Can I reduce my fertilizer applications?

If you have results from manure and soil tests, you can credit manure nutrients or calculate an application rate.

Credit manure nutrients: Multiply the amount of nutrients in the manure times the amount of manure applied times the proportion of the manure nutrients available. In other words:     test value: Pounds of nutrients per ton or per 1000 gallons of manure X application rate: Tons or 1000 gallons of manure applied per acre X % availability: % of nutrient available in first year (see below) = nutrient credit: pounds of nutrient per acre available in first growing season. Subtract this number from the fertilizer recommendation based on your soil test and crop needs.

Calculate an application rate: Decide whether the application rate will be based on the N or P needs of the crop. Multiply the manure test value of that nutrient times the % available (see below). Divide the fertilizer recommendation for the nutrient (in lbs/acre) by the amount available in the manure (in lbs/ton or 1000gal). The result is the application rate in tons or gallons per acre. In other words: fertilizer recommendation manure test value X % available =  manure application rate

What proportion of manure P and K is available?

70% to 80% of the phosphorus and 70% to 90% of the potassium in animal wastes is available to plants during the first year.

What proportion of manure N is available?

Percent of nitrogen in applied manure that is available to crops:
SWINE Broadcast Injection   NI* <4d <12hr sweep knife Available 1st yr 35% 55% 75% 80% 70% Available 2nd yr 15% 15% 15% 15% 15% Total N lost 50% 30% 10% 5% 15%	POULTRY Broadcast   NI <4d <12hr   45% 55% 70%   25% 25% 25%   30% 20% 5%
DAIRY Broadcast Injection   NI <4d <12hr sweep knife Available 1st yr 20% 40% 55% 55% 50% Available 2nd yr 40% 40% 35% 40% 40% Total N lost 40% 20% 10% 5% 10%	BEEF Broadcast Injection   NI <4d <12hr sweep knife   25% 45% 60% 60% 50%   35% 35% 35% 35% 40%   40% 20% 5% 5% 10%
*NI = not incorporated <4d = incorporated within 4 days after application <12hr = incorporated within 12 hrs after application Source: Michael Schmitt, personal communication A couple of notes. . . The table above shows nitrogen availability for only the first two years after manure application. In reality, residual nitrogen from manure may continue to elevate soil nitrogen in subsequent years. This is best quantified by using the residual nitrogen soil test. The calculations that generated this table are based on the climate in Minnesota. Decomposition rates and nitrogen availability will differ elsewhere.

Is fall better than spring?

There is no single answer to the question of when to apply manure. On sandy soils, fall application can lead to significant nitrogen losses and groundwater contamination. On other soils, nitrogen losses after fall application are similar to losses after spring application. Practical concerns, such as time and equipment availability, usually determine when you apply manure. Large storage systems or composting give you more flexibility in timing an application. Below are some considerations about each season.

Spring

Because there is less time for decomposition, a smaller proportion of nutrients will be available in the first year. Total losses over several years may be somewhat lower than with fall application. Spring is a tight time for labor, so manure application may delay other fieldwork. Soil compaction is a serious risk when applying manure on fine-textured or wet soils.

Fall

Fall application gives microbes time to decompose the manure—making nutrients more available to crops and preventing germination concerns. On the other hand, there is more time for nitrogen loss. Avoid fall application on coarse-textured soils or in karst (limestone sinkhole) areas.

Winter

Labor is generally available and compaction is not a concern in the winter. Because incorporation is impossible, nutrient losses can be high. Nutrient runoff from snow melt or rain is a serious problem when manure is applied to frozen, sloping soil. However, some farmers have found that the mulch created by a manure crust prevents wind erosion.

Summer

Summer application requires a crop rotation that leaves a field available for manure. This could be a fallow field, a pasture, or a crop that is planted late or harvested early. High biological activity and plant growth in the summer reduce the loss of nutrients from the rooting zone.

Is it worthwhile to compost manure?

Composting manure has several advantages that may make it worth the extra effort, especially for organic growers.

• Composting raises the pH of manure and reduces odor.
• Nutrients are released more steadily over several years.
• Composting substantially reduces the bulk of manure. Its low water content and high nutrient concentration make compost easier to handle than uncomposted manure.
• Weed seeds, fly eggs, and pathogens can be reduced if high temperatures are generated during the composting process.
• Compost does a better job of encouraging the growth of the soil organisms that improve nutrient cycling and soil structure. The low pH and high salt level of fresh manure can disrupt soil life.