Outcome of Maintaining nutrient balances in systems utilizing soluble organic fertilizers

By using the same substrate in all the treatments (as opposed to the different substrates used in Miles (2000), it was possible to separate the effects on pH and EC of organic fertilizers from the effects of organic additions to the substrate in the previous experiments (vermicompost, bloodmeal, bonemeal, potassium sulfate, limestone, gypsum). Conventional fertilizers used for tomato production generally drive down substrate pH, while Miles (2000), suggested that organic fertilizers may have the opposite effect on soil pH. Thus, instead of adding lime at the generally recommended rate for conventional fertilizers of 10 lbs . yd-3. (6 kg . m-3), the rate of limestone used was less than that recommended for conventional practices, but more than was reported to be optimal for organic production (Miles, 2000). In the present experiment, we therefore expected below-optimal pH in the conventional treatments and above-optimal pH in the organic treatments.

However, without organic additions to the peat/perlite substrate, and with these three fertilizers, pH was slightly below-optimal in all treatments, with no significant differences between treatments in substrate pH after 1 month. In fact, by the end of the experiment, pH was closest to optimal (6.1) in the NCS blend. Low pH may have somewhat reduced availability of Ca and K, although changes in availability over this range are fairly small, and pH levels did not seem related to differences in tissue concentrations (Tables 2,3). In future experiments, we would somewhat increase the level of lime addition to the substrates, but it seems unlikely that higher lime levels would significantly change the results of this experiment. This suggests that substrate additions are a more important factor contributing to the high pH and initial EC values seen in our earlier study. Different fertilizers were used in that study, however, so it may have been a combination of fertilizers and substrate additions. In one experiment in the previous study, the vermicompost was omitted from the media without reducing the pH or EC, so the other additions were the most likely source of these high values. Overall, combining the results from all our work with organic production practices, we would recommend careful testing of all substrate/fertilizer combination for effects on substrate pH and EC and adjusting substrate pH with lime or sulfur as necessary. We recommend a minimal number of substrate additions to increase fertility. Overall fertility was easier to control with drip irrigation than with substrate additions, however. Even in our final study, where substrate additions were much lower, initial EC was high (4), then declined. This can lead to burning, then subsequent nutrient stress. For tomatoes grown in soilless media, constant feed with drip irrigation, or minor adjustments to control plant growth seem to work better.

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