Bulk Density Show Down

Bulk density is a measure of how porous your soil is.  It is the mass of soil without moisture divided by the volume of that soil.

Mass of Soil (dry)

Volume of Soil

A very high bulk density means the soil is very compacted, while a low bulk density means there is lots of pore space for air and water in the soil matrix.


As this handy table from the USDA shows, once you reach around 1.5 g/cm3 plant growth suffers.

Measuring bulk density on your own is extremely easy.  To demonstrate the process I pitted my lovingly tended vegetable garden against a riparian zone in a local park. What site would come out with the lower bulk density?  Would the minimal tillage and copious amounts of organic matter I dump in the veggie patch every season be enough to match the virgin uncultivated porosity of Glen Canyon's creeksides?

First, I had to gather my high tech soil sampling equipment.  Block of wood, plastic bag, hammer, and aluminum can. Check. 

It's important to know the volume of your sample.  If you're using a can that had food in it, the volume is usually written on the label.  If not, simply fill the can to its top with water and then measure this quantity of water in a measuring cup.  Once you're set with the volume of your can, cut both the top and bottom out.  It's nice to use a sturdy can with corrugated sides.  Unfortunately we can only ask so much of our aluminum cans and when you're dealing with really compacted soils it's possible the can will buckle.  In this case you actually do need a more sophisticated steel ring.

Once you're out into the field clear away any plant matter on the soil surface and pound the can straight into the ground using the block of wood to evenly distribute your impressive hammer blows.

Pound until the can is flush with the ground.

Now the trick is to remove the can from the ground without losing any soil.  I find it's easiest to dig out around the can a little bit.

Gently pluck the can from the ground and slip it into your sample bag without losing any soil.

I dry the soil in an oven at very low temperatures.  Whatever the lowest temperature you can use the oven at should be fine.  Once the soil is dry, you can weigh the sample.  Divide this weight by the volume of the can and you have your very own bulk density data.

So how did my vegetable patch fare against the riparian zone?

Veggie patch, 1.08g/cm3

Glen Park, 0.85g/cm3

The veggie patch came out the gates strong with a low 1.08g/cm3, but it's just not enough to match the sylvan sanctity of Glen Park's 0.85g/cm3.

Was my sample spot in the veggie patch too close to a pathway? Perhaps.  Could the different soil textures of the samples account for some of the difference? Probably.  Should I find a weaker competitor for the veggie patch next time? Definitely.

Ficus lyrata sighting!

Four Santigolds, two fiddle leaf figs, solid album

Phosphate and Nitrate Algal Bloom in the Yellow Sea... Again

Once again a massive algae bloom has occurred off the shores of Qingdao (青岛). You may recall that the last time this occurred in 2008 Qingdao was just months away from hosting the Olympic sailing competitions. A massive clean up effort cleared the waters in time for the events but the underlying problem of fertilizer runoff is still very present. Phosphate and nitrate runoff from excessive fertilizer use in Shandong Province empty out into the waters around Qingdao. Combine this heavy nutrient load with the right weather and a massive algae bloom (410 square km according to The Telegraph) is the result.

Zhang qiao pier

Recognize that pier now?

I'll admit that much of the world's population depends on synthetic nitrogen fertilizer and mined phosphorus to provide them with food. However, the excess amounts being applied to Chinese cropland is ridiculous. In a great National Geographic article about nitrogen fertilizer, Dan Charles describes how one rice farmer is using 530 lbs. of nitrogen per acre, while some vegetable growers are using 800 or even 1600 lbs. per acre!

We can see how the majority of nitrogen fertilizer used for crops comes from synthetic sources.  We  can also see that we are applying more than is taken up by crops.

Using the great Roots Tubers & Bananas Maps from CGIAR we can really get a sense for just how excessive fertilizer use in China is compared with the rest of the world.  

Phosphorus use

Nitrogen use

There are dramatic well documented health and environmental problems this type of fertilizer use causes.  What I think is even more concerning though is that these fertilizers come from non-renewable sources.  Phosphorus is currently mined in a few countries - there is not an endless supply in these mines.  The way that synthetic nitrogen fertilizer is produced requires enormous amounts of natural gas- although cheap now this will eventually become more expensive as reserves dwindle.  

As the sources of these fertilizers become scarcer prices will increase.  Poorer nations will be the first to be priced out of the market. Indeed, fertilizer is already difficult to procure for farmers in developing countries.  If we are concerned about the stability of world food production, and the political stability of countries who will be first affected when production falters, we need to take a closer look at fertilizer use.

Alternative sources of fertility are readily available and often times cheaper than chemical fertilizer when the appropriate infrastructures are set up.  In addition to switching to renewable fertility sources, regulation needs to be set up to manage fertilizer runoff.  Most people would agree that dumping toxic pollutants from a factory should not be allowed, so why should applying fertilizer in quantities that will obviously lead to nitrate run off be treated any differently?

Biochar and oats

I purchased a sack of biochar a while back from what I would consider the most reputable source in my state.  I took one section of a bed and incorporated biochar into the soil, the other section received no biochar.  I applied compost and azomite to both sections. Kynon oats were then planted in both sections.

It looks as though the biochar has locked up some nutrient as the oats which received no biochar performed much better.  I used biochar on some different crops but didn't see this type of pronounced affect.  It's also possible there was gopher damage to the one end where the biochar was applied.  If the biochar did cause this though, then it might not be a great soil amendment for short term results.  At the very least it would be good to know what nutrient was being locked up and apply that along with the biochar.