Monday, November 3, 2014

Sufficiency levels of soil nutrients

After getting all excited about soil nutrients thanks to Steve Solomon's book The Intelligent Gardener, I banded together with a few local gardener types and got a bunch of soil tests done.  Having run the numbers through Solomon's worksheets, thought about the results a bit and read a little more widely, I've come to see that there are many perspectives on soil fertility.

Solomon preaches the base-cation saturation ratio (BSCR) method, which claims that there is an optimal ratio of cations in a soil (68% Ca, 12% Mg for a start).  He further believes that plants grown on such a "remineralised" soil will have a greater nutrient density than those grown on sub-optimal soils.  Now I haven't had the time to delve deeply into the evidence base for either hypothesis, but a quick look suggests that it is a bit thin.  Plant breeding appears to be a more likely culprit for decreased nutritional density over the years, hence applying fertilisers to reach some optimal ratio may be an unnecessary expense.

Having said that, there's nothing like trying something out for yourself to see what happens, which is exactly what I intend to do.

The conventional/mainstream agronomic view is that it is not the ratio of nutrients that is important, but rather the amount.  (This approach is either based on sound science, or biased by vested interests such as fertiliser companies, depending on your point of view.)  So they have come up with thresholds below which a nutrient can be considered deficient.  A strong argument against this approach is that such a threshold may be enough to avoid gross deficiency that exhibits visible symptoms, but not more subtle ones.  This is one reason for the growing use of plant tissue analysis to identify small deficiencies.  By testing both soil and plants, a farmer can gradually converge to a highly productive soil without resorting to broad-brush rules, be they thresholds or optimal ratios.  This seems to me to be a much more adaptive approach, allowing for variability in crops, soils, local climate and management.  However, a farmer will generally be managing for profitability (balancing inputs with production), whereas a home gardener may be more interested in maximising nutrient density.  A profitable crop is not necessarily a nutritious one (if soil fertility does indeed influence nutrient density).

Given that home gardeners and small-scale growers can't afford to do plant tissue tests, what are they to do?  They could follow the Solomon approach, but it might be quite expensive.  In particular, he advocates very high levels of phosphorus, which is an expensive nutrient.  Here is a summary of nutrient thresholds that I have found by scouring the internet.  I've also given the optimal range provided by AgVita on their soil reports.  AgVita is a Tasmanian testing lab that uses the Mehlich 3 method (unfortunately they are very expensive!).

Soil Acidity (pH)

pH is probably the most important thing to get right in a soil.  A slightly acid soil is most favourable to plants, and has the least risk of nutrient deficiencies and toxicities. The below just applies to acid soils - I'm not sure what to do with alkaline soils (pH >7.5).  Note that all pH values are as measured in a 1:5 water solution, not CaCl2 solution

Target: pH 6.5 (good for mixed veg)
Application rate: depends on soil texture.  According to Bill Cotching, 1t/ha of 100% NV lime will raise pH over 0-10cm as follows:
Sand: 0.5-0.7
Loam: 0.3-0.5
Clay: 0.2-0.3
Ferrosol: 0.04-0.1

Minimum application: 2.5t/ha
Maximum application: 7.5t/ha
Incorporate in top 10cm, response in 1-3 years.

Some similar figures, from Land & Water Australia:
Subtract soil pH from target.
Divide by conversion factor:
Clay: 0.3
Clay loam: 0.4
Sandy clay loam: 0.5
Sandy loam: 0.6

Surface applied lime only moves ~1cm/year down profile.  Need a big dose to get down to subsoil.

A nice table matching CEC to pH is given here.  Note that these are pH (Ca) values, and lime rates are to lift to pH (Ca) 5.5 in the top 10cm.

Another way is to use the buffer pH, which is a measure of how much the soil's pH changes per unit of lime.  An example for New York State is given here.  Note that I think these figures are calibrated for local conditions, so may not be applicable on your particular soil.

pH (water): 5.5
buffer pH (Mehlich): 6.6
CEC: 16
texture: sandy loam

Cotching: 2 t/ha
LWA: 1.67 t/ha
NSW DPI: 2.8 t/ha
NYS: 1.1 t/ha

So quite a lot of variability there!  I think I'd start with 2.5 t/ha and see how we go (since they are for 0-10cm, and we want to get down to 15cm).

Phosphorus (P)

The first thing to be aware of with phosphorus is that there are many ways of measuring available P.  Here is a table correlating the different methods (thanks to AgVita).

The short of it is that optimal P, as measured by the Mehlich 3 test, is 40-70ppm.  Since we're going for high production and nutrition, it can't hurt to aim towards the high end of that range.

50-100 ppm (UD)
56 ppm (Colorado)
AgVita: 40-90 ppm

Potassium (K)

160 ppm
90-180 ppm (UD)
70-140 ppm (NJ)
180 ppm (Colorado)
AgVita: 245-355 ppm (upper end of range may be CEC dependent, 4-6%)

Calcium (Ca)

Ca:Mg > 1:1 (reduced stability in heavy clays)
Ca:Mg 3-5 (AgVita, as measured in meq)

700-900 ppm (NJ)

AgVita: 1620-2700 ppm (appears to be CEC dependent, 60-70%)

Magnesium (Mg)

K:Mg < 1.5:1 (Spectrum)
K:Mg 0.3-0.5 (AgVita, as measured in meq)

70-150 ppm (NJ)

AgVita: 200-400 ppm (CEC dependent, 12-20%)

Sulphur (S)

12 ppm (Minnesota)
AgVita: 12-45 ppm

Boron (B)

1 ppm (Minnesota)
0.7 ppm (NJ)
AgVita 2.2-6 ppm

Zinc (Zn)

1 ppm (Minnesota)
1.5 ppm (Colorado)
AgVita: 2.2-11 ppm

Iron (Fe)

10 ppm (Colorado)
AgVita: 35-230 ppm

Copper (Cu)

5 ppm (Minnesota)
AgVita: 2.5-10 ppm

Manganese (Mn)

10 ppm (not Mehlich extract though ...)
Only use soil test if OM < 6%.  If OM > 6%, then Mn level is determined by pH:
< 6.0: high
6.0-6.9: optimum
> 6.9: low
Foliar: 0.2-0.4 lbs/ac in 50-100 gal water (0.1-0.2 kg/200-400 L), 2-3 applications (Minnesota). Chelated form best.

Rutgers University uses an Activity Index:
      MnAI = 101.7 + 3.75Mn - 15.2pH

The index should be between 25-100 (ref).
They also have a table with sufficiency levels for different pHs, see here.  Anything above 8 ppm is fine, no matter what the pH.
Broadcast 20-30 lbs/ac or band 4-8 lbs/ac, or 0.5-1 lbs in 20 gal (0.2-0.5 kg/75L) water per acre (NJ).

AgVita: 18-70 ppm