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I did a soil test and as expected my sandy soil came back low in almost everything but calcium with 8.4 pH and very low organic matter.

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I'm not worried about micronutrients as I could probably foliar feed them and/or use chelated ones.

My issue is with N-P-K-Ca-Mg. I'm having a very hard time figuring out how to fix those deficiencies when some can block others.

Let's say I start with a good balanced 20-20-20 (20gr per seedling for example). If I add Calcium, it may compete with Potassium and Magnesium and the same goes with adding Magnesium. And I'm not even mentioning Na if I use saline water for irrigation.

I understand that I should add organic matter, but are there any recommendations on how to give fertilizers while avoiding as much interaction as possible.

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Competition and interaction are inevitable, but they are also not so bad (usually).

Note: also pH effects on how easily root could get nutrients.

The rule is just to give enough nutrient for the plant you have. So you may give some more of the less competitive nutrients (in case of interactions). It is not easy because every soil is different, until you get a balanced soil (and so also small fertilizer quantities).

In your case, I would start with organic matter. This is the most important part: it creates a good environment for the bacteria/fungi which should deliver fertilizers to the roots. And also organic matter should capture fertilizers (sandy soils are permeable to water, but also to soluble fertilizers). (Note: you should not target "saturation").

Then, start with some fertilizers (maybe some which long lasting, so that they will remain more in the terrain, also in case of rain). Try to search for acid fertilizers (if you want to correct your pH toward some more neutral).

But after this, you need observation. Competition about nutrients is also plant specific. If you notice that plants lacks some nutrients, just add them. And so you correct the competition.

If you are brave, you can google about fertilizers on many crops and orchards (less frequent about garden plants): universities make studies about that, and with formulas on how to balance all nutrients (with most interactions). Note: these are crop specific. On the other hand, I doubt this is useful (but if you have huge fields, so you may get to fertilize again, which may take few days, and you will have harvest a week late, so you will be hit financially).

And liquid/foliar fertilizers can help us very quickly (but too expensive for normal crops).

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