NUTRIENT MANAGEMENT IN SOYBEANS
SOYBEANS ARE THE largest row crop in the province and remove a tremendous amount of nutrients from the soil every year. A 50 bushel per acre crop will remove 40 pounds per acre of phosphorous and 70 pounds per acre of potassium. Despite this fact most soybean fields do not receive fertilizer. Soybeans can only yield well if nutrient levels are adequate. There is increasing evidence that very high yielding soybeans will only be achieved with excellent soil test levels. Fall is a good time to apply nutrients. Below are the big ones to consider:
Nitrogen deficiency is due to a lack of proper nodulation. This most often occurs in fields where soybeans are being grown for the first time. But nodulation can also be delayed if the weather is extremely cold like it was in 2009. Plant symptoms will include stunting, and a pale green colour to the plant, particularly in the older leaves. Applying nitrogen fertilizer is not economical. However, applying inoculants even in fields with a history of soybeans provides a return in Ontario. In 2009, fields with a history of soybeans yielded an extra two bushels per acre when inoculants were used.
There are no obvious visual symptoms of phosphorus deficiency, although the plants may be stunted and darker green in colour. Any soil with less than 10 to 12 parts per million will respond to phosphorous fertilizer.
Visual symptoms will generally show up in the older leaves as yellowing followed by browning of the leaf margins. If a leaf deficiency has been identified, take a soil sample to determine the level of potassium that should be added to the field for future crops. There is no way to supply enough potassium during the growing season to improve yields the year symptoms occur.
Manganese is the only micronutrient deficiency commonly found in Ontario. Symptoms are pale green to yellow leaves developing on the upper part of the plant with veins remaining dark green.
A foliar application of manganese is recommended when visual symptoms occur. Soil application is not successful because of the large amounts required. Trials conducted in 2009 showed up to eight bushels per acre response in fields that were truly manganese deficient. •