Maximizing Crop Productivity and Soil Fertility Benefits in Vegetable Systems Fertilized with Organic Amendments and Processed Organic Fertilizers.

Many vegetable systems have a high nutrient demand and low nutrient use efficiency resulting in excessive fertilization and nutrient surpluses. Using organic amendments (composts and manures) as the primary nutrient source can be challenging in these systems because their effects on soil fertility are typically more complex to predict than those from processed fertilizers (e.g., urea, blood meal). This is especially true when estimating the fraction of nitrogen (N) that is “plant-available” (PAN) and trying to balance N and phosphorus (P) inputs given the low N:P ratios found in organic amendments. We compared the effects of four systems on soil fertility – PAN availability, soil P – and productivity of vegetable crops with increasing N requirements (beet > cabbage > cauliflower). The four systems were:

1) Low compost (municipal compost matching crop P demand);

2) High compost (municipal compost matching crop N demand);

3) Manure (composted chicken manure matching crop N demand);

4) Hybrid (low compost + blood meal to match crop N demand).

We observed soil N mining and poor yields in the low compost system that was unsustainable despite a balanced P budget. Among other systems, soil N availability was substantially higher in manure and hybrid systems, although beet and cabbage yields were as high or higher in the high compost system. In contrast, cauliflower yields were 25% lower in the high compost system compared to the manure system, consistent with the greater N availability measured with manure. Both high compost and manure systems resulted in substantial P surpluses (48-134 kg P ha^-1), although we measured soil P build-up only with manure. Our results suggest that municipal compost can maintain crop productivity while limiting environmental impacts for crops with low N demand, whereas the use of processed fertilizers would provide significant benefits only for crops with a high N demand.