In preparation for refreshing a garden box for a winter season, I tested for nutrients and found it was very low in nitrogen. I added more soil and amendment (for other reasons -- neither has many nutrients) as well as blood meal (to add nitrogen). If I were to test it again for nitrogen, should I expect it to be improved? I did test again and it showed almost no nitrogen -- much less than before. But is that just because the nitrogen in the blood meal takes time to become available? Or could it be I've done something wrong?

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Blood meal is not a quick fix, it's a slow release product https://plantcaretoday.com/blood-meal-fertilizer.html, unlike some other proprietary chemical (as opposed to organic/natural) fertilizers, so testing the level is probably not going to be useful because it won't change much. There are other variables which might explain why the nitrogen level fell after application; the time gap between application of whatever other amendments you made might be a factor, as well as precisely what other materials you added. For instance, if you added some composted material which contained fresh woody pieces, and didn't retest for a couple of weeks to a month, the woody material will take up nitrogen as part of the decomposition process. Equally, other partially composted materials may take up some nitrogen (depending on composition) as they break down. High rainfall will also wash out some nutrients such as nitrogen.

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