Addressing mid-season fertility issues

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As the crop progresses, some nutritional problems often become more evident to farmers. Here are a few to consider.

For nitrogen (N), if sidedressing corn has not occurred, the PSNT or chlorophyll meter can still be used to assess the N status in fields just before maximum crop demand. These tests are most useful on fields with manure to determine if the manure is supplying adequate N or if additional sidedress N is necessary.

Research has shown that the accuracy of sidedress recommendations are improved dramatically with these tests compare to just following standard N recommendations. See Agronomy Facts 17, Presidedress Soil Nitrate Test for Corn for details on the PSNT or Agronomy Facts 53 The Early Season Chlorophyll Meter Test for Corn for details on the using the chlorophyll meter. It is critical with both of these tests that the instructions be followed exactly for the results to be valid. If an N deficiency is detected beyond normal sidedress time, if equipment is available to apply N in the taller crop, rescue applications of N can be very helpful, up until around tasseling time.

There have been a few reports of potassium (K) deficiency. With regular soil testing these can usually be avoided. It is important to remember that forage crops remove very large amounts of K from the soil which can deplete the soil rapidly. Thus, maintaining K with regular manure or fertilizer applications based on soil tests is critical. Also, realize that while K deficiency in plants is often due to soil K deficiency, other factors like soil compaction can also result in a plant K deficiency even in a soil with adequate K.

A common question this time of year with K deficiency is whether it will help to apply K now to growing corn. If the corn is not too tall to broadcast K fertilizer, there is a good chance that adding K will help the crop recover at least some from the deficiency. Besides, if the soil is deficient, a K application is necessary at some point anyway, so applying it to the growing crop may help that crop. But regardless, it will begin building up the soil K for the future.

Before making a K application, run a soil test to make sure the deficiency in the crop is due to low soil K and not some other soil limitation. Applying K to a soil already adequate in K but limited by some other soil factor will be of no help.

As we might expect, there has been some sulfur (S) deficiency observed on corn this year. This has strictly been on cash crop farms without manure so far. At this point, if sidedressing has not yet occurred adding ammonium thiosulfate (ATS) to UAN for sidedressing is an easy way to add some sulfur to fields that have been determined to be S deficient. A common fertilizer with UAN and ATS is 28-0-0-5S. For every 10 gal of UAN applied per acre you will get 5.4 lb S/A.

For smaller corn, ammonium sulfate could be broadcast over the crop. A rate of 100 lb ammonium sulfate/A will supply 24 lb S/A. Ammonium sulfate will cause some burning but the benefit will usually outweigh any injury effects.

Either way, applying 15-25 lb S/A would be a typical rate. Remember that soil testing for S is not very reliable. Plant tissue testing is a better tool for diagnosing S deficiency.

Plant analysis is a valuable tool for monitoring crop nutrition and diagnosing suspected plant nutrient deficiency problems. There are two ways to use plant analysis. The first method is to compare the analysis of the plant tissue with standard tables of interpretive values. Since the table values are for specific plant parts sampled at a specific stage of growth, it is critical that sampling guidelines be strictly followed. Information on sampling crops and plant analysis interpretation levels are available on the Penn State Agricultural Analytical Services Lab web site.

When a problem is observed, analysis of plants from the problem area and a nearby normal area, along with soil tests from both areas, can be very useful in diagnosing nutritional problems and determining if they are related to soil nutrient deficiency or some other soil or plant factor affecting nutrient uptake. The same plant part should be sampled in both the good and bad areas to make valid comparisons.

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