Meeting Nitrogen Demand in an Organic Cash Grain System Using Legume Cover Crops.

We used nutrient budgets to test the hypothesis that legume cover crops can meet the nitrogen demand for a cash grain system in upstate New York.  We also measured the trajectory of nitrogen, phosphorus, and potassium over 8 years in four cropping systems in a long-term organic grain experiment. 

We conducted input-output budgets for the three macro-nutrients, nitrogen, phosphorus and potassium, for two rotations in four cropping systems: (1) High Input – in which a farmer maximizes yields by adding extra nitrogen in the form of compost and corn starter, (2) Low Input – in which a farmer maximizes profit by using only legume cover crops for nitrogen inputs, with no additional amendments besides corn starter, (3) Weed Management – Same nutrient management as “Low Input,” but in this system a farmer prioritizes reducing weeds trough high tillage intensity, and (4) Reduced Tillage – In this system a farmer attempts to improve soil structure by growing crops using a “ridge till” reduced tillage method.  Compost additions to this system were variable in an attempt to compensate for poor cover crop performance.  In the High Input system, we expected excess nitrogen, phosphorus, and potassium.  In the Low Input and Weed Management systems, we expected a net nitrogen balance but deficits of phosphorus and potassium.  In the Reduced Tillage system, we expected a small surplus of nitrogen, and net balance of phosphorus and potassium.

We found support for our hypothesis that legume cover crops can meet the nitrogen demand of cash grains in upstate New York.  In the Low Input and Weed Management systems, nitrogen inputs from legume cover crops were roughly equivalent to nitrogen export in grain.  However, those two systems have a net export of phosphorus and potassium.  The High Input system and the Ridge Till system both have net surpluses of nitrogen, phosphorus, and potassium.  In the High Input system, the surpluses are due to excess inputs, and a farmer in this situation could save money and improve environmental quality by reducing inputs.  In the Ridge Till system the surpluses are due to low yields.  Here, avoiding crop failure will prevent nutrient surpluses.

Nutrient budgeting tools can help farmers optimize cover crop management and reduced tillage methods to maximize both environmental and economic sustainability, by determining the inputs necessary to maintain crop yields and soil health, while avoiding nutrient surpluses or deficits.