Calcium and Phosphorus

Calcium and phosphorus are two of the most important nutrients for plants.

Blossom end rotOne of the most common “disease” symptoms in tomatoes – blossom end rot – is really a sign of calcium deficiency.

Conventional gardening and farming largely ignore these, especially calcium, in favor of short-term remedies.

Organic gardeners often ignore them, too, in favor of organic matter such as compost.

We need organic matter, but we need the minerals, too.

These foundational minerals build the optimum environment which soil biology needs in order to flourish.


Calcium is one of the most important minerals for both plants and microbes, probably the most important.

Yet we know there’s really no such thing as one mineral being the most important because they’re all important. Interestingly, it is the most needed element by weight and volume for plants, and without sufficient calcium, nothing works.

While nitrogen and potassium get a lot of attention, more and more focus is being put on calcium, particularly in organic horticulture.

Calcium helps plant cells communicate with each other by physically moving between cell membranes.

Not only is it integral in the basic structure of plants, with a deficiency often showing up as thick, woody stems, its largely responsible for the availability of nutrients in plants and has a strong influence on microbial activity.

In the book Mainline Farming For Century 21, Dan Skow says “calcium is essential for its energy creation potential in the soil to release the other elements that cause a plant to grow.”

With enough calcium, roots and fine root hairs proliferate, stimulating soil microbes and building humus. This means that without enough calcium in the soil plants can’t access or utilize nutrients very well.

Bruised appleIf your fruit bruises easily, you may want to check your soil calcium levels.

Likewise, foliar fertilizers and microbial inoculants will have little effect if there’s a major calcium deficiency. You can waste a lot of time and money on fertilization if you don’t have sufficient calcium.

We’re looking for 60-75% calcium on a base saturation test, the lower end of the range on sandier soil.

If your calcium is less than 60% on a base saturation test, there’s a good chance your soil will be compacted and riddled with grassy weeds, your soil food web will be unhealthy, your plants will be sick, and your fruit will be weak and easily bruised.

In fact, the list of things that fall into place when the calcium to magnesium ratio is in line is really too long to put down here.

Microbes need this ratio to be in line in order to create a soil that is free of compaction. The microbes need to be there in order for the calcium to be available, too. You can pile on bags and bags of calcium and you won’t get anywhere if you don’t have the humus and microbes to make use of it.

A calcium shortage needs to be improved before other nutrient ratios will be fixed.

For example, adding sulfur in the form of gypsum or ammonium sulfate can bind with excess magnesium and leach it, but you need sufficient calcium for this to happen efficiently.

Calcium is associated with nitrogen fixation and amino acid formation, so low calcium means the nitrogen cycle in the soil will also be less efficient and nitrogen will more easily leach.

Lots of calciumPiling on tons of calcium to make up for dead soil is exactly what conventional agriculture does.

Some gardeners and farmers are so in love with calcium they think you can’t have too much, so they may apply it annually without much thought. This is a bad idea because if you use too much calcium, other nutrients will become much less available or even get leached out of the soil.

And too much calcium can also create so much air space in the soil that it’s difficult to keep it wet.

Let’s look at the main calcium sources we could bring in if a soil test and garden observations tell us we need it.

Liquid Calcium and Micronized Calcium

The most common calcium sources are to follow, but first I wanted to mention liquid calcium because it’s one of the most important.

Liquid calcium is becoming more and more popular in ecological agriculture as part of foliar spray mixtures and soil applications. There are many forms, but the most common is generally from liquefied calcium nitrate, which is 9-0-0 with 11% calcium.

It’s not technically considered organic, but it’s one of the rare synthetic products that’s worth using. There are organic versions available, too, such as one from calcium lignosulfonate and others from micronized calcium carbonate.

There are also inferior versions such as liquid lime and calcium chloride that I avoid.

Liquid calcium products are applied in very small doses because they’re so readily available to be used by microbes and plants and can be spread out so evenly that not much is needed. Often, 1/3-2/3 cup per 1,000 square feet is all that’s used (or even less).

These products can be difficult to find, but if you need calcium, I recommend you take some time to try to find one. The price is often $20-$30 for a quart.

They should be mixed with sugar (such as molasses), microbial inoculants, and some of the biostimulants you’ll be reading about soon, especially liquid fish or sea minerals, and fulvic acid. Some products have some of these ingredients included already.

Liquid calcium is useful in spring and especially in fall, to stimulate bacteria that break down organic matter residue.

Calcitic Lime (30% Calcium and 4% Magnesium – Varies)

Calcitic lime, also known as high-calcium lime, calcium carbonate, or ag lime — or aragonite, which is actually ground seashells — is the main rock fertilizer used to increase calcium levels, especially when the base saturation test is low.

CalciteCalcite formations in Turkey.

As mentioned, some people are moving to liquid calcium as the first choice, but be forewarned it’s not always able to bring calcium levels up high enough.

Calcitic lime is not to be confused with hydrated lime or quicklime, which aren’t approved in organic gardening standards. They can be beneficial, but they can burn crops, as well as your skin.

Calcitic lime contains approximately 25-35% calcium and a few percentage points of magnesium. Some soil labs may recommend 45 pounds per 1,000 square feet, while some soil scientists like Dr. Arden Andersen, author of Science in Agriculture, advocate starting out by using less, such as 10 pounds, perhaps applied more often.

Neal Kinsey of Kinsey Agricultural Services recommends calcitic lime as high as 180 pounds.

Personally, I stick to the lower end of the scale because every time we add any fertilizer, we impact the soil environment and the soil food web. As I’ve already mentioned, I prefer to use fertilizers slowly and work on the soil food web for further stimulating fertility.

I’ll use 10-45 pounds per 1,000 square feet in my garden in one year, and some people would advocate even less.

Actually, I follow both of these pieces of advice by keeping the application rates low and spreading them out during the course of the year. I may apply 10 pounds per 1,000 square feet, as often as twice in the spring about four weeks apart and twice in the fall.

There’s one circumstance in which I may apply calcitic lime without a soil test — when I see an explosion of weeds that indicate a calcium deficiency, such as dandelions and crabgrass. But I’d usualy take a soil test anyway.

In North America, calcitic lime should be between $7 and $20 for a 50-pound bag, so it’s definitely not too expensive for a typical residential garden.

The price depends on whether you buy it from a farm supplier or from a retailer.

Dolomite Lime (22% Calcium and 12% Magnesium – Varies)

I don’t know how dolomite lime has come to be used by so many gardeners. Maybe it’s just because the dolomite industry has done a wonderful marketing job.

Maybe it’s because it was promoted by some of the early organic proponents who have had a big influence on the rest of us. It’s in the same price range as calcitic lime, but personally, I almost never use it.

It’s approximately 22% calcium and 12% magnesium, a 1.8:1 calcium to magnesium ratio, whereas calcitic lime is approximately 30% calcium and 4% magnesium, a 7.5:1 ratio.

Too much magnesium in the soil can cause nitrogen to volatilize into the air and soil to compact. Dolomite generally brings in too much magnesium for what we need.

If you had a soil with extremely low magnesium in relation to calcium, such as a 12:1 calcium to magnesium ratio, then it might make sense to use dolomite.

DolomiteUnprocessed dolomite stone.

Gypsum (22% Calcium and 17% Sulfur)

Gypsum is calcium sulfate.

I’m not referring to drywall, which contains ingredients that are toxic to plants, nor the chemical version of calcium sulfate, which also shouldn’t be used.

Gypsum is a sedimentary rock that is fairly common and is mined in many places. It’s in the same price range as calcitic lime.

The sulfur is in the sulfate form containing one sulfur molecule and four oxygens, readily available to plants.

I use it for the sulfur when I need it. And I use it very often as a calcium source along with calcitic lime and soft rock phosphate. It helps bring more oxygen into an anaerobic soil, so it’s even used when calcium is adequate.


Phosphorus is the other most important mineral.

It’s generally present in the soil but is often unavailable in soils with a low organic matter content and a poor soil food web.

Having active biology in the soil is probably the most important factor in getting phosphorus into plants. Carey Reams taught that calcium and phosphorus are two of the most limiting elements in soils.

Phosphorus is the element P on the periodic table. Fertilizers show phosphate rather than phosphorus on their labels. Phosphate is the main form of phosphorus that plants use.

Actually, fertilizers show available phosphate, which is the phosphate that is more readily available to plants, not tied up. Available phosphate is P2O5, two atoms of phosphorus attached to five atoms of oxygen.

In NPK fertilizers, for example, the middle number represents available phosphate as a percentage of the weight of the bag. Multiplying that by 0.44 gives us the amount of phosphorus. If the middle number is 10, the phosphorus is 4.4% of the weight of the bag.

Soil tests may give you the phosphorus number or the phosphate number. The nutrient testing lesson showed how to tell which is which.

Along with magnesium, phosphorus is necessary for photosynthesis.

PhotosynthesisPhotosynthesis requires sufficient mineral nutrients to work.

It’s in every living cell. It’s the major catalyst in all living systems, which means its presence is vital for many other reactions to take place in the plant, and for many other nutrients to get utilized.

For example, it promotes more photosynthesis and higher brix. The phosphates not only help produce the sugar but also bring it to the roots where it’s excreted to soil microbes. Then, the microbes make more nutrients that become available to the plant, so the plant can make more sugar.

It also circulates throughout the plant, up and down, carrying other nutrients to where they’re needed.

For example, calcium bonds with phosphorus to create phosphate of calcium. The phosphate brings the calcium through the plant and drops it where it belongs. It transports all nutrients throughout the plant with the exception of nitrogen.

Phosphorus is also needed in order for carbon and nitrogen to be made into all of the things they’re made into — amino acids, proteins, enzymes, vitamins and all of the things that are the basis for plant health and the health of every living thing on earth.

According to many in the ecological agriculture world, the ratio of phosphate to potash is ideally 2:1 in general, not 1:2 as many labs recommend. And it should be more like 4:1 for grasses and leaf crops, such as lettuce and greens.

Phosphorus cycleThe phosphorus cycle relies on living organisms.

When it gets lower than this, plants will suffer and be more prone to insect and disease damage, and there will be more broadleaf weeds such as plantain.

The only way to maintain adequate available phosphorus levels is to have a biologically active soil. Fungi are the main harvesters of phosphorus from the soil environment.

Most forms of phosphorus are not soluble in water, so it’s mostly not found floating around in the soil solution.

It’s an anion, so it can attach to positive charges on organic matter. More often, it binds with calcium to form calcium phosphate, or with other cations like magnesium and iron.

Because phosphorus doesn’t move down into the soil, it’s nice to incorporate phosphorus fertilizers into the top few inches.

A big problem is that there’s not a whole lot of phosphorus left in the world. Like oil, it has peaked, perhaps about 20 years ago.

Phosphate mineAn open pit phosphate mine.

Should we be using what’s left? I don’t know. Someday, we’ll all have to become biodynamic gardeners so we can use the energies we have on site and use homeopathic doses for our gardens and farms. We should start that now.

At the same time, there are two other methods of maintaining phosphorus levels and availability in the soil.

The first is that we need to learn to make high-quality compost that will supply phosphorus and microbes that make phosphorus available. We should even be composting human manure to recycle that phosphorus, among other nutrients.

The second is that we need to promote mycorrhizal fungi in our soil and bacteria that specialize in moving phosphorus. The fungi are one of the most important pieces of the puzzle in most of our soils when it comes to getting phosphorus into the plant.

Bone meal has been a main phosphorus source of gardeners for a long time. I don’t use it anymore due to the potential of spreading the prions associated with mad cow disease. Many ecological gardeners are still recommending it, though.

Bone mealBone meal may contain bits of nervous tissue, which carries the hard-to-destroy prions associated with mad cow disease and CJD.

Another one I use is molasses.

If a conventional soil test indicates there’s a lot of phosphorus in the soil, but a Reams test indicates there isn’t enough available phosphorus, molasses can be applied to the soil at 2/3 cup per 1,000 square feet, mixed with as much water as you need, to help “loosen” the phosphorus.

Rock Phosphate (Mainly Phosphorus and Calcium)

There are two main sources of phosphate we use in organic gardening.

Both of them come from the same parent material, which is ancient animal bones. They contain a lot of both phosphorus and calcium, although the amounts can vary greatly depending on the source.

They are called hard rock phosphate and soft rock phosphate, and they cost $20 to $40 for 50 pounds, twice as much as many of the calcium sources.

Diammonium phosphateRock phosphate in granular form looks almost identical to diammonium phosphate, a synthetic fertilizer with high levels of nitrogen. Don’t mix them up!

They can be difficult to source, but some farm supply stores have them.

I wish the most important products were more readily available, but I’m sorry to say that’s not always the case. I still have to recommend them because they’re the most effective. Unfortunately, it may be a bit of work for you to find a couple of them. It’s work for me, too.

Hard rock phosphate is the more commonly available of the two, containing approximately 30% phosphorus and 30% calcium.

It’s mostly tricalcium phosphate and is difficult for plants to get out of the soil because the phosphorus is bound tightly with the calcium. The calcium will become available over time, but we don’t use it to rapidly change the ratio of calcium on the cation exchange sites.

The phosphorus is only 3% available and takes many years to become available to plants. It’s often ground to a fine dust and granulated with chelators that are supposed to make it more available to plants, but it’s still not the best.

I’ll use it in a pinch, though, in order to build up long-term phosphorus reserves. In my experience, it won’t give as effective a result as soft rock phosphate.

If you do use it, be sure you’re also focusing on coaxing together the most active, healthy soil food web you possibly can. Of course, it’s a good idea to do this anyway.

Hard rock phosphate is also treated with acids to create chemical fertilizers like superphosphate (0-20-0) and triple superphosphate (046-0).

These are more available to plants, but 85-90% of the application will be leached, wasting money and polluting the environment, not to mention the other soil problems these fertilizers can cause.

Soft rock phosphate, on the other hand, is a by-product of hard rock phosphate mining.

Old phosphorite mineAn old phosphorite mine near Ulgase Vllage, Estonia.

It’s so much better than hard rock phosphate that I’ve driven hours just to go get a couple hundred pounds. It can be difficult to find.

Traditionally separated from the hard rock as an impurity and trucked away to holding ponds, it’s now considered superior to the hard rock form because it’s colloidal, meaning it’s in a form that won’t leach and doesn’t tie up as tightly as hard rock phosphate.

Like hard rock phosphate, it may only be listed as 3% available, but over a few years, it’s 100% available to plants.

It does combine with calcium, but not in the unbreakable bond that hard rock creates. It’s also a good source of silicon and many trace minerals.

In addition to the colloidal type, there’s reactive soft rock phosphate.

There’s a myth that this type is not as good, and that it ties up calcium, but neither of these is true. In reality, it may actually be more available to soil microbes and contain less heavy metals.

I say go with what you can get, colloidal or reactive. Either will help loosen up your soil like a loaf of bread rising in the oven.

Loose soilLoose soil is easier for young sprouts to penetrate, as well as having better air holding capacity which supports healthy microbes.

Like calcitic lime, it’s often recommended at 10-45 pounds per 1,000 square feet. I like to spread this out into at least two applications throughout the year or add some of it to the compost.

You can get a powder form and sometimes a granular form. The best products come from Idaho, North Carolina and Tennessee. I really like the powder, although it’s a bit tricky to spread.

Pick a calm day. In the book The Non-Toxic Farming Handbook, Phil Wheeler and Ron Ward advise to lay colloidal phosphate and then calcitic lime in order to create an energy that kills surface weed seeds.


  • Calcium and phosphorus are two of the most important soil nutrients.
  • One of our first goals should be balancing the calcium to magnesium and phosphate to potash ratios.
  • Liquid calcium, calcitic lime and gypsum are three of the best sources of calcium, while soft rock phosphate is the best source of phosphorus.
  • Dolomite lime, in my view, is usually inappropriate as a fertilizer.
  • If you can’t find them locally, which can be a challenge, I sell some of these products here.