Last week I talked about taking soil samples around my garden and sending them into a lab to have them analysed in a soil test. Today I’m going to talk about the results (shown above). This is a long one, and it’s gonna get geeky, so I hope you have a cup of coffee and some time to sit with me while I figure this all out.
I know dirt doesn’t seem like a very interesting subject. That’s certainly how I felt when I first began studying soil science. But the absolute honest truth is no matter how great you are as a gardener, how much time you spend doing it, or how much money you pour into it, you will always be somewhat disappointed by poor soils.
I did the tested my soil so I can grow the best possible crops. Now it’s time to work out how to optimise them for plant growth.
Quick disclaimer: I figured out the following recommendations myself. These are not based on anyone’s advice, and I may be wrong about stuff. Feel free to comment at the end if you know more than I do and see a problem I don’t.
Soil test results
First of all – just to be clear – these results come from undisturbed soils around the gardens, rather than the beds we grow in. I wanted to get our base readings – what we’ve started from. The garden beds themselves have been amended with manure, compost, green manures, and lime already.
I’ll start with the good news because there’s not much of it. The soil test showed we’re in a good range for magnesium, and have a pH of 6.44.
But we have a lot to work on. Our nitrogen, calcium, and phosphorus are too low. And our sodium and potassium are too high.
I don’t think anything here is going to be too difficult to make some improvements on. Compost, manure, and worm castings applied regularly should improve most of these numbers over time. But they are so low to begin with that I’m going to give each bed some additional amendments to improve how my soils function.
I’m balancing a few concerns here. I want to
- prioritise organic-based options
- add as few things as possible
- keep costs down
- avoid making the world to suck more because of my choices.
Gardening is also a long-term activity. The decisions I make now will be checked up on in a couple of years with a follow-up test. Adjusting one thing can affect another, so not everything needs to be solved right now. Overall improvement may take decades.
So with those criteria in mind, let’s get into it.
pH is probably the first factor to consider when thinking about what I want to do. pH has a major effect on a plant’s ability to take up nutrients to begin with. Each plant has a different ‘ideal’ pH, but in general, being between 6 and 7 is good.
We have a pH of 6.44. This is some of the best news from the soil test. Often areas that were once New Zealand native bush are very acidic, but my garden is near perfect.
Our measurement means we have some room to move either way. That’s worth knowing when applying amendments which might change or be affected by the pH.
Ideal: 40-80 (Nitrate), 40-80 (Ammonia)
Our reading: 2 (Nitrate), 11 (Ammonia)
Nitrogen is a macronutrient found everywhere, but it needs to be ‘fixed’ in the soil for the plants to access it. Plants need heaps of nitrogen. It helps with chloroform production, which is the plant’s main source of energy. Too much nitrogen makes your plants very leafy, but with too little, they’ll struggle to grow at all.
Nitrogen is measured in the test as both nitrate and ammonia. Nitrate can be fixed by adding fertiliser, but ammonia is really a measurement of the bacteria and fungi in my soil who produce it as they digest organic matter. The soil test shows my garden naturally sucks at both.
If I add synthetic nitrogen fertilisers, it’s likely to hurt the ammonia content long-term. Synthetic fertilisers are rarely good for developing microbial life. I have to stick to organic sources of nitrogen if I want to boost the ammonia.
But I know the former owners did a lot of work on this in the existing garden area. And I know I’ve been working on it too. It’s likely the developed beds are better than the tested soil already. As we established each bed, we’ve hoed in several buckets of cow manure. Then, each bed has had an additional layer of compost.
To boost nitrogen, we’ll continue to add a compost layer twice a year. At the moment we use about 40 litres per square meter annually.
We’ll leave it on top, rather than working it in. And I’ll include some nitrogen-fixing crops like broad beans and red clover into my crop rotation as green manure. These techniques will feed and encourage microbial life, naturally increasing the ammonia content along the way.
I’ll see how stuff grows, but if I feel anything needs a boost, I’ll give it fish meal or a seaweed fertiliser. Overall, this is a pretty easy one to adjust.
Our reading: 336
Potassium keeps plants strong and helps them resist disease. It helps a lot with plant growth by forming and moving starches, sugars and oils in plants; and can improve fruit quality. Too much of it can mean the plants have trouble getting other nutrients they need, making them more susceptible to pests and disease.
The soil test showed that our potassium is too high. It’s good to know though – most fertilisers contain high amounts of potassium, so this information informs what products I’ll choose for other deficiencies.
But, I’ve still got heaps, so how do I fix that?
Well, our potassium readings actually explain a few things. Vegetables that are high in potassium include broccoli, pak choi, tomatoes, potatoes, and cucumbers. These happen to be all the crops I’ve done really well in this year – I’ve even written blogs on my broccoli, tomatoes, and luffa (a relative of cucumber). They’ve all been pretty suceptable to pests or disease too.
Continuing to grow these crops in my rotation, while taking care to avoid adding more potassium through other fertilisers will help reduce the potassium levels over time.
Our reading: 801
Calcium is essential for root health, growth of new roots and root hairs, and the development of leaves. Without calcium, the plant can’t access nutrients, even if they are available in the soil.
The soil test showed we need to improve the calcium content of our soil. The most common methods for doing this are applying lime or gypsum. Lime is pure calcium carbonate. Adding it will increase the soil pH. We have room to move there, and there’s 5kg of it sitting in the shed.
Gypsum is calcium sulphate. It also adds sulphur (a micronutrient required by crops like brassicas and onion/garlic families especially), and is helpful for conditioning clay soils. It does not increase the pH, and will also help leach sodium from the soil. It’s more expensive, but will probably be worth the investment. It’s likely that once our bag of lime is finished, I’ll change over to gypsum.
But our calcium is at less than half the recommended level. It’s going to need a push, and I don’t think regular applications of either lime or gypsum will make a big enough difference by themselves. Even with our chickens, we’re probably not going to be adding enough egg shells to our soils to change the readings much either.
Because we are so low, and because calcium is so important, I think our beds will benefit from a one-off dose of EF Nano Cal at a rate of 200g/m². We’ll then supplement with a biannual dose of lime or gypsum in spring and autumn at 100g/m².
Our reading: 97
We’ve got salty soils. Who would have known? We’re not a huge distance away from the ocean, but we’re very protected from it. Sometimes artificial fertilisers can elevate salt levels in soils, so this is another potential explanation.
Sodium is bad for plants as it changes the flow of water and dries them out. That’s not great in a place prone to drought. When you have high-salinity soils, your plants may be stunted or dry out fast. I’m pretty sure I’ve seen this in some beds before.
I found a little formula that uses the sodium, calcium, and magnesium measurements to help work out how much salt your plants can access. We scored as slightly saline, so it’s not a major problem, but it’s a problem.
All the compost and manure going into the bed will help even it out. Using gypsum to address my calcium will also help dissolve and leach the salts. But beyond that, I have two options to address this issue: grow salt-tolerant plants; or flush the salts out with water.
Flushing the salts out is going to require that I look into drainage. Our garden bed is on a slope, so I think it should be a pretty straight-forward process to organise that, but I’m not convinced it’s really a priority. Right now, I’m concerned with holding moisture, not draining it away. If I build a drainage system, it won’t be until after the entire garden is set up and a follow-up soil test has been done. But I will plan for it as I build.
There is one vegetable I know of that I can call on in this situation. New Zealand Spinach is a native plant naturally found near the ocean, indicating it should be pretty salt tolerant. I wonder if I could sell it at farmer’s markets?
Ideal: 100-200 (Phosphorus), 200-400 (Phosphate)
Our reading: 67 (Phosphorus), 155 (Phosphate)
Like nitrogen and potassium, phosphorus is a macronutrient. It’s essential for plant growth. It helps transfer energy from sunlight to plants, stimulates early root and plant growth, and helps flowers, fruit, and seeds develop. It also increases a plant’s resilience – something they need in our very windy garden.
We’re running a little low on phosphorus. But fixing the levels the conventional way produces a moral dilemma.
You see, New Zealand’s soils are naturally deficient in phosphorus. It’s not strange that ours are. New Zealand farmers really hoon through phosphate while growing the meat, fruit, and vegetables that sustain a decent chunk of our economy.
Over the last 100 years, Kiwi farmers have entirely exhausted supplies from Christmas Island and Nauru. The US refuses to export it to us. Our main supplier, as it stands, is Morocco. But most of Morocco’s sources actually come from West Africa, and there are concerns that Morocco shouldn’t be the one profiting from it.
Organic growers are allowed to add RPR (Reactive Phosphate Rock) to their soils to fix phosphorus deficiency. But as well as the potential dubious origins of the rock, RPR also works better if it has an acidic soil (a pH below 6) to react with. Our soils are a touch too alkaline to get the best from phosphate rock as an amendment.
But we’re not too deficient in it. Our numbers for phosphorus are better than for calcium or nitrogen. We might be able to get away with a softer approach. So I’m looking at Tui Blood and Bone.
Tui Blood and Bone specifically also contains nitrogen, and no potassium, meaning it ticks a couple more boxes there, too. Different brands have different recipes though – Yates blood and bone contains a small amount of potassium, while “Number 8” contains less phosphorus. It’s always worth reading the labels, and knowing what I needed made the decision easy.
I’m going to add Tui Blood and Bone at a rate of 170g/m² on an annual basis. If the phosphorus is still too low in later soil tests, I’ll look at RPR options again. Hopefully we’ll be paying the right people for it by then.
The other stuff
There are other readings in our results. Some ratios that relate to aeration and compaction, paramagnetism and humus. Again, our soils perform pretty poorly.
I haven’t read much about paramagnetism before this test, but supposedly it stimulates root growth.
Kay Baxter from Koanga recommends including 400g/m² of EF BAS 50 when starting a new bed. It’s an organic product which is basically volcanic rock dust. It specifically aims to increase the paramagnetism of your soil.
The packaging describes paramagnetism as:
“…the antenna–like characteristics that allow a soil to attract maximum energy from electromagnetic radiation in the atmosphere.“
As well as improving our paramagnetism, adding BAS-50 will also give us the benefit of a whole lot of minerals that are likely leached or eroded from our soils. To do this on the entire garden works out at a one-off cost of about 50 cents per m².
At this point, it’s pretty clear that our soils have not been particularly loved over the past 100 years. If the minerals were there to begin with, they’ve either drained or eroded away into the Whangaroa harbour by now. Our soils aren’t in the best condition, and the entire purpose of this exercise is to set them up to be as productive as possible.
I believe that the minerals, at least, will be worth it. This product contains calcium, magnesium, sillicon, and a whole lot of micronutrients. Any extra benefits from the improvement in paramagnetism would really be a bonus. So I’m going to give it a try. The worst thing that happens is I purchased a product that doesn’t work. I don’t think basalt dust is likely to have any specifically negative effects on the soil.
But I do like to experiment and test things out. So I’m going to see if there’s really anything to this product, or if I’ve wasted my $50.
This week, I’ve been constructing two new beds for my strawberries. By the time I plant the beds, they will have both been ammeded with equal additives as per my new ‘recipe’ (below). But I will only add BAS 50 to one of them.
In theory, we’ll get better strawberries – and probably more of them – from the BAS 50 bed. We can measure that by tracking the size and health of plants, doing taste-testing experiments, weighing the crops, and brix testing (a measurement of the sugars in the fruit).
I’ll update on this experiment in the future and report back on how it goes. Sign up for the mailing list at the bottom of the page if you’re keen to stay in touch about it.
Humus is essentially what is left after organic matter breaks down completely. It’s pure carbon.
Normally, humus is collected over decades as compost and other organic matter is broken down by worms and microorganisms until there’s nothing left but a tiny speck of carbon.
That tiny speck of carbon will hold water and nutrients like a sponge, rather than letting them just leach away. It’s like a savings account for your soil. The more humus you have in your garden, the better your plants will grow.
Unsurprisingly, the humus levels in my garden suck too according to the soil test. There is some humus in the NanoCal product we’re using for calcium. But we can cheat the long natural process by adding biochar, which is another form of pure carbon.
Biochar is produced by heating wood to a very high temperature so all the gasses burn away, leaving only the carbon behind. It’s basically a fancy charcoal.
Biochar production is something I’ve planned to set up here eventually, but I’m not ready for it yet. (Though I have signed up for this online webinar in biochar production in a couple of weeks if it’s something you’re interested in).
I had to shop around a bit to find something that fit my budget, but I found a supplier who sells 40 litres of biochar for $30 including postage.
The catch was that the biochar was unactivated. If I poured it straight into my soil as I received it, it would spend two years sucking up every nutrient it can find in my soil. This would deprive my plants of the nutrients, and is the opposite of what I’m going for.
Thankfully, science had worked out the biochar can be ‘charged’ by soaking it in organic matter for a while. This ensures the carbon is already filled with nutrients and moisture before going into the soil.
I’ve used ‘worm juice’ from the worm farm to do that, and there’s a little Twitter thread about the process with pictures and video.
The new recipe
By the time I’ve finished creating the current garden plan, we will have 80m² of garden bed. As I’ve worked through my soil test results, I’ve devised a ‘recipe’ to help us improve in future. There’s a note about my suppliers at the end of the page.
Each bed will receive a one-off dose of:
- EF BAS 50 at 400g/m² – to increase the minerals available in the soil and the paramagnetism ($50, incl shipping)
- Biochar at 400g/m² – to increase humus, nutrient storage, water-holding capacity, and improve soil texture ($30, incl shipping)
- EF Nano Cal at 200g/m² – to improve calcium levels ($38, incl shipping)
- Several buckets of cow manure worked in with the rotary hoe to improve organic matter levels and nitrogen (free, thanks cows).
Once a year, each bed will receive:
- Blood and bone at 170g/m² – to improve phosporus, calcium, and nitrogen ($25/year if I buy it in 25kg bags via Farmlands).
Twice a year, each bed will receive:
- Compost at a rate of at least 20l/m² – left on top to improve everything, basically (3 cubic metres of compost, annually – $88/m³ if I’m buying it from our local landscape supplier, free if I’m making it)
- Either lime or gypsum at a rate of 100g/m² – to improve calcium, condition soils, and reduce sodium (gypsum) ($20 annually using lime, $32 annually using gypsum).
Total once-off cost: $118
Total annual cost: $45-$326, depending on how I do it.
Green manures and ‘no dig’
It’s not all about what I add to the soil though. There are some changes in my behaviour as a gardener that will be important too. In horticulture, these are called “cultural methods”. They are the practices the gardener applies to their garden, rather than the powders and potions.
I will start taking my green manures a little more seriously. I’ve already got a whole bunch of seeds I can use, so for a few years at least, this will be free.
Various green manures do different things. Some have roots that break up the soil. Others dig deep into the subsoil and pull up nutrients that may not be available in the topsoil. Clovers and legumes help produce nitrogen. Many provide habitat for predatory insects and bees. They all support microbial life, and eventually, they all improve the humus levels of the soil too.
In Northland, I’m lucky enough to grow and eat from my garden year-round. Usually, I like to fill every bed with something tasty. But it’s time to make room for green manures.
Another practice I am already doing (kind of) is the ‘no dig’ method. We have dug a lot with a rotary hoe while establishing each bed, but that will be the sum total of our digging. This is the method used by Charles Dowding for establishing beds, and it’s what I’m aiming for over time.
The main benefit of this method is it allows webs of fungi to grow beneath the soil. This improves a crop’s ability to reach the nutrients in the soil. It also improves nitrogen production and overall soil health and structure.
I won’t know how effective this regime will be for at least 2 years. It takes time for organic matter to break down and populations of fungi and microorganisms to develop. 2 years feels like the minimum for all the initial work in the garden to begin having a measurable effect.
I’ve set a reminder in my calendar for June 2022. That’s when I’ll so a soil test on the garden beds themselves. I’ll be able to assess what all this effort has actually done then.
Good soils take time – and a fair chunk of money. For me, this whole exercise is an investment I know I’ll be here to profit from in the future. For a lot of reasons, not everyone has that.
So just a little note to you, the home gardener who may be renting, or gardening in containers (if you’ve made it this far). So many of these readings are improved by just adding compost and organic matter.
If you can’t afford or justify a soil test at your place, make sure you add lots of mulch (grass clippings will do) and home-made compost or worm castings at least once a year. Green manures can be a cost-effective investment as well.
Any goods that were shipped went to my Dad’s house in Whangarei, where I picked them up. All postage costs are for an urban address.
I got my soil test done, and sourced EF BAS 50 and EF NanoCal via Environmental Fertilisers in Paeroa. They were efficient and very helpful the entire way. The fertilisers arrived within 36 hours of being dispatched. Many of their products are certified for organic use.
I sourced my biochar from Fertile Fields Ltd in Taranaki. Again, they were very responsive, friendly, and it arrived overnight. They also have a range of products certified for organic use.
Everything else I source via Mitre10 or Farmlands. If you can buy in bulk, Farmlands is often significantly cheaper.
All of these suppliers have been paid by me for their services, this post is not sponsored in any way. There is no affilliation.
Thanks for the great write up on your soil tests.
I did notice that when talking about RPR you say
“Organic growers are allowed to add RPR (Reactive Phosphate Rock) to their soils to fix potassium deficiency. But as well as the potential dubious origins of the rock, RPR also works better if it has an acidic soil (a pH below 6) to react with. Our soils are a touch too alkaline to get the best from phosphate rock as an amendment.”
Did you mean to fix phosphate deficiency or potassium?
Oh! I see what I did wrong there. I meant phosphate. Whoops! Will get that fixed now, thanks!
thanks for this