Posts Tagged ‘soybean nutrient management’

Nitrogen Fertilization and Irrigated Soybean Production

Friday, August 24th, 2012

Richard Taylor, Extension Agronomist; rtaylor@udel.edu

A number of people have been asking about applying nitrogen (N) fertilizer to irrigated soybeans so I thought I would make a few comments about the practice in case they might help you in making the decision as to whether to spend the money in hopes of getting a yield response.

To begin with, I have tried this practice on both full-season and double-cropped soybeans at one time or another. I’ve tried applications of N at both 25 and 50 lbs N/acre at R2 (full flower) and R4 (full pod) and for double-cropped soybean I’ve tried these rates broadcast at planting. I never got a significant response to the treatments although for double-cropped soybeans I was close to seeing an increase in early plant height and pod set. For yield, the treatments were all within a bushel or two of each other.

That being said, I should point out that significant responses have sometimes been reported from down South but only when the N was applied through an irrigation system (for the reports I’ve seen) and when both boron (B) and N were applied in combination. At the time of the research that I conducted, we did not have the capability to apply N to my plots through the irrigation system. I had to apply the N with a back-pack CO2 sprayer while walking through the soybeans. I did have the studies irrigated immediately after applying the treatments to minimize the chance of foliar burn. I remember hearing from the southern researchers that they felt that the leaf damage caused by walking on N or applying N with a ground rig would negate the slight yield response that they were able to obtain using fertigation. I also did not apply B along with the N which may have also reduced the chances of obtaining a positive yield response since B is important in sugar transport and in helping flower set.

I understand that some people suggesting that N should be applied to irrigated soybeans are suggesting the inclusion of sulfur (S) (probably as ammonium sulfate) along with the N. This makes some sense from a biological point-of-view in that the plant requires enough S to make the S-containing amino acids required for protein synthesis. However by the time soybeans reach the full bloom or full pod stage, the root system has reached or will soon reach its deepest penetration of the soil. Even the sandy soils in Sussex County, Delaware, were found to have large quantities of S (typically 300 to 500 lbs S/acre) stored in the clay lenses found in the 1 to 2 foot depth of soil and soybean roots should be able to tap into this S reserve by reproductive stage.

Let me summarize below some of my thoughts on trying to increase soybean yields with N fertilizer.

● If soybeans already have matured to the full seed stage (R6) where a full size seed is found in a pod at one of the four upper most nodes with a fully expanded leaf, it is much too late for N application to increase yield potential in my opinion. All the research that I’ve seen involves the application of N at full bloom (R2) to full pod (R4) stage.

● I doubt that the addition of S as ammonium sulfate is going to increase your chance of obtaining any return on your investment since soybeans are very likely to have more than an adequate supply of S available by this time of year due to root growth. An exception would be where there is a root restricting compaction layer in the top 12 inches of soil but in this case the chance that fertilizer will improve yield is very low.

● If your expected yield potential is not at least 60 to 70 bushels/acre, N fertilization will not help. Next year, try using either the liquid seed Bradyrhizobia inoculants or some of the new graphite soybean inoculants since the new strains available can really help increase your yield potential.

● If you still plan to apply N fertilizer to your soybean crop, be sure to add about 0.5 pound of boron per acre. The data I’ve seen where a yield response was obtained with late season (R2 or R4) N application were always where B had been included with the N.

● I would suggest limiting any N application to no more than 30 lb N/acre since levels higher than this have often been shown to reduce the nitrogen fixing activity of the soybean nodules. If this occurs, you’ll be trading dollars essentially since the nodules will either stop N fixation or reduce fixation to a degree where the plant will need the N you apply just to produce the original crop’s yield potential. Some studies with early season manure applications have shown yield reductions because the crop ran out of N during the reproductive stages and had to reinitiate nodulation because the crop ran out of available N.

● Do not consider N applications on non-irrigated soybeans. Keep in mind that in the case of a dryland soybean crop, the overall limiting factor is water availability not nutrient availability.

● Also if your field has a history of soybean cyst nematode (SCN) infestations, do not add N fertilizer since SCN will be your yield limiting factor not N or S or B fertilizer.

Watch for Potassium (K) Deficiency Symptoms in Soybeans

Thursday, August 4th, 2011

Richard Taylor, Extension Agronomist; rtaylor@udel.edu

Although it shouldn’t be a surprise, a few fields in the state have started showing potassium (K) deficiency symptoms. In the past, this condition usually shows up in the eastern sections of New Castle County at least every few years but reports this year indicate that other areas of the state are showing the symptoms. Since potash prices have been quite high the past few years, many growers have been cutting back K application rates or skipping a year or two between applications. These practices have lead to low soil test levels of K and now to at least some fields, usually with very light sandy soil and low organic matter content, showing K deficiency symptoms.

Many dryland corn fields have long had the corn leaves fire up or turn necrotic and even a lot of the irrigated fields I’ve been in during the past week are showing dead or firing lower leaves. The full dent growth stage is evident in even the irrigated fields probably due to the large number of heat units or degree days we’ve had this summer. So in the case of corn, we’re probably too late to notice K deficiency symptoms but for soybeans that’s not the case.

In soybeans the deficiency shows up as yellowing along the trifoliate leaflet edges on the oldest leaves of the plant (Photo 1). The yellowing of the edges intensifies and expands and can move up the plant (Photos 2 and 3). The reason the deficiency begins on the lower leaves is that the element K is mobile in the plant so that when there is an insufficient supply the plant will cannibalize the lower leaves and mobilize the K up to the new growing leaves. If the deficiency persists or worsens as the plant becomes larger and begins the reproductive phase, the yellowing gradually turns brown and the leaf edges die and become ragged (Photo 4).

Photo 1. Note the very beginning of leaf yellowing along the leaflet margins on the very lowest leaves.

Photo 2. Note that leaf yellowing along the leaflet margins has expanded and has moved up the plant closer to the terminal.

Photo 3. Leaf edges are almost fully involved as the deficiency worsens.

Photo 4. At this stage, the leaf edges begin to show necrosis while other leaf edges have turned completely necrotic.

What can be done if you find deficiency symptoms? Various liquid formulations of K are available that could be applied as a foliar spray although the experiences I and other researchers have had suggests that the foliar burn from these products could easily cancel out the benefit of adding K. A problem with foliar sprays is that it is difficult to apply enough to the plant to supply the plant’s need for K. If you increase the concentration or the amount of solution K applied, the burn potential increases and will cancel out the benefit of foliar K. Some work at Rutgers University (unpublished) showed that both potassium chloride and potassium carbonate significantly burned soybean plants and cancelled any yield increase.

Where does this leave the producer with a K deficiency? Essentially, we’re back to broadcasting or flying on muriate of potash (0-0-60) at about 100 lbs/acre and then either irrigating the field if irrigation is available or hoping for a rain event to begin to dissolve the fertilizer and move it into the upper soil layer where many of the soybean roots are present. Again with the element being a plant mobile nutrient, the K will be moved by the plant to the growing points which will be the terminal where new leaves are being formed and to the reproductive sinks (flowers, pods, and seeds). This should be effective in helping to reduce the yield reduction possible with K deficiency.

If you can, look at the soil test to make sure there is adequate phosphorus (P) available for the crop. It’s been shown that the addition of both P and K can have a synergistic effect where the yield increase is greater when both nutrients are applied versus just applying a single nutrient. If the soil test level of P is already high, as it is on many Delaware soils, then there likely will not be a benefit to adding both nutrients so an accurate soil test is essential when considering the combination product.