clues about the need for supplemental fertilizer or po- 

 tentially toxic metal content. 



Table I illustrates the analyses conducted on com- 

 post by the UNH Analytical Services Laboratory and 

 typical ranges observed in samples submitted for assay 

 in recent years. Additional tests (e.g., organic-N and 

 trace metals) will be done at UNH upon request. 



Below is a brief interpretation of the most critical 

 measurements. 



Acidity or Basicity (pH) 



The pH of compost measures the acidity (or basicity) of 

 the product. This can range from (very acid) to 14 

 (very alkaline) with pH 7.0 as the neutral point The pH 

 of most composts is generally near neutrality and pro- 

 vides an important clue about the compost inputs and 

 process. If pH exceeds 8.5, the compost mix probably 

 included wood ashes or kiln dust as a feedstock. Plant 

 growth is likely to be hindered at extremely high pH 

 (>8.5) unless the compost is diluted with soil, peat, or 

 another amendment, bringing the pH into a more 

 ideal — for most container plants — range of 5.5 to 6.5 



Soluble Salts 



Compost which shows high soluble salt readings (e.g., 2-3 

 mmhos/cm) is probably "nutrient-rich," but high salts, 

 because of phytotoxicity, will preclude its direct use as 

 a plant growth medium. Often, high salts are accompa- 



nied by high levels of soluble potassium (K), since K 

 salts are notable for their high salt index. If manure has 

 been a feedstock, high K readings are common. Reduc- 

 tion of salts to tolerable levels may require that the 

 compost be diluted with low-salt materials (e.g., 1:3 

 compost: soil v/v basis) prior to its greenhouse use. If 

 the compost is only being used as a mulch, rains will 

 usually leach excess salts away from the root zone. 

 Plants are extremely sensitive to salts during the germi- 

 nation and seedling stages of growth, greater salt toler- 

 ance being exhibited by mature plants. 



Ammonia (NH^-N) 



Extreme caution must exist if a compost tests for high lev- 

 els of ammonia-N (NH^-N). High NH^-N tests are common 

 if composts contain significant quantities of rapidly decay- 

 ing N-rich organic matter such as manure or wastewater 

 treatment biosolids. Composts having high ammonia (NH^- 

 N) are often viewed as lacking "stability" or "maturity". Se- 

 vere plant injury can result at ammonia levels in excess of 

 100 ppm NH^-N if compost is used directly as a growing 

 medium This has been observed in ornamental plants 

 grown commercially in New Hampshire. If the compost is 

 diluted with other materials or used as a mulch, ammonia 

 injury is less likely. When composting proceeds under 

 good aeration or the finished compost is stored and 

 turned frequently, the ammonia levels usually drop rap- 

 idly, since it is converted to nitrate-N (NO,-N), which is the 



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