Seed Bed: Are you getting the most from your manure?

Tuesday, August 4, 2009

Not all manures are created equal. Knowing their different characteristics, and when and how to use them, will help you get more value from your applications

by KEITH REID

You are probably sick of hearing the truism that "Manure is a valuable resource," especially if you have just spent a couple of days emptying the manure pit – again. And while it is hard to argue with the fundamental truth of this statement, we knowthere is a huge variation in the value of manure to different operations.

However, applying some knowledge about the nature of this material can help you to capture a bit more of that value for yourself.

The first thing to realize is that not all manures are created equal. The nitrogen (N) in manure is divided between the ammonium form, which is the same as fertilizer N, and the organic form, which must break down to release mineral N before crops can use it. In general, liquid manure has a higher proportion of ammonium N than solid manure. Within these classes, manure from cattle and sheep has the highest proportion of organic N, while swine and poultry manure has more ammonium N. This proportion is also influenced by the type and amount of bedding material that is used, and the treatment and storage of the manure. Digested manure, for example, will have a much higher ammonium N proportion than the raw manure entering the digester. 

Optimum application timing. When the manure is applied will not make much difference to the availability of phosphorus or potassium from the manure, but it will make a big difference to nitrogen. Liquid swine manure, which has most of the N in the ammonium form, will have the greatest N availability if it is applied close to the time the crop needs the nitrogen. This may be immediately pre-planted in the spring, or even side-dressed into the standing crop.

Compaction is often a concern with spring applications, but the use of a drag-line rather than tankers for application can help to alleviate this.

In contrast, solid beef manure, with most of the N in the organic form, will benefit from more time for N mineralization to occur. It may actually have the greatest N availability from application in late summer or early fall of the year before. 

Optimum placement. Getting manure quickly covered with soil will help retain ammonium N, irrespective of manure type, but the relative amount of ammonium and organic N will have a big impact on how important this is.

If the manure (or compost) has very little ammonium N, then losing a large portion of the ammonium by leaving the manure on the surface will have very little impact on the amount of N that is available to the crop. Injection, or immediate incorporation, is much more important for liquid manure with a high ammonium N content.

Of course, retaining the ammonia from a spring application translates directly into reduced fertilizer N requirement, while retaining ammonium in the fall may just be trading one form of N loss (volatilization) for another (leaching or denitrification).

Optimum field selection. The field that will show the greatest benefit from manure application is one growing a crop which can use all of the nutrients applied in the manure. Traditionally, this has meant a crop like corn, on a field that was low in P and K.  More often, however, choices need to be made between fields with a broader range of crops and nutrient requirements, and these should be taken into account in choosing the best field to apply.

Take the example of a liquid dairy manure, applied in the spring at a rate of 5,000 gallons per acre. A crop that could use all of the nutrients in this application would get a benefit of $193 per acre, but this farm has one field with very high P and K soil tests, which is growing corn, and another field at the back of the farm growing alfalfa, but with very low soil tests.

The value of the manure on the corn would only be $58 per acre, since there would be no direct value of the P and K, while there would be a value of $135 per acre to the alfalfa even though it does not need any of the nitrogen.

On this farm, the manure should be applied to the alfalfa first, and only the left-over manure applied to the corn ground.

One additional factor difficult to quantify is the value of the organic matter in the manure. Fields low in organic matter will generally show greater response to added manure, both in improved soil quality and in crop yields, but these increases are not as consistent as nutrient responses.

Optimum rate. The biggest challenge with manure, aside from the volumes that need to be handled, is that you cannot change the nutrient con-centrations to match crop requirements. There is also variability of nutrient content and availability within the manure and variation in application rates.

Growers who depend completely on manure to meet their crop requirements for nitrogen are often over-applying phosphorus and potassium, and may still fall short of the N requirements in some parts of the field.

One technique to overcome this is to use manure for only a portion of the N rates (up to 75 per cent), and balance the rest with mineral fertilizer. This combination will often generate higher yields than either manure or fertilizer on its own.

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, based in Stratford. Email: keith.reid@ontario.ca