Helping Plants to Need Less Irrigation
Dry though the maritime Northwest summer is, we enter the growing
season with our full depth of soil at field capacity. Except on
clayey soils in extraordinarily frosty, high-elevation locations, we
usually can till and plant before the soil has had a chance to lose
much moisture.
There are a number of things we can do to make soil moisture more
available to our summer vegetables. The most obvious step is
thorough weeding. Next, we can keep the surface fluffed up with a
rotary tiller or hoe during April and May, to break its capillary
connection with deeper soil and accelerate the formation of a dry
dust mulch. Usually, weeding forces us to do this anyway. Also, if
it should rain during summer, we can hoe or rotary till a day or two
later and again help a new dust mulch to develop.
Building Bigger Root Systems
Without irrigation, most of the plant's water supply is obtained by
expansion into new earth that hasn't been desiccated by other
competing roots. Eliminating any obstacles to rapid growth of root
systems is the key to success. So, keep in mind a few facts about
how roots grow and prosper.
The air supply in soil limits or allows root growth. Unlike the
leaves, roots do not perform photosynthesis, breaking down carbon
dioxide gas into atmospheric oxygen and carbon. Yet root cells must
breathe oxygen. This is obtained from the air held in spaces between
soil particles. Many other soil-dwelling life forms from bacteria to
moles compete for this same oxygen. Consequently, soil oxygen levels
are lower than in the atmosphere. A slow exchange of gases does
occur between soil air and free atmosphere, but deeper in the soil
there will inevitably be less oxygen. Different plant species have
varying degrees of root tolerance for lack of oxygen, but they all
stop growing at some depth. Moisture reserves below the roots'
maximum depth become relatively inaccessible.
Soil compaction reduces the overall supply and exchange of soil air.
Compacted soil also acts as a mechanical barrier to root system
expansion. When gardening with unlimited irrigation or where rain
falls frequently, it is quite possible to have satisfactory growth
when only the surface 6 or 7 inches of soil facilitates root
development. When gardening with limited water, China's the limit,
because if soil conditions permit, many vegetable species are
capable of reaching 4, 5, and 8 eight feet down to find moisture and
nutrition.
Evaluating Potential Rooting Ability
One of the most instructive things a water-wise gardener can do is
to rent or borrow a hand-operated fence post auger and bore a
3-foot-deep hole. It can be even more educational to buy a short
section of ordinary water pipe to extend the auger's reach another 2
or 3 feet down. In soil free of stones, using an auger is more
instructive than using a conventional posthole digger or shoveling
out a small pit, because where soil is loose, the hole deepens
rapidly. Where any layer is even slightly compacted, one turns and
turns the bit without much effect. Augers also lift the materials
more or less as they are stratified. If your soil is somewhat stony
(like much upland soil north of Centralia left by the Vashon
Glacier), the more usual fence-post digger or common shovel works
better.
If you find more than 4 feet of soil, the site holds a dry-gardening
potential that increases with the additional depth. Some soils along
the floodplains of rivers or in broad valleys like the Willamette or
Skagit can be over 20 feet deep, and hold far more water than the
deepest roots could draw or capillary flow could raise during an
entire growing season. Gently sloping land can often carry 5 to 7
feet of open, usable soil. However, soils on steep hillsides become
increasingly thin and fragile with increasing slope.
Whether an urban, suburban, or rural gardener, you should make no
assumptions about the depth and openness of the soil at your
disposal. Dig a test hole. If you find less than 2 unfortunate feet
of open earth before hitting an impermeable obstacle such as rock or
gravel, not much water storage can occur and the only use this book
will hold for you is to guide your move to a more likely gardening
location or encourage the house hunter to seek further. Of course,
you can still garden quite successfully on thin soil in the
conventional, irrigated manner. _Growing Vegetables West of the
Cascades_ will be an excellent guide for this type of situation.
Eliminating Plowpan
Deep though the soil may be, any restriction of root expansion
greatly limits the ability of plants to aggressively find water. A
compacted subsoil or even a thin compressed layer such as plowpan
may function as such a barrier. Though moisture will still rise
slowly by capillarity and recharge soil above plowpan, plants obtain
much more water by rooting into unoccupied, damp soil. Soils close
to rivers or on floodplains may appear loose and infinitely deep but
may hide subsoil streaks of droughty gravel that effectively stops
root growth. Some of these conditions are correctable and some are
not.
Plowpan is very commonly encountered by homesteaders on farm soils
and may be found in suburbia too, but fortunately it is the easiest
obstacle to remedy. Traditionally, American croplands have been
tilled with the moldboard plow. As this implement first cuts and
then flips a 6-or 7-inch-deep slice of soil over, the sole--the part
supporting the plow's weight--presses heavily on the earth about 7
inches below the surface. With each subsequent plowing the plow sole
rides at the same 7-inch depth and an even more compacted layer
develops. Once formed plowpan prevents the crop from rooting into
the subsoil. Since winter rains leach nutrients from the topsoil and
deposit them in the subsoil, plowpan prevents access to these
nutrients and effectively impoverishes the field. So wise farmers
periodically use a subsoil plow to fracture the pan.
Plowpan can seem as firm as a rammed-earth house; once established,
it can last a long, long time. My own garden land is part of what
was once an old wheat farm, one of the first homesteads of the
Oregon Territory. From about 1860 through the 1930s, the field
produced small grains. After wheat became unprofitable, probably
because of changing market conditions and soil exhaustion, the field
became an unplowed pasture. Then in the 1970s it grew daffodil
bulbs, occasioning more plowing. All through the '80s my soil again
rested under grass. In 1987, when I began using the land, there was
still a 2-inch-thick, very hard layer starting about 7 inches down.
Below 9 inches the open earth is soft as butter as far as I've ever
dug.
On a garden-sized plot, plowpan or compacted subsoil is easily
opened with a spading fork or a very sharp common shovel. After
normal rotary tilling, either tool can fairly easily be wiggled 12
inches into the earth and small bites of plowpan loosened. Once this
laborious chore is accomplished the first time, deep tillage will be
far easier. In fact, it becomes so easy that I've been looking for a
custom-made fork with longer tines.
Curing Clayey Soils
In humid climates like ours, sandy soils may seem very open and
friable on the surface but frequently hold some unpleasant subsoil
surprises. Over geologic time spans, mineral grains are slowly
destroyed by weak soil acids and clay is formed from the breakdown
products. Then heavy winter rainfall transports these minuscule clay
particles deeper into the earth, where they concentrate. It is not
unusual to find a sandy topsoil underlaid with a dense, cement-like,
clayey sand subsoil extending down several feet. If very impervious,
a thick, dense deposition like this may be called hardpan.
The spading fork cannot cure this condition as simply as it can
eliminate thin plowpan. Here is one situation where, if I had a
neighbor with a large tractor and subsoil plow, I'd hire him to
fracture my land 3 or 4 feet deep. Painstakingly double or even
triple digging will also loosen this layer. Another possible
strategy for a smaller garden would be to rent a gasoline-powered
posthole auger, spread manure or compost an inch or two thick, and
then bore numerous, almost adjoining holes 4 feet deep all over the
garden.
Clayey subsoil can supply surprisingly larger amounts of moisture
than the granular sandy surface might imply, but only if the earth
is opened deeply and becomes more accessible to root growth.
Fortunately, once root development increases at greater depths, the
organic matter content and accessibility of this clayey layer can be
maintained through intelligent green manuring, postponing for years
the need to subsoil again. Green manuring is discussed in detail
shortly.
Other sites may have gooey, very fine clay topsoils, almost
inevitably with gooey, very fine clay subsoils as well. Though
incorporation of extraordinarily large quantities of organic matter
can turn the top few inches into something that behaves a little
like loam, it is quite impractical to work in humus to a depth of 4
or 5 feet. Root development will still be limited to the surface
layer. Very fine clays don't make likely dry gardens.
Not all clay soils are "fine clay soils," totally compacted and
airless. For example, on the gentler slopes of the geologic old
Cascades, those 50-million-year-old black basalts that form the
Cascades foothills and appear in other places throughout the
maritime Northwest, a deep, friable, red clay soil called (in
Oregon) Jori often forms. Jori clays can be 6 to 8 feet deep and are
sufficiently porous and well drained to have been used for highly
productive orchard crops. Water-wise gardeners can do wonders with
Joris and other similar soils, though clays never grow the best root
crops.
Spotting a Likely Site
Observing the condition of wild plants can reveal a good site to
garden without much irrigation. Where Himalaya or Evergreen
blackberries grow 2 feet tall and produce small, dull-tasting fruit,
there is not much available soil moisture. Where they grow 6 feet
tall and the berries are sweet and good sized, there is deep, open
soil. When the berry vines are 8 or more feet tall and the fruits
are especially huge, usually there is both deep, loose soil and a
higher than usual amount of fertility.
Other native vegetation can also reveal a lot about soil moisture
reserves. For years I wondered at the short leaders and sad
appearance of Douglas fir in the vicinity of Yelm, Washington. Were
they due to extreme soil infertility? Then I learned that conifer
trees respond more to summertime soil moisture than to fertility. I
obtained a soil survey of Thurston County and discovered that much
of that area was very sandy with gravelly subsoil. Eureka!
The Soil Conservation Service (SCS), a U.S. Government agency, has
probably put a soil auger into your very land or a plot close by.
Its tests have been correlated and mapped; the soils underlying the
maritime Northwest have been named and categorized by texture,
depth, and ability to provide available moisture. The maps are
precise and detailed enough to approximately locate a city or
suburban lot. In 1987, when I was in the market for a new homestead,
I first went to my county SCS office, mapped out locations where the
soil was suitable, and then went hunting. Most counties have their
own office.
Using Humus to Increase Soil Moisture
Maintaining topsoil humus content in the 4 to 5 percent range is
vital to plant health, vital to growing more nutritious food, and
essential to bringing the soil into that state of easy workability
and cooperation known as good tilth. Humus is a spongy substance
capable of holding several times more available moisture than clay.
There are also new synthetic, long-lasting soil amendments that hold
and release even more moisture than humus. Garden books frequently
recommend tilling in extraordinarily large amounts of organic matter
to increase a soil's water-holding capacity in the top few inches.
Humus can improve many aspects of soil but will not reduce a
garden's overall need for irrigation, because it is simply not
practical to maintain sufficient humus deeply enough. Rotary tilling
only blends amendments into the top 6 or 7 inches of soil. Rigorous
double digging by actually trenching out 12 inches and then spading
up the next foot theoretically allows one to mix in significant
amounts of organic matter to nearly 24 inches. But plants can use
water from far deeper than that. Let's realistically consider how
much soil moisture reserves might be increased by double digging and
incorporating large quantities of organic matter.
A healthy topsoil organic matter level in our climate is about 4
percent. This rapidly declines to less than 0.5 percent in the
subsoil. Suppose inches-thick layers of compost were spread and, by
double digging, the organic matter content of a very sandy soil were
amended to 10 percent down to 2 feet. If that soil contained little
clay, its water-holding ability in the top 2 feet could be doubled.
Referring to the chart "Available Moisture" in Chapter 2, we see
that sandy soil can release up to 1 inch of water per foot. By dint
of massive amendment we might add 1 inch of available moisture per
foot of soil to the reserve. That's 2 extra inches of water, enough
to increase the time an ordinary garden can last between heavy
irrigations by a week or 10 days.
If the soil in question were a silty clay, it would naturally make 2
1/2 inches available per foot. A massive humus amendment would
increase that to 3 1/2 inches in the top foot or two, relatively not
as much benefit as in sandy soil. And I seriously doubt that many
gardeners would be willing to thoroughly double dig to an honest 24
inches.
Trying to maintain organic matter levels above 10 percent is an
almost self-defeating process. The higher the humus level gets, the
more rapidly organic matter tends to decay. Finding or making enough
well-finished compost to cover the garden several inches deep (what
it takes to lift humus levels to 10 percent) is enough of a job.
Double digging just as much more into the second foot is even more
effort. But having to repeat that chore every year or two becomes
downright discouraging. No, either your soil naturally holds enough
moisture to permit dry gardening, or it doesn't.
Keeping the Subsoil Open with Green Manuring
When roots decay, fresh organic matter and large, long-lasting
passageways can be left deep in the soil, allowing easier air
movement and facilitating entry of other roots. But no cover crop
that I am aware of will effectively penetrate firm plowpan or other
resistant physical obstacles. Such a barrier forces all plants to
root almost exclusively in the topsoil. However, once the subsoil
has been mechanically fractured the first time, and if recompaction
is avoided by shunning heavy tractors and other machinery, green
manure crops can maintain the openness of the subsoil.
To accomplish this, correct green manure species selection is
essential. Lawn grasses tend to be shallow rooting, while most
regionally adapted pasture grasses can reach down about 3 feet at
best. However, orchard grass (called coltsfoot in English farming
books) will grow down 4 or more feet while leaving a massive amount
of decaying organic matter in the subsoil after the sod is tilled
in. Sweet clover, a biennial legume that sprouts one spring then
winters over to bloom the next summer, may go down 8 feet. Red
clover, a perennial species, may thickly invade the top 5 feet.
Other useful subsoil busters include densely sown Umbelliferae such
as carrots, parsley, and parsnip. The chicory family also makes very
large and penetrating taproots.
Though seed for wild chicory is hard to obtain, cheap varieties of
endive (a semicivilized relative) are easily available. And several
pounds of your own excellent parsley or parsnip seed can be easily
produced by letting about 10 row feet of overwintering roots form
seed. Orchard grass and red clover can be had quite inexpensively at
many farm supply stores. Sweet clover is not currently grown by our
region's farmers and so can only be found by mail from Johnny's
Selected Seeds (see Chapter 5 for their address). Poppy seed used
for cooking will often sprout. Sown densely in October, it forms a
thick carpet of frilly spring greens underlaid with countless
massive taproots that decompose very rapidly if the plants are
tilled in in April before flower stalks begin to appear. Beware if
using poppies as a green manure crop: be sure to till them in early
to avoid trouble with the DEA or other authorities.
For country gardeners, the best rotations include several years of
perennial grass-legume-herb mixtures to maintain the openness of the
subsoil followed by a few years of vegetables and then back (see
Newman Turner's book in more reading). I plan my own garden this
way. In October, after a few inches of rain has softened the earth,
I spread 50 pounds of agricultural lime per 1,000 square feet and
break the thick pasture sod covering next year's garden plot by
shallow rotary tilling. Early the next spring I broadcast a
concoction I call "complete organic fertilizer" (see _Growing
Vegetables West of the Cascades_ or the _Territorial Seed Company
Catalog_), till again after the soil dries down a bit, and then use
a spading fork to open the subsoil before making a seedbed. The
first time around, I had to break the century-old plowpan--forking
compacted earth a foot deep is a lot of work. In subsequent
rotations it is much much easier.
For a couple of years, vegetables will grow vigorously on this new
ground supported only with a complete organic fertilizer. But
vegetable gardening makes humus levels decline rapidly. So every few
years I start a new garden on another plot and replant the old
garden to green manures. I never remove vegetation during the long
rebuilding under green manures, but merely mow it once or twice a
year and allow the organic matter content of the soil to redevelop.
If there ever were a place where chemical fertilizers might be
appropriate around a garden, it would be to affordably enhance the
growth of biomass during green manuring.
Were I a serious city vegetable gardener, I'd consider growing
vegetables in the front yard for a few years and then switching to
the back yard. Having lots of space, as I do now, I keep three or
four garden plots available, one in vegetables and the others
restoring their organic matter content under grass.
Mulching
Gardening under a permanent thick mulch of crude organic matter is
recommended by Ruth Stout (see the listing for her book in More
Reading) and her disciples as a surefire way to drought-proof
gardens while eliminating virtually any need for tillage, weeding,
and fertilizing. I have attempted the method in both Southern
California and western Oregon--with disastrous results in both
locations. What follows in this section is addressed to gardeners
who have already read glowing reports about mulching.
Permanent mulching with vegetation actually does not reduce
summertime moisture loss any better than mulching with dry soil,
sometimes called "dust mulching." True, while the surface layer
stays moist, water will steadily be wicked up by capillarity and be
evaporated from the soil's surface. If frequent light sprinkling
keeps the surface perpetually moist, subsoil moisture loss can occur
all summer, so unmulched soil could eventually become desiccated
many feet deep. However, capillary movement only happens when soil
is damp. Once even a thin layer of soil has become quite dry it
almost completely prevents any further movement. West of the
Cascades, this happens all by itself in late spring. One hot, sunny
day follows another, and soon the earth's surface seems parched.
Unfortunately, by the time a dusty layer forms, quite a bit of soil
water may have risen from the depths and been lost. The gardener can
significantly reduce spring moisture loss by frequently hoeing weeds
until the top inch or two of earth is dry and powdery. This effort
will probably be necessary in any case, because weeds will germinate
prolifically until the surface layer is sufficiently desiccated. On
the off chance it should rain hard during summer, it is very wise to
again hoe a few times to rapidly restore the dust mulch. If hand
cultivation seems very hard work, I suggest you learn to sharpen
your hoe.
A mulch of dry hay, grass clippings, leaves, and the like will also
retard rapid surface evaporation. Gardeners think mulching prevents
moisture loss better than bare earth because under mulch the soil
stays damp right to the surface. However, dig down 4 to 6 inches
under a dust mulch and the earth is just as damp as under hay. And,
soil moisture studies have proved that overall moisture loss using
vegetation mulch slightly exceeds loss under a dust mulch.
West of the Cascades, the question of which method is superior is a
bit complex, with pros and cons on both sides. Without a long winter
freeze to set populations back, permanent thick mulch quickly breeds
so many slugs, earwhigs, and sowbugs that it cannot be maintained
for more than one year before vegetable gardening becomes very
difficult. Laying down a fairly thin mulch in June after the soil
has warmed up well, raking up what remains of the mulch early the
next spring, and composting it prevents destructive insect
population levels from developing while simultaneously reducing
surface compaction by winter rains and beneficially enhancing the
survival and multiplication of earthworms. But a thin mulch also
enhances the summer germination of weed seeds without being thick
enough to suppress their emergence. And any mulch, even a thin one,
makes hoeing virtually impossible, while hand weeding through mulch
is tedious.
Mulch has some unqualified pluses in hotter climates. Most of the
organic matter in soil and consequently most of the available
nitrogen is found in the surface few inches. Levels of other mineral
nutrients are usually two or three times as high in the topsoil as
well. However, if the surface few inches of soil becomes completely
desiccated, no root activity will occur there and the plants are
forced to feed deeper, in soil far less fertile. Keeping the topsoil
damp does greatly improve the growth of some shallow-feeding species
such as lettuce and radishes. But with our climate's cool nights,
most vegetables need the soil as warm as possible, and the cooling
effect of mulch can be as much a hindrance as a help. I've tried
mulching quite a few species while dry gardening and found little or
no improvement in plant growth with most of them. Probably, the
enhancement of nutrition compensates for the harm from lowering soil
temperature. Fertigation is better all around.
Windbreaks
Plants transpire more moisture when the sun shines, when
temperatures are high, and when the wind blows; it is just like
drying laundry. Windbreaks also help the garden grow in winter by
increasing temperature. Many other garden books discuss windbreaks,
and I conclude that I have a better use for the small amount of
words my publisher allows me than to repeat this data; Binda
Colebrook's [i]Winter Gardening in the Maritime Northwest[i]
(Sasquatch Books, 1989) is especially good on this topic.
Fertilizing, Fertigating and Foliar Spraying
In our heavily leached region almost no soil is naturally rich,
while fertilizers, manures, and potent composts mainly improve the
topsoil. But the water-wise gardener must get nutrition down deep,
where the soil stays damp through the summer.
If plants with enough remaining elbow room stop growing in summer
and begin to appear gnarly, it is just as likely due to lack of
nutrition as lack of water. Several things can be done to limit or
prevent midsummer stunting. First, before sowing or transplanting
large species like tomato, squash or big brassicas, dig out a small
pit about 12 inches deep and below that blend in a handful or two of
organic fertilizer. Then fill the hole back in. This double-digging
process places concentrated fertility mixed 18 to 24 inches below
the seeds or seedlings.
Foliar feeding is another water-wise technique that keeps plants
growing through the summer. Soluble nutrients sprayed on plant
leaves are rapidly taken into the vascular system. Unfortunately,
dilute nutrient solutions that won't burn leaves only provoke a
strong growth response for 3 to 5 days. Optimally, foliar nutrition
must be applied weekly or even more frequently. To efficiently spray
a garden larger than a few hundred square feet, I suggest buying an
industrial-grade, 3-gallon backpack sprayer with a side-handle pump.
Approximate cost as of this writing was $80. The store that sells it
(probably a farm supply store) will also support you with a complete
assortment of inexpensive nozzles that can vary the rate of emission
and the spray pattern. High-quality equipment like this outlasts
many, many cheaper and smaller sprayers designed for the consumer
market, and replacement parts are also available. Keep in mind that
consumer merchandise is designed to be consumed; stuff made for
farming is built to last.
Increasing Soil Fertility Saves Water
Does crop growth equal water use? Most people would say this
statement seems likely to be true.
Actually, faster-growing crops use much less soil moisture than
slower-growing ones. As early as 1882 it was determined that less
water is required to produce a pound of plant material when soil is
fertilized than when it is not fertilized. One experiment required
1,100 pounds of water to grow 1 pound of dry matter on infertile
soil, but only 575 pounds of water to produce a pound of dry matter
on rich land. Perhaps the single most important thing a water-wise
gardener can do is to increase the fertility of the soil, especially
the subsoil.
_Poor plant nutrition increases the water cost of every pound of dry
matter produced._
Using foliar fertilizers requires a little caution and forethought.
Spinach, beet, and chard leaves seem particularly sensitive to
foliars (and even to organic insecticides) and may be damaged by
even half-strength applications. And the cabbage family coats its
leaf surfaces with a waxy, moisture-retentive sealant that makes
sprays bead up and run off rather than stick and be absorbed. Mixing
foliar feed solutions with a little spreader/sticker, Safer's Soap,
or, if bugs are also a problem, with a liquid organic insecticide
like Red Arrow (a pyrethrum-rotenone mix), eliminates surface
tension and allows the fertilizer to have an effect on brassicas.
Sadly, in terms of nutrient balance, the poorest foliar sprays are
organic. That's because it is nearly impossible to get significant
quantities of phosphorus or calcium into solution using any
combination of fish emulsion and seaweed or liquid kelp. The most
useful possible organic foliar is 1/2 to 1 tablespoon each of fish
emulsion and liquid seaweed concentrate per gallon of water.
Foliar spraying and fertigation are two occasions when I am
comfortable supplementing my organic fertilizers with water-soluble
chemical fertilizers. The best and most expensive brand is
Rapid-Gro. Less costly concoctions such as Peters 20-20-20 or the
other "Grows," don't provide as complete trace mineral support or
use as many sources of nutrition. One thing fertilizer makers find
expensive to accomplish is concocting a mixture of soluble nutrients
that also contains calcium, a vital plant food. If you dissolve
calcium nitrate into a solution containing other soluble plant
nutrients, many of them will precipitate out because few calcium
compounds are soluble. Even Rapid-Gro doesn't attempt to supply
calcium. Recently I've discovered better-quality hydroponic nutrient
solutions that do use chemicals that provide soluble calcium. These
also make excellent foliar sprays. Brands of hydroponic nutrient
solutions seem to appear and vanish rapidly. I've had great luck
with Dyna-Gro 7-9-5. All these chemicals are mixed at about 1
tablespoon per gallon.
Vegetables That:
Like foliars
Asparagus Carrots Melons Squash
Beans Cauliflower Peas Tomatoes
Broccoli Brussels sprouts Cucumbers
Cabbage Eggplant Radishes
Kale Rutabagas Potatoes
Don't like foliars
Beets Leeks Onions Spinach
Chard Lettuce Peppers
Like fertigation
Brussels sprouts Kale Savoy cabbage
Cucumbers Melons Squash
Eggplant Peppers Tomatoes
Fertigation every two to four weeks is the best technique for
maximizing yield while minimizing water use. I usually make my first
fertigation late in June and continue periodically through early
September. I use six or seven plastic 5-gallon "drip system"
buckets, (see below) set one by each plant, and fill them all with a
hose each time I work in the garden. Doing 12 or 14 plants each time
I'm in the garden, it takes no special effort to rotate through them
all more or less every three weeks.
To make a drip bucket, drill a 3/16-inch hole through the side of a
4-to-6-gallon plastic bucket about 1/4-inch up from the bottom, or
in the bottom at the edge. The empty bucket is placed so that the
fertilized water drains out close to the stem of a plant. It is then
filled with liquid fertilizer solution. It takes 5 to 10 minutes for
5 gallons to pass through a small opening, and because of the slow
flow rate, water penetrates deeply into the subsoil without wetting
much of the surface. Each fertigation makes the plant grow very
rapidly for two to three weeks, more I suspect as a result of
improved nutrition than from added moisture. Exactly how and when to
fertigate each species is explained in Chapter 5.
Organic gardeners may fertigate with combinations of fish emulsion
and seaweed at the same dilution used for foliar spraying, or with
compost/manure tea. Determining the correct strength to make compost
tea is a matter of trial and error. I usually rely on weak Rapid-Gro
mixed at half the recommended dilution. The strength of the
fertilizer you need depends on how much and deeply you placed
nutrition in the subsoil.