Greenhouse Planting Guide: Soil, Sowing, Crop Sequence

Planting under cover is settled in the soil before a seed goes in. The crop is never rained on, so salt arriving with fertilizer and irrigation water, and the residues a previous crop leaves behind, stay where they were put. The work runs in a fixed order: test the ground, correct it, prepare and ridge the beds, sow or transplant, hold the water and nutrient rhythm, then hand the house on to the next crop.

Electrical conductivity comes first, because it says how much salt the house is already carrying. A covered soil has no rain to leach it, so conductivity creeps up season by season. Take several cores across the house, sample topsoil and working layer separately and mix each batch, and sample after the old crop has been cleared.

The same report gives pH, organic matter, available nitrogen, phosphorus and potassium, and notes any disease or nematode history, which limits what may be planted next as much as any number does. The test then decides the amendment: sour ground is corrected towards the range the crop prefers, ground short of organic matter takes compost or well rotted manure, and ground carrying too much salt is leached before anything goes in. Which material, and how much, follows the report and your climate.

Prepare the ground when it is moist but workable; soil worked wet smears and compacts, and a compacted layer under the bed cannot be put right later. Loosen to the depth the crop roots need, break the clods down and level the surface, so water spreads along the bed instead of running to one end.

Ridges run along the length of the house, not across it: they drain with the fall of the site and keep row spacing even with the glazing above, so light reaches every row and air moves between them. Height and width belong to the crop and to the way the house is worked, tall narrow ridges warming and draining better on heavy ground, flatter beds holding water on light ground. Passages between beds are sized for the trolley that uses them and double as the path surplus water takes. Drip tape goes down before the mulch film, in the same pass, because lifting a film afterwards disturbs roots.

Raising the seedlings in trays of clean medium under the same roof takes pressure off the beds. The tray sets the pitch of the whole operation, so it is chosen before the sowing date: seeding trays hold the medium, keep roots separated and let the cell block be moved in one piece.

Seedlings are hardened off by lowering the temperature and opening the ventilation over the last few days. At transplanting, set the block so that its surface sits level with the bed: buried stems invite rot, and a block left standing proud dries out. Spacing follows the spread the crop will reach, the training method, and the light and air the house can deliver. Water in so block and soil are joined, and hold a shade net over new transplants for a few days where the sun is strong.

Under cover, irrigation is the only water the crop receives and the only means of washing salt out of the root zone, so little and often is the rule that follows: a bed kept evenly moist roots more deeply than one flooded and then left to dry. Young plants are held a little drier to push roots out, flowering is kept level so that flowers do not drop, and the heaviest demand comes while the fruit is swelling. Carrying the fertilizer in the water puts it where the roots are, which is why a drip irrigation system is the normal choice on a commercial bed.

Conductivity and pH in the root zone are worth reading through the season, since both drift as the crop feeds and the weather changes. Cold weather slows transpiration and the beds hold water far longer than the calendar suggests; hot weather shortens the interval. Air humidity and soil moisture are two different things, because a heavy irrigation with the vents shut leaves the leaves wet for hours. The volumes and concentrations that suit your soil and your climate are what a general guide cannot fix for you.

A change of crop is prepared during the last weeks of the old one, not after it. Clear the plants and as much of the root mass as can be lifted, disinfect or rest the house over the gap, work in the organic matter the soil test called for and prepare the beds again. That gap has to cover clearing, any treatment and the preparation itself, so it is planned into the sowing date rather than fitted around it.

What the previous crop leaves behind is not only a disease question. Work carried out on this subject found that planting corn, tomato, cabbage or celery ahead of a cucumber crop lowered both the salt content and the electrical conductivity of the soil. Corn was strongest of the four, with salinity down 49.03% and conductivity down 60%; cabbage and celery stubble gave 27.80% and 28.48%, then 23.55% and 24.43%; tomato stubble gave 16.99% and 16.33%.

A second set of trials took tomato as the following crop and found corn, spinach, celery and garlic all reduced salinity, conductivity and nitrate nitrogen: corn stubble by 49%, 70.3% and 20.98%, spinach stubble by 38.31%, 54.76% and 10.09%. Corn stubble also gave the fastest early growth, tallest plants and lightest disease pressure; tomato stubble gave short internodes, 57.5% fruit setting, 16.4 fruits and 2.35 kg per plant against 20.1 fruits, 3.35 kg and 80% setting after corn. The same trials recorded quality and soil biology: vitamin C ranged from 2.43 to 2.72 mg per 100 g by preceding crop, soluble solids ranked cabbage, celery, corn and tomato with nitrate and nitrite reversed, and microbial counts shifted under every preceding crop except tomato.

Salt is one reason to choose a preceding crop, but not the only one: crop length, market window and soil-borne disease count as heavily. Our factory has built greenhouses for above 35 years; send the internal span and length, the crops planned and the way the house will be worked, and we will advise on structure and irrigation layout. Call +86 191 5068 3942 or write to sales@cngreenhouses.com. This article sits in the planting technology section; the water end of the routine belongs with the greenhouse irrigation system range.

Independent references help when a routine is written for a site: the NGMA represents the greenhouse manufacturing and supply side, and HortWeek carries current commercial horticulture news.

Q: What should be tested in the soil before a greenhouse crop goes in?
Conductivity first, then pH, organic matter and the available nutrients, sampled from the topsoil and the working layer separately once the old crop is cleared.

Q: How can salt and conductivity be brought down under cover?
By leaching, because no rain does it for you: irrigate until the excess salt is carried below the root zone and keep the outlet draining. The preceding crop does part of the work as well.

Q: Are ridges worth the extra work compared with a flat bed?
On heavy ground or an early crop, yes, because a ridge warms and drains better; on light ground watered often, a flatter bed holds water longer.

Q: How deep should transplants be set?
So that the surface of the root block ends up level with the surface of the bed. Deeper buries the stem and invites rot; prouder dries the block out.

Q: How often should a greenhouse crop be watered and fed?
Little and often, with the concentration carried in the water and read back from the conductivity of the root zone. Expect a shorter interval as the fruit swells.

Q: Can the same crop be planted in the same house season after season?
That is the pattern that builds a soil problem, since the same roots, the same pathogens and the same nutrient uptake repeat. Rotating with a different family breaks the cycle and can pull salt back down between crops.