zn.

 

Mobile nutrients in the soil after failed corn

A very large portion of those mobile nutrients that were not taken up by the 2011 corn and/or wheat crops are likely still present in the top foot or two of soil. With the low rainfall in most of the state south of I-70, very little of the N will have been lost. In the K-State Soil Testing Lab, we are already seeing higher-than-normal soil test levels for N, reflecting an accumulation of unused nitrate N in the soil. Any unused sulfur (S) or chloride (Cl) would also be present in that top foot or two of the soil profile.

So the first tool a farmer should think about when planning his 2012 fertilizer program is a deep profile soil test for N, S, and Cl.

 

Immobile nutrients in the soil after failed corn

What about P, K, or Zn? Where these nutrients were applied to the 2011 corn crop, will they still be available for crops in 2012? When immobile nutrients such as P, K, and Zn are applied to the soil, they interact with different portions of the soil and are retained. Note the word “retained,” not “fixed.”

Phosphorus. Phosphorus reacts with the clay surfaces and the iron and aluminum coatings found on the soil particles and is sorbed to those surfaces. Sorption reactions occur in stages, and the initial stages are highly reversible. Sorbed phosphorus can be desorbed and go into soil solution, replacing the P taken up by plants. This is a buffering system which maintains a constant small quantity of P in the soil solution and supplies the P needed for good crop growth. This is how we store P in the soil and build soil test values, with little worry about that P being lost. Sorbed P is the primary P fraction in soils measured by a soil test. But the soil test only reflects a fraction of the total P present in the soil. In most Kansas soils, we have an 18:1 buffer factor. If we add 18 pounds of P2O5 and it reacts with the soil, becoming sorbed to the clays and other minerals present, the soil test will increase 1 ppm. If we remove 18 pounds P2O5 through crop uptake, the soil test value will drop 1 ppm.

So how does this relate to planning for 2012? Any P applied in 2010 or 2011 for this year’s crop that was not taken up by the corn was sorbed onto clays and other minerals. This creates a new equilibrium in the soil, and will increase the soil test values for P. The higher soil test values will result in a lower P fertilizer recommendation.

Potassium. Potassium is a charged cation, K+, which is attracted to and retained on the soil’s cation exchange capacity (CEC). Like sorbed P, exchangeable K maintains a constant supply of K in the soil solution to support plant growth. Also like P, this exchangeable K can be measured by a soil test, and it is a highly buffered system. With K, every 4-8 pounds K2O added will increase the soil test 1 ppm, and every 4-8 pounds removed will lower the soil test 1 ppm. The buffer factor is a function of CEC and soil minerals present. On low-CEC sandy soils this factor is closer to 4, while on high-CEC silty clay loams the value will be closer to 8. Any K applied and not taken up by the 2011 corn crop would have been retained on the CEC in the surface soil and remains available for 2012. And, the higher K soil test values will result in lower K fertilizer recommendations for 2012.

Zinc. With zinc, a third mechanism, chelation, occurs and retains applied zinc. Soil organic matter is a strong natural chelating agent, much like some of the synthetic compounds we buy as fertilizer sources. Zinc sulfate added to soil slowly dissolves. A portion reacts with the organic matter and is retained in soluble, natural organic matter chelates. The vast majority of the zinc that moves to plant roots for uptake is present as a natural soil organic matter chelate. Again, this can be measured by a soil test, and there is a common buffer factor of about 10:1 with our DTPA soil test. If we add 1 pound of Zn, the DTPA soil test value will increase by about 0.1ppm.

 

Testing for soil nutrients

The bottom line for soil nutrients is that any N, P, K, S, Zn, and Cl added as fertilizer and not taken up by crops is still likely there, and can be measured by soil tests. The mobile nutrients (N, S and Cl) will need to be measured using a deep profile test, while the immobile nutrients (P, K, and Zn) can be measured using a surface sample.

 

What about the nutrients taken up by the 2011 crop?

We sampled 15 fields across the eastern two-thirds of Kansas in mid-July to determine the actual condition of the 2011corn crop, yield potential, and nutrient content of the corn plants. These fields ranged from those that had essentially died prior to tasseling to some with the potential for outstanding yield. The full results from these measurements can be found in e-Update 312, from August 18, 2011.  

The plants fell into three basic groups. The corn from Edwards and Reno counties was the most severely stressed of those sampled, with only 1,200 to 4,500 pounds of dry matter present. The plants from Franklin and Cherokee counties had more normal vegetative growth, but little or no grain yield due to poor pollination. The remaining sites from Riley, Shawnee, and Republic counties had varying levels of growth and yield, but took up normal or slightly reduced levels of nutrients. 

The severely damaged samples from Reno and Edwards counties had high nutrient concentrations but very low total nutrient uptake per acre because of the low level of dry matter produced. In those fields a large portion of the applied nutrients are likely still present in the soil, and potentially available for the 2012 crop. The fields in the other areas had varying levels of dry matter and grain yield, and in many cases near-normal nutrient uptake per acre. In this situation, residual nutrients in the soil would likely be elevated, but not to the degree found in the extreme drought areas in south central and southwest Kansas.

The majority of the nitrogen, phosphorus, and sulfur in plant material is present as protein and other organic compounds. For these nutrients to become available to plants, these compounds must be broken down and the N and P mineralized. This process will normally take three or more years to run to completion, with the C:N ratio being the primary factor controlling the rate of release. Corn stalks are normally a very high C:N material, with a C:N ratio around 60:1. In high C:N materials, very little net N mineralization will occur until the organisms utilizing this material as a food stuff reduce the carbon content of the residue to a C:N ratio of roughly 25:1. 

In the drought-damaged crops, especially the severely damaged ones with reduced vegetative growth, the N content is much higher than normal, since there is little or no grain present. The C:N ratio in many of these severely damaged crops is less than 35:1. As a result, net mineralization will occur much more quickly -- a matter of months rather than years. In very severely damaged corn where N content is around 2% or more, such as at the Edwards and Reno county sites, roughly half of this N, P, and S is likely to be available for a summer crop planted next spring. In the fields with more normal vegetation but little or no grain yield (Cherokee and Franklin counties), the N will remain in the vegetation and enhance decomposition -- but likely not as quickly as where vegetative growth was more severely damaged by drought.

Potassium and chloride exist in plant cells as free ions. When the plant dies and those cells rupture, the K and Cl rapidly leach from the crop residues and return to the soil to be “recycled” in the next crop. These two nutrients will likely be available quickly for the 2012 crop and can be measured by soil tests later this winter and next spring. If the crop was taken off for silage or forage, these nutrients will have been removed.

Wheat planted this fall into these residues will not benefit nearly as much from the N, P, and S present in the corn vegetation as will next summer’s corn or sorghum crops. With wheat, there is not as much time for soil organisms to break down the residues and mineralize these nutrients.

 

Measuring nutrient levels on fields of failed corn

For those planting wheat this fall in these failed crop fields, a profile soil test for N, S, and Cl is a must. P and K applications should also be made based on a surface soil sample. For those planting corn or sorghum next spring, it would be best to wait until late winter or early spring to take the profile sample to get a better feel for the amount of the residual N which will be remaining in the soil. Mobile N can be moved below the root zone, especially in sandy soils if we get a wet winter.

Another potentially valuable tool to consider is the use of a crop sensor to help estimate the amount of the N being mineralized from the 2011 crop residues. Kansas has good recommendation systems for both wheat and sorghum to help interpret sensor data. The rate of mineralization will depend greatly on soil moisture and soil temperatures during March through June. A sensor-based N management system can help take some of the risk out trying to take credit for mineralized N.

 

Summary

A significant amount of residual nutrients will be present in many fields where this year’s crops failed. In severe situations, only a fraction of the nutrients applied were actually taken up by this year’s crop. Many of the nutrients remain in the soil and can be measured using soil tests. This is especially true for the mobile nutrients such as N, S, and Cl. But to get a good estimate of the amounts present, a profile soil test to a depth of 24 inches will be required.

Many of the nutrients taken up by this year’s crop will also be available, especially the K and Cl, which are not incorporated into organic compounds. However the N, P, and S must be mineralized as the vegetation decays. This process will be likely be faster than normal, and will increase the availability of these nutrients. But the exact rate of mineralization will depend on the weather, and is difficult to estimate. Crop sensors can help take some of the risk out of crediting these mineralized nutrients.


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