Crop Scene Investigation - 23 Solved: Why did Arthur's soybeans get 'the streaks?'

Tuesday, March 2, 2010

by BERNARD TOBIN

The streaks in Arthur's soybean field were caused by the high levels of straw in the manure that he applied randomly on the field before planting soybeans.

Generally, manure makes a positive contribution to a soybean crop. But the excessive levels of straw in Arthur's manure, and the heavy rate of application, caused a high carbon (C) to nitrogen (N) ratio, which actually starved the soybean plants of N.

Before organic N can be taken up by the plant, it needs to be converted to inorganic forms. This process of mineralization is the work of soil microbes as a by-product of organic matter decomposition. The higher the C:N ratio of the cover crop residue – or, in this case, the heavily-strawed manure – the more slowly decomposition will occur, which leads to tying up of N, so making it unavailable to the crop.

Manure can play a key role in crop production, explains Pioneer Hi-Bred agronomist Scott Fife. "But any time you have really heavy residue on the top of your soil, it's going to act as an insulation barrier and that means the soil will take longer to warm up in spring. That is going to hinder emergence, especially in this case with the high carbon-to-nitrogen ratio.

"All those bugs and earthworms need to work on breaking down organic matter to release nitrogen for that early-season crop. But that can't happen if they are tied up breaking down straw," explains Fife.

"When it comes to manure, you need to spread it carefully and evenly.

It is a good thing to add all this organic matter back into the soil, but tillage is a good idea when your residue is really heavy, even if it is just a minimum till," he says.

Congratulations to Ray Brubacher, Massey, for his correct answer.

When he submitted his winning response Ray posed a question of his own: "I'm guessing that after nodulation started the problem disappeared?" Ray also wondered about the impact on final yield. 

Fife says the problem did disappear after nodulation began but the impact on yield "is impossible to know for sure due to the random nature of the problem and lack of yield maps." Fife adds: "this would have had a negative effect on yield in a 'high yield' type environment with maximum inputs where every stress makes a difference but likely had very little if any effect in this 'average yielding' situation." BF