PSNT now more than ever on manured fields
If you reduce nitrogen based on manure, 2011 will be an important year to take a PSNT and double check for N credit.
The real value of the PSNT is in years like this; we have no idea what the weather has done to alter past manure nitrogen credits.
Farmers who have used the presideress nitrate test (PSNT) in the past may have found a pretty consistent pattern of nitrogen credits for corn on their farm. Fall, winter and spring manure applications generally show various nitrogen credits based on the time of year manure is applied, but the credit is generally consistent from the same time frame, on the same farm with the same manure and the same general handling and hauling pattern.
Spring applied manure generally provides most nitrogen credit compared to other application timings. The nitrogen credit, however, is usually not realized for several months after application, when the corn plant begins active growth and nutrient uptake. The time period between application and plant uptake is always a vulnerable time for N loss. This year there are four competing factors determining the N credit from manure applications:
- Did the soil warm up enough for the ammonium form (and mineralization of the organic fraction) of nitrogen to convert to nitrate, which is leachable? This may have happened last fall. We know that this spring, air temperatures have been cool, so has your soil ever had time to warm up enough and for long enough, to convert ammonium to nitrate? Soil temperatures need to be above 50 F for several days to cause this conversion. There have been some random short bursts of warm weather.
- If it did convert to nitrate, have you had rain to the point of saturation, and then leaching? This is more apt to happen on sandy soils or tile drained soils.
- Have we had saturated soils (and warm enough temperatures) to the point that nitrogen has denitrified, thus losing nitrogen up into the air? This is more apt to happen on heavy clay soils, but has been apparent on all soil types, including sands this spring.
- Has surface applied manure moved off fields with runoff rain water or soil erosion? Runoff and in-field ponding has also been obvious this year. If manure was surface applied, it may have concentrated in a low area and be gone from the rest of the field.
Each field has a range of soil types, drainage patterns and residue levels that can impact nitrogen credits from manure applications. The presidedress nitrate soil test can quantify the amount of nitrogen remaining in the field at sidedress time to help producers manage the nitrogen fertility levels for the growing crop. PSNT is not very accurate if the entire nitrogen fertilizer program has been pre-plant applied or applied at planting. It works best for those who planned to sidedress the majority of their nitrogen in corn.