Dairy manure applications and soil health implications

Dairy manure applications can potentially improve soil health by adding organic matter (OM) to the soil. However, intensive dairy manure applications can cause salt accumulations on arid, irrigated soils, impairing soil health, which can reduce crop growth and yield. Soil organic matter, a major contributor to soil health, increased by 0.02% for every ton of manure-derived organic matter applied. While soil OM increases typically improve soil health, salt accumulations from manure applications had antagonistic effects on soil health. As manure application rates and frequencies increased, soil properties became increasingly saline-sodic, as indicated by elevated EC and SAR values. Aggregate stability also decreased at the heaviest annual manure application rate, likely a consequence of clay dispersion caused by sodium in the added manure. One concern with some of the new soil health tests is that they do not account for the negative effects of salt accumulations. For example, the Haney “soil health score” increased with increasing organic matter and nutrient content, not taking into account the fact that EC and SAR increased to levels above the recommended salinity/sodicity thresholds for salt sensitive and even salt tolerant crops. The decline in aggregate stability also revealed that soil structure was compromised at the higher soil health scores. A newly proposed N mineralization estimation procedure is the “Haney – Additional N” test, which relates carbon dioxide respiration measurements to soil organic C to organic N ratios to estimate how much N will be available to plants in addition to the nitrate and ammonium traditionally measured in preplant soil tests. Unfortunately, the “Haney – Additional N” test severely under-predicted mineralizable N pools by 20-fold. Thus, use of this test for estimating N fertilizer applications would drastically under-estimate plant available N in the soil, which would lead to greatly over-estimating the N requirement of a crop to be grown on that field. Including salinity and sodicity parameters should be considered in future soil health evaluation programs, especially in semi-arid irrigated regions like southern Idaho where saline and sodic soil conditions can occur