Note to readers, attached is the complete final report for FNE99-264
The Martens are organic farmers. They sampled the soil under six of their fields at intervals, from February to December of 1999. The samples were analyzed for available P (weak and strong Bray extractions), Ca, Mg, K, Zn, Cu, B, Mn, Fe, S, and for organic matter. Their suspicion was that variations of weather and season may substantially impact nutrient availability.
The results of their analyses show levels of available P doubling, in cropped soils with no additions of fertilizer, between early February and late May. This was presumably an effect of burgeoning microbial activity, as the soil warmed in the spring. Levels of available P remained high through the summer, even though one might have expected the drought of 1999 to impact soil microbial activity, then fell off again as the soil cooled in the fall.
Soil levels of Ca and Zn declined over the summer, in response to crop uptake. This may be true of B and Cu as well. Most other nutrients displayed no such trend, probably because they are present in quantities far greater than the crop requires. S levels rose sharply after applications of gypsum (calcium sulfate), but fell again just as abruptly, showing that even strongly organic soils like the Martens’ have difficulty retaining the sulfate anion against elution. Available Fe showed a steady decline from mid-March through mid-August. This probably reflects oxidation from Fe++ to the insoluble Fe+++ form, during the prolonged drought.
Organic matter rose from a low of 3.2% in mid-March to a high of 4.3% in mid-August, then fell again to 3.5% by late October. The steady rise during the summer shows an accumulation by growing plants in excess of the degradative activity of soil microbes. Dominance of the latter in the fall, when plants have essentially stopped growing, accounts for the late season decline.
The trend with regard to phosphorus is the Martens’ most important finding. It shows that under organic conditions, and probably under others as well to a lesser extent, a low early-season reading of P availability does not necessarily call for application of P fertilizer. The ability of soil microbes to liberate P from unavailable forms is considerable.
This project continues as FNE00-313.