Soil negative charge and nutrient holding ability

Some soils have much higher leaching losses than others. (Leaching occurs when downward-moving water carries nutrients with it out of the root zone).

Factors Affecting Leaching Losses

Soil texture, negative charge, and amount of rainfall or watering determine a soil's leaching potential:

• Sandy soils are very susceptible to leaching losses for 2 reasons. First, they tend to have a low negative charge. which means little ability to hold on to plus-charged nutrients. Second, a given amount of water will penetrate more deeply than on finer textured soils with higher water-holding capacity (see Chapter 2).

• Clayey soils and those high in humus have lower leaching losses, due to higher negative charge and better water-holding capacity (up to twice as much as sandy soils. Remember, however, that "tropical" clays (e.g. kaolin and hydrous oxide clays) have a very low negative charge.

• The higher the rainfall, the higher the leaching losses.

• The "worst-case" scenario for leaching would be a sandy soil low in humus, under high rainfall.

How Negative Charge Helps a Soil Hold Nutrients

Clay and humus particles have a negative charge. The available forms of plant nutrients exist as ions which are molecules with a positive (plus or +) charge or negative (minus or -) charge. The minus-charged clay and humus particles act like little magnets to attract and hold those plus-charged ions like potassium (K+), calcium (Ca++), and magnesium (Mg++), which gives them some resistance to leaching. The nice thing is that the plus-charged ions (called cations) are available to roots even when held by the clay and humus particles. Cations will still leach somewhat, but not nearly as much as the anions.

Unfortunately, the minus-charged nutrient anions like nitrate (NO3-) and sulfate (SO.--) aren't so lucky. Since like charges repel, they're not held by the minus-charged clay and humus particles; instead, they end up floating around freely in the soil water, which makes them very prone to leaching in most cases.

TABLE 6-1 Some Common Plant Nutrients and Their Susceptibility to Leaching

+ Charged Nutrients (Cations) (fairly resistant to leaching)	- Charged Nutrients (Anions) (easily lost by leaching)
Ammonium nitrogen (NH4+)	Nitrate nitrogen (NO3)
Potassium (K+)	Sulfate (SO)
Calcium (Ca++) (Mg++)	NOTE: Leaching losses of potassium can be a problem on sandy soils under high Magnesium rainfall.

What about Phosphorus?: It's an exception. Even though its 2 soil ionic forms (H2PO4-, HPO4--) have a minus charge, they hardly move at all in the soil, because they readily form insoluble, immobile compounds with iron, aluminum, calcium, and magnesium. While this keeps phosphorus from leaching, roots have trouble absorbing it in this form. This "tie-up" is called phosphorus fixation and can be a serious problem on many soils, especially when phosphorus fertilizers aren't applied correctly.

NOTE: Don't confuse phosphorus fixation with nitrogen fixation (the process by which rhizobia bacteria associated with legumes convert atmospheric N into usable form for these plants).

How Soil Negative Charge is Measured: Cation Exchange Capacity (C.E.C.)

The exchange capacity of a soil ( also called cation exchange capacity or C.E.C.) is a measure of its negative charge or the amount of plus-charged nutrients it can hold against leaching.

A soil's C.E.C. depends on its clay and humus content, since they're the only 2 soil particles with a minus charge. Soils with a low C.E.C. are especially prone to leaching and have poor nutrient-retaining ability. Even soils of the same texture can vary markedly in C.E.C. due to variations in humus content and the type of clay minerals they contain (see Table 6-3).

Table 6-2 illustrates the marked variations in C.E.C. between humus and clay, as well as among different types of clay. Note the very high charge of humus, which is why it can easily account for the major portion of the C.E.C. in many soils, even when present at typical normal levels of just 2-4% (by weight). These differences explain why C.E.C. varies so much among soils (even those of the same texture), as shown in Table 6-3.

TABLE 6-2 The Relative Cation Exchange Capacity of Clay and Humus Particles

	C.E.C.*
Humus	150-200
"Temperate" -type	15-100
clay minerals
"Tropical" -type	2-15
clay minerals

TABLE 6-3 Typical Variations in Cation Exchange Capacity among Soils

Soil Name	C.E.C. of the Topsoil*
Hilo clay (Hawaii)	67
Cecil clay (Alabama)	4.8
Susquehanna clay (Alabama)	34.2
Greenville sandy loam (Alabama)	2.3
Colma sandy loam (California)	17.1

*Don't worry about what the actual numbers mean; it's the comparison that counts. (If you're familiar with chemistry, C.F.C. is measured in terms of milk-equivalents of cations per 100 grams of soil.)