Dilution Can Be a Solution For Managing Phosphorus

For years we have been advocating the importance of proper agitation of liquid manure for the purpose of manure analysis. That message hasn't changed. However, for farms where the fields closest to the barn have high soil phosphorus levels, here's a different approach to manure application.

Take the top portion of the liquid manure storage and apply the manure - without agitation - to the fields with the highest phosphorus soil test levels.

Of course, as always, it is essential to take an analysis of what was applied. In many situations application of phosphorus dilute manure will help increase the application rates to fields closest to the barn, where application of manure is fastest. Then manure application of the bottom portion, which is lower in water content and higher in phosphorus levels, can be applied at a lower application rate to fields further from the storage. The additional benefit is the reduction of water being transported a greater distance.

You are probably wondering how this affects the other nutrients in the manure? The phosphorus in manure is associated with the solids portion of the manure, while the total nitrogen content doesn't change as much. The ammonium portion of the nitrogen is almost all in the liquids while the organic nitrogen is highest in the solids. Potash is more closely tied to the liquid portion of the manure.

Some example samples ) taken from the same manure storage are shown in Table 1. The more diluted manure was applied without agitation to pre-plant cornfields nearest the storage at a rate to meet about two-thirds of the nitrogen needs of the corn crop, which was 3,500 gallons/acre (gal/ac). At that application rate the phosphorus applied is less than what the crop will remove. Over the long term this should begin to reduce the P soil test. The rate that would be applied from the well agitated manure to the same field would be limited to just over 2,000 gal/ac (phosphorus limit) if applied every year, or 3,000 gal/ac to meet the same nitrogen needs. At the 2,000 to 3,000 gal/ac rate the soil test for phosphorus would continue to increase.

Once the un-agitated manure from the top of the storage has been applied in the desired fields, the remainder of the storage should be well agitated, sampled and applied at rates to meet crop needs.