Economics of conservation tillage and soil productivity in North Central Iowa: a control theory approach

The major difficulty in the soil productivity issue is quantifying the user cost of soil erosion. Previous research has failed to answer the question of what soil is worth for crop production. The price of a unit of soil is a costate variable computed in this study by a deterministic optimal control model and found to range from 30.00 to 65.00 per cm. per acre in North Central Iowa, depending upon the tillage method and whether the farmland is managed by society, an owner, or a renter.;Another difficulty is the availability of data showing the long-term effects of soil erosion upon yields. The degradation of yields is critical in determining user costs. Previous studies have relied on data from farmer surveys or expert opinion. In this study, yields were simulated over time by an agronomic model that provides an index of corn yields based upon root growth and nutrient availability as soil erosion alters the bulk density, water holding capacity, and depth of topsoil.;Because the user cost of erosion is greatest in the present, the optimal time to adopt a soil conserving technique is now or never. This finding contradicts the conclusions of some previous research which used a nonoptimal simulation over time to predict that conserving practices would be adopted in the future. The findings in this study are also qualitatively different from previous theoretical work which postulated a continuous erosion control variable. The farmer's decision is modeled as a choice between discrete tillage practices and a change in the user cost does not necessarily imply a different tillage practice or rate of erosion. Alternate land tenure arrangements may alter the user cost, but rates of soil erosion may be the same.;Finally, previous research which does model the discrete nature of the tillage decision has either been static or not truly dynamic and has been unable to provide user costs. The methodology developed here provides reliable estimates for this most important aspect of the soil productivity issue.