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I have some russet potatoes that seem to be doing well growing in a plastic tub.
(This photo was taken near sunset; they do get 8+ hours open sun.)
I have a choice between these three fertilizers, and was about to go for the 7-4-5 but figured I'd run it by.. the world. which is the right one?
The best fertilizer for potatoes in the choices listed above is the tomato and vegetable fertilizer. Generally, you should have the most phosphorous, then nitrogen, and the least potassium in the ratio. Potatoes are an underground crop, which means they need lots of phosphorous. In a container, they will require more nitrogen than in the ground, because it washes away easily during waterings, and most growing mixes do not contain much.
Too much nitrogen is harmful to potatoes, causing lush top growth at the expense of tubers. In the above question, the tomato and vegetable fertilizer appears to have the closest to ideal ratio, because you have the plants in a tub.
If you were growing them in the ground, the ideal ratio would be different, and it would also differ according to soil makeup and existing nutrient levels. Where I am, the best fertilizer seems to be a slow-release 6-3-8 potato fertilizer. I don't have the best soil around, but I average 2 1/2 lbs. of harvest per foot of row. That's quite good, actually.
Your plants look as if they have gone without fertilizer for a long time. You should start fertilizing now if you want them to produce. Put on 1 1/2 times the rate which is instructed, because the plants are crowded. This will help them catch up a little.
