Plant-available nitrogen is highly dependent on application rates, method of application, timing and type of manure. Michigan State University experts offer some points to consider when contemplating application methods with an eye toward maximizing manure’s fertilizer benefits.

The ammonium form of nitrogen in manure is readily available to plants. However, most of the ammonium will volatilize when surface-applied during hot, dry conditions, such as application on wheat stubble in late summer. Under cool, damp soil conditions, such as spring and late fall, more of the ammonium nitrogen is retained in the soil. Injecting manure into the soil or incorporating within hours of application greatly improves the retention of the ammonium fraction, as will a light rain after applications. According to MSU experts, manure injected in the spring should retain almost 100% of the ammonium in the manure.

Producers can add up both the estimated ammonium fraction of manure with the amount of mineralized nitrogen for a total nitrogen credit to the current crop. A pre-side-dress nitrate test (PSNT) is the best method to measure the total nitrogen from manure applications, according to MSU experts. It will measure both the ammonium and organic fraction from manure as they are converted to nitrates in the spring. Fall stalk tests can be an indicator of too much or insufficient nitrogen and can be used to double-check manure and nitrogen rates on corn.

Try combining manure and inorganic fertilizer applications so that the future crop’s needs are met and not exceeded. Reducing or eliminating commercial fertilizer use on any fields where soil tests indicate that these nutrients are not needed, or where manure has already been applied to supply the necessary nutrient amounts, reduces the risk of nutrient loss. Matching the combination of commercial fertilizer and manure will save money, without sacrificing yield. To gain the greatest value from manure nutrients, it must be applied to fields that need the phosphorus and potassium.

When planning manure use for profitable crop production, MSU experts suggest applying manure to the lowest-testing fields for greatest benefit from all manure nutrients. Producers should take time to conduct soil tests in less than 20-acre increments every three years. Manure should also be tested, with manure samples taken annually, or at least frequently enough to monitor changes that may occur over time.

Application rates should be adjusted in order to supply crop needs and meet water-holding capacity of soils. Manure spreaders should be calibrated to apply the desired rate and to allow monitoring of application accuracy.

Train farm employees and family members to make sure the desired manure nutrient rates are consistently applied to the fields. The MSU manure management team strongly encourages producers to apply manure evenly and consistently, and to treat it like the fertilizer it is. Do not apply manure in a manner that creates ponding or soil erosion. Manure applications should be evaluated and recorded.

More manure application information is available at