How to Grow More Vegetables

bodies of microbial life. As humus is formed, nitrogen
becomes part of its structure. This stabilizes nitrogen in
the soil because the humus compounds are resistant to
decomposition. They are worked on slowly by soil
organisms, but the nitrogen and other essential nutrients
are protected from too-rapid solubility and dissipation.
Organic matter includes humus and some undecomposed
organic matter.
Humus also acts as a site of nutrient adsorption
(accumulation of nutrients on its surface) and exchange
for plants in the soil. The surfaces of humus particles
carry a negative electric charge. Many of the plant
nutrients—such as calcium, sodium, magnesium,
potassium, and most trace minerals—carry a positive
electrical charge in the soil solution and are thereby
attracted to and adhere to the surface of humus. Some of
the plant nutrients—such as phosphorus, sulfur, and the
form of nitrogen that is available to plants—are not
positively charged. Fortunately, a good supply of these
nutrients becomes available to the plants through
biological transformation in the compost pile and soil.
As plant roots grow through the soil in search of
nutrients, they feed on the humus. Each plant root is
surrounded by a halo of hydrogen ions that are a by-
product of the roots’ respiration. These ions also carry a
positive electric charge. The root actually “bargains” with
the humus, exchanging some of its positively charged
hydrogen ions for positively charged nutrient ions stuck
onto the surface of the humus. An active exchange is set