Manure and organic amendments provide nutrients, organic matter (OM) and contribute to increased biological activity. Manure pH influences what happens to the nutrients once they are applied to soil. Crop available nutrients are influenced by both manure and soil pH.
A manure analysis provides value information for nutrient management planning. Typically, dry matter (DM), total nitrogen (N) and ammonium nitrogen (NH4-N), total phosphorus (P) and potassium (K) are tested. Results, when combined with soil test information, can help to set application rates and determine additional fertilizer needs. Including pH and C:N ratio in the analysis gives insight to manure nutrient availability/loss and nutrient interaction with the soil.
In the last issue of Manure Manager, we covered Part II of our “Measure the Treasure” series, focused on how C:N ratio affects N availability from the organic N portion of manure. The balance between carbon and nitrogen in the manure helps to predict whether organic nitrogen will be mineralized (released) or immobilized (tied up) during the growing season how much plant available N could be anticipated for the current crop.
Manure pH is an indicator of how quickly ammonium N (NH4N) may be lost through volatilization. Knowing the pH supports better decisions around manure rate, timing, and impact of incorporation for maximizing available N. Manure pH can also influence phosphorus and micronutrient availability, especially in soils with very high or low pH levels.
pH affects nitrogen availability and microbial activity
pH drives the balance between ammonium (NH4+ and ammonia (NH3). As manure pH moves above 7.5 there is a shift to a higher proportion as NH3 gas, which accelerates volatilization losses. These losses are boosted in warm, windy conditions with surface applied manure.
Liquid manure with high NH4-N and high pH applied on a warm windy day can lose a significant portion of nitrogen within a few hours of application while losses for manure with more neutral pH (<7) applied in the same conditions occur more slowly. In Ontario field trials studying spring/late summer applied manure, the NH4-N loss in the first few hours was higher with manure pH 7.9 than accumulated NH4-N loss in a week with manure at pH 7 or lower. Solid manure tends to have higher pH levels (often >8), however the NH4-N levels were also low.
Manure mineralization occurs through microbial processes. Microbial populations, both in manure and in the soil are pH sensitive which impacts how quickly nutrients become plant available. This can affect the timing of nutrient supply relative to crop demand. Manure that is neutral to slightly alkaline (high pH) typically mineralizes N and P more quickly, while manure pH 6 or less can slow microbial activity and delay nutrient release.
Managing high manure pH
Although it is difficult to change manure pH, knowing the pH allows management considerations that can reduce rapid volatilization losses and prevent crop N deficiency. They include:
- Application timing of liquid manure to when conditions are cooler or application can occur into standing crops;
- Placement of manure by injecting, rapid incorporation or application into standing crops;
- Adjusting supplemental nitrogen for crops relying on manure N when conditions during application were favourable for high N loss;
- And, potentially, the use of nitrogen (nitrification) inhibitors (currently being researched).
pH affect on phosphorus and micronutrient availability
Manure phosphorus, mainly in organic form, must be mineralized by microorganisms to become available. Soil pH has the bigger influence, however manure pH can impact areas with manure/soil contact by slowing phosphorus availability.
In calcium-rich, high-pH soils, calcium binds with P, reducing P availability to crops. For example, the use of alkaline-stabilized biosolids (e.g., N-Viro or ashed biosolids) precision applied improves low pH soils but applied to field areas with high pH (eroded knolls) can reduce phosphorus solubility. Availability of micronutrients such as zinc, manganese, copper and iron can also be reduced in high pH conditions.
Manure pH affects how much N is lost to volatilization, how phosphorus behaves in the soil, micronutrient availability and microbial mineralization rates. Including pH in an manure analysis can help fine tune nutrient availability and identify when additional fertilizer may be required. •
