What to Test For; What the Numbers Tell You.
The information below is to help you decide what to test for:
THE FOLLOWING TEST IS USUALLY ONLY RUN ONCE ON A FIELD, IF YOU DON'T ALREADY HAVE THE ANSWER FROM A SOILS MAP.
SATURATION PERCENTAGE - A ONE TIME TEST
SATURATION PERCENTAGE: This test gives you a measure of how much sand, silt or clay is in your soil. Knowing this helps in scheduling irrigations. And herbicide recommendations often depend on knowing the amount of sand, silt and clay found in your fields.
Table 1. SATURATION PERCENTAGE (SP)
Below 20 Sand or loamy sand
20 - 35 Sandy loam
35 - 50 Loam or silt loam
50 - 65 Clay loam
65 - 135 Clay (some clays go up to 150) Above 135 Usually organic (peat or muck)
SOIL TESTS TO RUN REGULARLY
Make the following soil tests before planting every crop except where noted.
2. Potassium (K)
3. Zinc (Zn)
4. Boron (B) may be run every 2-3 years.
5. EC e
6. SAR or ESP
WHAT THE PHOSPHORUS SOIL TEST MEANS
PHOSPHORUS (P): Phosphorus builds up in fields when you add large amounts of manures or other phosphate fertilizer every year. Many fields contain so much phosphorus now that crops won't grow better with more overdoses of phosphate fertilizers. See Table 2 for interpretation of soil tests for phosphorus.
Table 2. Phosphorus (P) Olsen bicarbonate test
SOIL TEST, PARTS PER MILLION (PPM) PHOSPHORUS
|Corn||Lettuce||Potatoes||Canning tomatoes||Other field and warm season veg.||Other cool season vegetables||Response to phosphorus fertilizer|
|Below 6||Below 20||Below 18||Below 6||Below 8||Below 15||Highly responsive|
|6 - 12||20 - 30||1 8 - 35||6 - 12||8 - 15||15 - 30||Probably responsive|
|Above 12||Above 30||Above 35||Above 12||Above 15||Above 30||Not responsive|
For all crops on peat and muck soils, use water soluble phosphorus tests.
Responsive - less than 1 ppm
Not responsive - greater than 1 ppm
WHAT THE POTASSIUM SOIL TEST MEANS
POTASSIUM (K): Many California soils are rich in available potassium. It's a waste of your money to add potash fertilizer to these soils. But you can't be positive your soil is rich in potash unless you have it tested.
Table 3. Potassium (K), Ammonium or Sodium Acetate Test, for mineral soils
|Soil test, parts per million K||Responsive to potassium fertilizer|
|Celery, potatoes||Other vegetables *|
|Below 100||See below||Highly responsive|
|100 - 150||See below||Probably responsive|
|Above 150||100 - 120||Not responsive|
Except for celery or potatoes, don't expect any help from potassium fertilizer if your soil tests above 100-120 ppm K.** If your soil tests below 100-120 ppm potassium,** have the lab run a second test for potassium. It's called the "boiling nitric acid test".
If this nitric acid test is above 250 ppm potassium, expect no benefit from potassium fertilizer. If your soil is below 250 ppm K, based on nitric acid extraction, your crop might benefit from potassium fertilizer.
Table 3A. Potassium (K) Boiling Nitric Acid Test
|Soil test, parts per million K||Response to potassium fertilizer|
|Below 250||Probably responsive|
|Above 250||Not responsiv|
Instead of the boiling nitric acid test, some labs may use a sulfuric acid test for potassium. Here's what the numbers mean for the sulfuric acid test:
Table 3B. Potassium (K) Sulfuric Acid Test
|Soil test, parts per million K||Response to potassium fertilizer|
|Below 500||Probably responsive|
|Above 50||Probably not responsive|
WHAT THE ZINC SOIL TEST MEANS
Table 4. ZINC (Zn) DTPA or Dithizone test
Soil test, parts per million ZnOnion (mineral soils)CornPotatoesCanning tomatoesOther warm season vegetablesOther cool season vegetables Below 0.3Below 0.3Below 0.3Below 0.3Below 0.2Below 0.5Highly responsive0.3 - 0.80.3 - 0.60.3 - 0.70.3 - 0.70.2 - 0.50.5 - 1.0Probably responsiveAbove 0.8Above 0.6Above 0.8Above 0.7Above 0.5Above 1.0Not responsive *
* In some UC trials where soils had a high pH and heavy phosphate fertilizer history, lettuce and carrot yields were better when fertilized with zinc--even when the soil test was between 1 to 3 ppm zinc.
WHAT THE BORON SOIL TEST MEANS
BORON: Plants need a small amount of boron to grow, but too much boron makes crops grow poorly. This test may be run every 2-3 years.
Table 5. BORON (B) ppm in soil test
Below 0.1 Deficient
0.1 - 0.5 Satisfactory for all crops
1 Sensitive crops may show visible injury but yields may not be affected
5 Semi-tolerant crops may show visible injury
10 Tolerant crops may show visible injury
WHAT THE PH SOIL TEST MEANS
This test tells how acid or alkaline your soil is. a See Table 6 for explanation of pH readings.
Table 6. SOIL ph Chart
1.0 - 3.0 Extremely acid; not found in agricultural soils.
3.0 - 4.5 Acid; found occasionally in humid regions.
4.5 - 5.0 Acid; suitable for rhododendrons, azaleas, blueberries. Calcium, magnesium and molybdenum deficiency hazard on certain soils. Most vegetable fields should be limed (see potato pH below).
5.0 - 5.5 Acid; found in unlimed soils of humid regions; suitable for potatoes; calcium, magnesium and molybdenum deficiency hazard on certain soils.
5.5 - 6.0 Acid; should be limed for vegetables other than potatoes. Suitable for grasses. Calcium, magnesium and molybdenum deficiency hazard on certain soils.
6.0 - 7.0 Slightly acid to neutral; good for almost all vegetable crops.
7.0 - 7.9 Alkaline; most southern California soils are in this pH range.
Increased chance for zinc deficiency. Iron, boron, copper and manganese deficiencies are also related to this pH, but have been seldom found on vegetables in California.
7.9 - 8.3 Alkaline; adapted to growth of most crops. Increased chance for zinc deficiency. Iron, boron, copper and manganese deficiencies are also related to this pH but aren't common on California vegetables.
8.3 - 10 Alkaline; problem with excess sodium. Have gypsum requirement test run by lab to find amount of gypsum or sulfur or sulfuric acid or lime sulfur to apply before leaching to remove sodium.
10 - 14 Extremely alkaline; not found in agricultural soils.
DOES YOUR SOIL NEED LIME?
No lime applications are necessary or desirable on most of California's soils, since they commonly have a pH of 6.0 to 8.3. But if you have soils in the 4.0 to 5.5 range, they should be limed for growing most vegetables. Fields in this 4.0 to 5.5 range are given enough lime to raise the pH to 6.5. But on potatoes, pH is held between 5.2 - 5.5 if scab is a problem.
Table 9 shows roughly how much lime to apply on acid soils. For a more accurate guide, have the lab run a lime requirement test on very acid soils.
Table 7. Approximate tons of limestone to raise the pH of a 6 inch plow layer one pH unit.
|Texture of plow layer||Ground limestone required|
|Sand||1 ton per acre|
|Loam||2 tons per acre|
|Clay||4 tons per acre|
Use double these amounts to raise the pH one unit if you mix in the lime 12 inches deep.
Table 8. Tons of alternate liming materials to use per ton of finely 1 ground limestone required.
|Material||Tons to use|
|Calcium carbonate (pure)||1|
|Magnesium carbonate (pure)||0.84|
|Hydrated lime or calcium hydrate(pure)||0.74|
|Magnesium hydrate (pure)||0.58|
|Dolomitic hydrated lime||0.58|
|Calcium oxide (pure)||0.56|
1 Most should pass through a 100 mesh screen.
Lime needs to be mixed thoroughly in the plow layer because it moves slowly in the soil. One effective way to mix it is: spread half of it, disc and plow; then spread the remainder and disc again.
How Long Does Lime Take to Work?
Figure 2 to 6 or more months for lime to react in the soil. Hydrated lime is faster acting than ground limestone while most dolomitic limestones are slower.
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