One way to determine the value of straw left in the field is to look at the nutrients that are available in the straw. There are several online tools available to estimate the amounts of N, P, and K that will be removed if straw is bailed. For every bushel of wheat, you can assume that you will produce somewhere between 70 to 85 lbs. of straw. This estimate will be closer to the lower end of the range with shorter varieties and/or when grain yields are high and closer to the upper end of the range with taller varieties and/or when grain yields are low.
The amount of N, P, and K removed in straw are listed in Table 1. For a 50 bushels wheat crop, you will remove just about 2 ton of straw. This is equal to 28 lbs. of N, 6.6 lbs. of P2O5, and 48 lbs. of K20. Removing straw does not necessarily mean that you will mine your soils. Only if of the amount of nutrient removed is greater than the amount of nutrients applied, will you mine the soil. You can estimate how much you are mining by subtracting the total amount of nutrients removed in the grains and straw from the amount of fertilizer applied.
Table 1 - Pounds of N, P, and K removed per 2000 lbs. of wheat straw.
Quantifying the value of Soil Organic Matter (SOM) is more difficult that the calculations for soil organic matter. Higher SOM levels mean better soil aggregation, improved water infiltration, less compactability, less erodibility, and a generally higher level of productivity due to more available water and nutrients.
Each percent decrease in SOM equates to about 1 inch decrease in water holding capacity , about a loss of 1,000 lbs. of N, and decreases in availability of P and S. Removing straw doesn’t immediately result in loss of SOM as you will never remove all organic matter when you bail. Figure 1 shows how much straw can be removed to maintain the SOM using reduced tillage and moldboard plow.
Figure 1 - Amount of straw that can be removed without loss of Soil Organic Matter (Johnson et al. 2006).