Tone Ag Consulting Ltd.

With the latest revision to Manitoba’s manure regulations, manure application is restricted on fields with high soil test phosphorus.  If a field regularly receives manure on a nitrogen basis, the soil test phosphorus levels will rise.

N:P2O5 Ratios

The biggest factor in rising soil test phosphorus is the N:P2O5 ratio of the manure versus the N:P2O5 ratio needed by the plant.  The N:P2O5 Ratio is the proportion of Nitrogen to Phosphate.  I have converted the ratios to N:P2O5 because P2O5 is the form of phosphorus used by the plant (multiply phosphorus by 2.3 to get the phosphate value). Here are some typical N:P2O5 Removal Ratios for various crops.

Adapted from FPG for Pig Producers page 149.

Here are some typical manure values

Adapted from Tri-Provincial Manure Application and Use Guidelines page 4

Thus your typical crop removes 2-4 times as much nitrogen as it does phosphate.  However, manure only supplies about 1.5 times as much nitrogen as it does phosphate.  Here’s an example.  A farmer wants to plant spring wheat and his agronomist recommends that he apply 100 lbs. of N per acre for optimum crop growth.  The farmer uses liquid dairy manure to fertilize his field to the recommendation.  Using the manure ratios above, 100 divided 1.6 means that he will be applying 62.5 lbs. of P2O5.  The crop grows and is harvested and it removed exactly 100 lbs. of N per acre.  Using the N:P2O5 removal ratio for spring wheat, the crop removes 100/2.5 = 40 lbs. of P2O5.  This leaves 62.5-40 = 22.5 lbs. of excess P2O5 in the field.  This is why soil test phosphorous levels rise on manured fields.

Phosphorus levels rise on manured fields because manure supplies nitrogen and phosphate in roughly equal amounts and crops use less phosphate than nitrogen.

What can you do about it?  Here are some of the options.

• Reduce the nutrients in the manure.  I will be writing more about this later, but the key to this approach is tailoring the animal’s diet to ensure that there is a minimum of wasted nutrients.
• Apply manure based on the crop’s phosphorus needs.  In order to produce a good crop, you will have to supplement the crop with commercial fertilizer.
• Apply the manure based on the crop’s nitrogen needs.  In order to keep the phosphorus levels under control, you will have to rotate which fields receive manure.  As a rough estimate, fields should only receive manure one year in three to five.
• Separate the manure into nitrogen-rich and phosphorus-rich portions.  The phosphorus-rich portion will also have fairly high N.  Apply nitrogen-rich portions to fields with low nitrogen and high-phosphorus fields, apply phosphorus-rich portions to fields that are low in both nitrogen and phosphorus.  By concentrating the phosphorus, you may be able to export it to fields that are further away without paying prohibitive hauling costs.

All of these approaches can be useful and most of them can be used together.  I would strongly recommend taking a good look at your animals’ diet as the first step.  This doesn’t involve anything complicated and can even cut your feed costs!   Give us a call and we can give you a hand.