Soil Fertility and Nutrient Use: Adjustment to Fertilizer Recommendations

The general fertilizer recommendations in this book apply to situations where no organic sources of nutrients have been applied to the field. If manure or biosolids are applied to the land, or if legumes are plowed down, reduce the fertilizer rates to adjust for the nutrients applied in the organic form.

• Adjustment for Legumes Plowed Down
• Adjustments for Manure Application

Adjustment for Legumes Plowed Down

When sod containing perennial legumes such as alfalfa, birdsfoot trefoil and clover are plowed under, they supply an appreciable amount of nitrogen to the following crop. Table 9-7, Adjustment of Nitrogen Requirement, Where Crops Containing Legumes Are Plowed Down, opposite page, shows reductions that should be made in nitrogen fertilizer applications to crops following sod containing legumes.

Adjustments for Manure Application

A large number of Ontario farms produce livestock, generating over 33 million tonnes of manure annually. Proper management of the nutrients from manure is essential for optimum economic benefit to the farmer with minimal impacts on the environment.

Estimating Nutrients Available to the Crop From Average Values

The best way of determining the amount of each nutrient from manure is to analyze a sample. Unfortunately, this is not always possible, as in the case of a new barn. In this case, average values will provide an estimate of the nutrients available to the crop.
Table 9-8, Typical Amounts of Available Nitrogen, Phosphate and Potash From Different Types of Organic Nutrient Sources, provides estimates of available nutrients from various types of manure. It is based on the average results from manure analyses at the five accredited labs in Ontario. Nitrogen is reported as available N under various application systems. Phosphate and potash values are reported as nutrients available to replace fertilizer nutrients. These values can be used as the starting point in crediting nutrients from manure application.

The availability of manure N to the crop depends on the proportion of ammonium and organic N in the manure, as well as the timings of application and incorporation. The ammonium nitrogen in manure is chemically the same form of nitrogen as in many mineral fertilizers and is immediately available to the crop. Unfortunately, the ammonium form is also subject to loss by volatilization if not incorporated immediately. The balance of the nitrogen in manure is in the organic form, which becomes available to crops gradually as the organic compounds break down.

More precise estimates of available nutrients can be made by accounting for the actual timing and conditions for manure application, and the lag time before incorporation. The worksheet, Calculating Available Nutrients from Spring-Applied Manure Using a Manure Analysis, or the NMAN software, can help with this process.