No. 18 Article 9/July 24, 2009

Identifying Nutrient Deficiencies in Soybean

At the end of June I wrote about manganese deficiency in soybeans in high pH soils. Today I will address how to recognize other nutrient deficiencies in soybean and possible ways to correct any problems. Nitrogen (N), potassium (K), and iron (Fe) are additional nutrients that are known to cause deficiency problems for soybean in Illinois.

Wet conditions and cloudy (low light intensity) days combined with unusually low temperatures recently are causing some soybean fields or parts of fields to show nutrient deficiencies. As I have written previously, the fact that plants look deficient does not mean that you should get overly concerned and immediately seek to solve the apparent problem by adding additional fertilizer. Most of the deficiencies we are seeing this year relate to growing season conditions and not to inadequate soil fertility.

As growing conditions improve, most symptoms will disappear without additional fertilization. However, since growing season conditions this year accentuate problems that might not be as evident in other years, this is a good opportunity to learn about field conditions and management practices that can be adjusted to prevent or at least lessen problems in the future.

Some of the most obvious nutrient problems I have observed this year are in fields with insufficient drainage, compacted soils, and marginal nutrient levels. Fields with insufficient drainage show a pattern of better crop growth in the area where tiles are installed and problem areas where tiles are not installed or where the distance between tiles is too large to effectively drain the soil. I have seen deficiency symptoms in fields where a compacted layer is present or where the crop was planted in wet soil. Planting under such conditions created sidewall compaction. Drainage and compaction problems limit root exploration of the soil and limit the capacity of roots to take up nutrients. Fields with marginal fertility that might not show the deficiency under good growing season conditions are showing it this year because the soil conditions have made nutrients slightly less available and because the roots of the crop are not as active to compensate for the reduction in nutrient availability.

Once nutrients enter the plant, some are mobile and others are not. Mobile nutrients will cause deficiency symptoms to develop in the older leaves of the crop. This is because nutrients present in the older leaves will move to the new leaves to maintain the new growth. On the other hand, immobile nutrients will cause the new leaves to show greater deficiency symptoms while older leaves might be completely green. Deficiency symptoms for many of the essential nutrients have not been verified, or are very rare, in Illinois, so I will not address them here. Mobile nutrients that are known to cause deficiencies in soybean include N and K. Besides Mn, Fe is the other immobile nutrient.

N deficiency makes older leaves turn pale or yellowish-green. Deficiency develops commonly because of poor nodulation, which can be the result of insufficient inoculum in the soil or wet or cool soil temperatures. Typically, if soybean has been grown in the field in the recent past, there should not be a problem related to insufficient bacteria present in the soil to properly inoculate the roots. If you suspect this is the problem, however, it would be important for the future to plant inoculated seeds. Most often, inoculation problems are related to poor growth conditions. Soybeans do not grow very well in wet soils; ensuring adequate soil drainage is probably one of the best ways to reduce the problem of poor nodulation.

Nitrogen deficiency.

K deficiency is observed as yellowing or browning and necrosis (death) of the edge of older leaves. When the problem persists, this deficiency will continue to move up from older to newer leaves, while the top leaves may look completely green. Even though rapid K uptake does not occur until reproductive stages, under marginal soil K levels or inadequate growing season conditions, K deficiencies develop more often at early stages of development when the root system is small. Soybean cyst nematode problems can often be confused with K deficiency. The best way to determine the cause in these cases is to test plant tissues for nutrient levels and send root samples to be evaluated for cyst nematode presence.

Potassium deficiency.

Soybean cyst nematode damage.

Fe deficiency is observed as yellow coloration between leaf veins. Under more severe conditions, it is possible to see completely yellow leaves. Since the nutrient is immobile in the plant, the symptoms appear in the new leaves. The problem is most often observed in high-pH soils (pH >7.5), in soils with drainage problems, and when Fe-inefficient varieties are used.

Iron deficiency.

--Fabián G. Fernández

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