I've been thinking about soil fertility as I do every spring. Yes, it's still wet and cold but mildly so compared to most of the nation. I have to mow the lawn all winter long since it never gets so bad that it won't grow a little.

The problem that I've been working is that I have mildly acidic soil and nearly everything makes it worse. Doing nothing at all makes it worse since plants exude hydrogen ions to stay in charge balance as they take up nutrients. The faster the plants grow the more this is so. Anything I do to increase fertility and help plants grow better makes it worse since they grow faster.

As acidity changes so does nutrient availability. Some nutrients become more available and others become less available depending on their charges. Some are freed from their association with minerals while others get locked up. To assess soil you have to think ahead and anticipate how nutrient availability will change over time. There are frequently discussed and well known changes in nitrogen availability as organic matter decomposes and frees it, rhizobia draw it down from the air, and other bacteria consume it and send it back to the air. Phosphorus is less often discussed.

Some fertilisers have a liming effect built in, for example Reactive Phosphate Rock (RPR). Phosphate rocks that are sufficiently soluble to apply directly to the soil (RPR) have a reasonable amount of calcium carbonate in their structure (calcium carbonate = lime). Added to this, the process of converting the phosphorus in RPR into the soluble form that is available for plant uptake consumes Hydrogen ions and this also reduces acidity. It is estimated that the liming value of 1 kg of phosphorus from RPR is roughly equivalent to 3.2 kg’s of lime.

Not all rock phosphate behaves this way. It depends on its chemical composition. The word reactive is important since it is used to distinguish such rock from similar but much less reactive forms - often called hard rock phosphate - and it only works when the soil is a bit acidic. If it isn't acidic you likely wouldn't want the liming effect anyway.

There may be costs as well as benefits. The increase in calcium may interfere with magnesium uptake. Everything you do to make something better makes something else worse. The trick is to find some balance and try to maintain it over time as conditions change during growth and nutrient uptake.

Other rock dusts often used are dolomite, calcium magnesium carbonate (CaMg(CO3)2); and calcite, calcium carbonate (CaCO3). They don't provide phosphorus but they may alter the availability of phosphorus locked in the soil. It depends on initial conditions.

Phosphorus availability is commonly lower in strongly acidic and alkaline soils because of increased P reactivity with soil and formation of insoluble compounds with aluminum and iron in acid soils and with calcium in alkaline soils. The pH associated with the maximum P availability in soils usually is between roughly pH 6.0 to 7.0. . .

You may have plenty of phosphorus in your soil but not much that is available as a plant nutrient if the PH is too high or low. This means that you might be able to achieve the effect of adding phosphorus by just getting the soil PH right. In my acidic soils - which get ever more acidic with plant growth - I can free up some phosphorus for plants by liming to raise PH. Lime is cheaper than RPR and locally available, which saves shipping costs too.

This is one of the reasons for my interest in biochar and clean wood ashes. Both can raise PH as well as providing some residual nutrients depending on the feedstocks used and processing details. You have to test it to really know what you are getting just as you do any soil amendment no matter whether it is compost, manure, rock dusts or biochar. Since everything affects everything else you can't ignore the details. And with biochar some of the effects are long lasting though variable over time.