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Soil Considerations for 2021 (09/24/20)

Prevent plant acres to 2021 crop. Sampling before soybean. Gypsum for amending soil pH and an amendment for reducing soil salt.

Prevent plant acres to 2021 crop-

- Fields tilled, but no cover crop- soil test by zone. Sampling can be done at any time.

Growing corn after a ‘black fallow’ prevent planting, or a radish-only cover crop in prevent planting

may result in ‘fallow syndrome’, which is P/Zn deficiency caused by limited mycorrhizae. Corn must

have a starter P/Zn application at planting to overcome this condition. A broadcast P application is

not enough.

- Fields in cover crop – wait until cover crop is terminated, then sample. Nitrate test will decrease with

cover crop growth. Prevent plant with a grass cover crop should not have a problem with fallow

syndrome.

 

Sampling before soybean

 Many growers don’t sample before soybean, since soybeans make their own N thanks to symbiotic N-fixing bacteria (Bradyrhizobium) that growers inoculate their seed with, or have previously grown well-nodulated soybean in the same field. But this is a mistake if iron deficiency chlorosis (IDC) is possible. IDC is possible in eastern ND if soil conditions are favorable. The IDC is a result of soil carbonates that can greatly reduce the ability of soybean to take up iron (Fe) if the soil moisture is high enough to dissolve the carbonates. To screen fields for susceptibility to IDC, soil pH is the first indicator. Soil pH less than 7 means IDC is not possible. If soil pH is greater than 7, the IDC is related to the amount of soil carbonates. In addition to carbonates, the soluble salt content of the soil (EC) increases the stress on the soybean and the IDC severity. If EC is greater than 2, the chances of profitable soybean production are low even if IDC is not a problem. In addition, high soil nitrate (>80 lb N/acre) can increase the severity of IDC.

 The list of soil test items to consider before soybean are:

   Soil pH

   Soil test P and K

   Soil carbonates (optional, but good to know)

   Soil EC (essential)

The soil sampling should be conducted by zone.

Knowing these soil factors will help growers determine if planting soybean in a field in 2021 is a good idea, and if IDC tolerant varieties need to be considered.

 

Gypsum for amending soil pH and an amendment for reducing soil salt-

Under extremely low soil pH (less than 4.5), gypsum can be used to raise the soil pH to about 4.5. However, 4.5 is still too low for profitable growth of all North Dakota crops. Limestone is the appropriate amendment to alleviate acid soil pH. The reaction of limestone with acid forms water and carbon dioxide, neutralizing the acid. The reaction of gypsum with acid, forms sulfuric acid, which is an acid, and does nothing to increase pH to crop productive levels.

People also confuse the activity of gypsum in amending sodic soils when considering how to increase the productivity of saline soils. Gypsum is a salt, and it is impossible to neutralize a salt with a salt. If sodium is a problem, gypsum can help, and there is an NDSU gypsum replacement app that can help determine the rate to apply. But gypsum has no activity on salts. To determine whether the soil issue is salinity or sodicity, sample the area at a series of depths and analyze for soil EC and either SAR, or its proxy- the % sodium as a part of the soil base saturation.

 

Dave Franzen

Extension Soil Specialist

701-799-2565

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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