Wheat Straw Options

Wheat harvest

Wheat harvest is progressing depending on rainfall. Both yields and quality appear to be good this year. Some farmers are wondering what to do with the wheat straw? Should they keep the straw on the field to build soil organic matter (SOM) or should they sell it? What is the value of the wheat straw and how many nutrients are being lost? Does straw residue hurt the next crop? 

Straw is a valuable resource in high demand for bedding or mulch and livestock farmers even use straw to add fiber to their livestock rations Some farmers chop it and let it lie on the field, others bale it. If planting double crop soybeans or cover crops, removing the straw may be beneficial. High carbon straw may slow down plant growth (especially chaff) and it also attracts pests like slugs and voles. 

If selling straw, make sure to sell it for what it is worth. Straw has value as bedding but it also contains soil nutrients. At a minimum, straw sellers should consider the value of nutrients leaving the farm plus plus the harvesting cost. The nutrient value of straw varies but 11 pounds of nitrogen (N), 3 pounds of phosphorus (P2O5) and 20 pounds of potassium (K2O) per ton can be lost on fields that yield 2 to 2.5 tons straw per acre. Based on fertilizer values of $.60 (N)-$.60 (P2O5)-$.25 (K2O) per pound, the nutrient fertilizer value is about $13.40 per ton of straw removed. On a typical field yielding 2 to 2.5 tons wheat straw per acre, that is $26.80 to $33.50 of nutrients removed per acre. Some secondary and micro-nutrients are also removed and they also add value. Nutrient content varies based on environmental conditions and growing season. 

We use to think the straw was beneficial to building soil organic matter (SOM). However, we now know that it is mainly the roots, root exudates, and soil microbes that build SOM. Dr Christine Jones recently clarified how soil organic matter degrades and is sequestered. The top layer of soil is where most organic matter input occurs from plant residues, leaf litter, and other organic materials. This organic matter is decomposed by microbes and is not as stable as deeper layers, composed of primarily roots. 

In this top layer, there is a high rate of carbon mineralization where microbes break down complex organic compounds into simpler substances, releasing carbon dioxide (CO₂) as a byproduct. A wide range of decomposer organisms, including bacteria, fungi, and actinomycetes, actively break down organic matter, releasing nutrients that can be taken up by plants. There is a high turnover rate of organic matter in this top layer leading to a dynamic and constantly changing microbial community focused on breaking down organic inputs. This upper organic layer provides the carbon dioxide for photosynthesis for plants to grow. 

The stabilized carbon is where most soil nutrients are stored and is where soil structure is improved. In the lower layers of the soil (below 8 inches), carbon is incorporated into more stable forms. This is largely due to the lower availability of fresh organic matter and the distinct microbial communities adapted to these conditions and here the main source of carbon is root exudates derived from photosynthesis. Dead microbial cells and their byproducts, becomes a significant component of SOM in these layers. This carbon becomes stabilized through interactions with soil minerals. These associations protect the organic carbon from further decomposition forming stable soil aggregates that improve aeration, water infiltration, water holding capacity and improved root growth to lower depths. 

The higher the plant diversity, the better this process will progress. When selecting cover crops to stimulate long-term stable carbon, include as many plant families as possible into the mix dominated by forbs. Forbs are broad leaf plants with soft not woody stems. Many wildflowers are considered forbs. The prairies were dominated by forbs and grass and were examples of deep well drained soils. The two most important benefits derived from deepening the soil aggregation is the improved water infiltration and water holding capacity as well as the access to deeper available mineral nutrients down to 40 inches deep. 

Farmers wanting to add forbs as a cover crop should add clovers, vetches, sunflower, buckwheat, rape, and kale. Sunflower, kale and rape are deep rooted cover crops while the buckwheat has short shallow roots and the clover and vetches are intermediate. Some agricultural sites consider radish a forb, others sites do not; but radish are another good deep rooted cover crop that breaks up soil compaction. Cover crops keep soil from eroding, improve nutrient recycling, and can build up your SOM.