Soil science paper has me stumped

You're referring to Formula #6 on p. 386?

Q = (qrir * wwir) + (qrk * wwk) + (qrbd * wwbd) + (qroc * wwoc) + (qrpH * wwpH)

The small q's (which range in values from 0 to 1) tell you about how good the soil is based on a variety of criteria. For example qrk describes soil erodability from worst (where qrk = 0) to best (where qrk = 1).

The formula gives us an overall measure of soil quality, Q, which is a weighted average of the small q's. Why do they use a weighted average instead of a simple average? It's because some of the small q's are more important than others. For example, qrk, the soil's score for soil erodibility, is important, so we set the value of wwk to .25 in the formula for determining Q. The soil's score for bulk density, qrbd, is less important, so we set the value of wrdb to .10. The w's are all values that you have to multiply the q's by in order to determine the weighted average.

Then I think we use this Q (which will always have a value between 0 for bad soil and 1 for good soil) to estimate the T-value. The authors ran some kind of statistical test to see how well it did, compared to a tried-and-true method of estimating T-values, and apparently it passed.

Would you like to know how they came up with scores for soil erodibility, etc.? (Remember, a score of 1.0 for soil erodibility means that the soil is less erodible.)

[NOTE TO OTHERS READING: the "T-value" mentioned in the article should not be confused with the "t-values" you read about in stats courses.]

CAVEAT--I have really only skimmed the article so far. I can't really give an opinion yet on the quality of this research, or anything. I read just enough to be able to explain some of the equations.

I shall dispense with all the formulas. Plant trees. Restrict goats and farming. Farming should be crop rotational and have trees surrounding the farmed areas. Goats eat everything killing any chance of plant growth. I wouldn't be surprised that goats and sheep maybe the reason there are deserts in Asia, Europe and Africa.

Table 3 (p.387) shows how to use Q, along with soil depth, to estimate the "soil loss tolerance limit" or SLTL.

The interacting factors, permeability or percolation,bulk density,soil erodibility (K factor which is another spacey one), organic C, and pH, have all been established via research in various agronomy labs.
We use the K factor values for design bases (in order to develop granular soil mixes on slopes in order to prevent run off and sheet werosion.

(I think it s all bullshit too, because we always find some soils at wide variance to calculated values)

This comes via the Universal SOil Loss Equation. If you post a query to Penn State U wrt "Modelling soil loss and erodability in tropical environments" Maybe you can get the relationships established to satisfy you. PS , when you find out, please post here cause Id really like to know.

How technical will you have to be? Will you have to explain the T-value formula? or can you tell WHAT it is and that is can be conducted to reveal valuable information.

To me, the paragraphs that sum up the main idea are

If the erosion exceeds the T-value,
it adversely affects productivity and must be brought
down within the permissible rate to ensure sustainability
of a production system. Conservation objectives for soil
loss tolerance are based on maintaining a suitable seedbed
and nutrient supply in the surface soil, maintaining
adequate depth and quality of the root zone, and minimizing
unfavourable changes in water availability throughout
the soil15.

And . . .

The concept of tolerance limit or permissible soil loss
was evolved to prevent land degradation which may otherwise
impact the livelihood of the people due to landlessness.
A framework (model) has been developed for
determining soil loss tolerance (T-value) that ensures
long-term sustainability of the soil resource, by preventing
excessive erosion through appropriate conservation
measures. The framework or the methodology for computing
the T-value has been applied and tested using data
from representative benchmark soils of India. The Tvalue
varied from 2.5 to 12.5 Mg ha–1 yr–1 depending on
the soil quality and soil depth in different physiographic
regions of the country.

. . . Site-specific T-values would help in devising a suitable strategy for
identifying the best management practices to bring soil
loss within permissible limits in a given area or region.

Thanks, Oylok, for your response! Yes, I think, I now get the bit about some factors being more important than others.

Thanks for taking the time to respond!

Nah, not very technical. I just have explain the model they used. I'm getting to work on this again today and let's see what comes out of it.

What exactly do you want me to post? I didn't do that research obviously!

Both RUSLE and USLE can be expressed as follows:
A = R * K * LS * C * P
Where
A = estimated average soil loss in tons per acre per year
R = rainfall-runoff erosivity factor
K = soil erodibility factor
L = slope length factor
S = slope steepness factor
C = cover-management factor
P = support practice factor

Another factor for soils is called "T value" which stands for "Tolerable Soil Loss." It
is not directly used in RUSLE equation, but is often used along with RUSLE for
conservation planning. Soil loss tolerance (T) is the maximum amount of soil loss in
tons per acre per year, that can be tolerated and still permit a high level of crop
productivity to be sustained economically and indefinitely.

http://140.112.190.56/2010/2010%20Chap.4(%E4%B8%8A%E8%AA%B2%E8%B3%87%E6%96%99)/Reader(Universal%20Soil%20Loss%20Equation).pdf

It appears to be tolerances. You calculate A the soil loss and the T values are what you would tolerate. Example you calculate how much it cost to make a handbag. But to make a profit you set limits (tolerances of cost). If it costs $20 to make a handbag. You can sell it for $25. You cannot tolerate a cost of $21 as $20 is the limit.

In manufacturing there are tolerances. A rod can 5 inches in diameter +/_ .01" tolerance. That means the rod over 5.01" will be rejected as it will not get into the hole. A diameter of less than 4.99" would be too loose so it is rejected.

http://books.google.ca/books?id=gWHsuGTcF34C&pg=PA226&lpg=PA226&dq=t+value+equation+in+soil+erosion&source=bl&ots=KJ75njDe4q&sig=BS5sHUUZDQ3Cmmg0myopU9hLnNU&hl=en&ei=7AZjTcf7DIiisQPU0NCXDg&sa=X&oi=book_result&ct=result&resnum=5&ved=0CDAQ6AEwBA#v=onepage&q=t%20value%20equation%20in%20soil%20erosion&f=false

http://www.crcnetbase.com/doi/abs/10.1201/NOE0849338304.ch128

http://www.agronomy.lsu.edu/courses/agro2051/Chapter%2017.pdf

T values are human values based on experience or knowledge as to what is tolerable in soil erosion.