Many organic vegetable producers have been relying heavily on livestock composts as a source of fertility on farm. Often, high rates of compost are applied to meet the nitrogen (N) needs of crops. When this strategy is implemented it can lead to over application of phosphorus (P) and potassium (K). As an example, a grower may apply poultry manure at 6 tons ac-1 for 3 consecutive years. This contributes 225 lbs ac-1 of P per year, where vegetable crop removal of P ranges from 10-80 lbs ac-1 per season. In this scenario, there is an over application of P, leading to an excess of 900 lbs ac-1 in 3 years in cases where the soil already had sufficient P levels. After multiple seasons of using composts, P levels may accumulate in the soil to the point where applying additional P poses an environmental risk to nearby waterways. Phosphorus loading and associated risk depends on soil type, slope, and proximity to water. However, with impending water quality regulations, farmers will be required to account for their nutrient balance.


Vermont, University of Vermont

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