Soil Tests

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.

1. Phosphorus
2. Potassium (K)
3. Zinc (Zn)
4. Boron (B) may be run every 2-3 years.
5. EC e
6. SAR or ESP
7. pH

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

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

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.

^* For processing tomatoes, potassium fertilizer should be applied if this soil test is below 80 ppm K.

^** Celery or potato growers should note detailed information for those crops in pages that follow.

Table 3A. Potassium (K) Boiling Nitric Acid Test

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

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

pH Test

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.

^a The normal range for California soils is 5.0 to 8.4.

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.

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.

¹ Most should pass through a 100 mesh screen.

Applying Lime

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.

Continue to Fertilizing Vegetables

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