Phosphate and potash increase fruit yield on tomatoes?
It really depends on the state of your soil. If the pH is off, the plants don't get the right amount of moisture, or the soil is compact or low in organic matter, extra fertilizer may increase yields, but then, it might not. The only way to know for sure is to have your soil tested once a year before you add fertilizer. That way you can avoid nutrient imbalances that can interfere with growth and yields, and can actually save yourself the trouble of buying, hauling, and applying fertilizer that your plants already have plenty of in the soil.
Generally, though, tomatoes require quite a large food supply over the season. You are on the right track thinking that potassium and phosphorous will help with fruit set and fruit health; steer clear of fertilizers with a very high nitrogen content. Too much nitrogen results in tall, dark green plants with few tomatoes. Tomatoes can benefit from a side dressing of fertilizer a few times throughout the growing season.
Generally, this side dressing is applied when the first tomatoes have just formed and every three weeks after that. When side dressing apply the fertilizer by making a circular furrow approximately 5 to 6 inches away from the main stem of the tomato, or in a trough alongside a row of plants. Work the fertilizer into the top 1 to 2 inches of the soil. The next rain or watering will carry the fertilizer to the root zone of the tomatoes. Consult your soil test and the fertilizer label to determine how much to apply.
From ArcaMax Publishing Home and Garden.