One of the key tools we can use in season
is plant tissue analysis. Because not a lot of nutrient uptake has occurred before squaring
and there’s not a lot of pressure on the plant, we usually wait until about the start of the
reproductive stage, unless there’s unusual symptoms there that may be related to something
like a trace element deficiency. The critical period is to get an understanding
of what’s happening between squaring and flowering and the first boll being set, because once
those bolls are set, the nutrient demand goes exponential and it’s just hard to get some
nutrients into the plant. You’ve got two options with plant tissue testing
– to take the leaf or the petiole. The leaf is best for your major nutrients in total,
and certainly for trace elements. We can use the petioles by themselves for N and K.
The technique that we tend to use is to make sure that the crop is not stressed in any
way – obviously at that time of year it’s usually water stress, so we pick a time between
when it’s out of any waterlogging stress and before it starts to stress at the other end.
The other stresses that can be important is cloud cover. We want the plant to be metabolising
as quickly as possible, so if we’ve got overhead cloud or we’ve had a period of cloud then
sometimes it’s better to wait a day or two. Early on cold stress days you would avoid,
and again leave it a couple of days afterwards for the plant to be actually cracking on and
growing as it should be. Anything that affects normal plant growth
that you could expect at that time of year is going to either cause a dilution or an
accumulation of nutrient. We want to know what it’s like growing normally with the supply
that it’s got, and is that supply going to be adequate?
If it’s just a general look at the health of the crop, I’d generally direct people to
where the moisture monitoring equipment is located, because then we can start to relate
the data coming out of the plant samples with what’s happening with moisture at a local
place. Where there are unusual patches in the crop
and you think they’re nutrient-related, take some good and bad, because then we can use
those to calibrate what’s going on. Being consistent in the leaf that you take
from the plant is important – we want things of similar maturity, both in the plants that
we sample across the field and if we come back to sample later. Leaves with similar
maturity have similar nutrient contents, so if there’s a change in the soil supply it
will reflect in the leaf. The way to select the right one – looking
at the youngest mature leaf, we don’t want that one, it’s not maximum size, it hasn’t
reached its full potential and nutrient requirement. We’re looking for the one that is maximum
size – it’s generally the 3rd or 4th depending on the time of year that you take them, so
we’ve got one here that’s very small, we’ve got a small one here, one here, and the next
is this one here. The technique is to remove the leaf and petiole – if you want to send
the both away separately for different readings you can, but the leaf sample MUST have the
petiole removed. One mistake that people make is that they
don’t do that in the field, they pick all the leaves and take them to the lab and do
it – it continues conductance of material out of the petiole into the leaf and you end
up with a false reading. Laboratories require to run a full scan of
nutrients about 20g of dry material. So if we know the leaf’s about 90% water, you’re
looking at a minimum of about 200g of fresh material.
The reason why I don’t say how many leaves is because obviously early in the season they’re
small; later in the season it takes fewer. Try to get a representative sample of the
leaves from the area and sufficient quantity to satisfy the laboratory’s requirement. For
petioles, it’s generally around the 30-40 petioles. Like anything, if it’s very variable,
the more the better. Maintain the sample at or below the temperature
it left the field, so taking it and throwing it on the dashboard of the car and the temperature
gets to 50 or 60 degrees… Higher temperatures tend to cause things to bleed, so we end up
with some of the nutrient content coming out, and we don’t want it on the bag, we want it
in the plant. If it’s not too hot, then the lab’s going to dry it out anyway, so drying
out’s not a problem. It’s just about getting it there quickly because generally when we
take plant tissue samples, we want the results for an action in-crop, which needs to be as
quick as possible.