Soil pH is arguably most important factor in yield. It’s not so much that you’re adding a lot of nutrients, but that your plant can absorbs it even if you pay top dollar for high-end fertilizer. The way I think about it: pH is a dimmer switch, not an on/off button. It makes minerals accessible or locks them out of reach based off how acidic it is; it determines mineral availability.
As the chart indicates (above), different nutrients becomes available on each side of pH scale. Phosphate binds with iron and aluminum in acid soils, so most folks believe that applying phosphate will make their leaves green again. No such luck. All of that phosphorus sits there unavailable because it is bound up. In alkaline soils, phosphorus gets bound up with calcium.
Why Soil pH Is Important for Plants
To get the best availability of all nutrients, you want to be somewhere between pH 6.0-7.0. It is not a random number. That is the area where microbial action are best and where cation exchange works well. Hit that spot and plants has the opportunity to take up magnesium, calcium, potassium and nitrogen simultaneousy.
Soil bacteria is very important to nitrogen availability. Why? They convert organic materials into forms plants use, and these organisms operate less efficient in acidic soils. If you’re growing legumes, this becomes even more important, as rhizobial bacteria don’t like anything below pH 6.0.
Phosphorus availability is in the smallest sweet spot of any nutrient, leading gardeners (and growers) to fault it when yield disappoints. More likely, however, the cause was a change in pH outside that small window. Potassium remain soluble in a wider pH range and doesn’t get quite so picky… Unless acidity is extreme enough to displace potassium in sandy soils.
Magnesium and other so-called secondary nutrients (calcium) acts opposite: The higher the pH, the more available they are. So in addition to that other benefit, raising soil pH away from toxic levels of aluminum or manganese; liming also improve access to those nutrient. Tomatoes gets blossom-end rot if they don’t have enough calcium, which goes into their cell walls. And magnesium forms the core of each molecule of chlorophyll. Without magnesium, there is no photosynthesis. Look back at the nutrient bars above, you’ll notice how calcium availability widens as you move to the right along the chart toward neutral or just a bit on the alkaline side.
Lime is slow to react with soil particles. It may take many months to fully incorporate in the root zone, which is why timing is such an issue. If you amend in the fall, it has all winter to react before spring planting. Heavy clay resist changes in pH and buffers it. Sandy soils swing quickly, holding little organic matter or nutrients. This makes them more likely to need careful monitoring, as they can go from bad to worse fast. Test regularly (every couple of years) rather than let things gradualy drift toward nutrient lockout.
To manage soil, you must first manage what happens underground. If it’s not right for the plant to eat, you could of made it. Work WITH chemistry (keep things in their best pH range), don’t fight against it. Leaves will tell the tale; so will good root growth.