Manure Rate for a Nitrogen Target

Manure is a fertiliser with two complications: its analysis varies enormously between sources and even between loads, and only part of its nitrogen becomes available in the season it is applied. Both have to be in the calculation or the answer is decorative.

Availability is the term that surprises people. Manure nitrogen exists as ammonium, which is immediately available, and as organic nitrogen, which mineralises over years. First-season availability typically runs 30–50% for solid manures and higher for slurries injected promptly, but it collapses if the manure is surface-applied and left — ammonia volatilises within hours, and a hot day can take half the available nitrogen before it is incorporated.

The nutrient-management trap in this calculation is that meeting a nitrogen target with manure almost always over-applies phosphorus. Manure carries phosphorus and nitrogen in roughly equal measure while a crop removes several times more nitrogen than phosphorus, so an application sized for nitrogen delivers two to four times the phosphorus removed. Repeated annually, soil phosphorus climbs to levels that pose a runoff risk, which is why nutrient management regulations in most jurisdictions cap manure on the phosphorus figure and let the grower top up nitrogen from fertiliser.

Test the manure rather than using book values. Between-farm variation in nitrogen content is easily threefold depending on species, bedding, diet and how much rainfall has run through the pile, and a book figure can be out by more than any other term in this equation.