Soil texture and productivity
Change what the soil is made of and watch the four properties that decide how well plants grow. No mix wins on all four.
Make a soil
The three mineral fractions are always rescaled to total 100%, so moving one moves the others.
What that soil does
75
Primary productivity potential, as a percentage of the best this model allows
Limited by: nutrient retention
Nutrients are not held against percolating water, so they leach below the roots.
40% sand, 40% silt, 20% clay, 4% organic matter. Productivity potential 75 out of 100, limited by nutrient retention. Nutrients are not held against percolating water, so they leach below the roots.
Try these
- Work through the five presets and record the productivity potential of each. Put them in order.
- Compare Loam with Loam, no humus. Only one thing changed. Which properties fell, and by how much?
- Build a soil where the limiting factor is drainage. Now build one where it is nutrient retention. Write down both mixes and explain why each fails.
- Sandy and Clay both score badly, but for opposite reasons. Explain each in one sentence.
- Take any soil that is limited by nutrient retention and fix it without touching the sand, silt or clay sliders. Explain what you did.
This is a model, not measurement. The bars show the direction and size of each effect as the proportions change, so that the trade-offs are visible. They are not readings from a real soil, and a real soil is also affected by climate, slope, depth and management. Particle sizes follow the USDA classification: sand 0.05 to 2 mm, silt 0.002 to 0.05 mm, clay below 0.002 mm.
One thing to notice on the first bar: a clay soil holds the most water in total, but it grips some of it so tightly that roots cannot pull it out, so the bar shows water that is actually available to a plant rather than total water.
Press Sandy, then Clay. Both score badly, and every one of their four bars is the opposite way round. Nothing about a soil is good or bad on its own: the clay that holds the nutrients is the same clay that stops the water draining, and the sand that lets water through is the same sand that lets the nutrients go with it.
Why the score is not an average
Productivity here is dragged down by whichever property is shortest, not lifted by the three that are fine. A soil that waterlogs is a soil that waterlogs, however well it holds nutrients. That is why the readout names the limiting property rather than just giving a number.
The fourth slider is the one you can change
A farmer cannot alter the sand, silt and clay under their feet. They can alter the organic matter, and it is the only input that improves three properties at once. Compare Loam with Loam, no humus, and the case for compost, manure and crop residues makes itself.