Feeding and watering olives

Olives will grow and fruit in a wide range of soils, even under arid conditions. Tree spacing is less in modern groves than it was historically, when trees were frequently planted around the Mediterranean at 12 x 12 metres spacing. Ignoring super high-density groves where the trees are close spaced for “over the top” harvesting with machines, it is currently usual to plant at distances between 5 x 5 to 7 x 7 metres.

Temperature constraints are that damage will occur below minus 6ºC.   Severe damage, and perhaps trees killed, occurs very little below that, but, like many species of fruit and nuts, olives have a chilling requirement  – i.e so many hours of temperatures below 7ºC,  (45ºF for those who use Fahrenheit) before they will flower the following spring. How many hours each cultivar needs is variable, and it seems extremely little research has been done. No doubt cultivars evolved to suit the environment in which they are found naturally, and a failure to reach the chilling requirement of any cultivar means that flowers will not bloom. A mean daily temperature below 12ºC for several weeks seems sufficient to meet the chilling requirement of all cultivars. How many weeks is open to debate.

As with all plants, better quality soil, adequate nutrients and adequate water lead to much more production. Soil is usually a “given” for most people, and, within the temperature ranges required, almost any soil can be made to grow olives. Millions of trees have been planted in individually terraced planting spaces on very steep hillsides. If looking to buy a new property specifically for planting olives, then somewhere with a good depth of soil, say at least a metre and not solid clay, and plenty of available water for irrigation are the main requirements.

With limited knowledge of olives, and a desire to begin a non-animal enterprise when we first bought our present property, I was led to plant a grove at 6 x 6 metres spacing. I am quite happy with that and have planted more since at the same spacing. This works out at 277 trees/ha of planted area. Over the last 10 years I have read an enormous number of articles, research papers and advice from a huge number of people and organisations involved in olive production in probably every country in the world where olives are grown, as well as spoken with several knowledgeable people. I have made decisions, based on all this information, that I use to provide food and water for my trees. Whilst at least one expert agrees with each individual decision - because I have chosen to follow at least one expert’s advice in everything I have decided, some will disagree with my overall management, but then they disagree with each other, ranging from extremely little fertiliser to extremely high levels, and from allowing quite severe moisture deficiencies (at times) to maintaining full moisture profile of the soil.

Note that being in the northern hemisphere the calendar year begins a few days after mid-winter, so southern hemisphere information that refers to dates is in the reverse production cycle position. Based on soil analyses, fertilisers used and crops grown in the groves since the previous analysis, I apply granular fertiliser as soon as practicable in the year. I top up phosphorus(P) and potassium(K) if necessary. I apply Nitrogen(N) according to a personal assessment of how much to apply. I may have grown a winter leguminous crop for harvesting or for incorporation as a green manure. Alternatively, a crop of maize may have been grown the previous summer and the stover left on the surface as an overwintering mulch; or a summer leguminous crop may have been sown and either harvested or ploughed in. I am also conscious of the requirement for adequate trace elements without fertilising to excess.  I have used foliar feeds, sometimes called foliar fertilisers, this year and propose to make them an integral part of my fertiliser programme. I will use high N early in the year and low N as the fruit matures. I may apply a second dose of N granular fertiliser in the spring too – again depending on a personal assessment of need. I prefer to split fertiliser into smaller doses rather than apply a full season’s requirements at once, but this is not always practicable. I will aim to have a pH of 7 or slightly above. Somewhere in the range of 6.5 to 8 seems to be preferred, but as I said, the experts do not always agree on the ideals

The groves are irrigated, either through drip irrigation or overhead sprays. The drip system is a single line with four litres/hour drippers at one metre spacing and laid on the ground. The different irrigation methods determine what I can and cannot grow in the summer. With overhead I can grow crops between the rows. With drippers I am limited in summer, but not winter. I still intend to grow comfrey on these lines (to be cut and used as a mulch for the trees) but this might be delayed for another couple of years because the weed seed bank in the soil is still too high.

There is nothing I can do about the texture of the soil I have except to increase the Organic Matter(OM) content. Higher OM means a soil that is more capable of retaining both nutrients and water. Many semi-arid and arid soils have an OM content of 1% or even less but, from anecdotal information only, it seems that the highest yields come from trees grown on land with OM of several percentage points. I will also avoid bare soil as much as possible, endeavouring to maintain some ground cover to stop the possibility of water erosion in winter despite the fact that I have not experienced this happening, and to ameliorate high soil temperatures in summer.  This means cultivating as infrequently as possible – anathema to some growers who insist on permanent bare land cultivated regularly throughout the year.

The sole reason behind my decisions is the way in which the olive tree grows and produces its fruit. The following three paragraphs are a combination of information in a University of Cordoba, Spain publication and a paper by Razouk et al, with the long title of “Optimal time of supplemental irrigation during fruit development of rainfed olive trees in Morocco” with some additional comments based on other information I have sourced.

Yield of olives is the result of three main developmental processes that occur from flowering to harvest:  fruit set, fruit growth and oil accumulation in the fruit pulp. Vegetative growth is critical in terms of olive fruit production, because flowers are borne in inflorescences at the axil of leaves of one-year old wood. This means that flowering and fruit set originate in the axillary buds of last year’s growth. So, the big problem is ensuring that this year’s fruit crop has sufficient nutrients to yield a heavy crop of large (for the variety) olives, but at the same time producing sufficient growth of new wood for next year’s crop. In less than optimum conditions one or the other, or both, will suffer. This is one of the reasons why there is so much biennial bearing in olives. New growth is restricted by the demands of a heavy crop on the tree, but flourishes when the previous year’s poor growth carries a small or nil crop, and the tree produces another good crop in the following year. A vicious circle.

The reproductive cycle from flower bud initiation to fruit ripening takes 15-18 months depending on cultivar and growing conditions, as initiation starts in one summer and fruits from this initiation ripen in the autumn or winter of the following year. The terminal bud of a shoot is almost always vegetative and shoot growth starts with this bud break in spring, when temperatures rise above 12ºC. Growth continues as long as temperatures are below 35ºC, and there are no soil water deficit or other environmental stresses. Growth begins pre-flowering, but natural soil water supplies are almost always sufficient to avoid the need for irrigation at this time. Critical stages for the tree are flowering in April/May; a time of rapid growth about mid June; pit hardening about the end of July or early August, and a second rapid growth about mid September. The first rapid growth stage occurs just as most Mediterranean soils are becoming water deficient for the summer. Pit hardening and the second rapid growth stage are well and truly in the dry summer and early autumn before the winter rains begin. Some fortunate areas receive some summer rain. Pit hardening is about the time of flower initiation for next year’s crop too, so a lot of stress on the tree at this stage. The growth stages and fruit development were the factors considered by Razouk and his associates.

So, back to why I do what I do. I rely on soil analyses in preference to leaf analyses. The reason for this is simple – the chemical and nutritional composition of any particular leaf changes throughout the year, and younger leaves have a different composition to older leaves. Because of these changes I believe that leaf analysis is of less value than soil analysis for long term planning. We could obviously become super technical and do all sorts of tests, but that is for research scientists, not your average grower. Some sources of information, particularly those selling foliar feeds, insist on using leaf analysis (taken in July each year) as the decision maker in fertiliser requirements. Following the work of some researchers, maintaining the soil at adequate levels of all plant nutrients is recommended – bearing in mind that an excess of some can prevent an uptake of others, even although they are available. I take an approach that if levels of the macro nutrients are considered adequate by most authorities I am probably offering my plants the best that can be reasonably done by any farmer.

I agree with those who say that applying foliar feeds shows an almost immediate response, whereas granular (or liquid) fertiliser applied to the soil may not. At the same time it is not a lasting effect – similar to the application of prilled lime, which is a very finely ground limestone that is formed into prills the same as granular fertilisers and break down rapidly after application to the land. This gives a quick fix to a low pH but does not have the ongoing pH correcting ability of coarser ground limestone. The same applies to the foliar feeds, they raise nutrient levels within the aerial part of the tree for a short period of time. I do, though, go along with the school of thought that foliar feeds assist the plants to assimilate nutrients from the soil. P in particular can be a problem for plants to take up because it is fairly immobile in the soil, and it appears that applying foliar N can assist with that problem, although it also seems sensible to have some P in the feed. Foliar feeds do not replace the soil nutrients, they complement the soil supply. You cannot feed a tree just through its leaves.

I apply N fertilisers to the soil so that the tree has the nutrients to produce fresh young growth in early spring, and will apply a high N foliar feed, containing P, K and trace elements, as a booster pre-flowering, possibly a second high N foliar feed too. Some authorities believe that Boron(B) should be applied pre-flowering. Both compound fertilisers and foliar feeds that I use contain some B and I am not inclined to add more as a special feed unless I apply only N to the soil when I might mix some additional B with the foliar feed. B is toxic to most living things at fairly low levels – it is used by people who do not like “poisons” in order to kill ants. It certainly is a poison (as are most things) above certain levels. If I think the pH level needs raised, I will include lime when filling the fertiliser spreader with the granular N or compound fertiliser. Lime spreads easier when mixed with granular fertiliser.

Provided the trees are growing well there will be no further fertilisers applied specifically for the olives until late summer, but any crops grown between the rows of trees will be fertilised independently of the needs of the olives, and some of this is available to the trees. After pit hardening I will apply at least one and probably two foliar feeds with lower N and higher levels of P and K, plus trace minerals. It is convenient to apply these foliar feeds with sprays against olive fruit fly – a necessity where I live. There are so many untended groves that all olive pests and diseases are able to proliferate and a failure to treat against them results in enormous crop losses, sometimes total failure. It is also necessary to spray against a fungus that can cause total crop loss too and this is done nearer harvest. Again it is convenient to add a foliar feed with these sprays if desired.

I use the overhead spray equipment for irrigation primarily because I have it and it suits me to use it in a grove which I can summer crop. The drippers were installed to meet the needs of a new grove, but using existing pumps and underground main lines. I have the dripper lines on the surface because I prefer them where I can see them and I make an inspection as soon as I begin an irrigation. That way I can spot any problems along the lines and fix them immediately without switching off the pumps. An underground line that suffers damage will not show up until the water begins to puddle on the surface, which undoubtedly means some trees have missed out on their water, and it is necessary to dig up the line to mend it. There is also the problem of where to bury the line. In the early years the trees occupy only a small area of the soil so the drippers need to be relatively close. As they grow, the lines are moved back from the trees and the trees make use of additional drippers as they spread their root zone.

Not all the water I currently pump is utilised by the trees, but it will be in time, and until then I make use of drippers outside the root zone by sowing pumpkins and squashes as  feed for the goats, as well as some household use and also tomatoes, cucumbers, melons, etc. immediately adjacent to a dripper. In previous years I have also sown the drought hardy black eyed peas between the rows and they have benefitted from the water from the drippers, particularly those peas close to the drip line. The site is a very slight slope so the water naturally travels downhill and the drip line is uphill of the row of trees it supplies. Both types of irrigation are left in their permanent positions. Hand harvesting means there is no need to move the lines off the fields.

Irrigation commences around mid-June every year for the whole property, which makes it convenient for the first irrigation requirement of the olives – unless there has been a particularly dry spring when an earlier start to the irrigation season may be required. There is always plenty of water available in the early summer, and this first irrigation has been found to be the most beneficial from an economic perspective. It assists both the current crop and the ability of the tree to produce growth for next year. It is more valuable than the water needed at pit hardening and the second rapid growth stage. This is a most fortuitous situation, because those relying on natural water supplies, as I do, may run out of water if the winter and spring rains were low or ceased early.

My strategy then is to maintain a good soil moisture level as long as possible, and of course other summer crops grown in the groves need this water. My theory behind this is that if I run out of irrigation water, then the more there is in the soil the longer it will be before moisture stress affects the crop. I know from my 2012 maize crop that it was successful without irrigation from silking onwards. The olives did not receive their pit hardening and mid-September water that year. Provided I have the water I will continue to irrigate past the second flush of growth, but in 2013 the last irrigation was just after pit hardening. Reducing irrigation earlier in the season does not prolong my season, being dependent upon a river continuing to flow.

For those with water storage, Razouk showed that a large quantity applied at the first rapid growth stage (he used 500 litres per tree, on mature trees) can be followed by the next at pit hardening, and preferably a third at mid-September. If there is storage for only one irrigation, or natural supplies only into the early summer, then apply water in June. Whether water is available or not I will cease irrigation at the end of September. This allows time for the fruit to mature without containing too much water - which would result in a lower oil content, giving a poor yield of oil for the fruit weight. This is extremely important for anyone paying a contractor crusher to produce their oil and charging, as apparently many do, on the weight of fresh fruit rather than yield of oil.