ISSUE 3 May 21, 1998
Some field peas were seeded this spring without Rhizobium inoculation either by design or because inoculant was in short supply. The fortunate producer may find, particularly if the field has grown field peas in the past, that wild types of Rhizobium bacteria might be present and the field pea may show no signs of N deficiency. Just because field pea is a legume does not mean that it can manufacture N alone. Field pea requires high amounts of N, and if properly inoculated with the right Rhizobium bacteria will usually provide its own N without supplement.
However, if soil N levels are low, (less than 100 lb N/a) and field pea specific Rhizobia bacteria are not present, field pea will become N deficient. N deficient field peas will show yellowing on lower leaves, with more serious deficiency moving up the plant, giving the plants a generally yellow to light green cast.
If deficiencies are found, it will be necessary to top-dress field peas with N. The preferred top-dress material is ammonium nitrate because it has relatively low ammonia volatility compared to urea. Urea is much easier to find commercially and is lower in price than ammonium nitrate. Liquid products, such as 28%, or 20% solutions have greater leaf burning potential and at high rates they should be used with caution. Application of liquids in a straight-stream band rather than a broadcast spray is one way to limit damage from foliar burn. Apply liquids during cool days, preferably when there is dew on the leaves to dilute the fertilizer. Expect some burn with liquid materials.
The problems we have had this season in obtaining specialized products for rotational crops, such as inoculum for field peas or ammonium sulfate for canola underscores the importance in future years of ordering early. The agricultural manufacturing and distribution network is not used to, nor will it soon be used to, the new Freedom to Farm rotational options that producers now have available. It puts pressure on producers to try to line up associated supplies earlier than in the past.
All crops in North Dakota require boron, an essential element. Some crops are more sensitive to low boron levels than others. Two indicator crops that are particularly sensitive to low boron are alfalfa and sunflower. However, there have been only very rare instances in North Dakota where boron deficiency in alfalfa has been suspected. In sunflower, deficiencies have been sought for decades by North Dakota soil scientists, but none have been documented.
The lack of documented deficiency is a reason why our recommendations for boron from a soil test become confused. In order for a region to establish critical levels for soil tests, where a response could be expected above a soil test level, there first must be a response to application. Since there have been no responses to boron in North Dakota, there are no reliable guidelines for use at various soil test levels. Soil testing laboratories are asked to analyze for boron and give recommendations based on the results. The reliable response curves in the US come from the southeast, Georgia, Alabama, Florida, etc., where boron levels are low and responses are common for many crops. The critical soil test level is about 1 ppm for hot water soluble boron. Many soils in the southeastern US contain no pool of active minerals that replace boron lost to leaching and crop removal. In North Dakota, our soils are mostly derived from marine sediments. Marine sediments are minerals that settled down through oceans, which are relatively high in boron. As a result, our soils have a store of high boron-containing minerals not found in the southeast. So although a soil test contains less than 1 ppm boron, the number probably means a different response probability than a similar test in Georgia. North Dakota soil scientists are simply not able to predict response because none have been found here.
So should a producer worry about boron? Probably not. Our soils have the ability to produce over 2,000 lb/acre of sunflower oilseed with no boron deficiency symptoms in years with good rainfall. Boron deficient soil would not be expected to achieve these yields. If a producer is curious about boron responses on a farm, try a small strip and apply no more than 1 lb B/acre, so that any boron toxicity from too much is limited if adequate levels are already present. Boron is one nutrient where the line between adequate and too much is very thin. However, our limits to greater sunflower production appear to be related much more to excessive salts, high water tables, inadequate N, weed control problems and insect challenges than lack of boron.
NDSU Extension Soil Specialist